"Human influence on the climate system is clear."
With these words, Thomas Stocker of the University of Bern in Germany summed up the new assessment of climate science by the Intergovernmental Panel on Climate Change (IPCC). Stocker was one of the report's co-chairs.
The new report, published last Friday, reaffirms the findings of the previous assessment in 2007
But there is always more to learn, and the report's individual chapters contain a lot of new science. New Scientist breaks down the most important new findings.
Ice and sea
The past six years have been a golden age for ice studies. "Polar regions have been changing very rapidly, providing data for our projections on sea ice, snow cover, ice sheets and sea level rise," says David Vaughan of the British Antarctic Survey in Cambridge, UK, the lead author of the cryosphere chapter.
New understanding of how big ice sheets on Greenland and Antarctica might break up has forced the IPCC to almost double its estimates of likely sea level rise by the end of the century – to as much as 1 metre.
But the authors controversially dismissed the work of scientists who think it could be 2 metres or more. "Those predictions are based on extrapolation, but we have better information than that now, based on knowledge of ice processes," says Tony Payne of the University of Bristol, UK, lead author on sea level change.
Vaughan agrees. "We are now more confident that ice sheet collapse isn't going to happen in the next few decades." But the Antarctic ice sheets could have an Achilles heel, such as Pine Island glacier. "If anything scary happens it will be in Antarctica," says Payne.
Up in the air
Another key issue is the effect of clouds. "IPCC hasn't done a good job on clouds before," says clouds lead author Piers Forster of the University of Leeds, UK. "They were a big unknown in modelling warming."
In 2007, it was uncertain even whether clouds cooled or warmed the planet overall. "But we now believe that they are a positive feedback on temperature," he says. "Their warming effect will intensify with global warming."
Forster's chapter also reports on another important uncertainty: the cooling effect of smoke and other aerosols, which some argued almost negated the warming effect of greenhouse gases in the short term.
It now seems aerosols are counteracting less global warming than previously thought. This is good news for attempts to clean up smog. If the aerosols had been keeping a lid on warming, cleaning up smog could have produced a dangerous surge in warming. This now seems less likely to happen, so we can save lives by getting rid of air pollution without worrying too much about this potential downside.
Short-term blips
An important emerging issue, according to Stocker, is whether the unexpected hiatus in atmospheric warming over the past 15 years is a blip or evidence of a longer term trend.
The oceans could provide the explanation, says Stephen Rintoul of Australia's CSIRO research organisation, the oceans lead author. Temperature sensors in the oceans suggested that the surface layers joined the hiatus after 2003. "But the deeper ocean shows no slowing in warming, and sea levels continue to rise – which we believe is still mostly down to thermal expansion," says Rintoul. If heat is being redistributed in the oceans, the cooler surface could be cooling the air above.
This report also has more than previous ones on the effects of "near term" warming up to 2050, says Rowan Sutton of the University of Reading, UK.
It now seems this warming will probably be distinguishable from natural variability much faster in tropical regions than in mid-latitudes. That is a crucial threshold. "Once natural systems exceed the bounds of natural variability, there is greater potential for rapid and unpredictable change," says Sutton.
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