NERC Climate change challenge
Summary of the debate
The Natural Environment Research Council's Chief Executive, Professor Alan Thorpe, challenged climate change sceptics to an online public debate in December 2006. Alan said, "If you don't believe the science then please tell us why, or if you are confused about it, then ask a question. In either case we will do our best to respond."
A panel of climate change experts answered questions about the science of climate change and discussed a range of scientific theories that try to explain the recent global warming. The challenge ran for six weeks and had 365 postings. Here we present a brief summary of the debate - this doesn't attempt to cover every individual point raised but groups the discussion into thirteen broad topics raised by the contributors. The first nine topics are related to the scientific results. The last four topics are concerned with the methods used to produce and present them.
The web discussion forum has provided the opportunity for sceptics of the science of human-induced climate change to be frank about their views. It is important for NERC to provide such a forum even though it gives a platform to only one side - the minority one - of the debate.
Many sceptical correspondents were knowledgeable about aspects of the scientific literature and have tried to use the (erroneous) argument that uncertainties in one aspect of the science means that the whole idea about human influence on climate must therefore be disregarded. Many correspondents are explicit that they are motivated by trying to show that mitigation policies to reduce carbon dioxide emissions are wrong. On the other hand the science panel only addressed the science.
Some sceptics argue that the scientists want to promote (mitigation) policies. This is simply not the case. The science that NERC funds generates knowledge and understanding of the climate system for its own sake. The main cause of recent climate change - namely the large and rapid input to the atmosphere of greenhouse gases by human activities - has not happened before. Natural causes of past climate change, whilst of interest and importance, do not address what is happening as a result of the injection of carbon dioxide by humans.
The web debate has been a new experience for NERC and one that has been valuable. Debate about an environmental issue as politically charged as climate change is both difficult and hugely important.
Terminology: Throughout the summary we have used the prefix "palaeo-" to mean "prior to the last 1000 years" and the term "forcing" to mean an external factor that affects climate. "CO2" will be used consistently for carbon dioxide. We have used "man-made" instead of the word "anthropogenic" ("anthropogenic global warming" or AGW for short was used extensively in the Challenge).
Navigate from the following list to topics raised during the debate:
- Greenhouse effect
- Increased CO2
- Recent increase in global temperature
- "Palaeo"-record and prehistoric climate changes
- Climate models
- Regional variations in climate change
- Ice sheets and glaciers
- A New Ice Age?
The scientific results
Sceptic view: Some contributors questioned the underlying physics of the greenhouse effect.
The sun's energy enters the climate system at the Earth's surface as light, while energy is lost from the Earth surface as heat. Greenhouse gas molecules such as water vapour, methane and CO2, in the atmosphere can absorb this heat. This energy is then re-radiated warming the lower atmosphere and surface of the planet.
The greenhouse effect has been established for well over a century and the physics is well understood - this is not "just a theory", it is an accepted scientific principle. Without the greenhouse effect the planet would be over 30°C cooler than it is today. It is the extra greenhouse gases humans are adding that has changed climate over the last 50 years or more. Scientists have refined the science of the greenhouse effect through better observations and understanding.
Sceptic view: Contributors have disputed the relative importance of the various greenhouse gases.
Water vapour and CO2 are the most important greenhouse gases. However, air temperature controls the amount of water vapour in the atmosphere; human activities provide only a very small input of water vapour. The levels of CO2 have varied in the past via natural processes, for example weathering of rocks. Human activities are now rapidly and substantially changing CO2 levels. The direct response of the greenhouse effect to a change in CO2 has been known for over a hundred years (although the calculations have been refined since then). Feedbacks (such as warmer air containing more water vapour) also contribute to the total temperature response.
Sceptic view: Some contributors referred to variety of CO2 measurements before 1950, in an attempt to show that current CO2 levels are not unusual.
There are reliable instrument observations of the amount of CO2 since 1958. They show a significant, continuing increase with annual fluctuations superimposed. The instruments used to take CO2 measurements before 1958 are not considered accurate enough to give an indication of the global average CO2 levels. However, scientists can measure the amount of CO2 contained in tiny bubbles trapped in ice cores that stretch back many hundreds of thousands of years. These show that current levels are unprecedented in recent geological history. A history of CO2 can be found on the UN Environmental Programme/ GRID-Arendal website. The majority of the additional CO2 is man-made, from sources such as fossil fuel burning and deforestation.
Sceptic view: Enhanced CO2 has positive effects, mainly an increase in photosynthesis in plants.
It is true that increased levels of carbon dioxide can promote the growth of certain plant types. However carbon dioxide can affect climate, such as rainfall, and that can have various effects on plants, some of some of which will be negative. In particular, when global average temperatures rise by more than 2°C we predict significant negative impacts.
Sceptic view: The current increase in atmospheric CO2 is a response to the increased temperatures.
CO2 changes have followed temperature changes in the past 500,000 years, until industrialisation occurred. So the CO2 responded to, but also amplified, past temperature changes. However, the current situation is different. We know that human activities have emitted large quantities of CO2 to the atmosphere. We know that CO2 levels before 1750 are only responsible for very small global average temperature changes over the past 10,000 years. The natural factors that might have contributed to the warming over the last 50 years, including sun cycles, have been analysed and shown to contribute in a very small way. Though the contribution to the warming from natural factors is small, all factors are included in climate models.
- changes over the past 100 years are sometimes called the "Hockey Stick" because of the graph's shape
Sceptic view: The quality and location of thermometers have changed, and the warming trend is a consequence of these changes instead of climate changes.
Instrument observations show the global mean temperature has increased by greater than 0·7°C (1·25°F) in the past century. The temperature change is not a straight line, but shows two distinct warming periods: firstly from 1920 to 1940 and then a stronger increase from 1975 onwards. Thermometers and their local environments have changed, however these effects have been corrected for in a rigorous way. The global climate observing system uses an established set of climate monitoring principles to remove any systematic errors.
Sceptic view: Temperatures and their measurements fluctuate naturally over time and current changes are not exceptional.
There are records of the global mean temperature over the past 1000 years, which are inferred from proxies such as tree-rings and stalactites. These show that the temperature has varied throughout the last millennium, but have never climbed as high as current levels when averaged over periods such as 50 years. The "mediaeval warm period" (around 1000 AD) and the "little ice age" (around 1650 AD) are examples of these variations. There are many different published studies, all of which reach the conclusion that recent period of higher global temperatures are unprecedented. There have been comments, repeated from elsewhere on the internet, suggesting that one of these studies by Michael Mann is scientifically flawed - in an attempt to discredit this conclusion. A US National Academy of Sciences report found that overall the conclusions of that study were sound although some statements about temperatures in particular years, for example, were not warranted.
Sceptic view: The world has been warmer in the past, when there were no humans releasing CO2.
The world has seen many climate variations in the past. Indeed there were times, 55 million years ago, when the Arctic had a more tropical climate and there was probably no ice at all. There are long records of temperature and CO2 from ice cores and other sources. During the past 500,000 years, the Earth has experienced a series of ice ages with ice sheets covering large portions of the northern hemisphere. These changes are probably driven by variations in the amount of solar radiation reaching the Earth at different latitudes. The CO2 levels also change during the ice ages, acting as one of many complex feedbacks on the solar-induced variations. What is happening now is different as humans are adding greenhouse gases. The natural variations continue but their combined effect over the last 50 years is very small.
Sceptic view: There is no evidence in the palaeo-record of a CO2 increase causing a temperature increase.
It is hard to imagine a natural process that would simply release a large quantity of CO2 by itself. It is far more likely that the climate changes for some other reason (there can be many), producing a change in CO2 that then feeds back on climate. In other words, an "analogue" for the current situation of human input of greenhouse gases has not happened in the palaeo record. Knowledge of the past does confirm that CO2 plays a role in determining the climate, even if it did not cause the main changes then.
Sceptic view: Climate models are too simple and do not include important factors.
Considerable misunderstanding has come to light in the challenge about climate models.. The atmosphere and ocean are governed by many interacting and well-established physical principles (primarily from geophysical fluid dynamics and thermodynamics). It is not practical to solve these equations by hand, so computers are used instead. Climate models are an expression of these physical equations on a discrete mesh. Some physical processes are too local to be explicit on this grid resolution, so representations are used to include their effects. Many processes are incorporated into climate models and at least one correspondent has inaccurately claimed each of the following is missing: variations/oscillations in the sun's input, emissions from volcanic eruptions, vertical motion in the ocean and atmosphere, weather systems in the atmosphere and eddies in the ocean, land cover changes, the sulphur cycle, sea level changes, carbon uptake of ocean life, emissions of aerosols. All of these processes are included in the most sophisticated climate models.
Sceptic view: No one can predict the climate many years into the future, because meteorologists cannot predict weather much more than a week ahead.
Climate models are based on the same physical equations as weather forecasting models, which are very successful and validated every day. The details of each weather system (timing etc) are no longer predictable beyond a week or two. However the average over the weather systems - the climate - is predictable for a lot longer. This is because the climate is determined by climate forcings such as the greenhouse effect, solar input etc. So the average weather can be described by the climate models even though the precise location etc of each weather system cannot. We know this because of comparison of the modelled climate with the measurements.
A climate forcing is something that causes a change in the climate, such as a change in the sun's energy reaching the earth, volcanic eruptions blocking out sunlight and a change in the amount of CO2.
Sceptic view: Climate models can be readily adjusted to produce any result deemed appropriate.
No climate model is perfect; however the large-scale features are well represented and we have high confidence in them. Climate models can replicate past climates, though we accept as we go further back in time there remain some uncertainties as to exactly what the climate was like. . The values of some parameters, such as the rate at which cloud droplets are formed, are known to within a certain tolerance and this represents an uncertainty in the models. However the model output can be compared with observations and parameter settings can be refined - sometimes pejoratively called "tuning". But the climate system, and the model, is so complicated that it is impossible to "fix" the results by artificially changing such parameters.
Sceptic view: Scientists are downplaying the degree of uncertainty in climate science.
We have been clear in our replies that we accept that there is still uncertainty in some areas of the science that we need to reduce. The lack of complete understanding about some aspects of the climate system does not mean that there is uncertainty in whether there will be increase in global temperature caused by greenhouse gases. It means, however, that the local consequences of climate change are not certain. The future amount of greenhouse gas in the atmosphere from human emissions, causes most uncertainty in the magnitude of climate change in predictions for the next century. Scientists are constantly working to further understand the climate system, and reduce the uncertainties.
Sceptic view: Recent local and/or brief cool periods are evidence against climate change. ("Why is it snowing in summer in Tasmania? Where is global warming?").
The climate system is never in a completely constant state; it varies naturally. However by averaging over time periods such as months or years we can disentangle the climate trend from the day-to-day variations. On smaller scales, the amount of natural variability increases. Just as a single thermometer reading does not represent the global mean temperature, the trend at an individual location does not represent the trend of a larger region. It is only by combining a large amount of observations that we can view the big picture of global climate change. For example, one extreme weather event is not, on its own, evidence one way or the other for global warming.
Sceptic view: The growth of an individual glacier or ice-sheet is a "clear contradiction… of global warming"
Ice sheets and glaciers are created and survive because over the course of many years the amount of snowfall is greater than the amount of melting. As temperatures rise under climate change, melting increases and the amount of precipitation that falls as snow decreases. Mountain glaciers and snow cover have declined on average in both hemispheres. The amount of global average rainfall is set to rise slightly and combined with the effects of natural variability (see above) has meant that some glaciers are actually growing at the moment. However, they are only a small part of the total number of glaciers and the changes in rainfall will fall a long way short of the amount required to offset the projected increase in temperatures. The large ice sheets over Greenland and Antarctica react to the same factors. Their response times are much longer, because of their greater size. Scientists project the East Antarctic ice sheet will initially grow because of increased snowfall, but, as the temperature increases further, it too may shrink.
Sceptic view: We will soon be entering a new ice age
The palaeo-record shows that for the last few million years the Earth has cycled between cool periods (ice ages) and warmer period, such as the last 10,000 years (called interglacials). The exact causes of past climate changes between ice ages and interglacials are complex and not fully understood, however changes in solar input are thought to play an important part. In the 1960s scientists published work relating to the solar cycle, suggesting we should be moving towards the next ice age. However, in recent decades, the detection of global warming and advances in our knowledge about climate science have made it clear that the current elevated CO2 levels from human input of greenhouse gases is causing much more rapid warming than the slow changes associated with ice ages.
Gathering and presenting results
Sceptic view: Climate research does not use the scientific method and is removed "from the realm of science."
We can only observe the past and current climate system, rather than make controlled real-life experiments (although human emission of carbon dioxide does represent a scientifically-uncontrolled experiment). Therefore, we have to build a model that encapsulates our knowledge about the physical processes in the climate system and then compare that to the observations. If our knowledge is correct then the model will replicate the observations. Climate models can replicate the observations of past and present climate and the path between them. This accusation probably stems from misconceptions about climate models.
Sceptic view: The process of publishing research allows too many errors and is biased.
Climate science is a very important and topical field, and hence has been receiving a lot of attention and scrutiny. There is a vast amount of information about climate science available, especially on the internet. However, this information is of varying quality and reliability. The best available method to identify the highest quality science is called peer review. This consists of having independent experts (peers) reading the work critically to determine whether it is credible science and should be published or not. This process is not infallible, and very rarely fraud and mistakes do occur. Published papers are open to public scrutiny so if the paper is not of sufficient quality it is quickly exposed as such. This is the best method available to establish scientific quality and it allows science to progress in a timely fashion.
Sceptic view: There is no consensus about climate change
Consensus does not mean that every single scientist agrees with man-made climate change, but that the vast majority does agree. The strongest evidence of this majority is the recent report of the Intergovernmental Panel on Climate Change (IPCC). This panel consists of over 1000 of the leading climate scientists from around the world. They have produced a comprehensive summary of the state of climate science. They have not performed new research, but rather they have collected the results from the peer-reviewed literature.
Sceptic view: The IPCC is biased and ignores disagreements
The IPCC does not attempt to force a consensus where none exists. The rule is: if there is controversy in the literature then the IPCC reports must reflect that there is controversy. The IPCC's fourth assessment report will be published later this year, but the executive summary was released on 2 February.
Sceptic view: Preventing further climate change is a waste of money
NERC does not determine government policy, but it is committed to providing the best possible guidance on the science of climate change. However, the recent Stern Review provides an economic assessment, in which lower-probability, high-impact climate changes mean that the world should allocate the necessary resources to reduce greenhouse gases.
Sceptic view: NERC only funds research that will agree with government policy
NERC is independent from the government (although it receives its resources from them) and determines which science to fund entirely by itself - the so-called Haldane principle. It funds projects based on the quality of the science proposals received rather than their anticipated results. NERC is deliberately set up in this manner to remove any possible bias.