Our understanding of climate change began with intense debates amongst 19th century scientists about whether northern Europe had been covered by ice thousands of years ago. In the 1820s Jean Baptiste Joseph Fourier discovered that "greenhouse gasses" trap heat radiated from the Earth's surface after it has absorbed energy from the sun. In 1859 John Tyndall suggested that ice ages were caused by a decrease in the amount of atmospheric carbon dioxide. In 1896 Svente Arrhenius showed that doubling the carbon dioxide content of the air would gradually raise global temperatures by 5-6C - a remarkably prescient result that was virtually ignored by scientists obsessed with explaining the ice ages.
The idea of global warming languished until 1938, when Guy S Callender suggested that the warming trend revealed in the 19th century had been caused by a 10% increase in atmospheric carbon dioxide from the burning of fossil fuels. At this point scientists were not alarmed, as they were confident that most of the carbon dioxide emitted by humans had dissolved safely in the oceans. However, this notion was dispelled in 1957 by Hans Suess and Roger Revelle, who discovered a complex chemical buffering system which prevents sea water from holding on to much atmospheric carbon dioxide.
The possibility that humans could contribute to global warming was now being taken seriously by scientists, and by the early 1960s some had begun to raise the spectre of severe climate change within a century. They had started to collect evidence to test the idea that global temperatures were increasing alongside greenhouse gas emissions, and to construct mathematical models to predict future climates.
In 1958 Charles Keeling began long-term measurements of atmospheric carbon dioxide at the Mauna Loa observatory in Hawaii. Looked at now, the figures show an indisputable annual increase, with roughly 30% more of the gas relative to pre-industrial levels in today's atmosphere - higher than at any time in the last 700,000 years. Temperature readings reveal an average warming of 0.5-0.6C over the last 150 years.
Climate change sceptics have pointed out that these records could have been due to creeping urbanisation around weather stations, but it is now widely accepted that this 'urban heat island effect' is relatively unimportant and that it doesn't explain why most of the warming has been detected far away from cities, over the oceans and the poles.
Since the 1960s, evidence of global warming has continued to accumulate. In 1998 Michael Mann and colleagues published a detailed analysis of global average temperature over the last millennium known as the "hockey stick graph", revealing a rapid temperature increase since the industrial revolution. Despite concerted efforts to find fault with Mann's methodology, his basic result is now accepted as sound. Then, in 2005, just as the Kyoto Protocol for limiting greenhouse gas emissions was ratified, James Hansen and his team detected a dramatic warming of the world's oceans - just as expected in a warming world.
There is now little doubt that the temperature increase over the last 150 years is real, but debate still surrounds the causes. We know that the warming during the first half of the last century was almost certainly due to a more vigorous output of solar energy, and some scientists have suggested that increased solar activity and greater volcanic emissions of carbon dioxide are responsible for all of the increase. But others point out that during the last 50 years the sun and volcanoes have been less active and could not have caused the warming over that period.
By 2005 a widespread scientific consensus had emerged that serious, large-scale disruption could occur around 2050, once average global temperature increase exceeds about 2C, leading to abrupt and irreversible changes. These include the melting of a large proportion of the Greenland ice cap (now already under way), the reconfiguration of the global oceanic circulation, the disappearance of the Amazon forest, the emission of methane from permafrost and undersea methane hydrates, and the release of carbon dioxide from soils.
This new theory of "abrupt climate change" has overturned earlier predictions of gradual change, and has prompted some scientists to warn that unmitigated climate change could lead to the complete collapse of civilisation. Fears have been fuelled by the possibility that smoke, hazes and particles from burning vegetation and fossil fuels could be masking global warming by bouncing solar energy back to space. This "global dimming" effect is diminishing as we clean up air pollution. As a result global average temperature could rise by as much as 10 degrees Celsius by the close of the century - a catastrophic increase.
A more conservative assessment by the Intergovernmental Panel on Climate Change (IPCC) in 2001 indicated that with unabated carbon emissions, global temperature could rise gradually to around 5.8C by 2100. An increase of this nature would still threaten the lives of millions of people, particularly in the global south, due to sea level rise and extreme weather events.
Although some people still deny that climate change is a problem we can do something about, last year the UK government indicated that it was on board. The Stern Review showed that without immediate and relatively inexpensive action, climate change would lead to severe and permanent global economic depression by 2050. There is now a strong scientific and economic consensus about the severity of the climate crisis.
· Stephan Harding is Coordinator of the MSc in Holistic Science at Schumacher College in Devon, UK. He is the author of Animate Earth: Science, Intuition and Gaia. To order a copy for £9.95 with free UK p&p call 0870 836 0875 or go to theguardian.com/bookshop.