Seekers after good news on climate change will have to look beyond The Economist’s commentary reprinted in Saturday’s Strib, reporting that “some scientists are arguing that man-made climate change is not quite so bad a threat as it appeared to be a few years ago” and has, in fact, “plateaued.”
I admit those conclusions grabbed my attention, all the more so because I admire The Economist’s analytical approach to climate issues and to environmental subjects in general.
You can still find publications willing to report that “some scientists” think global warming is a fiction, a conspiracy, a hoax. But The Economist prefers real scientists and real data, and so I began to dig into the devilish details behind these conclusions, both in the full-length analysis from which the commentary was derived and elsewhere.
Here’s what I found:
Unlike the commentary, the analysis reaches no conclusion that “climate change has plateaued.” It does report, and discuss, two important but brief and recent temperature trends:
- “The five-year mean global temperature has been flat for a decade.” That’s a key measure, to be sure, and the quote is from NASA’s James Hansen, perhaps the most influential single scientist in this field.
- “Ed Hawkins, of the University of Reading, in Britain, points out that surface temperatures since 2005 are already at the low end of the range of projections derived from 20 climate models. If they remain flat, they will fall outside the models’ range within a few years.”
A puzzle, not a reversal
That’s not The Economist’s take, not by a mile:
The mismatch between rising greenhouse-gas emissions and not-rising temperatures is among the biggest puzzles in climate science just now. It does not mean global warming is a delusion. Flat though they are, temperatures in the first decade of the 21st century remain almost 1°C above their level in the first decade of the 20th. But the puzzle does need explaining.
One possible explanation is that something out of the ordinary has been happening in this relatively short time frame. Another is that “climate sensitivity” — the degree to which greenhouse-gas levels drive global temperatures upward — is being offset by factors we still don’t fully understand.
Heat absorption by seawater is once such factor, and it may be more potent than was thought. While the oceans’ surface temperatures have also leveled off in recent years, recent research shows that the deep ocean — waters more than two kilometers down — has been warming more than we noticed, and may greatly increase this offset.
A second offset is reflective “aerosols” like volcanic dust and sulfates. But here the research trends seem to be moving in an unhelpful direction.
Based on a leaked draft of the Intergovernmental Panel on Climate Change’s next report, due out in about six months, The Economist says aerosols appear to be helping less than was thought and some of them — like carbon soot — are actually making warming worse.
‘Faustian bargain’ with carbon
Hansen himself, in a recent blog, refers to aerosol-induced optimism as a “Faustian bargain” in which we hope that atmospheric carbon loads lasting for millennia will somehow be canceled by aerosols that “fall out after about five days.”
A third factor is natural variability, a sort of background level of warming and cooling that occurs independent of human-caused atmospheric changes. Without going anywhere near the claims of some deniers that all recently observed warming is natural, The Economist reports that
There is some evidence that the natural variability of temperatures may be somewhat greater than the IPCC has thought. A recent paper by Ka-Kit Tung and Jiansong Zhou in the Proceedings of the National Academy of Sciences links temperature changes from 1750 to natural changes (such as sea temperatures in the Atlantic Ocean) and suggests that “the anthropogenic global-warming trends might have been overestimated by a factor of two in the second half of the 20th century.”
It is possible, therefore, that both the rise in temperatures in the 1990s and the flattening in the 2000s have been caused in part by natural variability.
How long this plateau may last is of course a matter of much controversy, but it’s important to note that every scientist and study referenced in The Economist’s analysis predicts that global temperatures will continue to rise.
Their disagreements are over the amount of that increase, arising primarily from differences in how they approach modeling:
One type of model — general-circulation models, or GCMs — use a bottom-up approach. These divide the Earth and its atmosphere into a grid which generates an enormous number of calculations in order to imitate the climate system and the multiple influences upon it. The advantage of such complex models is that they are extremely detailed. Their disadvantage is that they do not respond to new temperature readings. They simulate the way the climate works over the long run, without taking account of what current observations are. Their sensitivity is based upon how accurately they describe the processes and feedbacks in the climate system.
The other type — energy-balance models — are simpler. They are top-down, treating the Earth as a single unit or as two hemispheres, and representing the whole climate with a few equations reflecting things such as changes in greenhouse gases, volcanic aerosols and global temperatures. Such models do not try to describe the complexities of the climate. That is a drawback. But they have an advantage, too: unlike the GCMs, they explicitly use temperature data to estimate the sensitivity of the climate system, so they respond to actual climate observations.
Different models, different numbers
The closest we have to consensus projections on climate come from the IPCC, which uses the first type of modeling. Based on the leaked draft, The Economist says the panel’s 2013 report will retain its 2007 assessment that the most likely warming scenario is for an increase of 3º C —”extremely damaging” — while raising the upper, worst-case limit from 4.5º C to 6º C or even 7º C .
Other recent projections —almost all of which use the second method—put the most likely extent of warming in the range of 1.6º C to 1.9º C and the upper limits at 3º C to 4.5º C.
Taking all this evidence together, The Economist analysis concludes that
a small reduction in estimates of climate sensitivity would seem to be justified: a downwards nudge on various best estimates from 3°C to 2.5°C, perhaps; a lower ceiling (around 4.5°C), certainly. If climate scientists were credit-rating agencies, climate sensitivity would be on negative watch. But it would not yet be downgraded.
If that counts as good news with you, then you are more susceptible to soothing than I.