By Dr. Roger Pielke Sr.

Andy Lacis has posted two guest contributions on my weblog;

Guest Post “CO2: The Thermostat That Controls Earth’s Temperature” By Andy Lacis

Further Comment By Andy Lacis On CO2 As A Climate Thermostat

I very much appreciate this collegial interaction.

Today, I want to comment on his conclusions.

First, I agree with Andy’s conclusion that if CO2 were removed from the Earth’s atmosphere, the climate system would rapidly cool. I also concur that CO2 is a first order climate forcing and is a non-condensing greenhouse gas forcing.

The more interesting question, however, is how this applies both to how the Earth’s climate system actually evolved, and how incremental increases in CO2 above what was present in pre-industral times alter the climate.

With respect to the early Earth atmosphere, CO2 was emitted from volcanic eruptions but so was water vapor. The two acted together to warm the climate. Indeed, this is one explanation proposed to explain the warm, wet period in the earlier atmosphere of Mars and Venus. While, the model experiment presented by Andy and colleagues is quite interesting, it does not reflect the real climate system.

The second issue is, of course, directly relevant to our future climate. As I posted in

Comment On The Science Paper “Atmospheric CO2: Principal Control Knob Governing Earth’s Temperature” By Lacis Et Al 2010

we have examined the effect of incremental increases in CO2 (and water vapor) as described in detail in

Relative Roles of CO2 and Water Vapor in Radiative Forcing

Further Analysis Of Radiative Forcing By Norm Woods

In regards to the effect of an incremental effect on radiative flux of an increase in the atmospheric concentration of CO2, there is an informative figure at Watts Up With That in a post by David Archibald titled The Logarithmic Effect of Carbon Dioxide. The figure is from 2006 by Willis Eschenbach which was posted on Climate Audit.

What is of importance to our future climate is the added downwelling radiative fluxes as given by the green and black lines. The Lacis and colleagues study examined the effect of the radiative forcing from red line.

The issue with respect to our future climate is how will it be altered in response to these incremental increases, part of which (particularly in the humid parts of the world) overlaps with water vapor absorption).

In terms of how environmentally and societally important resources are altered, as I have often posted on (e.g. see), in terms of climate, this involves how droughts, floods, tropical cyclones, heat waves, etc are altered. This means the focus should be on alterations in regional ocean and atmospheric circulations, mesoscale weather patterns, and so forth rather than on trends in the global average surface temperatures. The addition of CO2 is one factor (both radiatively and biogeochemically) but is not the single ”control” of these climate metrics.

The equilibrium temperature of Earth is just one of these metrics, and, indeed is not adequate to explain how regional and local climate could change. In fact, even with respect to global warming and cooling, the use of ocean heat content is a much more robust way to diagnose these climate system heat changes than a global average surface temperature trend, as discussed most recently in

Pielke Sr., R.A., 2008: A broader view of the role of humans in the climate system. Physics Today, 61, Vol. 11, 54-55.

Andy’s posts (and paper) do clearly show that

“ there is a clear demonstration that without the radiative forcing provided by the non-condensing GHGs, the terrestrial greenhouse effect collapses because there is no structural temperature support to restrain the current climate water vapor from condensing and precipitating.”

However, there needs to be a recognition that the human influence on the climate system, including global warming and cooling, involves much more than the non-condensing greenhouse gases, and that the role of natural climate forcings and variability remain incompletely understood. We have discussed this in our paper

Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R. Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W. Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell, W. Rossow, J. Schaake, J. Smith, S. Sorooshian, and E. Wood, 2009: Climate change: The need to consider human forcings besides greenhouse gases. Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American Geophysical Union.

I invite Andy to discuss where he agrees, and where he disagrees, with our conclusions and recommendations in the above paper.