"The global concentrations of carbon dioxide, methane and nitrous oxide – all greenhouse gases – have increased markedly since 1750," said Susan Solomon, co-chair of Working Group I, at the press conference for the report’s release. "There can be no question that the increases in these greenhouse gases are dominated by human activity. We can now be very confident that the net human effect since 1750 has been one of warming – these gases are not being cancelled out by the aerosol effect by any means."

Atmospheric aerosols formed as a result of pollution by sulphates, organic carbon, black carbon, nitrate and dust can cool the climate but improvements in in situ, ground-based and satellite measurements have led to increased knowledge about the size of the effect.

"In terms of direct observations of recent climate change, warming of the climate system is now unequivocal," said Solomon. "That's evident in observations of global average air and ocean temperatures, melting of snow and ice, and global average sea level. These are changing in the way that one could expect if the world is warming, so clearly it must be warming."

The working group also reports evidence that humans have influenced other aspects of climate besides air temperature, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.

But some aspects of climate don't appear to have changed. Although the third assessment report found a decrease in diurnal temperature range, the fourth looked at updated data and found that the range has not altered from 1979 to 2004 as both day- and night-time temperature have risen at about the same rate.

There were also no significant trends in the extent of Antarctic sea ice, despite changes from year to year and localized changes. And there’s not enough evidence to show trends in the meridional overturning circulation – also known as the ocean conveyor belt – or in small-scale phenomena, such as tornadoes, hail, lightning and dust storms.

Produced by around 600 authors, the IPCC working group I report looks at the predictions of 23 climate models – 12 more than used in the third assessment report. The models were developed by 16 modelling groups in 11 countries and each was run between four and eight times.

"The ability to do the multiple runs allows us to be much more quantitative," said Solomon. "What’s really exciting to us is that, because of the very extensive database of model results that is now available, we can give best-guess estimates to how much the temperature would warm if we followed one of the emissions scenarios."

This is the first time that the IPCC is providing such estimates. The report says that a greenhouse gas level of 650 ppm would be likely to warm the climate by around 3.6 °C; a level of 750 ppm would lead to a warming of 4.3 °C; 1000 ppm would lead to 5.5 °C; and 1200 ppm to 6.3 °C. To put this in context, the global atmospheric concentration of carbon dioxide in 2005 was 379 ppm while pre-industrial levels were roughly 280 ppm.

"If we were to emit greenhouse gases above or at current rates, that would lead to bigger changes in the coming century than we saw in the last century," said Solomon.

The fourth assessment report also contains new summaries regarding patterns of climate change. "There is higher confidence now in projected patterns of warming, and in wind-pattern changes, changes of precipitation and at least some aspects of extremes and of ice," said Solomon. "We expect the pattern to be one of drying in tropical regions and more precipitation at higher latitudes. We can’t project those in detail but the broad patterns can be better projected now."

The IPCC predicts that sea levels by the end of the century will be 28–58 cm higher than 1989–1999 levels as a result of ocean expansion and glacier melt. The 2001 report gave a best estimate of 9–88 cm.

But these predictions don't take into account rapid changes in ice sheets or carbon-cycle feedbacks. According to Solomon, that's because there's no information in the scientific literature on rapid ice discharge that might occur in future, making it difficult to include data values in the models. Sea-level rises of up to 1 m by 2100 "cannot be ruled out if ice sheets continue to melt as temperature rises".

"When it comes to sea-level rise there is improved information on glaciers but there is a big uncertainty on the melting of the ice sheets," said Solomon. "Some factors have been qualified but others remain uncertain."

The report predicts that sea ice will shrink in both the Arctic and the Antarctic regions. If human emissions grow to the higher end of current estimates, large areas of the Arctic Ocean could lose year-round ice cover by the end of the century.

According to the working group's best estimate, melting of the Greenland ice sheet would be a slow process. "If temperatures were to exceed some value – somewhere between 1.9 and 4.6 °C warmer than pre-industrial levels – and were sustained for millenia, eventually one would expect the Greenland ice sheet to melt," said Solomon. "That would raise sea level by 7 m – comparable to what we can infer happened 125 000 years ago when polar temperatures were similar."

While the fourth assessment report has resolved a number of uncertainties, including reconciling the temperature record of the lower atmosphere from satellite measurements with the ground-based record, uncertainties still remain. These include the roles played by clouds, glaciers, ice caps, oceans, deforestation and other land-use changes, and the linking of climate and biogeochemical cycles.

A recent paper in Sciencexpress compares climate model projections in the 2001 IPCC report to actual measurements. Stefan Rahmstorf and colleagues found that atmospheric carbon dioxide concentrations followed the projections almost exactly, global-mean surface temperatures were in the upper part of the predicted range, and the observed sea level has been rising faster than the models projected.

The IPCC says that its reports are "not policy prescriptive but highly policy relevant".

Following the release of the IPCC working group I report, Yvo de Boer, executive secretary of the United Nations Framework Convention on Climate Change, has called for speedy and decisive international action to combat climate change. "The findings, which governments have agreed upon, leave no doubt as to the dangers mankind is facing and must be acted upon without delay," he said. "Any notion that we do not know enough to move decisively against climate change has been clearly dispelled."

The Working Group 1 Summary for Policymakers (PDF) is currently available on the web. The full report – “Climate Change 2007: the Physical Science Basis” – will be published by Cambridge University Press later in the year.

The IPCC working group II will report on climate impacts and adaptation on 6 April 2007 while working group III’s report on mitigation is due on 4 May. A synthesis report integrating all aspects of climate change will come out on 16 November.