Two climate science experts go head to head
YES, says Chris Rapley, director of the Science Museum and Professor of Climate Science at University College London
Human beings have shaped the world to suit their needs. Through agriculture and engineering we have assured our supplies of food and water, and many of us enjoy lives of unprecedented richness and security. Two key factors have underpinned this success — the recent unusual stability of the global climate system and our exploitation of cheap and convenient energy from coal, oil and natural gas.
Early in the industrial age, such scientist-thinkers as Joseph Fourier and John Tyndall pondered the consequences of the massive burning of carbon-based fuels. They understood that water vapour, carbon dioxide and other minor constituents of the atmosphere warm the surface of the Earth, allowing life to exist and prosper. They reasoned that by increasing the atmospheric concentration of these greenhouse gases, we would increase the warming effect.
Analysis of air trapped for millennia in Antarctic ice shows that the growth in atmospheric CO2 between the last Ice Age and the Industrial Revolution has already been matched by what we have generated since. And we can detect the resultant gradual warming. Eleven of the past 13 years have been the hottest since records began, with measurable impacts on the distribution of vegetation and ice cover. The global mean sea level, stable for several thousand years, has begun to rise as seawater warms and expands, and because of the melting of ice on land.
Of course climate is affected by other factors. These include “natural” oscillations, the changing Sun, small variations in the Earth’s orbit, the wandering of the continents, even the evolution of life.
It is always possible to find a local temperature record or a wobble in a graph that shows a cooling, or to postulate some novel process that will compensate for enhanced greenhouse warming and make everything OK. Would that it were true. But the overall pattern of results shows otherwise.
The evidence is sufficiently compelling for the science academies of America, Brazil, Canada, China, France, Germany, India, Italy, Japan, Russia and the United Kingdom to conclude: “There is now strong evidence that significant global warming is occurring. It is likely that most of the warming in recent decades can be attributed to human activities.”
Changes in climate threaten food and water supplies, as well as world security. We need to manage these risks and reduce their likelihood. Yet most of us are utterly dependent on fossil fuel energy. This is why the problem is so difficult and generates such controversy.
The negotiations in Copenhagen this month, and into the future, are critical. The words of John F. Kennedy have never been more relevant: “Our problems are man-made, therefore they may be solved by Man.”
Prove It! All the Evidence You Need to Believe in Climate Change is on now at the Science Museum, London, and is free to visit
NO, says Richard S. Lindzen, Alfred P. Sloan Professor of Atmospheric Sciences, Massachusetts Institute of Technology
Or rather, we can’t tell. “Global warming” refers to changes in an index known as the global mean temperature anomaly. This index has increased irregularly by about 0.75C since the Industrial Revolution began, but it always shows some warming or cooling, and fluctuations of 0.5C are common. Claims of record-breaking years hinge on fluctuations of tenths of a degree. Such changes go unnoticed because local fluctuations are much larger and significantly uncorrelated with the global index. Nevertheless, when the UN Intergovernmental Panel on Climate Change issued its statement that man was likely responsible for most of the warming since 1957, it was essentially referring to this index. The statement was hardly alarming and was consistent with Man having a small impact.
So how was the claim arrived at? It was argued that climate models could not account for this slight warming unless forced to do so, and the only “forcing” the modellers could think of was Man. Making this assertion assumed that their models already represented a major natural source of variability — the atmosphere-ocean system, manifested in phenomena such as El Niño, the Pacific decadal oscillation and the Atlantic multi-decadal oscillation.
Within a year of the 2007 IPCC report, scientists from modelling centres in the UK and Germany acknowledged that the failure of the models to predict the relatively static temperatures of recent years was due precisely to the models’ failure to adequately display natural variability. This hardly proves that Man made no contribution, but it certainly does invalidate the claimed attribution.
In any case, are variations of tenths of a degree in global mean temperature what people are worried about? Not likely. The concern is with larger warming in the future as well as melting Arctic sea ice, rising sea levels, suffering polar bears etc. For the first item, one might ask how models predicting large warming were made consistent with the modest warming of the past 52 years. The answer from scientists at the US National
Centre for Atmospheric Research in Colorado is that each model was separately “adjusted” by a cooling caused by largely unknown aerosols — the aerosols being considered part of Man’s contribution.
For the more dramatic associations, we could argue about what is really going on, about the cherry-picking of data etc, but that would be beside the point. The important point is that these are complex phenomena that depend on the confluence of many factors, of which global warming is generally not the most important. Consider the following scenario: Person A kicked up some dirt, leaving an indentation in the ground into which a rock fell, and B tripped on this rock and bumped into C, who was carrying a carton of eggs that dropped and broke. Would any rational person conclude that the best way to prevent this would be to prohibit kicking dirt? Yet this is precisely the “logic” that will dominate Copenhagen.