The masthead for The Economist magazine. Graphic: The Economist

STOCKHOLM, 27 September 2013 (The Economist) – It has been a long time coming. But then the fifth assessment of the state of the global climate by the Intergovernmental Panel on Climate Change (IPCC), a United Nations body, was a behemoth of an undertaking. It runs to thousands of pages, involved hundreds of scientists and was exhaustively checked and triple-checked by hundreds of other boffins and government officials to whom they report—and whose policies are often based on what they read.

The first tranche of the multi-volume report—an executive summary of the physical science—was released in Stockholm on September 27th. And it is categorical in its conclusion: climate change has not stopped and man is the main cause.

It may be the last report of its kind: a growing chorus of experts thinks a more frequent, less bally-hooed and more up-to-date assessments would be more useful. It is certainly the first since negotiations for a global treaty reining in carbon emissions collapsed in Copenhagen in 2009; the first since questions were raised about the integrity of the IPCC itself following mistaken claims about the speed of glacier melt in the Himalayas and, most important, the first since evidence became incontrovertible that global surface air temperatures have risen much less quickly in the past 15 years than the IPCC had expected. A lot is riding on its findings, from the public credibility of climate science to the chances of a new global treaty.

The report is more definitive than in the past about the role of people in causing climate change. It say that it is "extremely likely"—IPCC speak for having a probability of over 95%—that man is responsible. This contrasts with the tentative tone of the early IPCC reports. “The observed increase [in surface air temperatures] could be largely due to this natural variability,” said the first one, in 1990. The next report in 1995 merely suggested a link between rising temperatures and human activity. That link was deemed “likely” (which means probability of 66%) in 2001, and “very likely” (90%) in 2007. […]

Global warming is, then, continuing unabated in the watery world. It is not clear whether the trend itself has changed dramatically since 1990 or whether the rise is due to improved measurements, which have enabled scientists to gauge more exactly what has been going on. Probably the latter. The new assessment says that, since the fourth report in 2007, "instrumental biases in upper-ocean temperature records have been identified and reduced, enhancing confidence in the assessment of change."

Either way, the trend is worrying. Since water, like almost everything else, expands as it gets hotter, its rising temperature causes sea levels to rise. It is "very likely", the report adds, “that the mean rate of global averaged sea level rise was 1.7mm a year between 1901 and 2010, 2.0mm a year between 1971 and 2010 and 3.2mm a year between 1993 and 2010.” The rate of sea-level rise all but doubled between the start of the 20th century and its end. That is a significant change and one that the first IPCC assessment report in 1990 had little inkling of. That report reckoned that “the average rate of rise over the last 100 years has been 1.0-2.0 mm a year. There is no firm evidence of acceleration in sea level rise during this century.” The rate is now thought to be higher—and growing.

New instruments are providing better information about the rate at which ice sheets and glaciers are melting, too. In particular, the launch of the twin GRACE satellites has provided more detail about how much ice there actually is. GRACE, which stands for Gravity Recovery and Climate Experiment, enables the mass of objects on Earth to be worked out more precisely by measuring tiny changes in their gravitation pull. The report says that “the average rate of ice loss from glaciers around the world, excluding glaciers on the periphery of the ice sheets, was very likely 226Gt [trillion tonnes] a year over the period 1971-2009 and very likely 275Gt a year over the period 1993-2009.”

In other words, it has speeded up. The Greenland ice sheet, the Antarctic sea ice and the Arctic sea ice have all lost mass (got thinner). The extent of the Arctic sea ice has shrunk by 3.5-4.1% a decade in 1979-2012, more than was estimated in 2007, and the summer sea-ice minimum is shrinking by about 10% a decade, though this year’s summer ice melt was smaller than last year’s.

What does that mean for the future? The report uses four new sets of scenarios for greenhouse-gas concentrations to claim that “global surface temperature change for the end of the 21st century is projected to be likely to exceed 1.5ºC relative to 1850 to 1900 in all but the lowest scenario considered, and likely to exceed 2ºC for the two high scenarios.” The 2ºC mark is widely considered to be the dividing line between warming which is just about tolerable and that which is dangerous. [more]

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