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

A new paper has appeared in Science magazine that concludes that CO2 is the dominate control of the Earth’s climate system. It is also yet another model sensitivity study (climate process study) in which only a subset of the real world climate system is simulated.

 The paper is

Andrew A. Lacis, Gavin A. Schmidt, David Rind, and Reto A. Ruedy, 2010: Atmospheric CO2: Principal Control Knob Governing Earth’s Temperature. 15 Ocotober 2010 Vol 330 Science. http://www.sciencemag.org

The abstract reads

“Ample physical evidence shows that carbon dioxide (CO2) is the single most important climate-relevant greenhouse gas in Earth’s atmosphere. This is because CO2, like ozone, N2O, CH4, and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current climate temperatures, whereas water vapor can and does. Noncondensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapor and clouds via feedback processes that account for the remaining 75% of the greenhouse effect. Without the radiative forcing supplied by CO2 and the other noncondensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state.”

Lacis et al 2010 is correct that the human addition of CO2 is a first order climate forcing as we reported on 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.

However, this paper perpetuates the narrow view that the since this gas does not condense and precipitate, it is THE dominate forcing since it “sustains the current levels of atmospheric water vapor and clouds via feedback processes.”

Text in their paper includes

It often is stated that water vapor is the chief greenhouse gas (GHG) in the atmosphere. For example, it has been asserted that “about 98% of the natural greenhouse effect is due to water vapour and stratiform clouds with CO2 contributing less than 2%”

An improved understanding of the relative importance of the different contributors to the greenhouse effect comes from radiative flux experiments that we performed using Goddard Institute for Space Studies (GISS) ModelE

“CO2 is the key atmospheric gas that exerts principal control over the strength of the terrestrial greenhouse effect. Water vapor and clouds are fast-acting feedback effects, and as such are controlled by the radiative forcings supplied by the noncondensing GHGs. There is telling evidence that atmospheric CO2 also governs the temperature of Earth on geological time scales…”

The paper is an interesting model experiment, but it really does not present any new insight beyond what we already know. Quite frankly, this would be a good Master’s thesis study to show why CO2 is an important climate forcing as well as provide insight into the water cycle feedback. However, it presentation as a major new research insight by Science is puzzling, unless the magazine wants to promote the message at the end of the Lacis et al paper that

“The anthropogenic radiative forcings that fuel the growing terrestrial greenhouse effect continue unabated. The continuing high rate of atmospheric CO2 increase is particularly worrisome, because the present CO2 level of 390 ppm is far in excess of the 280 ppm that is more typical for the interglacial maximum, and still the atmospheric CO2 control knob is now being turned faster than at any time in the geological record. The concern is that we are well past even the 300- to 350-ppm target level for atmospheric CO2, beyond which dangerous anthropogenic interference in the climate system would exceed the 25% risk tolerance for impending degradation of land and ocean ecosystems, sea-level rise, and inevitable disruption of socioeconomic and food producing infrastructure. Furthermore, the atmospheric residence time of CO2 is exceedingly long, being measured in thousands of years. This makes the reduction and control of atmospheric CO2 a serious and pressing issue, worthy of real-time attention.”

My conclusion is that their paper does not present new scientific insight but is actually an op-ed presented in the guise of a research paper by Science magazine

They also do not present (and show why they should be refuted) alternative published perspectives so as we present in Pielke et al (2009) that

“In addition to greenhouse gas emissions, other first-order human climate forcings are important to understanding the future behavior of Earth’s climate. These forcings are spatially heterogeneous and include the effect of aerosols on clouds and associated precipitation [e.g., Rosenfeld et al., 2008], the influence of aerosol deposition (e.g., black carbon (soot) [Flanner et al. 2007] and reactive nitrogen [Galloway et al., 2004]), and the role of changes in land use/land cover [e.g., Takata et al., 2009]. Among their effects is their role in altering atmospheric and ocean circulation features away from what they would be in the natural climate system [NRC, 2005]. As with CO2, the lengths of time that they affect the climate are estimated to be on multidecadal time scales and longer.”

The authors of this paper are welcome to present a guest post on my weblog (or could do that on Real Climate) as to what is actually new about their findings. Absent that rebuttal, we should interpret this article has simply a repackaging of their perspective that CO2 is the dominant climate forcing.

It is well known that CO2 is a first order greenhouse gas and is essential for providing the Earth with a habitable climate. We discuss this in our book

Cotton, W.R. and R.A. Pielke, 2007: Human impacts on weather and climate, Cambridge University Press, 330 pp.

For example, on page 158 we wrote

“In the absence of ….greenhouse gases, the average surface temperature of the Earth would be over 30C cooler than it is today.”

and

“The major greenhouse gas is water vapor which varies naturally in space and time due to the Earth’s hydrological cycle….[the] second most important greenhouse gas is carbon dioxide. In contrast to water vapor, carbon dioxide is rather uniformly distributed throughout the troposphere, although the radiative forcing associated with it is more heterogeneous as a result of spatial (e.g. latitudinal) and temporal variations in tropospheric temperature and water vapor concentrations, and in surface emissions and absorption.”

On page 174 in our Section “Water vapor feedbacks”, we write

“…any changes in water vapor concentration in response to other greenhouse gases would substantially alter the net greenhouse heating”.

In Section 8.2.8 on pages 166 to 175 of our book we present a section titled “Assessment of the relative effect of carbon dioxide and water vapor” with this text reproduced and discussed further in the weblog posts 

Relative Roles of CO2 and Water Vapor in Radiative Forcing

Further Analysis Of Radiative Forcing By Norm Woods

In Cotton and Pielke and in these posts, we present an analysis of the role of 1X and 2X CO2 as a radiative forcing for three representative atmospheric soundings [tropical; subarctic summer; subarctic winter].

We found, as should be expected since the radiative forcing of CO2 is a logarithmic function of its atmospheric concentration, that the largest effect of changing its concentration is from zero CO2 to 1X CO2 relative to 1X CO2 to 2X CO2.  For example, as we report in our book and on my weblog posts, Norm Woods found that for a tropical sounding

“the downwelling longwave flux at the surface when the CO2 concentration changes from 360ppm to 560ppm is 0.09 Watts per meter squared, as contrasted with a change of 0.41 Watts per meter squared when the concentration changes to 360ppm from 0 ppm. The reason for this relative insensitivity to added CO2 in the tropics is due to the high concentrations of water vapor which results in additional long wave flux changes due to CO2 being very muted”

For a subarctic summer sounding

“the corresponding values are 2.94 Watts per meter squared when changing the CO2 concentrations to 360 ppm from 0, and 0.47 Watts per meter squared when changing the CO2 concentrations to 560 ppm from 360 ppm.”

For a subarctic winter sounding

“the change is 14.43 Watts per meter squared when the CO2 concentrations are changed to 360 ppm from 0, and 1.09 Watts per meter squared when the CO2 concentrations are changed to 560 ppm from 360 ppm.”

The Lacis et al 201o paper accurately reports that CO2 is a first order climate forcing. However, this is not a new finding.

There is a further (presumably unintended by the authors) bottom line message, however, from the Lacis et al 2010 Science paper.

 While, they have reconfirmed the importance of CO2 as a first-order climate forcing, they have not added anything that is new. Thus, in terms of further model predictions using the GISS model (or other IPCC model) what are they going to add that is policy relevant beyond what has already been achieved with their model?

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