Didn’t global warming stop in 1998?

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.

What happened to the global cooling that was predicted in the 1970s?

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.

Isn’t the so-called ‘hockey stick’ curve of temperature against time wrong?

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.

Hasn’t the risk of a rise in sea level been overstated?

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?

Could the Urban Heat Island effect account for some of the reported global rise in temperature?

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.’

Isn't it true that balloon and satellite observations of temperature don't support global warming?

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 actually follow rises in temperature so how can carbon dioxide cause global warming?

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.

Couldn't climate change be a conspiracy or exaggeration by scientists to get funding for research?

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.

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