TCS Daily

Winter Weather Wonder

By Willie Soon - January 28, 2004 12:00 AM

With record-breaking cold-spells striking North America, Siberia, Turkey and even Bangladesh, one would think that the rhetoric on global warming would momentarily soften. On the contrary, during the same week when many of America's homeless and the poor struggled with the reality of cold spells, the article "Global warming evidence is mounting" pronounced that 2003 was another record warm year.

Experts from the National Climatic Data Center (NCDC) and National Center for Atmospheric Research (NCAR) complained about the unfairness in suggesting that one cold spell would disprove the CO2-global warming theory. One NCAR climate expert said that "Mother Nature keeps reminding us that [global warming] is going on. The evidence never really comes out to contradict it, even though the man on the street says, `It's bloody freezing out here'."

Meanwhile, one reporter spun the recent cold snap this way: "Global warming may be playing a role in Americans' sense that January was especially cold. Because winters have been milder in the 1990s and 2000s, cold snaps feel colder, as people are unaccustomed to them." Is it true that the global warming of the 1990s and 2000-2003 is causing us to "feel " colder and to be incapable of being objective about the past one to two weeks of cold air outbreaks? That's unlikely. The cold spells are reality - not just relative reality. Air temperatures for much of the U.S. Northeast were at or near the all-time record lows.

What Does Adding CO2 Actually Do?

But all this recent chatter about CO2-global warming theory raises a serious scientific question: What does adding carbon dioxide to the atmosphere really do to the weather and climate of the world?

And by good chance, this is an opportune time to seek better understanding. The January 1 issue of the Journal of Climate includes a thought-provoking article, "A mechanism of tropical precipitation change due to CO2 increase," by Masato Sugi of the Meteorological Research Institute and Jun Yoshimura of the Frontier Research System for Global Change of Japan.

It turns out that the question about the effects of carbon dioxide should not - indeed, they simply cannot -- stand alone. Assessing the role played by CO2 in climate change can be truly meaningful only if one includes water vapor -- the most important greenhouse gas in the Earth's atmosphere -- to the puzzle.

Sugi and Yoshimura sought to explain why, in their model, the frequency of tropical cyclones decreases when the atmospheric CO2 is increased. It seems counter-intuitive, at least if one's intuition has been trained by the frequent popular suggestion that more CO2 leads to more cyclones! (The Intergovernmental Panel on Climate Change Third Assessment Report (IPCC TAR) for example, despite the caveats on the lack of observational data for assessment of change during the 20th century, predicted a "likely" increase in the tropical cyclone mean and peak precipitation intensities "over some areas" in the 21st century.) But Sugi and Yoshimura found that the root cause of the modeled tendency is a reduction of radiative cooling (hence a net warming) in the lower troposphere because of added absorption owing to an intrinsic overlap of CO2 and water vapor absorption bands in the infrared portion of the electromagnetic spectrum (specifically, around the 15 ┬Ám wavelength).

Apparently as more CO2 is added to the atmosphere, the lower troposphere is increasingly warmed through extra absorption of infrared (or longwave) radiation. This reduces the vertical temperature gradient between the sea surface and lower troposphere -- with less of a temperature contrast, it will be less probable to initiate a violent energetic event like a cyclone.

Sugi and Yoshimura also found only a little net increase in tropical precipitation or rainfall despite a significant increase in atmospheric moisture. The net effect is a significant reduction in the strength of tropical circulation in their model, and hence a decrease in the frequency for spawning more tropical cyclones.

But what is this intrinsic overlap of CO2 and water vapor absorption bands?

Figure 1 first illustrates two hypothetical scenarios: a tropical atmosphere[1] loaded with water vapor alone (panel a) and CO2 alone (panel b). Figure 1 shows that the primary role of water vapor and CO2 in the atmosphere is mainly to cool the layer of air in the atmosphere (except for the slight warming in the upper troposphere for the CO2 case). In case you are confused about what is being referred to by the almost universal acceptance of CO2-global atmospheric warming notion in the popular media, you have the right reason to be so.

Figure 1: A comparison of the radiative effects in the two hypothetical scenarios of having (a) water vapor (H2O) alone and (b) carbon dioxide (CO2) alone in the tropical (30N-30S) atmosphere. Note that the principal effect of having water vapor or carbon dioxide is to cool, rather than warm, the column of air in the lower and middle troposphere. The expected global warming by "trapped radiation" because of added CO2, more popularly conceived and discussed, is a secondary effect more correctly explained by the overlap effects of water vapor and carbon dioxide absorption bands discussed in this CharTiFact. The radiative cooling effect by water vapor is dominant in lower atmosphere and almost none in the upper troposphere since very little water vapor molecules are available at great heights. The radiative effect of CO2 is of lesser importance when compared to the water vapor and its presence cools the lower layer of air but slightly warms the air in the upper troposphere. [Adapted from Masato Sugi and Jun Yoshimura, Journal of Climate, vol. 17, 238-243 (2004).]

But one need not hesitate to ask and find out.

The popular CO2-global warming notion is referring to the secondary effect of adding more CO2 (also known as the enhanced greenhouse effect) to the column of air in the atmosphere so that the combined cooling effects by CO2 and water vapor are reduced, hence a net warming of the atmosphere (and perhaps triggered down to the terrestrial and oceanic surface by additional physical mechanisms).

Consider this calculation from the paper by Sugi and Yoshimura: When atmospheric CO2 is doubled, the presence of CO2 will reduce the upward longwave radiation flux to the top of the atmosphere by 3.8 W/m2 in the tropics but at the same time, the extra CO2 in the lower atmosphere will increase the downward longwave flux by 6.2 W/m2. So the net effect of adding more CO2 is still to cool the atmosphere since there is a net flow of radiation from the atmosphere to the surface.[2]

So where is this net tropical or global warming from doubling CO2 that was promised to be true?

It is now time to consider the overlap effect of CO2 and water vapor absorption bands that is rooted in the intrinsic molecular properties of the two gases.

Instead of calculating the longwave fluxes going out of the atmosphere and going to the surface by considering only the hypothetical CO2-alone scenario, Sugi and Yoshimura now included the overlap effect of CO2 and water vapor in a revised evaluation of the problem of adding more CO2 in the atmosphere. Now they found that a CO2 doubling reduces the upward longwave radiation flux to the top of the tropical atmosphere by 2.6 W/m2 but it increases the downward surface radiation by only 0.6 W/m2. In this case -- rather than the previous two hypothetical scenarios -- the increase in downward radiation flux to the surface is significantly reduced from before because the downward radiation flux is now disproportionately absorbed by the water vapor in the lower troposphere before reaching surface.[3] So indeed the net effect of doubling CO2 is to warm the atmosphere somewhere in between the top-of-the atmosphere and surface (i.e., lower and middle troposphere or about a few kilometer above surface[4]) since there is now less reduction in the longwave radiation cooling.

With this research, we are getting a deeper understanding of the effects of adding CO2 to the Earth's atmosphere. The lower and middle troposphere will warm not because of the greenhouse effect of CO2 per se but instead because of the overlap effects of CO2 and water vapor absorption bands -- which are truly inseparable in its physical presence and impact for the Earth's atmosphere. Consideration of the radiative effect or absorption properties of CO2 alone is clearly not realistic and highly misleading.

Sugi and Yoshimura remarked that "the reduction of radiative cooling associated with the CO2 increase is a result of overlap effect of CO2 and water vapor absorption bands and the coexistence of CO2 and water vapor in the earth's atmosphere is essential. ... Since the overlap effect is a unique mechanism in the greenhouse warming due to CO2 increase, it enables us to distinguish the CO2-induced climate change from other climate changes."

Additional Implications

What additional implications might such a finding have on the popular depiction of CO2-global warming?

The two authors first warn that "The effect of CO2 and the other greenhouse gases are not the same ... A simplified treatment of greenhouse gases as CO2 equivalent [such as those routinely done by many assessment and impact studies including those by the IPCC] may cause a problem in estimating the impact of greenhouse gas increase on the tropical precipitation and the earth's climate."

Because they were able to find the physical mechanisms for the weakening of tropical circulation and reduced tropical cyclones from added CO2, Sugi and Yoshimura were also able to confidently conclude that the recently reported strengthening or intensification of tropical circulation (associated with the decadal variability in the imbalances of the tropical-mean radiative energy) by a team of NASA scientists is therefore "not directly caused by the greenhouse warming due to CO2 increase."

So adding CO2 in the atmosphere may warm the Earth a little, because of the intrinsic absorption properties of both CO2 and water vapor, but it will not necessarily bring about more tropical cyclones. Further, increasing CO2 does not square with the intensification of tropical circulation observed over the past two decades. It is certainly not a stretch to suppose that increasing atmospheric CO2 is not the cause of the January 2004's cold winds and snows.

[1] Sugi and Yoshimura were focusing on the impact of adding atmospheric carbon dioxide to the generation of tropical cyclones, so most of their calculations and analyses were performed strictly for the Tropics instead of the full globe.

[2] Further note that even if the full 6.2 W/m2 of infrared radiation energy is added to the surface, it is very small compared to sunlight energy deposited at the surface or the atmospheric latent heat energy produced when water vapor condensed to make rain. This is why we are truly being misguided to be thought to think that CO2 infrared radiative forcing is the most significant factor for maintaining the mean climate and for inducing its changes. It is not. As the distinguished meteorologist Reid A. Bryson of Wisconsin commented "Every few days I see another paper where the author demonstrates that he has no knowledge even of the present climate, much less what makes it change."

[3] This is also the reason why the lower troposphere must warm first relative to the surface when carbon dioxide is increased/doubled, often noted by many careful scientists from around the world.

[4] A simple but rough rule of thumb for converting the pressure-scale height (in millibar or mb) to an actual height (in meter) is through 1 mb ≈ 10 meter.


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