Guest essay By Roger A. Pielke Sr.

My son and Kevin Trenberth did an interview for Colorado Public Radio on February 17th. The entire interview is worth listening to, but here I want to comment on a specific statement that Kevin made that is scientifically inaccurate.

The entire interview (well worth listening too) is titled

Is climate change causing extreme weather? Experts disagree – click the listen button at http://www.cpr.org/news/story/climate-change-causing-extreme-weather-experts-disagree for the interview

In the discussion on added heat during droughts that is due to the increase of atmospheric CO2, Kevin Trenberth said

“You can add up how much of that heat there is and over a six month period it’s equivalent to running a very small microwave over every square foot at full power for about ½ hour”.

The interviewer [Ryan Warner] seemed to be impressed by this analog. The analog of a microwave is an effective image, but it is scientifically wrong for several reasons. Public Radio listeners and Mr. Warner were misled by this analog.

· First, the reduction of long wave radiation emitted to Space due to the added CO2 occurs over the six month time period, not in a short duration burst. Clearly, a short ½ burst of such heat would have a very different effect than when this heat is distributed across a six month time period.

· Second, the effect of long wave radiative flux divergence on surface temperatures from added CO2 (or other greenhouse gas including water vapor) is much larger at night. This is because during daylight, most of the time, vertical turbulent mixing dominates. The atmospheric boundary layer is typically much deeper during the daytime, so that added heat from the increase of CO2 is distributed through a much deeper depth. While the effect on nighttime minimum temperatures can be significant as we showed in our paper

McNider, R.T., G.J. Steeneveld, B. Holtslag, R. Pielke Sr, S. Mackaro, A. Pour Biazar, J.T. Walters, U.S. Nair, and J.R. Christy, 2012: Response and sensitivity of the nocturnal boundary layer over land to added longwave radiative forcing. J. Geophys. Res., 117, D14106, doi:10.1029/2012JD017578. Copyright (2012) American Geophysical Union. http://pielkeclimatesci.files.wordpress.com/2013/02/r-371.pdf

the effect on daytime maximum temperatures (and thus on increasing the heat stress in a drought) will be much less. Kevin did not properly inform the audience how the added heat would be processed differently during the day and night.

· Third, we examined this issue for a seasonal time scale in our paper

Eastman, J.L., M.B. Coughenour, and R.A. Pielke, 2001: The effects of CO2 and landscape change using a coupled plant and meteorological model. Global Change Biology, 7, 797-815 http://pielkeclimatesci.files.wordpress.com/2009/10/r-229.pdf

We concluded based on our model sensitivity runs that the radiative forcing effect of doubled atmospheric concentrations of CO2 on regional temperatures over a growing season are minimal (e.g. see Tables 8 and 9). This is especially true for daytime temperatures. Indeed, the biogeochemical effect on the regional weather from added CO2[which Kevin did not mention] was a much larger effect, as was land use change.

The ½ hour of added heat from the microwave forcing that Kevin presented, when properly input over the entire growing season would only result in a trivial effect on maximum temperature (ie. The hottest part of the day)!

Thus, while added CO2 and other human and natural climate forcings certainly can have an effect on large scale circulation features which could exacerbate droughts and fires, the analogy to a microwave that Kevin presented to convince the audience regarding the importance of added surface heating from the radiative effect of the increase of atmospheric CO2 is scientifically incorrect.

Indeed, when we perform model sensitivity experiments, we find that biogeochemical effect of added CO2 on plants (and the feedback to weather) and of land use change are much larger effects on this time and spatial scale.

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