By Alex Kirby
BBC News Online environment correspondent
Most climate scientists are convinced they are right to warn us the prospect ahead is alarming unless we act soon.
Parts of the Antarctic ice shelf have broken away
They accept there are uncertainties but say human activities are having a clear effect on natural climate change, and that the Earth could warm dangerously.
Their critics say the evidence so far is not conclusive, and think the human impact is so small as to be negligible.
But recent findings suggest there are real causes for concern at the speed with which the Earth is now heating up.
Rapid carbon build-up
The Intergovernmental Panel on Climate Change is starting work on its fourth assessment report, which should ready by 2007.
One rapidly changing phenomenon is the atmospheric concentration of carbon dioxide (CO2), the main greenhouse gas given off by human activities.
Analysis of an ice core drilled from the Antarctic shows the level fluctuated over the last 500,000 years between about 200 parts per million (ppm) during ice ages to more like 270ppm in warmer inter-glacial periods.
Before the start of the Industrial Revolution about 200 years ago, the level of CO2 in the atmosphere was around 270-280ppm.
It reached 360ppm in the 1990s and recently climbed to a high of 379ppm. The year-on-year average rise is currently 2ppm.
Peat bogs are rich in carbon and methane
There is concern that Greenland's ice sheet could disappear within the next 1,000 years if global warming continues at its present rate.
Studies forecast an 8C increase in Greenland's temperature by 2350, and researchers believe that if the ice cap melts, global average sea level will rise by about 7m (23ft).
Even if global warming were halted, they say, the rise could be irreversible. This is because it can take decades or even centuries for actions to produce effects.
Another worry is whether peatlands could release vast amounts of CO2 into the atmosphere.
The release, triggered by the higher atmospheric carbon levels, would be an example of what is called "positive feedback", when warming itself causes a further temperature rise.
Scientists say the rate of release is accelerating at 6% a year, which they think means that by 2060 the peat could account for greater carbon emissions than the burning of fossil fuels.
The permafrost of northern Europe and North America is known to contain large quantities of methane, a potent greenhouse gas, which could also be released as global warming thaws the tundra.
Professor Mike Hulme, executive director of the Tyndall Centre for Climate Change Research, told BBC News Online: "The evidence that feedbacks are occurring is one of the most striking recent findings.
The part clouds play is still unclear
"Other pieces of evidence are last year's heatwave in Europe, and the suggestion that climate change could mean a million species will be at risk of extinction by 2050."
Dr Geoff Jenkins, of the Met Office's Hadley Centre for Climate Prediction and Research, told BBC News Online: "Over the last few decades there's been much more evidence for the human influence on climate.
"We've reached the point where it's only by including human activity that we can explain what's happening.
"The feedbacks mean that by the end of this century we'll have lost a lot of the free buffering that nature provides.
"As wetlands grow wetter and hotter, for instance, by 2100 they'll probably account for as much methane as human activities."
Both he and Professor Hulme agree, though, that many uncertainties remain. These include the role played by clouds and solar radiation.