Glaciers are retreating. Ocean temperatures are going up. Animal, bird, fish and plant species are being found in new environments in which temperatures were once too low to allow them to thrive. The reasons for this trend are less certain. The Earth’s climate changes naturally all the time. Witness the many occasions over millions of years in which the planet has slipped into and out of ice ages.

Peaks and troughs in solar activity and fluctuations in the concentrations of atmospheric greenhouse gases that keep the world warm enough for life occur independently of humanity and drive climate change.

The big question is the extent to which natural variation and human activity are responsible for the current warming pattern. Scientists know that the world is getting warmer of its own accord.

The expert consensus, however, is that while natural climate change accounts for some warming, it cannot account for it all. The other influence on climate is the transformation of greenhouse gas levels that began with the industrial revolution and the large-scale burning of fossil fuels.

In pre-industrial times, concentrations of carbon dioxide, the most important greenhouse gas after water vapour, stood at about 280 parts per million. Today the concentration is about 375ppm.

This matters, because evidence from ice cores drilled from Antarctica and Greenland has shown that carbon dioxide levels have a clear influence on global temperatures. Most scientists are convinced that there is a direct and causal link between the extraordinary rise in carbon dioxide levels, caused by human activity, and the rise in world temperatures.

The combined evidence from many fields of research has become so overwhelming that the national science academies of all the G8 nations, along with those of China, India and Brazil, last month issued an unprecedented joint statement.

“There is now strong evidence that significant global warming is occurring,” the document said. “The evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures, and from phenomena such as increasing sea levels, retreating glaciers and changes to many physical and biological systems. It is likely that most of the warming in recent decades can be attributed to human activities.”

The biggest question of all is what this means for the future — and it is here that the greatest controversies arise. The Intergovernmental Panel on Climate Change, a collection of climate experts who examine all the evidence, predicts that world temperatures will rise by between 1.4C (2.5F) and 5.8C (10.4F) by 2100. The projections, however, rely on computer models, the accuracy of which are disputed by a vocal minority of scientists.

One, Richard Lindzen, professor of atmospheric science at Massachusetts Institute of Technology, says that rising water vapour in the atmosphere should increase cloud cover, ensuring that more heat is reflected back into space, protecting the Earth from severe heating. Others, notably the Danish economist Bjørn Lomborg, accept that climate change is happening, that it is human induced, and that the computer models are probably right. He believes, however, that attempts to reduce greenhouse emissions are doomed to failure, largely because carbon dioxide lives so long in the atmosphere, taking between 50 and 200 years to dissipate.

A recent study from the US National Centre for Atmospheric Research found that even if greenhouse gas levels stabilised at 2000 levels, temperatures would rise by 0.5C (0.9F) and sea levels by 11cm by 2100.

Even at the lower end of IPCC range, the impact of rising temperatures is projected to be severe. Ice sheets in Antarctica and Greenland are expected to melt partially, causing a sea level rise of between 10cm and 90cm.

Flooding in coastal and low-lying regions, including parts of the Thames Estuary and East Anglia, would become commonplace. The Government’s recent Foresight study predicted that flooding could become 30 times more common in Britain by 2080. Extreme weather events are forecast to become more common, as warmer temperatures draw more water vapour into the atmosphere.

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