Quote

...The hostile criticism is coming from sceptics who question the reality of climate change. Critics have always been around, but in recent months their voices have become increasingly prominent and influential. One British newspaper called climate change a "global fraud" based on "left-wing, anti-American, anti-west ideology". A London-based think tank described the UK's chief scientific adviser, David King, as "an embarrassment" for believing that climate change is a bigger threat than terrorism. And the bestselling author Michael Crichton, in his much publicised new novel State of Fear, portrays global warming as an evil plot perpetrated by environmental extremists.

If the sceptics are to be believed, the evidence for global warming is full of holes and the field is riven with argument and uncertainty. The apparent scientific consensus over global warming only exists, they say, because it is enforced by a scientific establishment riding the gravy train, aided and abetted by governments keen to play the politics of fear. It's easy to dismiss such claims as politically motivated and with no basis in fact - especially as the majority of sceptics are economists, business people or politicians, not scientists (see "Meet the sceptics"). But there are nagging doubts. Could the sceptics be onto something? Are we, after all, being taken for a ride?

This is perhaps the most crucial scientific question of the 21st century. The winning side in the climate debate will shape economic, political and technological developments for years, even centuries, to come. With so much at stake, it is crucial that the right side wins. But which side is right? What is the evidence that human activity is warming the world, and how reliable is it?

First, the basic physics. It is beyond doubt that certain gases in the atmosphere, most importantly water vapour and carbon dioxide, trap infrared radiation emitted by the Earth's surface and so have a greenhouse effect. This in itself is no bad thing. Indeed, without them the planet would freeze. There is also no doubt that human activity is pumping CO2 into the atmosphere, and that this has caused a sustained year-on-year rise in CO2 concentrations. For almost 60 years, measurements at the Mauna Loa observatory in Hawaii have charted this rise, and it is largely uncontested that today's concentrations are about 35 per cent above pre-industrial levels (see Graph).

The effect this has on the planet is also measurable. In 2000, researchers based at Imperial College London examined satellite data covering almost three decades to plot changes in the amount of infrared radiation escaping from the atmosphere into space - an indirect measure of how much heat is being trapped. In the part of the infrared spectrum trapped by CO2 - wavelengths between 13 and 19 micrometres - they found that between 1970 and 1997 less and less radiation was escaping. They concluded that the increasing quantity of atmospheric CO2 was trapping energy that used to escape, and storing it in the atmosphere as heat. The results for the other greenhouse gases were similar.

These uncontested facts are enough to establish that "anthropogenic" greenhouse gas emissions are tending to make the atmosphere warmer. What's more, there is little doubt that the climate is changing right now. Temperature records from around the world going back 150 years suggest that 19 of the 20 warmest years - measured in terms of average global temperature, which takes account of all available thermometer data - have occurred since 1980, and that four of these occurred in the past seven years (see Graph).

The only serious question mark over this record is the possibility that measurements have been biased by the growth of cities near the sites where temperatures are measured, as cities retain more heat than rural areas. But some new research suggests there is no such bias. David Parker of the UK's Met Office divided the historical temperature data into two sets: one taken in calm weather and the other in windy weather. He reasoned that any effect due to nearby cities would be more pronounced in calm conditions, when the wind could not disperse the heat. There was no difference.

It is at this point, however, that uncertainty starts to creep in. Take the grand claim made by some climate researchers that the 1990s were the warmest decade in the warmest century of the past millennium. This claim is embodied in the famous "hockey stick" curve, produced by Michael Mann of the University of Virginia in 1998, based on "proxy" records of past temperature, such as air bubbles in ice cores and growth rings in tree and coral. (see "Areas of contention") Sceptics have attacked the findings over poor methodology used, and their criticism has been confirmed by climate modellers, who have recently recognised that such proxy studies systematically underestimate past variability. As one Met Office scientist put it: "We cannot make claims as to the 1990s being the warmest decade."

There is also room for uncertainty in inferences drawn from the rise in temperature over the past 150 years. The warming itself is real enough, but that doesn't necessarily mean that human activity is to blame. Sceptics say that the warming could be natural, and again they have a point. It is now recognised that up to 40 per cent of the climatic variation since 1890 is probably due to two natural phenomena. The first is solar cycles, which influence the amount of radiation reaching the Earth, and some scientist have argued that increased solar activity can account for most of the warming of the past 150 years. The second is the changing frequency of volcanic eruptions, which produce airborne particles that can shade and hence cool the planet for a year or more. This does not mean, however, that the sceptics can claim victory, as no known natural effects can explain the 0.5 °C warming seen in the past 30 years. In fact, natural changes alone would have caused a marginal global cooling (see Graph).

In the face of such evidence, the vast majority of scientists, even sceptical ones, now agree that our activities are making the planet warmer, and that we can expect more warming as we release more CO2 into the atmosphere. This leaves two critical questions. How much warming can we expect? And how much should we care about it? Here the uncertainties begin in earnest.

The concentration of CO2 in the atmosphere now stands at around 375 parts per million. A doubling of CO2 from pre-industrial levels of 280 parts per million, which could happen as early as 2050, will add only about 1 °C to average global temperatures, other things being equal. But if there's one thing we can count on, it is that other things will not be equal; some important things will change.

All experts agree that the planet is likely to respond in a variety of ways, some of which will dampen down the warming (negative feedback) while others will amplify it (positive feedback). Assessing the impacts of these feedbacks has been a central task of the UN's Intergovernmental Panel on Climate Change, a co-operative agency set up 17 years ago that has harnessed the work of thousands of scientists. Having spent countless hours of supercomputer time creating and refining models to simulate the planet's climate system, the IPCC concludes that the feedbacks will be overwhelmingly positive. The only question, it says, is just how big this positive feedback will be.

The latest IPCC assessment is that doubling CO2 levels will warm the world by anything from 1.4 to 5.8 °C. In other words, this predicts a rise in global temperature from pre-industrial levels of around 14.8 °C to between 16.2 and 20.6 °C. Even at the low end, this is probably the biggest fluctuation in temperature that has occurred in the history of human civilisation. But uncertainties within the IPCC models remain, and the sceptics charge that they are so great that this conclusion is not worth the paper it is written on. So what are the positive feedbacks and how much uncertainty surrounds them?

Melting of polar ice is almost certainly one. Where the ice melts, the new, darker surface absorbs more heat from the sun, and so warms the planet. This is already happening. The second major source of positive feedback is water vapour. As this is responsible for a bigger slice of today's greenhouse effect than any other gas, including CO2, any change in the amount of moisture in the atmosphere is critical. A warmer world will evaporate more water from the oceans, giving an extra push to warming. But there is a complication. Some of the water vapour will turn to cloud, and the net effect of cloudier skies on heat coming in and going out is far from clear. Clouds reflect energy from the sun back into space, but they also trap heat radiated from the surface, especially at night. Whether warming or cooling predominates depends on the type and height of clouds. The IPCC calculates that the combined effect of extra water vapour and clouds will increase warming, but accepts that clouds are the biggest source of uncertainty in the models.

Sceptics who pounce on such uncertainties should remember, however, that they cut both ways. Indeed, new research based on thousands of different climate simulation models run using the spare computing capacity of idling PCs, suggest that doubling CO2 levels could increase temperatures by as much as 11 °C (Nature, vol 434, p 403).

Recent analysis suggests that clouds could have a more powerful warming effect than once thought - possibly much more powerful (New Scientist, 24 July 2004, p 44). And there could be other surprise positive feedbacks that do not yet feature in the climate models. For instance, a release of some of the huge quantities of methane, a potent greenhouse gas, that are frozen into the Siberian permafrost and the ocean floor could have a catastrophic warming effect. And an end to ice formation in the Arctic could upset ocean currents and even shut down the Gulf Stream - the starting point for the blockbuster movie The Day After Tomorrow.

There are counterbalancing negative feedbacks, some of which are already in the models. These include the ability of the oceans to absorb heat from the atmosphere, and of some pollutants - such as the sulphate particles that make acid rain - to shade the planet. But both are double-edged. The models predict that the ocean's ability to absorb heat will decline as the surface warms, as mixing between less dense, warm surface waters and the denser cold depths becomes more difficult. Meanwhile, sulphate and other aerosols could already be masking far stronger underlying warming effects than are apparent from measured temperatures. Aerosols last only a few weeks in the atmosphere, while greenhouse gases last for decades. So efforts to cut pollution by using technologies such as scrubbers to remove sulphur dioxide from power station stacks could trigger a surge in temperatures.
“Efforts to cut aerosol pollution could trigger a surge in temperatures”

Sceptics also like to point out that most models do not yet include negative feedback from vegetation, which is already growing faster in a warmer world, and soaking up more CO2. But here they may be onto a loser, as the few climate models so far to include plants show that continued climate change is likely to damage their ability to absorb CO2, potentially turning a negative feedback into a positive one.
Achilles' heel?

More credible is the suggestion that some other important negative feedbacks have been left out. One prominent sceptic, meteorologist Richard Lindzen of the Massachusetts Institute of Technology, has made an interesting case that warming may dry out the upper levels of the innermost atmospheric layer, the troposphere, and less water means a weaker greenhouse effect. Lindzen, who is one of the few sceptics with a research track record that most climate scientists respect, says this drying effect could negate all the positive feedbacks and bring the warming effect of a doubling of CO2 levels back to 1 °C. While there is little data to back up his idea, some studies suggest that these outer reaches are not as warm as IPCC models predict (see "Areas of contention). This could be a mere wrinkle in the models or something more important. But if catastrophists have an Achilles' heel, this could be it.

Where does this leave us? Actually, with a surprising degree of consensus about the basic science of global warming - at least among scientists. As science historian Naomi Oreskes of the University of California, San Diego, wrote in Science late last year (vol 306, p 1686): "Politicians, economists, journalists and others may have the impression of confusion, disagreement or discord among climate scientists, but that impression is incorrect."

Her review of all 928 peer-reviewed papers on climate change published between 1993 and 2003 showed the consensus to be real and near universal. Even sceptical scientists now accept that we can expect some warming. They differ from the rest only in that they believe most climate models overestimate the positive feedback and underestimate the negative, and they predict that warming will be at the bottom end of the IPCC's scale

For the true hard-liners, of course, the scientific consensus must, by definition, be wrong. As far as they are concerned the thousands of scientists behind the IPCC models have either been seduced by their own doom-laden narrative or are engaged in a gigantic conspiracy. They say we are faced with what the philosopher of science Thomas Kuhn called a "paradigm problem".

"Most scientists spend their lives working to shore up the reigning world view - the dominant paradigm - and those who disagree are always much fewer in number," says climatologist Patrick Michaels of the University of Virginia in Charlottesville, a leading proponent of this view. The drive to conformity is accentuated by peer review, which ensures that only papers in support of the paradigm appear in the literature, Michaels says, and by public funding that gives money to research into the prevailing "paradigm of doom". Rebels who challenge prevailing orthodoxies are often proved right, he adds.

But even if you accept this sceptical view of how science is done, it doesn't mean the orthodoxy is always wrong. We know for sure that human activity is influencing the global environment, even if we don't know by how much. We might still get away with it: the sceptics could be right, and the majority of the world's climate scientists wrong. It would be a lucky break. But how lucky do you feel?...