Wednesday, September 4, 2013

The Charney Report

Source: National Academies Press
Later this month, the Intergovernmental Panel on Climate Change (IPCC) will be releasing the first working group report of its Fifth Assessment Report (AR5), entitled Working Group I: The Physical Science Basis.  Previous assessment reports, available here, were issued in 1990, 1995, 2001, and 2007.

Although one can learn quite a bit about the recent evolution of our understanding of climate from these reports, it is also informative to look back at what is known today as The Charney Report, which was prepared by an ad hoc team of scientists led by Jule Charney (hence the name) in 1979.  It is quite short (34 pages including front and back matter) and clearly written.  It includes four brief chapters: Summary and Conclusions (they got to the point quickly), Carbon in the Atmosphere, Physical Processes Important for Climate and Climate Modeling, and Models and Their Validity.  Basically, it was a Cliff Notes for climate change and a refreshing read compared to more recent climate reports, which often suffer from verbosity.

Here are a few quotes from their Summary and Conclusions.
"When it is assumed that the CO2 content of the atmosphere is doubled and statistical thermal equilibrium is achieved, the more realistic of the modeling efforts predict a global surface warming of between 2°C and 3.5°C, with greater increases at high latitudes."

"The primary effect of an increase of CO2 is to cause more absorption of thermal radiation from the earth’s surface and thus to increase the air temperature in the troposphere. A strong positive feedback mechanism is the accompanying increase of moisture, which is an even more powerful absorber of terrestrial radiation."

"We have examined with care all known negative feedback mechanisms, such as increase in low or middle cloud amount, and have concluded that the oversimplifications and inaccuracies in the models are not likely to have vitiated the principal conclusion that there will be appreciable warming. The known negative feedback mechanisms can reduce the warming, but they do not appear to be so strong as the positive moisture feedback."

"We estimate the most probable global warming for a doubling of CO2 to be near 3°C with a probable error of ±1.5°C. Our estimate is based primarily on our review of a series of calculations with three-dimensional models of the global atmospheric circulation, which is summarized in Chapter 4. We have also reviewed simpler models that appear to contain the main physical factors. These give qualitatively similar results."

"One of the major uncertainties has to do with the transfer of the increased heat into the oceans. It seems to us quite possible that the capacity of the deeper oceans to absorb heat has been seriously underestimated, especially that of the intermediate waters of the subtropical gyres lying below the mixed layer and above the main thermocline. If this is so, warming will proceed at a slower rate until these intermediate waters are brought to a temperature at which they can no longer absorb heat."

"The warming will be accompanied by shifts in the geographical distributions of the various climatic elements such as temperature, rainfall, evaporation, and soil moisture. The evidence is that the variations in these anomalies with latitude, longitude, and season will be at least as great as the globally averaged changes themselves, and it would be misleading to predict regional climatic changes on the basis of global or zonal averages alone."
How the Charney Report arrived at these estimates is quite interesting and described by Kerr (2004) in Science.
"As climate modeler Syukuro Manabe of Princeton University tells it, formal assessment of climate sensitivity got off to a shaky start. In the summer of 1979, the late Jule Charney convened a committee of fellow meteorological luminaries on Cape Cod to prepare a report for the National Academy of Sciences on the possible effects of increased amounts of atmospheric CO2 on climate. None of the committee members actually did greenhouse modeling themselves, so Charney called in the only two American researchers modeling greenhouse warming, Manabe and James Hansen of NASA's Goddard Institute for Climate Studies (GISS) in New York City."

"On the first day of deliberations, Manabe told the committee that his model warmed 2°C when CO2 was doubled. The next day Hansen said his model had recently gotten 4°C for a doubling. According to Manabe, Charney chose 0.5°C as a not-unreasonable margin of error, subtracted it from Manabe's number, and added it to Hansen's. Thus was born the 1.5°C-to-4.5°C range of likely climate sensitivity that has appeared in every greenhouse assessment since, including the three by the Intergovernmental Panel on Climate Change (IPCC). More than one researcher at the workshop called Charney's now-enshrined range and its attached best estimate of 3°C so much hand waving."
Despite the hand waving, these early estimates have thus far stood the test of time well.  Kerr (2004) goes on to conclude:
"Most [attendees of a workshop on climate sensitivity in 2004] polled by Science generally agreed on a most probable sensitivity of around 3°C, give or take a half- degree or so. With three complementary approaches—a collection of expert-designed independent models, a thoroughly varied single model, and paleoclimates over a range of time scales—all pointing to sensitivities in the same vicinity, the middle of the canonical range is looking like a good bet. Support for such a strong sensitivity ups the odds that the warming at the end of this century will be dangerous for flora, fauna, and humankind. Charney, it seems, could have said he told us so."
Indeed, today we have a much larger collection of estimates based on climate models and physical evidence (i.e., that derived from of past climate change during the Earth's history), with the IPCC Fourth Assessment Report stating:
"The equilibrium climate sensitivity is a measure of the climate system response to sustained radiative forcing. It is defined as the equilibrium global average surface warming following a doubling of CO2 concentration. Progress since the TAR enables an assessment that climate sensitivity is likely to be in the range of 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values."
Ultimately, Mother Nature will have the final say as it appears that at least a doubling of CO2 is inevitable.  Charney passed away in 1981, so he did not see the roughly 0.4ÂșC of warming that has occurred since then.  My guess is that if he were alive today, he'd stick with their estimate.


  1. So far most climate models are over hyping future warming. Case in point:

    1. I don't doubt that GCMs probably have overdone warming in the previous decade, but that is one terrible plot Spencer made! Why would he choose 20S to 20N mid-tropospheric temperature? Why not global surface temperature for which observations are much more accurate and abundant? Also, why are there no error bars on the observational balloon and satellite data? Balloons in the tropics are sparse (and biased to land locations and specific times of day) and satellite retrieved temperatures have errors that need to be mentioned (and are likely biased to clear sky conditions). Lastly, why only show CMIP5 RCP-8.5? That is the most extreme CO2 emission scenario, i.e. an upper bound. The IPCC has other scenarios that are all more moderate and thus they present an envelope of uncertainty, which Spencer ignores (as he also does for his observational retrievals).

      The previous decade has been dominated by La Nina conditions, which cause cooler global temperatures. Combined with deep ocean warming (which is predicted to saturate at some point), this is the reason that most scientists believe that warming has held steady over the past decade. Note that for such persistent La Nina conditions in a non-changing climate, we would expect global mean temperature to decrease, not remain steady, so what is keeping it steady? Our predictive ability for ENSO is almost non-existent, so you can't expect GCMs to get these conditions right for any given decade. Until recently, many of them could not even simulate a proper ENSO, but that does not mean that we discount them for long-term (e.g., 100-year) climate change predictions.