The Kyoto protocol comes into force on 16 February, yet the sceptics tell us global warming is nothing but a fantasy dreamed up by climate scientists. Could these naysayers possibly be right?

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

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