How does science work and what makes scientists think they can predict what will happen?

Warnings about the likely undesirable consequences of climate change have come largely from environmental scientists. However, the language used by scientists to describe their observations and predictions is often hard for non-scientists to understand. A basic knowledge of how science works and how scientists come to their conclusions may help in understanding what they are saying.

The process of science generally starts with observations of the world. Scientists seek to observe the world in a dispassionate way by trying to avoid making assumptions about what they will see or what their measurements will or will not show. Painstaking observations and accurate recording are essential tools in scientific work – for example methodical astronomical observations in the 16th century led to an understanding of the orbits of planets around the sun. 
Science uses data to develop testable questions – known as hypotheses – about what is being observed. To be scientifically useful a hypothesis must be falsifiable which means there must be a logical possibility that it can be shown to be false by observation or experiment.  Beliefs are not falsifiable and cannot form the basis of a scientific hypothesis.  Sometimes hypotheses will come from a deep analysis of data; at other times they are in response to questions people want answered – for example ‘does this new drug work’? 
The scientific method involves checking if hypotheses hold up by making more measurements to see if they are consistent with the hypothesis – for example testing if mice are more likely get better when given a new drug. Scientists design experiments that aim to test hypotheses more widely. In climate science there is rarely the opportunity to carry out carefully controlled experiments in the laboratory, so measurements of the actual climate and past events, such as those that occurred in the repeated sequence of warm periods between the last ice ages, must be studied instead.
Scientific hypotheses can never be ‘proved’ because there always remains the possibility that contradictory data will be found later. However, as hypotheses remain consistent with more and more observations, they are often built up into theories – ideas that are felt to be strong enough to start making predictions. The process does not always proceed in this order – Newton and Einstein came up with many of their ideas based on abstract reasoning, but subsequent testing has shown them to be extremely valuable (and testable) theories in terms of predicting how the world works.
The world is a stochastic place  –many events are determined partly by chance which  means there is often a lot of ‘noise’ or randomness around the data we collect. For example, patients react differently to drugs, some get better and some do not. As a consequence modern science applies statistical techniques to assess data and results. These techniques become more important when it is impossible or impractical to conduct certain experiments, as is true to a large extent in environmental science. 
Statistics is a branch of mathematics. Statistical methods are used to test hypotheses and to determine how certain or uncertain conclusions are. Thus scientists often talk about things like ‘confidence levels’ –that is how confident we can be that a given hypothesis is supported by the data; they do not use statistics to claim that something has been proved (or disproved) absolutely. This might make science sound as if it can never answer any questions definitively. However, as evidence builds up, confidence in theories and predictions increases. 
People can find this apparent lack of certainty disconcerting, but in reality it is no different from the judgements people make in their own lives - for example: in judging the probability that a horse with good form will win a race; or the probability that something bad will happen when they take a risk.
Anyone can undertake scientific investigations, and most important scientific theories and discoveries are freely available in scientific journals for anyone to study. However, to publish a paper in a reputable scientific journal, scientists are expected to follow rigorous methods and be subject to peer review. Results of experiments should be reproducible by others. Many scientific frauds or misinterpretations were finally exposed when others failed to duplicate the claimed results. Journals use other reputable scientists, who may be working on competing hypotheses, to study and comment on the validity of draft papers and articles before they are published.  In this peer review process, papers are often reviewed by two or three scientists working in different institutions.  These checks are not perfect but are thought to weed out most poor-quality work and eliminate errors.
These days, scientists frequently use computer modelling which allows them to make more precise predictions based on measurements and theories. Things that are modelled range from the behaviour of individual atoms up to modelling the Earth’s climate. Scientists make efforts to check the results of models by, for example, trying to duplicate the results with an independently constructed model. Of course computer modelling is also used and trusted in many other applications – for example in modern engineering design and aircraft flight systems.

Can we be sure that the scientists are right even though thousands of scientists accept man-made global 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.

Isn't the Intergovernmental Panel on Climate Change (IPCC) a ‘political concoction’ which shouldn’t be trusted?

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.

Wasn’t the ‘ Oregon declaration’ signed by over 31,000 scientists who opposed the reality of man-made global warming?

Science is not a democracy.

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.

Aren’t computer climate models unreliable?

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.

The weather forecasts are often wrong a few days ahead, so why should I believe climate forecasts years ahead?

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.

Is there more uncertainty in the IPCC’s predictions than most scientists would admit to?

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.

Is the science too uncertain for us to be able to make any sensible decisions about 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.

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