Frequently Asked Questions - Climate Change - Questions and answers

Climate change is a far more serious and widespread problem than any previous threats to mankind.

Many previous environmental and heath problems have indeed turned out not to follow the worst-case scenario outlined by some experts e.g. the pesticide DDT, BSE (bovine spongiform encephalitis), Salmonella in eggs and the millennium computer bug. Nearly all these scares have prompted responses, such as national legislation, to reduce their risk. However, there have been few controlled experiments where we could compare how bad some of these problems might have been if there hadn’t been the appropriate government action in response to the scientific data and public concern.

Climate change as a problem is far more serious than any of these previous concerns because it is global, a lot of evidence suggests it is getting worse and it has potential impacts that would be worse than nearly all previous threats (perhaps with the exception of the total destruction that would follow from a full-scale nuclear war). Climate change might turn out not to be as serious as the vast majority of climate scientists are currently predicting, but we shouldn’t rely on this – the risks of doing so are too high. Just because some people are tempted to downplay the dangers and use special pleading, it makes little sense to deduce that the climate change problem isn’t important. It is worthwhile contemplating the motivation of those who promote such thinking, provide false comfort and potentially delay action at a moment when there is no time to lose.

Global warming is the most serious of all the problems facing mankind and therefore deserves the highest priority response now. Although the future is uncertain, global warming is practically certain to happen and we cannot afford to ignore it.

This question reveals a flawed view of the potential seriousness of the impact of climate change on human health and development, and on biodiversity. Climate change is already making the poorest poorer in developing countries and in the future is the greatest threat to economic development both here and abroad. Regional and international conflicts caused or made worse by climate change threaten the very existence of civilisation. Even the viability of the human race, at least in its current numbers, may be threatened by the end of the 21st century. In addition, the rapid poleward migration of the climatic and vegetation zones and drastic decreases in productivity of the oceans, and of many habitats on land, mean that climate change has been rated, above habitat destruction by man, pollution, over exploitation, and the effects of invasive species, as the greatest threat to biodiversity.

Even if there is some uncertainty about its exact scale and impact can we afford to take the risk of ignoring climate change?

True up to a point but in the long run the effects of drought and temperature rise are likely to overwhelm any short term increase in plant growth caused by increasing carbon dioxide concentrations.

Amazonia accounts for more than half the world’s rainforest and has been highly important in the past in removing an estimated 2 billion tonnes of carbon dioxide from the atmosphere each year. However recent droughts, particularly the harsh one of 2005, have killed many trees and drastically reduced the forest’s ability to take up carbon dioxide. Furthermore, drought has increased the rate at which carbon is lost from Amazonian soil to the atmosphere. These effects have combined to turn the Amazon forest from being a net remover (‘sink’) of carbon dioxide to a net producer, adding an estimated 5 billion tonnes of carbon dioxide to the atmosphere.

In addition, higher atmospheric carbon dioxide levels are making the oceans more acidic. Although acidification is seriously damaging corals, surprisingly, it does not appear to alter the ability of some planktonic plants to remove carbon dioxide from the atmosphere by photosynthesis and there appears to be no net change in how much carbon dioxide they remove.

Tropical rainforests and other mechanisms could provide positive or amplifying feedback mechanisms leading to progressively accelerating (runaway) climate change. Other possible positive feedbacks include the reduced ability of warmer sea water to dissolve and hence store carbon dioxide and the increased warming of the Arctic Ocean resulting from the decreased reflection of heat into space, as the ice melts, known as the albedo effect. Most seriously, melting of the permafrost and warming of the oceans could lead to accelerated release of huge quantities of methane, an even more powerful greenhouse gas than carbon dioxide, from sediments.

Global warming may provide some temporary benefit to the UK but eventually, rising temperatures will cause problems as will rising sea levels.

It is actually quite hard to use computer models to accurately predict the regional, let alone local, impact of global warming. However by running many different models and by using different assumptions about the future some consistent predictions have been obtained. Even though on average the Earth’s surface will warm, the amount of warming will vary regionally..There will be (probably temporary) winners as well as losers. Warmer weather may, to begin with, allow a longer growing season in temperate latitudes and reduce the need for heating buildings but eventually temperatures may rise to the point that they damage agriculture, allow the spread of ‘tropical’ diseases and cause deaths during heat waves. Elsewhere, reduced rainfall and increased temperatures will lead to the poleward expansion of deserts even into southern Europe. Rises in sea level will threaten low-lying coasts and islands at all latitudes.

Climate change happens slowly but steadily over tens of years; weather, whether hot or cold, wet or dry, is just natural variations about the gradually changing climate.

Global warming, and climate change, happen on much longer timescales than our weather. Weather is what happens today, tomorrow or next week. Climate change is defined by almost imperceptible changes or trends moving steadily in one direction that happen over years and ten of years or even longer. So, even in a warming climate we will still get individual weather systems which will bring ‘miserable' weather for a short while but this does not mean that the longer-term trends are absent. Although global mean temperatures have risen inexorably since 1850, including the last ten year period, changes in Pacific Ocean currents known as La Niña have produced periods of global cooling lasting for as much as three to four years.

There is indisputable evidence that the climate is changing and that global warming is really happening. The average global surface temperature has risen by 0.74°C in the past 100 years. Globally, eleven out of the twelve hottest years ever recorded have occurred since 1995. Here, in the UK, four out of five of the hottest years ever recorded over a 330-year period have occurred since the same date.

Rising sea level is one threat driven by global warming that is already affecting all coastal countries. Other impacts of global warming will also affect us, not least because the UK’s society and economy is increasingly inter-connected with those of other countries.

The 17th century philosopher John Donne wrote that ‘No man is an island’ meaning that we cannot ignore what is happening to other people because in the end what happens to them will affect us. The same is true of global warming. Just because some climate models suggest that temperature rises in the British Isles may be less than in some other parts of the world at the end of the 21st century this is no reason to suppose that we can ignore what is happening elsewhere. For example, we cannot ignore the effect of global warming on rising sea levels which will affect people living on low-lying coasts whether in Bangladesh, the Nile Delta, London or Portsmouth. Or, if desertification of North Africa and southern Europe causes large numbers of people to migrate northwards, and if indeed climate change in the British Isles is relatively benign, how are we to repel waves of these unfortunate people migrating in search of a better life? Also, we depend on other countries for many basic items for everyday living, including food. If those countries are affected, we will inevitably be affected by shortages and price rises. Global warming is a global problem that people everywhere must address.

If all Greenland’s ice melts, sea levels would rise by 7m (23 ft) which would have a major impact throughout the world. The melting caused by global warming is happening faster than expected.

It is estimated that the average temperature in Greenland needs to rise by only 3°C to melt the whole Greenland ice cap. Because, for various reasons, surface temperature is increasing twice as fast in the Arctic as at lower latitudes this is equivalent to a global average rise of about 1.5 °C. A global rise of 1.5°C by the end of the 21st century appears very likely even if manmade greenhouse gas emissions stopped tomorrow. Estimates of how long complete melting of the ice would take range from a few thousand years to a century or less depending on the dynamics of how the melt waters penetrate the ice and lubricate it at its base thereby speeding the flow of ice to the sea. In any event, if all Greenland’s ice melts it would raise sea levels by 7m (23 ft) and this would cause catastrophic coastal flooding world wide. Based on today’s population size and distribution, it would displace around half a billion people.

When ice sheets have been destabilised in the past, from smaller climate ‘forcings’ than humans are currently causing by global warming, sea level rises at rates of up to 3-5 meters per century have occurred. Today the rate of sea-level rise is less (only around 0.3 metres per century) but appears to be increasing. Scientists have recently shown that the loss of Greenland ice has accelerated over the last decade. They predict that this rate of loss could lead to sea level rises of 1 m or more by the end of the century, which is up to three times more than the average predicted by the International Panel on Climate Change (2007).

Whether green is an erroneous transcription of grunt ("ground"), which refers to shallow bays, is not known. The south-western portion of Greenland (not covered by glaciers today) is green in the summer and may have been even greener during warmer periods in the past when human habitation was more extensive.

Quite the contrary, action needs to be taken today if we are to avoid the unpleasant future consequences of global warming.

This argument is tempting but very wrong. Just because climate change is happening almost imperceptibly on a time scale that is much longer than the lifetime of elected governments it doesn’t mean that it can be ignored by ‘burying our heads in the sand’. We all know that if a large asteroid should appear to be on a collision course with the Earth it would require much less effort to deflect it from its path at an early stage than if we should wait until the last moment. Global warming is similar except that the ‘collision’ is going to happen whether we like it or not. Mankind’s choice today is how and by how much to mitigate the impact of that ‘collision’.

Manmade emissions of carbon dioxide are rising inexorably and have been accelerating. The root cause is a rising global population and our ‘addiction’ to fossil fuels and their use to fuel consumer economies and raise and maintain standards of living. To stabilise the Earth’s climate at a level which gives us a chance of avoiding disastrous runaway climate change requires a limit to be placed on the absolute amounts of greenhouse gas to be emitted in the coming century. We can either begin to cut back now or be faced with the daunting task of much higher, and possibly unachievable, cuts later.

The Stern Review (2006) stated ‘The benefits of strong, early action on climate change outweigh the costs’. Global warming needs to be tackled urgently and if we start now it is feasible to keep it in check.

It’s not too late but action needs to be taken today.

The levels of greenhouse gases, especially carbon dioxide, in the atmosphere continue to increase relentlessly, largely as a result of mankind burning fossil fuels. Climate scientists tell us that we have a window of opportunity in the next few years to begin to cut back greenhouse gas emissions substantially to avoid temperature increases of over 2°C, with respect to pre-industrial times. Average global temperature increases higher than 2°C are likely to be dangerous and may even trigger runaway changes in climate. If all governments, and particularly the governments of the major greenhouse gas emitters (China, USA, Russia, India and Japan and others), can be persuaded to take immediate and far-reaching action then mankind has a chance to avoid the worst effects of global warming. However a massive global response and a substantial investment of the world’s income each year will be required.

The UK government’s Stern Review (2006) stated ‘The benefits of strong, early action on climate change outweigh the costs’. Stern estimated that if we start to cut back on greenhouse gas emissions immediately, to prevent greenhouse gases in the atmosphere exceeding a concentration equivalent to 500 parts per million of carbon dioxide, the annual cost would be only about 1% of global Gross Domestic Product by 2050 (Stern revised this estimate to 2% in June 2008). Stern makes it clear that the cost, both economic and in terms of human suffering, of doing nothing are likely to be far in excess of these estimates. Doing nothing is likely to plunge the world into an economic crisis far deeper and longer than the current recession.

That’s right, population is a key part of the problem. However, the world population can’t be reduced rapidly enough to prevent global warming (although eventually population must decrease).

The greenhouse gas emissions causing global warming largely result from mankind’s use of fossil-fuel based energy. The amount of energy used per person typically depends on their ‘standard of living’. People in developed countries tend to use considerably more energy than those in developing countries. In fact a country’s Gross Domestic Product fairly closely correlates with the amount of energy it consumes. Therefore global emissions currently depend on both population and standard of living.

It is necessary to significantly reduce mankind’s emissions within the next few tens of years, and the sooner the better, if we are to avoid dangerous climate change towards the end of the 21st century. This means that we all must learn to live sustainably and cut back our greenhouse gas emissions drastically. In developed countries this can be done by changing behaviour and by adopting relevant technologies, without necessarily reducing our standard of living. In developing countries increasing standards of living, improvements in education (particularly of females) and increasing life expectancies will slowly help to reduce birth rates. This is a key part of the longer term solution.
 

The UK’s current emissions are only a small part of the global total, but developing countries look to developed countries to take the lead. The UK’s past emissions have helped to put us in the mess we are in today and much of China’s current emissions arise from producing goods for people like us to buy. The UK can also provide technology to help developing countries reduce their emissions.

It is true that today the UK emits only 2.2% of all the carbon dioxide currently produced by mankind however today’s global warming is actually being caused by the total amount of carbon dioxide that has been emitted in the last few hundred years. It has been estimated that the UK has emitted more carbon dioxide per person since 1750 that any other country because the global industrial revolution started in Britain. Even today the UK ranks tenth among all countries in emissions per person; on average a UK person emits eight times more carbon dioxide than someone in India and twice as much as someone in China. Thus it is clear that the UK has a moral responsibility to play its full part in reducing global emissions.

It should also be remembered that a significant fraction of the emissions in some developing countries are actually caused by the manufacture of cheap goods that we in the developed world like to import. So, for example, some of China’s emissions should also be attributed to the UK’s total. As consumers, we can demand lower carbon goods and services which will encourage businesses, wherever they are based, to reduce the environmental impact of their products.

There are opportunities for the developed world to assist developing countries in cutting back their emissions by introducing suitable technology. If developing countries can be helped to improve their standard of living and invest in education, particularly of females, this will help to reduce their higher birth rates, population growth, and demand for energy and other resources. Thus World Development and climate change must be tackled together.

By providing leadership, the UK and other developed countries potentially can have a major influence on reducing global emissions of greenhouse gases.
 

Many technologies already exist to help us but it is very unlikely that these, together with technologies yet to be developed, will be enough on their own. We also have to change ourselves, in our behaviour and expectations, to have an effect.

Whilst the maxim that ‘invention is the mother of necessity’ does have a good historical basis, we must ask (given how serious the threat from climate change is) whether it would be sensible to rely on technological silver bullets which don’t yet exist. So far, technology has helped us to extract oil as fast as we’ve consumed it and is providing new ways to find and extract it; likewise mankind has been able to innovate and increase food production to accommodate the world’s growing population. However, promising technologies can take longer to deliver than at first hoped, or worse, turn out not to be as useful as first thought. For example, nuclear fusion is still many years from providing a reliable source of energy, and possibly may never do so. But time is not on our side and the scientific consensus is that greenhouse gas emissions need to be reduced sharply within ten years. We therefore need to act now, rather than waiting to see if a technological silver bullet emerges.

We may need to remove carbon dioxide from the atmosphere by enhancing natural sinks and creating artificial ones as well as by reducing our emissions. However, technology already exists that could produce the massive global reductions in greenhouse gas emissions that are now required e.g. building insulation, low-energy vehicles and sources of renewable energy. The primary difficulties surround social issues of providing the political, institutional and individual will to implement these solutions.

A large part of the solution in developed countries is related to reductions in consumption – our society is currently driven by mass consumerism, advertising and the unsustainable expectations that it engenders. In turn, changing people’s behaviour doesn’t require any technology whatsoever.

Finally, we would be taking a great risk by relying solely on technology to provide last-ditch solutions to save ourselves from dangerous climate change. To risk all on geo-engineering solutions that interfere with the natural climate system would be very dangerous indeed. There is no technology in sight that promises to solve the problem. For example, tests which have tried to enhance the biological capture of carbon dioxide in the oceans have so far proved to be ineffective.
 

Yes, you can make a difference. Society’s collective response to global warming is the combined effect of the actions of each one of us. We must all take responsibility for reducing our greenhouse gas emissions at home, at work and when travelling.

Many of the changes that are needed to reduce the UK’s greenhouse gas emissions can be made individually; in 2006 household and road transport emissions together accounted for 57% of our total emissions. If we wait for the necessary changes in lifestyle to be forced upon us by the government, by high fuel costs or by climate-induced disasters we may find that it is more difficult, socially disruptive and costly to make the necessary adjustments. We can and must act now.

Very often we take on different roles in our daily lives and we can take action in each of these roles. We can switch from business person to private individual, from producer of goods and services to consumer, from pedestrian to driver, politician to voter. It is worth asking who these other people are that we are expecting to solve global warming for us, since deep down we know that this is our problem and we all occupy the same planet.

Most politicians and some sectors of industry have consistently lagged behind or ignored the scientific evidence for global warming and the growing desire for change at grassroots level. Politicians seem to require a certain baseline of support before they act and often require a demonstration that what they offer is possible, so they appear timid and lacking in leadership. Your demonstration of support for taking urgent, significant action to tackle climate change and your individual actions will help to create the groundswell needed for politicians to take action. Similarly, businesses respond to what they believe their customers want; the more we, as consumers, demonstrate support for a low-carbon society, the more businesses will act to help make it a reality.

Although we need political and technical support to underpin the radical changes in society that are required, the easiest way to promote these changes is to begin with ourselves and lead by setting an example to others (much of the technology that is needed is already well known and available to us). It is possible (even likely) that we will each influence those around us and promote substantially more change than our own individual impact. Our own personal changes may seem small but the change that is needed will necessarily be the sum of all of our changes. However, this is not an argument for everyone simultaneously making relatively small changes. These will add up to only an inadequate impact (“if everyone does a little, we’ll achieve a little”). Instead we each need to take meaningful action and encourage others to do the same.
 

There is little evidence to support this idea. Trends in global warming in the recent past have been in the opposite direction to that predicted by the idea that cosmic rays cause climate change.

Cosmic rays are fast moving particles that arrive from space, partly from the Sun, and release electric charge in the Earth’s atmosphere. Laboratory experiments hint that cosmic rays could play a role in the development of tiny particles that could help clouds to form. If, and this is uncertain, the same chain of processes occurs in the atmosphere it might lead to more clouds; clouds can have a cooling effect because they reflect the Sun’s heat back into space. When the Sun is more active its magnetic field is stronger and acts to deflect more cosmic rays away from the Earth. So the argument goes that a more active Sun would lead to fewer cosmic rays reaching the atmosphere, therefore fewer clouds and hence a warmer Earth. Observations have shown that, at most, the above chain of processes produces only a small effect. Even if the effect were greater, the Sun’s activity has changed so little over the last few tens of years that it could not explain the observed rises in global surface temperature over the same period. Indeed, since the late 1980s, trends in cosmic ray intensity have been in the opposite direction to that required to explain global warming.

No. Although the Sun’s output does vary, its effect on global warming is about one tenth of that caused by mankind’s emission of greenhouse gases.

The primary source of energy providing heat to the Earth’s living organisms, atmosphere and oceans is the Sun. The amount of energy reaching the Earth’s surface depends on the output of the Sun, the Earth’s orbit around the Sun and effects in the atmosphere (both cloudiness and tiny particles emitted by man and volcanic eruptions). The Sun undergoes small (less than ±1 part in 2000) but significant variations in its intensity on an 11-year cycle and possibly underwent similar, longer term, changes in the distant past that cannot be measured directly. When the Sun is more active, as indicated by a greater number of sunspots on its surface, it emits more light and heat. However measurements have shown that the amount of heat reaching the Earth’s surface depends much more on the concentration of particles (aerosols) in the atmosphere than on the Sun’s activity. While there is evidence of a link between solar activity and some of the warming of the early 20th century, satellite measurements have shown that changes in the last 30 years cannot account for the recent rises in global temperatures. However, greenhouse gases can explain the rise; the warming attributable to mankind’s emissions of these gases is about ten times greater than that from changes in the Sun’s output.

When global temperature changes over tens of years are considered, so that local and more temporary changes are averaged out, there is no doubt that global warming is happening.

It is true that the amount of carbon dioxide in the atmosphere has been rising slowly for the last 5,000 years, but this has been in response to the natural cycle driven by variations in the Earth’s orbit around the Sun. Superimposed on this are natural and local changes in climate caused for example by violent volcanic eruptions and by changes to current patterns in the oceans. However, following the industrial revolution in the 1700s, the concentration of carbon dioxide has increased very steeply. According to the latest Intergovernmental Panel on Climate Change report (2007), it is more than 90% certain that the world is already warming as a result of human activity.

The ‘Little Ice age’ of 17th -19th century Europe was not a global change, but was regional. The 20th century changes in surface temperature, by contrast, have been global. The global cooling from 1940 -1970 was caused by the injection of small particulates and aerosols into the atmosphere by polluting industries, now largely stopped by clean air legislation. Sulphur dioxide emitted during volcanic eruptions and a reduction in solar activity also contributed to this cooling.

The apparent global cooling since 1998 only appears when temperatures are averaged over less than ten years; the long-term (1998-2007) trend is still upwards. This is even clearer when a short-term warming caused by ocean currents in 1998, the El Niño effect, is removed.

The planet’s atmosphere is a chaotic system, with many complex and interconnected effects, which will cause some years to be warmer or cooler than others. So it is important to look at the trends over tens of years or longer. Some authors have examined temperature over unduly short periods of time, leading to a mistaken interpretation of the long term global trend, which is undeniably that of increasing temperature.

The changes, and rates of change, in climate seen today are unprecedented and can only be explained by the effect of mankind’s greenhouse gas emissions.

Climate change has been occurring on Earth for millions of years. For example, the slow movement of tectonic plates and the creation and destruction of mountain chains has caused relatively large changes in temperature and sea level and affected the exchange of carbon between the Earth’s crust and the surface.

The Earth’s climate has also exhibited natural cycles of cold and warm periods which occur over thousands of years due to predictable variations in the Earth's orbit around the Sun which affect the heat reaching the Earth’s surface. In the last few million years there have also been periodic ice ages (up to 10°C cooler than present) and warmer ‘interglacial’ periods (such as the last 11,000 years). During these glacial/interglacial periods, greenhouse gas concentrations and temperature rose and fell together. Regional fluctuations of 1.0-1.5°C have also occurred; for example, northern Europe was cold until the 7th century and temperatures rose 2°C higher than today in the Medieval Warm Period (900-1300 AD).

However, today, the global concentration of carbon dioxide in the atmosphere is the highest it has been for at least the last 650,000 years. Further, the rate of increase in carbon dioxide since 1750 appears unprecedented in more than 10,000 years. Computer modelling confirms that global warming during the past 50 years was mainly caused by greenhouse gases released by mankind. It is likely that the 20th century temperature increase in the northern hemisphere exceeded that of any other century for the last 1000 years. The average global temperature also appears to be increasing faster with time. The Intergovernmental Panel on Climate Change warns of potentially large-scale, abrupt and possibly irreversible changes in physical and biological systems due to this relatively recent and rapid rise in the concentrations of carbon dioxide and other greenhouse gases. As far as we know, what is happening today has no equal in the past.

Methane causes around a fifth of the global warming attributable to mankind. This could increase in future as global warming triggers the release of methane from new sources.

Methane is the major constituent of natural gas. It is derived from the breakdown of plant and other organic remains in rocks deep within the Earth but it is also emitted by rotting vegetation (including food in landfill sites), marshes, cattle and even rice paddies. Methane is a greenhouse gas like carbon dioxide, nitrous oxide and some gases containing fluorine and chlorine which are used in fridges and some fire extinguishers.

Methane is the second most important greenhouse gas after carbon dioxide and is currently responsible for around 18% of the global warming attributable to mankind (carbon dioxide contributes 63%). Methane is often picked out as a significant greenhouse gas because, over a hundred years, it is actually 25 times more powerful than carbon dioxide but fortunately, in the atmosphere, it has a much shorter lifetime and is over 200 times less concentrated. However methane can also be emitted in the summer months by the decay of plant and other organic-rich material, as the sub-Arctic permafrost melts, and it is also contained as an ice-like solid in some ocean sediments. Some scientists are concerned that as global warming accelerates the warming permafrost, and eventually the ocean sediments, may cause the emission of huge quantities of methane with disastrous consequences for an already warming world.

The amount of water vapour in the air depends on the weather and any excess is soon lost but carbon dioxide in the atmosphere lasts for over 100 years.

Whether water vapour or carbon dioxide is more important in causing global warming is not simple to answer. Currently about 50% of the greenhouse effect is due to water vapour, 25% due to clouds, 20% to carbon dioxide, with other greenhouse gases accounting for the remainder.

So why aren't climate scientists a lot more worried about water vapour than about carbon dioxide? The level of water vapour in the atmosphere is determined mainly by air temperature and any excess is lost in a few days as precipitation (mainly rain or snow). On the other hand the level of carbon dioxide is determined by the fine balance between sources and natural sinks, and it would take centuries to return to pre-industrial levels even if all emissions ceased tomorrow. Other, even more powerful, greenhouse gases than carbon dioxide are emitted by mankind but because they occur in far smaller quantities their net effect is much smaller than that of carbon dioxide. Carbon dioxide is responsible for about two-thirds of the warming caused by greenhouse gases emitted by mankind.

Water vapour will play a huge role in global warming in future. The amount of water vapour about 5 -10 kilometres up in the atmosphere is expected to double by 2100, as temperatures rise, resulting in a warming effect roughly twice as much as today’s. However, water vapour increases as a consequence of the increase in other greenhouse gases and is not a primary cause of global warming itself.

Man-made emissions of carbon dioxide have upset the Earth’s natural balance; natural sinks can no longer extract this extra greenhouse gas from the atmosphere so we have global warming.

Carbon dioxide is emitted by a variety of sources and absorbed by a variety of sinks that make up a natural carbon cycle. Carbon dioxide is emitted by the respiration (‘breathing’) of plants, fungi, animals and some bacteria. These natural carbon dioxide emissions amount to around 770 billion tonnes of carbon dioxide per year. Carbon dioxide is absorbed by photosynthesising plants on land and in the sea and directly by dissolving in the oceans. The natural exchanges have been kept in balance for many thousands of years by the above processes. For the last 500,000 years the concentration of carbon dioxide in the atmosphere has remained relatively steady between 180 and 300 parts per million.

However, since pre-industrial times (around 1750) mankind has emitted large quantities of carbon dioxide into the atmosphere by burning fossil fuels (coal, oil and gas). Since 2000 this has amounted to around 26 billion tonnes of carbon dioxide every year, i.e. only about 3% of the emissions produced naturally. Even so, this relatively small amount and the continuing relentless increase in the amount of carbon dioxide remaining in the atmosphere, almost 40% since 1750, means that the natural equilibrium of the Earth’s system has been upset. The sinks can no longer cope with the amounts of carbon dioxide being added to the atmosphere and global warming is the result.

Carbon dioxide, even in minute quantities, causes almost two-thirds of man-made global warming. Man-made emissions have upset the Earth’s delicate natural balance. Natural ‘sinks’ can no longer cope so rising carbon dioxide levels cause global warming.

It is true that carbon dioxide exists in small quantities in the atmosphere, about 387 parts per million, or 0.04%, by volume at the present time. Other gases such as nitrogen and oxygen exist in far greater amounts but they are not greenhouse gases and so do not contribute to global warming. Carbon dioxide is just one of several greenhouse gases (methane and nitrous oxide are others) that exist in small amounts in the atmosphere. Greenhouse gases, because of their particular molecular structure, contribute to the greenhouse effect by trapping heat in the atmosphere. However it is important to recognise that the total warming effect of a greenhouse gas is due to a combination of its abundance, its molecular structure and its lifetime in the atmosphere (greenhouse gases eventually disappear, unless replenished, because they react with other gases or, in the case of carbon dioxide, are removed by living organisms or by dissolving in water). It so happens that carbon dioxide comes out ahead of all other greenhouse gases when these three factors are considered and accounts for about two-thirds of the global warming caused by greenhouse gases that have been emitted by mankind. Normally carbon dioxide is absorbed by natural ‘sinks’, such as tiny organisms living in the surface waters of the oceans and the vegetation in tropical forests, however the recent and continuing relentless increase in atmospheric carbon dioxide, plus the destruction of tropical forests, means that the natural equilibrium of the Earth’s system has been upset. The sinks can no longer cope with the amounts of carbon dioxide being added to the atmosphere and so the atmosphere is warming.

Scientists work in an open and consensual way that allows for conflicting views. This makes it even more remarkable that today there is very strong agreement among them that global warming is real.

It is not entirely unreasonable to take the view that because nearly all the experts agree this doesn’t make them right, since they could just all be making the same mistake. However, the current agreement among climate scientists suggests that the chance of them all being wrong is extremely low. This is especially so because the way science works allows rival hypotheses to challenge the accepted view provided they rest on the basis of compelling new evidence. Thus far, no alternative hypothesis has been able to successfully challenge the agreed view on the basis of logic and evidence. So, at present, such a challenge appears highly unlikely to happen. What matters is our estimation of the truth, given what evidence and knowledge we have so far. It has to be said that this is overwhelmingly in favour of man-made global warming being caused by the excessive emissions of greenhouse gases.

This is wrong. Hundreds of experts from all over the world contribute to the main IPCC reports. Although the summaries are written by diplomats, scientists ensure they remain scientifically valid.

Hundreds of experts from all over the world contribute to the preparation of IPCC reports as authors, contributors and reviewers. The composition of the authoring teams reflects a range of views, expertise and geographical representation. Review by governments and experts are seen as essential elements of the preparation of IPCC reports. The IPCC’s work is also supported by the worldwide scientific community. For example, in June 2005, a joint statement of support was issued by the highly respected national science academies of Brazil, Canada, China, France, Germany, India, Italy, Japan, Russia, UK and the USA.

The only document produced by the IPCC which is not written directly by scientists is the Summary for Policymakers (around 30 pages). This is reviewed by diplomats from around the world whilst the lead authors of the report ensure that any changes remain scientifically valid. However the scientists have the final word. The IPCC thereby delivers reports that have formal approval of the world's governments as well as the authority of the scientific authors.

Scientists work in an open and consensual way that allows for conflicting views. This makes it even more remarkable that today there is very strong agreement among them that global warming is real.

The ‘Oregon declaration’ refers to a petition, that was compiled between 1999 and 2001 by the Oregon Institute of Science and Medicine (OISM), calling for the US Government to reject the Kyoto agreement and claiming that “proposed limits on greenhouse gases would harm the environment, hinder the advance of science and technology, and damage the health and welfare of mankind”. These claims contradicted the Intergovernmental Panel on Climate Change’s 2001 report. Critics of the petition claimed that it involved a degree of deception because it was made to look like a scientific article. They also alleged that false names appeared on to the petition – the signatories’ names appear on the OISM website, but without listing any institutional affiliations or even cities of residence, so it is very difficult to determine their credentials or even whether they exist at all. It may also be worth noting that it was part funded by the George C. Marshall Institute which has itself received significant funding from ExxonMobil, a company well known at the time for its denial of global warming.

The best way, around ten years on, to gauge the current scientific consensus is by examining the contents of peer-reviewed scientific publications. Before publication these articles have been thoroughly examined and critiqued by other experts, often anonymously, who work in the same field of research. This method ensures that articles that take unsupportable positions will be rejected. For example, in 2004, Naomi Oreskes, a science historian, was unable to find a single peer-reviewed scientific article that rejected the latest (2001) views of the Intergovernmental Panel on Climate Change (IPCC) whereas three quarters of the papers accepted these views.

A well-informed view on climate change does not require any particular belief but instead requires one to take account of the large amount of evidence in support of man-made climate change. For most people this means relying on the distillation of the evidence by leading scientists and scientific, governmental or well-reputed international bodies, such as the IPCC.

What many non-scientists find hard to understand is that, although there is a very strong agreement that man-made climate change poses a serious threat, science works by assuming that the current leading explanation is correct, only until it can be disproved. This approach, which allows for competing hypotheses and new data and an acceptance that we cannot know everything, does not imply that we know nothing at all and certainly does not imply that we should be inactive in the face of present and predicted changes to the climate.

Many models seem reliable because, starting with real observations, they successfully model climate change in the recent past. These different models also consistently predict rising temperatures in future.

The Earth’s climate system is highly complex with many interacting natural systems, together with changes that mankind is causing, that involve the Sun’s radiation, the atmosphere, the oceans, vegetation, ice and so on. There are also factors that can cause knock-on effects so that an increase in one component may cause a much larger increase in some other component of the system (so-called positive feedback mechanisms). Undoubtedly too there are parts of the system that we do not fully understand. All this means that climate is difficult to predict. Nevertheless, computer models are the best tool we have to make quantitative predictions of future climate. Further, projections of future climates rely on assumptions about the rate of greenhouse gas emissions and these assumptions may need to be altered to take into consideration changes in technology, economics, human population and human behaviour.

The validity of many climate models has been tested successfully against climate history; in other words, models that have been run forward from some time in the past, when conditions were reasonably well known, can successfully predict past changes in climate. This gives a degree of confidence in the reliability of the models in predicting future climate.

Weather forecasts try to predict how rapidly changing, rotating masses of air will affect certain places whereas climate forecasts, using other information, predict the average conditions over much larger areas.

Weather is what we experience today, tomorrow or in a few days time whereas climate represents the average conditions experienced over a number of years. Weather depends on the distribution of low-pressure and high-pressure weather systems across the Earth’s surface which form an apparently ever-changing pattern. Climate on the other hand is a steady or slowly changing feature of our environment. It is hard to be sure a climatic change has happened on shorter time scales because of the natural variability caused by the weather, ocean currents, volcanic eruptions and other factors.

Weather forecasts are made using computer models that start with real measurements (like pressure and temperature). The models use mathematical equations that include the movement of the air and transfer of heat between the air and the ground or the sea. Even so measurements are not available everywhere, some may be inaccurate and it is only possible to calculate future conditions at a grid of points which may be kilometres apart. Even if a series of models are run from almost the same starting point predictions begin to differ after a few days because of the chaotic nature of the atmosphere.

Climate forecasts are different because they are attempts to predict what the average conditions will be over much larger areas of the Earth’s surface many years in the future. Climate modelling is constrained by our knowledge, or estimates, of long-term changes in the Earth’s orbit, the output of the Sun, the distribution of vegetation and ice and even human activities as well as by the slowly warming or cooling deep ocean. Because these forecasts look so far ahead they smooth out the random variability of weather and can predict trends far into the future. Even so, climate forecasts can differ because of the different assumptions made.

Scientists working at the forefront of knowledge are used to uncertainty. Future temperature predictions have upper and lower limits within which the eventual temperature is very likely to lie. However, even the lowest forecast temperature would be bad news for mankind.

All science carries with it some uncertainty and scientists in particular are very used to this. It is true that the uncertainty attributed to temperature predictions means that scientists admit that it is possible that there may be both higher and lower temperatures than the central prediction (although it is important to notice even the ‘lower bound’ (minimum) temperatures which the Intergovernmental Panel on Climate Change (IPCC) predict will increase with increasing greenhouse gas concentrations). For example, the IPCC estimates, on the basis of certain assumptions, that the best estimate of the increase in global surface temperature by the last ten years of the 21st century will be 1.8°C compared to the period 1980-1999, with a likely possibility (meaning two chances out of three) that the temperature increase will lie between 1.1 and 2.9°C. The general scientific and political consensus is that increases of 2oC or more would be ‘dangerous’.

The larger uncertainties surround what the weather will be in particular places in the future.

Certain effects may have a strong influence on local or regional climate; the details are hard to predict accurately. Scientists are much more confident of the broader global warming trend.

Ocean currents are driven, amongst other factors, by water temperature and salinity (saltiness). In the northern North Atlantic water increases in density, due to winter cooling of the water, and salinity increases, due to evaporation from the surface - both effects cause the water to sink. This drives the ocean currents in the Atlantic Ocean. However, melting land ice, which dilutes the North Atlantic waters and makes them less dense, lowers the density of this North Atlantic water so reducing its ability to sink. Thus global warming may be capable of disrupting the Atlantic current system. Although the possible mechanisms of disruption are understood, the precise changes in the currents are difficult to predict with much confidence.

There is evidence to suggest that the Atlantic current system may have shut down when a huge volume of freshwater, derived from melting of the North American ice sheet at the end of an Ice Age, was suddenly released by the rupture of a natural dam. So, in 2005, climatologists were shocked by data that suggested that disruption of Atlantic currents might already be happening. However, subsequent measurements showed no clear trend. Relatively few scientists think there will be a rapid shutdown of ocean currents since most ocean models predict no more than a slowdown, probably towards the end of the 21st century. This could locally slow, or even reverse, some of the warming due to greenhouse gases emitted by mankind, which might even be welcome in an overheated Europe. Ultimately, both in the long run and on a global scale, local effects such as this will be insignificant compared with the much greater changes caused directly by global warming.

Scientific reports written in the 1970s suggested that cooling from industrial dust particles in the atmosphere might outweigh the effects of global warming. These were given disproportionate media attention, given that between 1965 and 1979, 44 scientific publications predicted warming, 20 were neutral and just 7 predicted cooling. Since that time there has been less particulate pollution and any cooling effect is much less marked.

There is always a possibility that certain effects will have a strong influence on local or regional climate and the details of these are indeed more difficult to predict accurately. However scientists are much more confident of the broader global trends.

Even under global warming, surface temperatures will not necessarily rise every year. But all the evidence suggests that the oceans are continuing to absorb more heat from the Sun so that the overall trend is that the Earth as a whole is still warming.

Different sources claim that 1998, 2005 or 2007 was the warmest year since records began. Nevertheless, the Earth as a whole has warmed since 1998. In the long term, what really matters is how much heat from the Sun is gained or lost by the entire planet. For example, falling surface temperatures do not prove that the entire planet is losing heat. We can say this because the oceans are getting warmer. Water stores over 1000 times more heat per unit volume than air. Since the 1960s, over 90% of the excess heat due to higher greenhouse gas levels has gone into the oceans, and just 3% into warming the atmosphere. Globally, this means that if the oceans soak up a bit more heat energy than normal, surface air temperatures can fall even though the total heat content of the planet is rising and vice versa.

This is why surface temperatures do not necessarily rise steadily year after year, even though the planet as a whole is heating up year on year. In the long term, some of the heat being soaked up by the oceans will inevitably spill back into the atmosphere, raising surface temperatures. But unless we see a simultaneous fall in both surface temperatures and ocean-heat content, claims that the "entire planet" is cooling are nonsense.

The dust from a big volcanic eruption could potentially trigger genuine cooling for a few years but in the current situation global warming will resume again once the dust has settled.

Global cooling is no longer expected. The original prediction was partly flawed. Decreases in air pollution in many countries means that now more sunlight reaches the Earth’s surface.

In the 1970s several scientific articles discussed the possibility of a new ice age at some point in the future. One of the sources of this idea may have been a 1971 paper by Stephen Schneider, a climate researcher in the USA. Schneider's article suggested that the cooling effect of dirty air, which scatters the Sun’s heat back into space, could outweigh the warming effect of the greenhouse gas carbon dioxide, potentially leading to an ice age if air pollution quadrupled.

This scenario was seen as plausible by many other scientists, as the Earth had been cooling since the 1940s. Furthermore, it had also become clear that the interglacial period in which we live had already lasted an unusually long time. However, Schneider soon realised he had overestimated the cooling effect of air pollution and underestimated the effect of carbon dioxide, meaning warming was more likely than cooling in the long run.

Since the 1980s however there have been quite successful attempts, at least in some countries, to clean up the air by reducing the emission of industrial pollutants. At the same time emissions of greenhouse gases have being increasing relentlessly. The net effect has been a strong increase in global warming. According to the latest Intergovernmental Panel on Climate Change report (2007), it is more than 90% certain that the world is already warming as a result of human activity.

When first published the ‘hockey stick’ curve was controversial. However, other scientists have since independently confirmed that the Earth’s surface has warmed significantly and very rapidly since 1900.

In 1998, three American scientists named Mann, Bradley and Hughes used data from ice cores and historic records, yearly growth lines in trees and corals plus thermometer records from the 20th century, to estimate the Northern Hemisphere’s average surface temperature for the last 1000 years. When the resulting curve was smoothed (averaged over a number of years) it showed a trend of declining temperatures until the early 20th century followed by a precipitous rise of about 1°C, with only one temporary dip, until the present day. Because of its shape this curve has become known as the ‘hockey stick’ curve. Sceptics hailed this curve as false or even a fraud. It is true that there were many possible problems with the curve; data from different sources had been merged and the older data probably have larger uncertainties. It was also unclear how much increasing atmospheric carbon dioxide may have changed tree ring growth and affected the authors’ ability to infer temperatures from these rings. However, in 2008 the same authors with others showed, with even more data, that even if the criticised tree ring data were excluded essentially the same interpretation of a rapid 20th century rise in temperature can be discerned. Other scientists carried out similar but independent studies and came up with their own curves not much different from the curve of Mann and his colleagues. Thus to a very large degree all the curves tell the same story - the Earth’s surface has warmed significantly and very rapidly since 1900.

Not at all. If anything, the dangers of rising sea level have been understated because the most recent science shows that the Greenland and Antarctic ice caps are melting faster than expected.

Although sea level rose by 120 m (390 ft) at the end of the last ice age, since then it remained more or less constant until the late 19th century. During the 20th century sea level began to rise at 1.7 millimetres (0.07 inches) per year and satellite measurements show that since 1993 the rate has been around 3 millimetres (0.12 inches) per year. Sea level is rising faster today, first, because climate change is warming the oceans which are therefore expanding, and second, because ice on land in parts of the Antarctic and Greenland and ice in glaciers is melting. The melting of Arctic sea ice and Antarctic ice shelves, which float in the ocean, does not contribute directly to sea level rise. However the destruction of ice shelves may indirectly contribute to sea level rise by removing a barrier which slowed the flow of ice from western Antarctic glaciers into the sea.

The Intergovernmental Panel on Climate Change (2007) reported that in the period 1993-2003 thermal expansion and melting land ice contributed about equally to the rise. Although locally there may be a net accumulation of ice and snow in some parts of the Antarctic and Greenland, resulting from increased snow fall, on average there is a net loss. In 2009 one scientist reported ‘As a result of the acceleration of outlet glaciers over large regions, the ice sheets in Greenland and Antarctica are already contributing more and faster to sea level rise than anticipated. If this trend continues, we are likely to witness sea level rise one meter or more by year 2100’. If sea level rose by 1 m (3.3 ft) globally it would affect over 100 million people and submerge over one million square kilometres (400,000 square miles) of land. It is certain that rising seas will adversely affect coastal states; the question is, how soon and to what extent will it do so?

No. The effects of expanding cities and changes in land-use on global temperatures are not significant as far as the average values over whole continents, or even Earth’s hemispheres, are concerned.

The Urban Heat Island effect refers to the fact that cities are usually warmer than the countryside. This is partly because heat from the Sun is stored during the day by the large areas of concrete and tarmac found in cities and is released at night; other factors in cities include higher levels of fine particles (aerosols) and less sunlight being reflected by roofs, car parks etc. This phenomenon also affects rainfall, cloudiness and the daily temperature difference between night and day. Some temperature recording locations, once in the open countryside, have been absorbed into an urban area as the adjacent city grew so that the temperature measurements could have been skewed towards warmer values. However it is relatively simple to check such suspect data for quality and consistency to avoid the Urban Heat Island effect. The IPCC (2007) stated that ‘…. effects of urbanisation and land use change on the global temperature record are negligible … as far as hemispheric- and continental-scale averages are concerned.’

This is an outdated view. Observations and analyses made in the early 1990s tended to support this view but since then the different observations have been brought into agreement.

It is true that in the early 1990s initial estimates of temperatures in the lowest part of the Earth's atmosphere, based on measurements taken by satellites and weather balloons, did not mesh with the temperature rises seen at the Earth's surface. However these discrepancies were found to be related to problems with how the data were gathered and analysed and have now largely been resolved.

Our understanding of mankind-induced global warming leads us to expect that both the lower atmosphere (troposphere), where most greenhouse gases are found, and the surface of the Earth should warm. Simultaneously the lower stratosphere, just above the greenhouse gas ‘blanket’, should cool.

At first, satellites and weather balloons seemed to show that virtually no warming was happening in the troposphere. However, this has been found to be due to errors in the data. Satellites were found, for example, to be slowing down and consequently orbiting slightly lower, leading to inconsistencies in their measurements. Variations between the instruments onboard different satellites (and weather balloons) also led to discrepancies. Furthermore, a mathematical error meant that satellites showed less warming in the troposphere. However, once the data were corrected, the troposphere temperature was shown to be broadly consistent with the Earth’s surface temperature.

Some discrepancies remain between the temperature that computer models predict at the surface and in the troposphere and what we actually observe. However, these discrepancies are less than the likely remaining uncertainties in the observations and the models.

Rises in carbon dioxide naturally followed rises in temperature in the ice ages; today the situation is different. Man-made carbon dioxide emissions are driving the global rise in temperature.

This question is based on a misunderstanding of how climate change is happening today. In the last two million years, changes in the Earth's orbit around the Sun caused the fluctuations in temperature that led to the ice ages which, in turn, drove changes in levels of carbon dioxide in the atmosphere. This sequence of events is inferred from ice cores which show that rises in temperature came first, and were then followed by rises in levels of carbon dioxide up to several hundred years later. The reasons for this, although not yet fully understood, are partly because the oceans emit carbon dioxide as they warm up and absorb it when they cool down and also because soil releases greenhouse gases as it warms up.

However, we know that the recent steep increase in atmospheric carbon dioxide (30 per cent since 1900) is not the result of natural factors. Analyses of the different carbon and oxygen isotopes (different physical forms of atoms) prove that the majority of this carbon dioxide has come from the burning of fossil fuels (carbon dioxide from other sources has a different mix of isotopes). The carbon dioxide from human sources is almost certainly (9 chances out of 10 that it is correct) responsible for most of the global warming over the last 50 years. There is much evidence that backs up this explanation which, contrary to the ice age situation, means that since about 1750 carbon dioxide emitted by mankind has been driving the warming and not vice versa; there is no known evidence that conflicts with this explanation.

This is a baseless accusation. Scientific proposals are reviewed by many independent reviewers and funding agencies and it is inconceivable that any worldwide ‘conspiracy or exaggeration’ could go unnoticed.

If you think that tens of thousands of scientists across the world are really taking part in a huge conspiracy, then it is probably a waste of time to try and convince you otherwise. However there are less strong versions of this viewpoint. One is that climate scientists spread alarm about global warming just to boost their research funding; but if that were the case then they have managed to fool multiple independent funding agencies, other, non-climate, scientists and even government advisors. For example, the 2007 Intergovernmental Panel on Climate Change report was written by 152 coordinating lead authors from over 30 countries and reviewed by over 600 experts, including government officials. Another argument is that because climate scientists depend on government funding this ensures that they are careful to stay ‘on message’. This argument is not supported by the fact that today the climate science community is increasingly expressing its frustration in public with politicians’ inability to comprehend and urgently act on the dangers posed by climate change. In addition both the above versions of the argument ignore that it has taken more than a century of research by several generations of scientists to reach today’s broad scientific agreement on climate change. This agreement has come about through a steadily growing body of evidence, from many different scientists in different countries, and the evidence has been published openly in the scientific literature. But, even so, evidence or ideas from a small minority of scientists, that appear to clash with the idea of human-induced global warming, are not suppressed or ignored but are still being presented and/or published.