Several people have said to me that they couldn’t quite believe what they were being told in Channel 4’s programme last week on climate change – and I promised yesterday in my interview on the Today programme to put the facts on my blog. Below I have set out what Defra scientists say about the 11 main allegations in the programme. You can read for yourself what the International Panel on Climate Change say or the statement of the Academies of Science of the 11 largest countries in the world.
I am convinced well beyond reasonable doubt that the swindle is not being perpetrated by the vast, vast majority of scientists in the world. There will always be people with conspiracy theories trying to do down the scientific consensus, and that is part of scientific and democratic debate, but the science of climate change looks like fact to me. If the effect of the programme, instead of making people think, is in fact to make them disregard the accepted science (in other words stop thinking) then that would be a real swindle.
The following key allegations are paraphrased from the programme “The Great Global Warming Swindle”.
1. The high temperatures seen in the last few decades are not unique. Temperatures are naturally variable and have been higher in the past, for example, during the Medieval Warm period (800-1300AD).
It is true that temperatures have been higher than today’s in the distant past. However, for the Northern Hemisphere at least, it is clear the rapid warming of the past half century has resulted in a level of warmth not seen in at least 500 years, and likely for at least the past 1300 years. For the Southern Hemisphere, long records of temperatures are more scarce and therefore it is difficult to draw such clear conclusions. The important characteristic about the current warmth is that it is global, whereas many previous warming periods have occurred over smaller areas.
Globally, eleven out of the last twelve years (1995-2006) rank among the warmest 12 years since records began in 1850. Over the past century, temperatures have risen by 0.74°C, with 0.4°C of this warming since 1970.
Climate models indicate that if greenhouse gas emissions continue unabated, by the middle of the century the world could reach a level of warmth not seen since the peak of the last interglacial period, around 125,000 years ago. At that time, sea levels were around 4 – 6m greater than today.
2. The key piece of evidence used by scientists to prove the warming effect of greenhouse gases is that in the past, on timescales of hundreds of millennia, global temperatures have followed carbon dioxide concentrations, i.e. when temperatures are high, carbon dioxide levels are high. But this is not true, changes in carbon dioxide concentrations actually lag behind changes in temperature.
Firstly, this is not the key piece of evidence for human-induced climate change. Basic physics tells us that an increase in greenhouse gas concentrations will have a warming effect on the climate. Levels of carbon dioxide alone now far exceed that at any time in at least the past 650,000 years. The trend and pattern of current warming is consistent with what we would expect for the observed rise in greenhouse gases.
Secondly, it is true that temperature changes appear to have preceded CO2 changes through glacial-interglacial cycles in the distant past. However, this just suggests that CO2 was not the initial driver of the glacial cycles. The evidence suggests that CO2 levels rose as a result of warming, possibly as the surface of the ocean warmed. As CO2 has a warming effect on the climate, it would then act as a feedback – stimulating additional warming.
Now human emissions are causing the rise in CO2 levels, and therefore, the resulting warming of the climate.
Aside: Climate models predict that the positive feedback effect between temperature and CO2 seen in the distant past could happen again in the future if global temperatures reach high enough levels.
3. The trend in carbon dioxide concentrations over the past century does not match that of temperature, and therefore, carbon dioxide can not be the key driver. For example, in the middle of the century, when emissions were growing rapidly, temperatures actually fell.
It is true that the trend in CO2 concentrations over the last century does not exactly match the trend in temperature. But we do not expect it to. This is for two key reasons. Firstly, there is a time lag between warming and changes in CO2, caused by the inertia in the climate system. Secondly, greenhouse gases are not the only determinant of temperature. Aerosols, which are also emitted from human activities, are also important and can be shown to explain much of the cooling seen in the middle of the 20th century.
Climate models represent the lag in the response of the climate, and the influence of many external factors. These show that the trend in temperatures does match what we would expect. Based on these analyses and others, the recent report of the IPCC concluded that most of the warming over at least the last 50 years has been caused by the rise in greenhouse gas concentrations.
4. Most of the 20th century warming occurred before 1940, when carbon dioxide emissions were still relatively low.
This first statement is not correct. Global temperatures did rise during the first few decades of this century, but much of the warming seen this century has occurred since around 1970 (0.4°C of the total 0.74°C warming). Global temperatures have risen almost continuously since 1950. The linear growth rate in temperature during the past 50 years is nearly twice that of the last 100 years. While the IPCC concluded that much of the warming over the past 50 years is very likely due to greenhouse gases, the cause of the warming in the first half of the century is not clear. Current thinking is that it was likely a mixture of natural and human factors.
5. The patterns of warming in the atmosphere do not match what we would expect for a warming caused by rising greenhouse gas concentrations and therefore, can not be the cause.
This is not correct. The patterns of warming in the atmosphere do match what we would expect. Previously reported discrepancies were due to problems with early versions of observed data from satellites and radiosondes. These errors were corrected a few years ago.
6. Human emissions are only a tiny fraction of total natural emissions and therefore can not be important in causing climate change.
It is true that human emissions are relatively small compared to natural emissions, particularly from ecosystems and the oceans. However, these natural emissions are in balance: the amount emitted is then reabsorbed. Human emissions tip the balance and lead to an accumulation of gases in the atmosphere. The human source can be shown through, for example, examining the chemical make-up and distribution of CO2 in the atmosphere.
7. There is no evidence that human emissions are causing the current warming trend.
This is not true. As stated in the recent IPCC report, a growing body of evidence demonstrates that increases in greenhouse gas concentrations, due to human emissions, have caused most of the warming observed over the past half century. Both the spatial patterns and trend of warming can only be explained by the inclusion of human emissions. It is very unlikely that the rapid increase in global temperatures seen over the past half century could be caused by natural factors alone. For example, the most recent report of the IPCC concludes that the warming effect of human emissions is around ten times that of solar variations.
8. Attribution studies rely on climate models, but these models are tuned to give the right results and therefore can not be trusted.
Climate models are an essential tool in understanding how the climate will respond to changes in greenhouse gas concentrations, and other external effects, such as solar output changes and volcanoes. The models use fundamental physics and chemistry to simulate processes within the climate system and are rigorously assessed to ensure their reliability.
9. Past changes in climate have been driven by natural factors, such as changes in solar radiation and cosmic rays. It is most likely that these same natural factors are causing the current warming.
It is true that natural factors have driven climate changes in the past. But it is considered very unlikely that the rapid increase in global temperatures seen over the past half century could be caused by natural factors alone. Both the spatial patterns and trend of warming can only be explained by the inclusion of human emissions.
It is clear that changes in solar radiation are a significant driver of the climate. However, there is strong evidence that changes in solar radiation could not have caused the rapid warming observed over the past half century. The warming effects of changes in solar output since pre-industrial are estimated to be less than around one tenth that of human emissions of greenhouse gases.
Despite recent claims, there is no scientifically robust evidence suggesting that cosmic ray variations have, or could even, play a significant role in recent warming. Variations in cosmic rays over the past few decades can not explain the long-term global warming trend.
10. Observed changes in sea ice are due to natural factors.
Sea ice extent does change naturally, but the current reduction in sea ice extent is in line with what we would expect in a warming world.
11. The scientific process is biased.
The IPCC is the most authoritative voice on climate change. Its assessments represent the consensus of thousands of scientists worldwide, based on peer-reviewed research. Objectivity is ensured by the broad and open review process and shared responsibility for the report. No one government, organisation or individual has sole responsibility for any part of the report.