by Andrew Lacis
The one year anniversary is soon approaching for the Science paper that we wrote a year ago to illustrate the nature of the terrestrial greenhouse effect. I describe here how this paper came to be.
The title of the paper: “Atmospheric CO2: Principal Control Knob Governing Earth’s Temperature“ is also the principal conclusion of the paper. The full text, including the Supporting Online Material, is downloadable from the GISS webpage. The abstract describes the physical basis and rationale for the key point that is being expressed in the title.
Ample physical evidence shows that carbon dioxide is the single most important climate-relevant greenhouse gas in the Earth’s atmosphere. This is because CO2, like ozone, N2O, CH4 and CFCs, 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 non-condensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate to an icebound Earth state.
The text of the paper provides the supporting science and illustrates the key role that atmospheric CO2 plays in the operation of the terrestrial greenhouse effect, perhaps more directly than has been done before. The paper also makes a bit of a digression in pointing out the erroneous and irresponsible assertion by Dick Lindzen that “about 98% of the natural greenhouse effect is due to water vapour and stratiform clouds withCO2 contributing less than 2%”. (Lindzen really should have made some effort to correct this misinformation, instead of letting people quote his erroneous result ad infinitum.)
The Supporting Online Material for the Science paper provides additional background information, including a table that lists the fractionalattributionof theterrestrialgreenhousecontributorsshown in Figure 1 of the Science paper. There is also the accompanying GRL paper by Gavin Schmidt et al. (2010) that describes the GCM modeling runs for the greenhouse attribution analysis.
This paper drew initial attention in the form of a NASA press release on ‘How Carbon Dioxide Controls Earth’s Temperature’.
Two related science briefs were also displayed on the GISS webpage: ‘CO2: The Thermostat that Controls Earth’s Temperature’, and ‘Taking a Measure of the Greenhouse Effect’.
Our Science paper also caught the attention of the climate skeptics crowd. Roy Spencer commented that “there was a very clever paper published in Science this past week . . . in an attempt to prove that carbon dioxide is the main driver of the climate system”.
But then Roy Spencer went astray by claiming that “After assuming that clouds and water vapor are no more than feedbacks upon temperature, the Lacis et al. paper then uses a climate experiment to ‘prove’ their paradigm that CO2 drives climate – by forcing the model with a CO2 change, resulting in a large temperature response!” followed by an erroneous (but nevertheless a useful point) comment that “Well, DUH! If they had forced the model with a water vapor change, it would have done the same thing.” (It would not.)
Roy’s claim about our “assuming” water vapor and clouds to be feedbacks might have been a valid point if we had used an old 1970s vintage 1-D model to perform our analysis. In current climate GCMs no such assumptions are being made. Atmospheric water vapor and cloud distributions are the direct results of the model physics interactions (via evaporation, transport, condensation, precipitation).
We later went on to demonstrate this very point by forcing the model with a water vapor change by instantaneously doubling (and zeroing out) water vapor in a couple of GCM runs. These GCM experiments further re-emphasized the key point of our Science paper that water vapor is indeed a fast feedback process in the climate system. Within about two-weeks time, the atmospheric water vapor distribution had essentially returned back to control run levels. During this two-week transition period, any water vapor excess (or deficit) relative to the equilibrium distribution did of course produce a radiative greenhouse heating (or cooling) effect, but this ‘virtual forcing’ was very transient in nature, without any lasting impact on the global temperature.
Roy Spencer concluded his analysis by stating that “the paper really tells us nothing new about (1) how much warming we can expect from adding more CO2 to the atmosphere, or (2) how much of recent warming was caused by CO2”. (So, nothing new, but nothing wrong!)
Roger Pielke, Sr. also commented on our Science paper, stating that “The paper is an interesting model experiment, but it really does not present any new insight beyond what we already know.” And further that “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.” (Nothing new, only an op-ed.)
I since have had some useful dialogue with Roger, and went on to post some commentary items on his climate blog. The first posting was basically the GISS webpage science brief. I then later posted a longer commentary on the Atmospheric CO2 Thermostat that included (1) a description of our recent water vapor feedback/forcing experiments, (2) an illustration describing high accuracy GCM LW radiation calculations, (3) a short discourse on the latitudinal nature of climate feedbacks, and (4) an overview of those things that we know well (about climate), and those things that we know less well.
In regard to questioning the lack of any ‘new scientific insight’, I scarcely think that Jim Hansen’s opinion (or mine) would really differ greatly from what Roy Spencer and Roger Pielke, Sr. have expressed. However, Jim Hansen did go on to say in addition, that “It’s about time that somebody wrote a paper like that”. Even though we have known and understood for decades the basic science of the terrestrial greenhouse effect, and water vapor feedback effects, in the current climate (politically speaking) of the frequently expressed irrational thinking, there is unfortunately a clear and pressing need to keep on repeating and explaining the most basic of global climate concepts.
So, if there really was ‘nothing new’ in our Science paper . . .
then how did this paper come to be?
First, let me state here what the paper is not. The paper is not a snowball Earth paper (although it is relevant.) Geological evidence suggests that tropical latitudes were glaciated some 600M years ago. Even with near zero CO2, and the Sun 3-4% dimmer than now, the tropics require albedo help from clouds and encroaching sea ice for global freeze-0ver – hence, an interesting science problem that needs careful cold-climate cloud and ocean dynamics modeling. Also, as Roy Spencer noted, this paper is not an explanation for past decadal temperature change (although it is relevant). Modeling of the recent decadal climate record would require careful accounting for all of the radiative forcings, and would also require accurate modeling of ocean dynamics to accurately simulate the climate system’s response time.
Instead, the aim of our Science paper was to illustrate as clearly and as simply as possible the basic operating principles of the terrestrial greenhouse effect in terms of the sustaining radiative forcing that is provided by the non-condensing greenhouse gases, which is further augmented by the feedback response of water vapor and clouds.
If you were to go and read the acknowledgment that is at the end of the Science paper, you would see the very standard “thank you” for helpful comments from numerous GISS colleagues, and a “thank you” for funding support from NASA program managers.
But, you would not see there any mention of Bishop Hill. Why so? And, would the Science editors have really let that happen?
It started with the following (February 9, 2010) posting on the Bishop Hill blog that stated: “While perusing some of the review comments to the IPCC’s Fourth Assessment Report, I came across the contributions of Andrew Lacis, a colleague of James Hansen’s at GISS. . . . Remember, this guy is mainstream, not a skeptic, and you may need to remind yourself of that fact several times as [you] read through his comment on the executive summary of the chapter:” This topic was also later picked up and commented on at WUWT.
I was being hailed as a fellow critic of the IPCC AR4 Report, and by some, as a newly-found hero of the climate change denier cause.
Back in 2005, having uploaded countless review suggestions for the IPCC AR4 Report, toward the end, I did make some intemperate remarks that were directed at the Executive Summary of Chapter 9.
I was irked by the persistent use of wishy-washy terminology such as ‘likely’ and ‘very likely’ that was totally uncalled for. One example: “It is likely that there has been a substantial anthropogenic contribution to surface temperature increases in every continent except Antarctica since the middle of the 20th century.”
Such ‘social sciences’ terminology might be allowable if there was no other available evidence for global warming except for the statistical analysis of a relatively short global temperature time-series (on which there is superimposed a substantial natural variability component). But the physical evidence for global warming is quite overwhelming, and it is downright irresponsible (and stupid) not to make use of it.
More specifically: (1) precise measurements show atmospheric CO2 has increased from its 280 ppm pre-industrial value to the current ~390 ppm; (2) there is available an accurate HITRAN tabulation of line absorption coefficients for all of the atmospheric absorbing gases; (3) we have available accurate radiation modeling techniques as well as capable global climate models; and (4) that 9 Gigatons of carbon (coal, gas, oil) are being burned each year (by us humans).
Based on this basic input data, the relevant physics is inescapably clear that the increase in atmospheric CO2 is indeed enhancing the strength of the terrestrial greenhouse effect, and thus causing global warming to happen – all directly attributable to human industrial activity. To characterize this fully documented global warming only as being ‘likely’ a ’substantial’ anthropogenic contribution is clearly resorting to unscientific understatement that does nothing to clarify or accurately portray our understanding of global climate change. Rather, using such under-whelming weasel words only adds to the deliberate public confusion regarding climate change. Unfortunately, such subtle misinformation is being actively promoted by the fossil fuel lobbyists and their growing multitude of dupes and minions.
The above was the essence of my response to Bishop Hill, as it came to be posted on Andy Revkin’s New York Times dotearth blog.
Blog commentaries, however, are too transient to have any lasting impact. It became increasingly clear that the current climate situation really called for a simple and convincing demonstration of just how the terrestrial greenhouse effect operates in maintaining the surface temperature of the Earth. Hence, our writing of the Science paper.
Basic conclusions that can be drawn from this exercise:
(1) The terrestrial greenhouse effect is comprised of two distinct components: (a) the non-condensing greenhouse gases that provide the ‘radiative forcing’ that sustains the terrestrial greenhouse effect; (b) the ‘feedback component’ by water vapor and clouds that acts to amplify the radiative effect of the non-condensing greenhouse gases.
(2) The radiative forcing by the non-condensing greenhouse gases is accurately known, and fully understood. Of the GHGs, atmospheric CO2 is the principal contributor, hence the principal control knob that governs the strength of the greenhouse effect and global temperature. The greenhouse physics, and the increase of atmospheric greenhouse gases as the fundamental basis for global warming, are well founded.
(3) Water vapor and clouds account for about 75% the strength of the terrestrial greenhouse effect, but are feedback effects that require sustained radiative forcing to maintain their atmospheric distribution. Their radiative effects are accurately known. The magnitude of their feedback sensitivity is also reliably known, to within order of 10%.
(4) The temporal record of global climate change can be separated into two distinct components: (a) global warming – this is the steady and predictable increase in the strength of the terrestrial greenhouse effect that is caused by the increase in atmospheric greenhouse gases resulting from human industrial activity; (b) natural variability – this is the unforced and mostly unpredictable inter-annual, regional, and decadal variability of the climate system that is superimposed upon the steadily increasing global warming component.
(5) Global warming, the climate change component that is driven by greenhouse gas increases, is the reason for concern because of its increasing impact on ecosystems and polar ice caps/sea level rise. Whether humans like it or not, and whether humans realize it or not, global warming has been so, and continues to be, fully under human control via fossil fuel burning. Smaller contributors such as changes in aerosols, solar irradiance, and sporadic large volcanoes exist. But aerosol forcing is also anthropogenic and/or short lived. Solar forcing is cyclical and small, while the GHG residence time is very long.
(6) Natural (unforced) climate variability is the principal reason for the uncertainty manifested in the largely unpredictable temperature and precipitation fluctuations that occur on regional spatial scales, and on inter-annual and decadal time scales. Arising from changes in advective energy transports and poorly understood interactions with ocean dynamics, this is where uncertainty reigns supreme. However, these advective transports must globally add to zero, and the unforced fluctuations are necessarily fluctuations about the global equilibrium reference point. Nature conserves energy very carefully. Hence, large deviations from the global equilibrium cannot be sustained. So, this unforced climate variability cannot significantly impact the long-term global temperature trend, but its effects on local and regional climate will remain the main source of uncertainty for the foreseeable future.
(7) Global climate change is far too complex to be understandable in one swoop. Fortunately, the global warming component, it being tied directly to the growing strength of the terrestrial greenhouse effect, is a uniquely radiative effect that can be addressed independently of the other climate complexities. The basic physics of the greenhouse effect is rooted to the conservation of global energy. Precise measurements of the rising concentration of atmospheric CO2 are irrefutable, leaving nodoubtthatglobalwarmingis happening. Geological evidence shows that 450 ppm of atmospheric CO2 is the critical level that is needed to sustain polar ice caps, although the time scale for the melting of polar ice caps is many centuries. That is the scientific perspective on global warming. Deciding what, if anything, to do about global warming is a political problem, but the politicians should keep the science in mind.
Moderation note: this is a technical thread. Comments will be moderated for relevance.