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 hundredds 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.
The latest iteration identifies radiative forcing, the difference between the amount of heat coming into the climate and the amount reflected back, as the immediate cause of warming. Radiative forcing is expressed in watts per square metre (W/m2), a unit of energy. A rise indicates that heat is building up in the system.
Total radiative forcing from man-made sources since 1750 (i.e., before industrialisation) has risen from 0.29-0.85 W/m2 in 1950 to 0.64-1.86 W/m2 in 1980 to 1.13-3.33 W/m2 in 2011. The average has jumped from 0.57 to 1.25 to 2.29, respectively—a four-fold increase in 60 years. The big change recently, the report points out, is that the cooling effect of aerosols seems to have been less strong than it used to be. But there is no sign that the rise in radiative forcing has slowed during the past 15 years of flat surface temperatures. The best estimate for total man-made radiative forcing in 2011 is 43% above 2005 levels.
Of course, more heat does not necessarily equal perceptible climate change. The IPCC admits the pause in the rise of surface air temperatures is real. “The rate of warming over the past 15 years,” it says, “[is] 0.05 ºC per decade...smaller than the rate calculated since 1951.” In its 2007 report the panel had said the rate of warming was 0.2ºC per decade in 1990-2005 (four times the current rate). It predicted that this would continue for the next two decades.
But it plays down the long-term significance of the shift, saying that “due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends.” The start of the recent 15-year trend, in 1998, was a year of a strong worldwide fluctuation in the climate known as El Niño. This produced a temperature spike.
Still, all the extra heat implied by higher radiative forcing has to go somewhere. It isn’t going into the air. It is possible that not all that much is going into the surface waters of the oceans, either. The report says that “it is about as likely as not that ocean heat content from 0-700 metres increased more slowly during 2003-2010 than during 1993-2002.” That only leaves one other heat sink: the deep oceans below 700 metres, where it could be locked up in the deep oceans without affecting other parts of the climate.
And indeed, most of the extra heat does go into the oceans, which is not surprising given that they cover two thirds of Earth’s surface and have a much greater capacity to absorb heat than the air does. “Ocean warming,” the report says, “is largest near the surface and the upper 75 metres warmed by 0.11 ºC per decade over the period 1971-2010.” It adds that more than 60% of the net energy increase in the climate system is stored in the upper ocean (0-700 metres)...and about 30% is stored in the ocean below 700 metres.
In fact, vast deeps are a plausible candidate to explain the pause in surface air temperatures. The trouble is that measurements deep down, while improving, remain patchy. The IPCC says that it is likely that the ocean warmed from 3,000 metres to the bottom in 1992-2005 and that heat will penetrate from the surface down. Moreover, in a report earlier this month in Nature (published too late to make it into the IPCC report), Yu Kosaka and Shang-Ping Xie of the Scripps Institute of Oceanography, in San Diego, suggests that a cooling trend in an area of the eastern equatorial Pacific ocean may be “the cause of the pause.” But at the moment, this conclusion remains tentative.
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 1.7 mm a year between 1901 and 2010, 2.0 mm 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 226 Gt [trillion tonnes] a year over the period 1971-2009 and very likely 275 Gt 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 1.5 ºC relative to 1850 to 1900 in all but the lowest scenario considered, and likely to exceed 2 ºC for the two high scenarios.” The 2 ºC mark is widely considered to be the dividing line between warming which is just about tolerable and that which is dangerous.
For the first time, the IPCC gives some credence to the possibility that Earth’s climate may not be responding to higher concentrations of greenhouse gases quite as sharply as was once thought. The response is referred to as “equilibrium climate sensitivity” and defined as the rise in surface temperatures in the long term which accompanies a doubling of the concentration of CO2 in the atmosphere. In its previous report, the IPCC put this at between 2ºC and 4.5 ºC, with a most likely figure of 3ºC. But recent work, partly influenced by the pause in temperatures, has suggested sensitivity might be somewhat lower. The IPCC’s new range of 1.5-4.5 ºC (the same as in its first report) reflects the new consensus (though some new research puts the upper bound of sensitivity below 4.5 ºC).
The IPCC also decided to scrap its central “best guess.” Perhaps this is meant to reflect uncertainty in the science. If so, some scientists argue, then perhaps it should not have increased its confidence that man is the main cause of global warming.
In theory, a lower climate sensitivity means temperatures would rise more slowly for any given amount of extra radiative forcing. Earth might hence have a little more time to adjust to a changing climate. But whether such breathing space actually exists depends on how many tonnes of greenhouse gases people are putting into the atmosphere. So, for the first time, the IPCC has set what is usually called a carbon budget. To have a two-thirds chance of keeping global warming below 2 ºC, it says, “will require cumulative CO2 emissions from all anthropogenic sources to stay between 0 and about 1,000 [trillion tonnes].”
The world has already blown through just over half that amount (531 trillion tonnes) by 2011. At current rates of greenhouse-gas emission, the rest of the budget will have been spent before 2040. The odds of keeping the eventual rise in global temperatures to below 2 ºC will lengthen—even if climate sensitivity is lower than was thought and even if the pause in surface air temperatures persists for a while. As Thomas Stocker, the co-chair of the report depressingly put it: “we are committed to climate change…for many centuries even if emissions of CO2 stop.”
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