"Are Scientists Overestimating — or Underestimating — Climate Change, Part I"
“Overturning the ‘Consensus’ in One Fell Swoop” gloats Planet Gore, which says the study “concludes that the Earth’s climate is only about one-third as sensitive to carbon dioxide as the IPCC assumes” and so we “should expect about a 0.6oC additional increase in temperature between now and 2070″ [0.1oC per decade] if CO2 concentrations hit 550 parts per million, double preindustrial levels.
Is this possible? Aren’t we already warming up 0.2oC per decade — a rate that is expected to rise? Has future global warming been wildly overestimated by the Intergovernmental Panel on Climate Change (IPCC) consensus?
Or, as I argue in my book, has future global warming been underestimated by the IPCC. This is perhaps the central issue in the climate change debate, so this will be a long post. To cut to the chase, it is not possible for one study to overturn the consensus, and in any case this inadequately researched, overly simplistic, and mistake-riddled study certainly doesn’t.
Climate sensitivity expert James Annan points out key mistakes that rip the guts out of Schwartz’s analysis. That is strike one. Now I’ll offer my 2 cents worth.
I have always considered it ironic that the Denyers — who don’t believe the consensus, which is based on hundreds of studies that they obviously reject out of hand — are so enamored of the very few studies that suggest the consensus overestimates climate change (while ignoring the great many studies that suggest an underestimate). Even more ironic, let me quote from the end of the Schwartz paper, which is painfully aware how dubious its main conclusion is:
Finally, as the present analysis rests on a simple single-compartment energy balance model, the question must inevitably arise whether the rather obdurate climate system might be amenable to determination of its key properties through empirical analysis based on such a simple model. In response to that question it might have to be said that it remains to be seen. In this context it is hoped that the present study might stimulate further work along these lines with more complex models…. Ultimately of course the climate models are essential to provide much more refined projections of climate change than would be available from the global mean quantities that result from an analysis of the present sort.
Yes, the Denyers routinely attack the IPCC consensus for using elaborate computer models that they claim are still far too simplistic to model the real climate — claiming those models omit key variables and negative feedbacks that would reduce future climate change. But now they would have us embrace a self-acknowledged “simple model” — one far more simplistic than the climate models the Denyers repeatedly denounce as too simplistic. That’s chutzpah.
There is both a simple reason and a more complicated reason why I firmly believe that IPCC scientists are underestimating future climate change (and hence that Schwartz is very wrong). First, the simple reason — Scientists have underestimated current climate change:
- “The recent [Arctic] sea-ice retreat is larger than in any of the (19) IPCC [climate] models” — and that was a Norwegian expert in 2005. The retreat has accelerated in the past two years.
- The ice sheets appear to be shrinking “100 years ahead of schedule.” That was Penn State climatologist Richard Alley in March 2006. In 2001, the IPCC thought that neither Greenland nor Antarctica would lose significant mass by 2100. They both already are.
- The temperature rise from 1990 to 2005 — 0.33°C — was “near the top end of the range” of IPCC climate model predictions.
- Sea-level rise from 1993 and 2006 — 3.3 millimetres per year as measured by satellites — was higher than the IPCC climate models predicted.
- The subtropics are expanding faster than the models project.
- Since 2000, carbon dioxide emissions have grown faster than any IPCC model had projected.
So what are the chances that the IPCC has overestimated the climate sensitivity by a factor of three as Schwartz’s overly simple model would have us believe — that the rate of warming in the next several decades will be under half that of the rate of the past 16 years? Zilch. Does Schwartz mention any of these data points? Not one. Shame on the JGR editors for letting this go by. Strike two.
Now on to the more complicated reason I am convinced scientists are underestimating future climate change.
First let’s define the “equilibrium climate sensitivity” as the “equilibrium change in global mean surface temperature following a doubling of the atmospheric (equivalent) CO2 concentration. Schwartz’s simple model deduces this is 1.1 ± 0.5 degrees° Kelvin (or °C). Schwartz notes that the IPCC’s 2007 Fourth Assessment Report concluded it was “2 to 4.5 K with a best estimate of about 3 K and … very unlikely to be less than 1.5
Planet Gore touts the fact that “Schwartz’s results … are based on the empirical relationship between trends in surface temperature and ocean heat content,” implying that Schwartz’s simple model is based on empirical data but other estimates of the climate sensitivity aren’t. In fact, as noted, Schwartz ignores the abundant real world data of the past two decades that the climate is more sensitive than the models.
And the IPCC’s numbers are derivable from empirical data from the past century as well as paleoclimate data as well as complex climate models. Back in 2004, Science magazine published an excellent article, Three Degrees of Consensus, which explains that “almost all the evidence points to 3°C” as the most likely climate sensitivity, as climate researcher Alan Robock explained. The article notes, for instance, that we have very good empirical data from the temperature dip and rebound following recent volcanoes:
From the magnitude and duration of the Pinatubo cooling, climate researcher Thomas Wigley of NCAR and his colleagues have recently estimated Earth’s sensitivity to a CO2 doubling as 3.0ºC. A similar calculation for the eruption of Agung in 1963 yielded a sensitivity of 2.8ºC. And estimates from the five largest eruptions of the 20th century would rule out a climate sensitivity of less than 1.5ºC.
Doh! Annan himself co-authored a terrific article in 2006, “Using multiple observationally-based constraints to estimate climate sensitivity,” which combines many different pieces of data and research to conclude that the climate sensitivity has a 68% chance of being 2.5-3.5ºC and a 95% chance of being 2-4ºC, as he explains in this blog post.
And climatologist Barry Pittock cites several articles that suggest a climate sensitivity range of “around 2º–6°C” in his important EOS article, “Are Scientists Underestimating Climate Change” (All Pittock’s citations can be found here). These include Annan’s paper, as well as:
- Forster, P. M. D., and J. M. Gregory (2006), The climate sensitivity and its components diagnosed from Earth Radiation Budget Data, J. Clim., 19, 39–52.
- Hegerl, G. C., T. J. Crowley, W. T. Hyde, and D. J. Frame (2006), Climate sensitivity constrained by temperature reconstructions over the past seven centuries, Nature, 440, 1029–1032.
- Murphy, J. M., D. M. H. Sexton, D. N. Barnett, G. S. Jones, M. J. Webb, M. Collins, and D. A. Stainforth (2004), Quantification of modelling uncertainties in a large ensemble of climate change simulations, Nature, 430, 768–772.
- Piani, C., D. J. Frame, D. A. Stainforth, and M. R. Allen (2005), Constraints on climate change from a multi-thousand member ensemble of simulations, Geophys. Res. Lett., 32, L23825, doi:10.1029/2005GL024452.
- Stainforth, D. A., et al. (2005), Uncertainty in predictions of the climate response to rising levels of greenhouse gases, Nature, 433, 403–406.
I list them all here because Schwartz doesn’t cite a single one of them in his paper. That’s right — he has written an article purporting to overturn perhaps the central number in the climate change debate, and he doesn’t cite any of these recent articles on the subject. I agree with Annan:
It’s surprising that Schwartz didn’t check his results with anyone working in the field, and disappointing that the editor in charge at JGR apparently couldn’t find any competent referees to look at it.
Strike Three. This paper is called out swinging. The Denyers can keep citing it, but it is too severely flawed to be considered a serious contribution to the scientific literature. The paper should be withdrawn.
Annan explains how Schwartz made a simple but important mistake — the paper “grossly underestimates [by a factor of 3] the time scale of response of climate models to a long-term forcing change” — that led his results to be off by a factor of 3.
So Schwartz’s paper is easily dismissed.
There’s a very important twist to this discussion, however, which gets to the heart of how the IPCC underestimates future climate change — and why “equilibrium climate sensitivity” is very poorly named. That will be the subject of Part II of “Are Scientists Overestimating — or Underestimating — Climate Change?”