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Carbon Change Controversies: a simple guide from The Royal SocietyThe Royal Society has produced this overview of the current state of scientific understanding of climate change to help non-experts better understand some of the debates in this complex area of science. This is not intended to provide exhaustive answers to every contentious argument that has been put forward by those who seek to distort and undermine the science of climate change and deny the seriousness of the potential consequences of global warming. Instead, the Society - as the UK's national academy of science - responds here to six key arguments that are currently in circulation by setting out, in simple terms, where the weight of scientific evidence lies.
What does the science say? It is true that the world has experienced warmer or colder periods in the past without any interference from humans. The ice ages are well-known examples of global changes to the climate. There have also been regional changes such as periods known as the 'Medieval Warm Period', when grapes were grown extensively in England, and the 'Little Ice Age', when the River Thames sometimes froze over. However, in contrast to these climate phases, the increase of three-quarters of a degree centigrade (0.74°C) in average global temperatures that we have seen over the last century is larger than can be accounted for by natural factors alone. The Earth's climate is complex and influenced by many things - particularly changes in the Earth's orbit in relation to the Sun, which has driven the cycles of ice ages in the past, as well as volcanic eruptions and variations in the energy being emitted from the Sun. But even when we take all these factors into account, we cannot explain the temperature rises that we have seen over the last 100 years both on land and in the oceans - for example, eleven of the last twelve years have been the hottest since records started in 1850. So what is causing this increase in average global temperature? The natural greenhouse gas effect keeps the Earth around 30°C warmer than it would otherwise be and, without it, the Earth would be extremely cold. It works because greenhouse gases such as carbon dioxide, methane, but mostly water vapour, act like a blanket around the Earth. These gases allow the Sun's rays to reach the Earth's surface but hinder the heat they create from escaping back into space. Indeed, the ability of carbon dioxide and other greenhouse gases to trap heat in this way has been understood for nearly 200 years and is regarded as firmly established science. It has been alleged that the increased level of carbon dioxide in the atmosphere is due to emissions from volcanoes, but these account for less than one per cent of the emissions due to human activities.
What does the science say? It is true that the fluctuations in temperatures that caused the ice ages were initiated by changes in the Earth's orbit around the Sun which, in turn, drove changes in levels of carbon dioxide in the atmosphere. This is backed up by data 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. These increased levels of greenhouse gases in the atmosphere then further enhanced warming, creating a 'positive feedback'. In contrast to this natural process, we know that the recent steep increase in the level of carbon dioxide - some 30 per cent in the last 100 years - is not the result of natural factors. This is because, by chemical analysis, we can tell that the majority of this carbon dioxide has come from the burning of fossil fuels. And, as set out in ' misleading argument 1 ', carbon dioxide from human sources is almost certainly responsible for most of the warming over the last 50 years. There is much evidence that backs up this explanation and none that conflicts with it. Warming caused by greenhouse gases from human sources could lead to the release of more greenhouse gases into the atmosphere by stimulating natural processes and creating a "positive feedback", as described above.
What does the science say? Modern climate models have become increasingly accurate in reproducing how the real climate 'works'. They are based on our understanding of basic scientific principles, observations of the climate and our understanding of how it functions. By creating computer simulations of how different components of the climate system - clouds, the Sun, oceans, the living world, pollutants in the atmosphere and so on - behave and interact, scientists have been able to reproduce the overall course of the climate in the last century. Using this understanding of the climate system, scientists are then able to project what is likely to happen in the future, based on various assumptions about human activities. It is important to note that computer models cannot exactly predict the future, since there are so many unknowns concerning what might happen. Scientists model a range of future possible climates using different scenarios of what the world will 'look like'. Each scenario makes different assumptions about important factors such as how the world's population may increase, what policies might be introduced to deal with climate change and how much carbon dioxide and other greenhouse gases humans will pump into the atmosphere. The resulting projection of the future climate for each scenario, gives various possibilities for the temperature but within a defined range. While climate models are now able to reproduce past and present changes in the global climate rather well, they are not, as yet, sufficiently well-developed to project accurately all the detail of the impacts we might see at regional or local levels. They do, however, give us a reliable guide to the direction of future climate change. The reliability also continues to be improved through the use of new techniques and technologies.
What does the science say? Changes in solar activity are one of the many factors that influence the climate but they cannot, on their own, account for all the changes in global average temperature we have seen in the 20th Century. Changes in the Sun's activity influence the Earth's climate through small but significant variations in its intensity. When it is in a more 'active' phase - as indicated by a greater number of sunspots on its surface - it emits more light and heat. While there is evidence of a link between solar activity and some of the warming in the early 20th Century, measurements from satellites show that there has been very little change in underlying solar activity in the last 30 years - there is even evidence of a detectable decline - and so this cannot account for the recent rises we have seen in global temperatures. The magnitude and pattern of changes to temperatures can only be understood by taking all of the relevant factors - both natural and human - into account. For example, major volcanic eruptions produce a cooling effect because they blast ash and other particles into the atmosphere where they persist for a few years and reduce the amount of the Sun's energy that reaches the Earth's surface. Also, burning fossil fuels produces particles called sulphate aerosols which tend to cool the climate in the same way. Over the first part of the 20th Century higher levels of solar activity combined with increases in human generated carbon dioxide to raise temperatures. Between 1940 and 1970 the carbon dioxide effect was probably offset by increasing amounts of sulphate aerosols in the atmosphere, and a slight downturn in solar activity, as well as enhanced volcanic activity. During this period global temperatures dropped. However, in the latter part of the 20th Century temperatures rose well above the levels of the 1940s. Strong measures taken to reduce sulphate pollution in some regions of the world meant that industrial aerosols began to provide less compensation for an increasing warming caused by carbon dioxide. The rising temperature during this period has been partly abated by occasional volcanic eruptions.
What does the science say? Any effect that cosmic rays could have on the climate is not yet very well understood but, if there is one, it is likely to be small. Cosmic rays are fast moving particles which come from space, and release electric charge in the atmosphere. Experiments done in a laboratory hint that cosmic rays could play a role in the development of tiny particles that could in turn play a part in the formation of clouds. If this happens in the same way in the atmosphere - which isn't proven - it might lead to more clouds, which generally have a cooling effect by reflecting the Sun's rays back into space. Whether the whole chain of processes actually occurs in the atmosphere is speculative, but some of the individual steps are plausible. It has been proposed that this process would act to enhance the influences of the Sun on the climate. We know that when the Sun is more active its magnetic field is stronger and this deflects cosmic rays away from the Earth. So the argument is that a more active Sun would lead to fewer cosmic rays reaching the Earth, resulting in fewer clouds and therefore a warmer Earth. However, observations of clouds and galactic cosmic rays show that, at most, the possible link between cosmic rays and clouds only produces a small effect. Even if cosmic rays were shown to have a more substantial impact, the level of solar activity has changed so little over the last few decades the process could not explain the recent rises in temperature that we have seen.
What does the science say? Under one of its mid-range estimates(*), the Intergovernmental Panel on Climate Change (IPCC) - the world's leading authority on climate change - has projected a global average temperature increase this century of 2 to 3 ºC. This would mean that the Earth will experience a larger climate change than it has experienced for at least 10,000 years. The impact and pace of this change would be difficult for many people and ecosystems to adapt to. In the short term, some parts of the world could initially benefit from climate change. For example, more northerly regions of the world may experience longer growing seasons for crops and crop yields may increase because increased levels of carbon dioxide in the atmosphere would have a fertilizing effect on plants. However the IPCC has pointed out that as climate change progresses it is likely that negative effects would begin to dominate almost everywhere. Increasing temperatures are likely, for example, to increase the frequency and severity of weather events such as heat waves, storms and flooding. Furthermore there are real concerns that, in the long term, rising levels of greenhouse gases in the atmosphere could set in motion large-scale and potentially abrupt changes in our planet's natural systems and some of these could be irreversible. Increasing temperatures could, for example, lead to the melting of large ice sheets with major consequences for low lying areas throughout the world. And the impacts of climate change will fall disproportionately upon developing countries and the poor, those who can least afford to adapt. Thus a changing climate will exacerbate inequalities in, for example, health and access to adequate food and clean water. Our scientific understanding of climate change is sufficiently sound to make us highly confident that greenhouse gas emissions are causing global warming. Science moves forward by challenge and debate and this will continue. However, none of the current criticisms of climate science, nor the alternative explanations of global warming are well enough founded to make not taking any action the wise choice. The science clearly points to the need for nations to take urgent steps to cut greenhouse gas emissions into the atmosphere, as much and as fast as possible, to reduce the more severe aspects of climate change. We must also prepare for the impacts of climate change, some of which are already inevitable. This document was compiled with the help of the Royal Society Climate Change Advisory Group and other leading experts. April 2007 |
