Climate change meltdown unlikely but human disaster looms, claims new research

But forecast global temperature rise of 4C heralds disaster for large swaths of planet with oceans absorbing most global warming
Fiona Harvey The Guardian 19 May 13;

Some of the most extreme predictions of global warming are unlikely to materialise, new scientific research has suggested, but the world is still likely to be in for a temperature rise of double that regarded as safe.

The researchers said that warming was most likely to reach about 4C above pre-industrial levels if the past decade's readings were taken into account.

That would still lead to catastrophe across large swaths of the Earth, causing droughts, storms, floods and heatwaves and with drastic effects on agricultural productivity leading to secondary effects such as mass migration.

Some climate change sceptics have suggested that because the highest global average temperature yet recorded was in 1998, climate change has stalled. The new study, which is published in the journal Nature Geoscience, shows a much longer "pause" would be needed to suggest that the world was not warming rapidly.

Alexander Otto, at the University of Oxford, lead author of the research, told the Guardian that there was much that climate scientists could still not fully factor into their models. He said that most of the recent warming had been absorbed by the oceans, but this would change as the seas heat up. The thermal expansion of the oceans is one of the main factors behind current and projected sea level rises.

The highest global average temperature ever recorded was in 1998, under the effects of a strong El Niño, a southern Pacific weather system associated with warmer and stormy weather, which oscillates with a milder system called La Niña. Since then, the trend of average global surface temperatures has shown a clear rise above the long-term averages – the 10 warmest years on record have been since 1998 – but climate sceptics have claimed that this represents a pause in warming.

Otto said that this most recent pattern could not be taken as evidence that climate change has stopped. "Given the noise in the climate and temperature system, you would need to see a much longer period of any pause in order to draw the conclusion that global warming was not occurring," he said. Such a period could be as long as 40 years of the climate record, he said.

Otto said the study found that most of the climate change models used by scientists were "pretty accurate". A comprehensive global study of climate change science is expected to be published in September by the Intergovernmental Panel on Climate Change, its first major report since 2007.

Jochem Marotzke, professor at the Max Planck Institute for Meteorology in Hamburg and a co-author of the paper, said: "It is important not to over-interpret a single decade, given what we know, and don't know, about natural climate variability. Over the past decade, the world as a whole has continued to warm, but the warming is mostly in the subsurface oceans rather than at the surface."

Other researchers also warned that there was little comfort to be taken from the new estimates – greenhouse gas emissions are rising at a far higher rate than had been predicted by this stage of the 21st century, and set to rise even further, so estimates for how much warming is likely will also have to be upped.

Richard Allan, reader in climate at the University of Reading, said: "This work has used observations to estimate Earth's current heating rate and demonstrate that simulations of climate change far in the future seem to be pretty accurate. However, the research also indicates that a minority of simulations may be responding more rapidly towards this overall warming than the observations indicate."

He said the effect of pollutants in the atmosphere, which reflect the sun's heat back into space, was particularly hard to measure.

He noted the inferred sensitivity of climate to a doubling of carbon dioxide concentrations based on this new study, suggesting a rise of 1.2C to 3.9C was consistent with the range from climate simulations of 2.2C to 4.7C. He said: "With work like this, our predictions become ever better."

Climate slowdown means extreme rates of warming 'not as likely'
Matt McGrath BBC News 19 May 13;

Scientists say the recent downturn in the rate of global warming will lead to lower temperature rises in the short-term.

Since 1998, there has been an unexplained "standstill" in the heating of the Earth's atmosphere.

Writing in Nature Geoscience, the researchers say this will reduce predicted warming in the coming decades.

But long-term, the expected temperature rises will not alter significantly.

The slowdown in the expected rate of global warming has been studied for several years now. Earlier this year, the UK Met Office lowered their five-year temperature forecast.

But this new paper gives the clearest picture yet of how any slowdown is likely to affect temperatures in both the short-term and long-term.

An international team of researchers looked at how the last decade would impact long-term, equilibrium climate sensitivity and the shorter term climate response.
Transient nature

Climate sensitivity looks to see what would happen if we doubled concentrations of CO2 in the atmosphere and let the Earth's oceans and ice sheets respond to it over several thousand years.

Transient climate response is much shorter term calculation again based on a doubling of CO2.

The Intergovernmental Panel on Climate Change reported in 2007 that the short-term temperature rise would most likely be 1-3C (1.8-5.4F).

But in this new analysis, by only including the temperatures from the last decade, the projected range would be 0.9-2.0C.

"The hottest of the models in the medium-term, they are actually looking less likely or inconsistent with the data from the last decade alone," said Dr Alexander Otto from the University of Oxford.

"The most extreme projections are looking less likely than before."

The authors calculate that over the coming decades global average temperatures will warm about 20% more slowly than expected.

But when it comes to the longer term picture, the authors say their work is consistent with previous estimates. The IPCC said that climate sensitivity was in the range of 2.0-4.5C.
Ocean storage

This latest research, including the decade of stalled temperature rises, produces a range of 0.9-5.0C.

"It is a bigger range of uncertainty," said Dr Otto.

"But it still includes the old range. We would all like climate sensitivity to be lower but it isn't."

The researchers say the difference between the lower short-term estimate and the more consistent long-term picture can be explained by the fact that the heat from the last decade has been absorbed into and is being stored by the world's oceans.

Not everyone agrees with this perspective.

Prof Steven Sherwood, from the University of New South Wales, says the conclusion about the oceans needs to be taken with a grain of salt for now.

"There is other research out there pointing out that this storage may be part of a natural cycle that will eventually reverse, either due to El Nino or the so-called Atlantic Multidecadal Oscillation, and therefore may not imply what the authors are suggesting," he said.

The authors say there are ongoing uncertainties surrounding the role of aerosols in the atmosphere and around the issue of clouds.

"We would expect a single decade to jump around a bit but the overall trend is independent of it, and people should be exactly as concerned as before about what climate change is doing," said Dr Otto.

Is there any succour in these findings for climate sceptics who say the slowdown over the past 14 years means the global warming is not real?

"None. No comfort whatsoever," he said.