My Comments on 2021 Report of Intergovernmental Panel on Climate Change, Working Group I


The 48 hr media cycle for the latest IPCC AR6 WG I) report is well past, but for anyone interested, here’s a short column with a few highlights in the Summary for Policymakers that caught my attention. The Summary plus full report are available here,

The Summary for Policymakers is 42 pages long and not that difficult to read for those with some science education.

The conclusions in the report are generally the same as in previous IPCC reports though with greater confidences for stated conclusions on climate change, and narrower projections of future temperature ranges. We’ll have more hot days, fewer cold ones, more intense rainfall events and higher evapotranspiration. Fewer tropical storms but more intense ones.

Global average temperature is now back up to the same peak reached about 6500 years ago. That agrees with what E.C. Pielou reported for Canada in her classic book, “After the Ice Age.” Of interest, Pielou also presented strong evidence that Western Canadian mountain glaciers largely melted at that time; what’s melting now with the current global warming are glaciers that mostly formed after peak temperatures 6500 years ago. (This is not to diminish the significance of present glacier loss to modern society.)

The new IPCC report places more emphasis than before, I think, on the cooling effect of aerosol compounds in the stratosphere (upper atmosphere). IPCC authors are quite confident that average temperature has increased about 1.1C since 1850-1900, but surprisingly uncertain on the extent to which this has been caused by GHG heating in the troposphere (lower atmosphere) and/or cooling by these upper atmosphere aerosols.

This part-sentence in Section A.1.3 is key, “It is likely that well-mixed GHGs contributed a warming of 1.0°C to 2.0°C, other human drivers (principally aerosols) contributed a cooling of 0.0°C to 0.8°C.” (“Likely” means 66% probability.)

This uncertainty also shows up in the IPCC WG I estimate of the warming caused by specified amounts of CO2 released into the atmosphere. In Section D.1.1, authors say, “Each 1000 GtCO2 of cumulative CO2 emissions is assessed to likely cause a 0.27°C to 0.63°C increase in global surface temperature with a best estimate of 0.45°C.” That’s a 2 1/2 times range in temperature response to given amount of CO2 emission, with a confidence level of just 66%.

IPCC projections on aerosol cooling also merit emphasis. If sulfur dioxide emissions decline with less coal burning and less air pollution, that could mean increased global warming because of less stratospheric cooling.

The net effect of the above, in my view, is some doubt about the precision of the IPCC’s projections of future temperature increases for given future amounts of GHG emissions. This is not to detract its overall message that human activities are causing global temperature changes and changing precipitation patterns compared to what would be expected solely from natural causes – and that a human priority should be to limit these changes to the extent possible while focusing also on adaptation.

I’ve one other comment: The IPCC report features five models of future climate changes to be expected with different levels of GHG emissions. But two of the five involve future rates of CO2 emissions far greater than what are being projected by most credible analysts – i.e., a 50% increase in rate of CO2 emissions by 2050 for one model and a near 100% increase by 2050 for the other. Discussions in the report of future climatic conditions with global warming are based too much, in my view, on these two unlikely models. Dr. Roger Pielke Jr. makes the same point though more eloquently and completely in a column here.

For those seeking more, here is one Twitter thread that I found very helpful: . You should also consider following @Peters_Glen who posts meaningful analyses almost daily. The Breakthrough Institute ( also has published several good articles on climate change, of particular interest to agriculture.

As for what this means for global agriculture and food supply, I offer three conclusions:

  1. Agriculture must devote more effort to technological changes, which reduce or even eliminate net GHG emissions, while still ensuring adequate supplies of nutritious food at reasonable costs for consumers.
  2. Plant breeding and the use of advanced genetic technology, to produce new cultivars more resistant to heat and drought, will be even more important in years ahead.
  3. Cultural techniques to store more water, including soil moisture reserves, from periods of intense rainfall for use during dry periods/seasons will become even more important than they already are.
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