December 8, 2010

CO2-induced Vegetation Growth Slows Global Warming

Filed under: Adaptation, Climate Models, Plants

We are continually deluged with talk about positive feedbacks leading to even higher levels of global warming, but aside from the great water vapor debate, we rarely hear much about negative feedbacks which could act to slow the rate of temperature rise.

Well that is about to change.

A new study has identified a negative feedback between carbon dioxide-enhanced vegetative growth and global warming—the denser that vegetation becomes, the greater the cooling influence it has on any global temperature rise. The enhanced vegetation doesn’t offset all of the projected warming, but a sizeable chunk of it—13% globally, 20% over land areas, and more than 50% over the eastern United States. And this negative feedback is not included in current climate models.

A research team of scientists from NOAA, NASA, and the University of Maryland, led by Lahouari Bounoua set out to study how changing vegetation characteristics induced both directly by enhanced atmospheric carbon dioxide levels (recall that CO2 is a plant fertilizer) as well as by climate changes that are favorable for plant growth (increased temperature and/or precipitation) may feedback on the projected climate changes. The authors note that there has been some previous work on this topic, but that their current work (just published in the peer-reviewed journal Geophysical Research Letters) uses a different modeling approach and includes plant responses not included in earlier studies.

The Bounoua team coupled a plant vegetation model to a climate model that allowed plant characteristics to change and in turn feedback on the climate system. The model was run for a case of doubled atmospheric CO2 concentration (700ppm).

The biggest changes to the plants were an increase in water use efficiency and an increase in the Leaf Area Index (LAI) (the LAI is the total area of all the upper surface of all the leaves on a tree divided by the area on the ground covered by that tree—the more and larger the leaves, the greater the LAI ). Figure 1 shows that the LAI increased by more than 50% at each of three different scales/regions—all global land areas, U.S.A., and the eastern United States. The increased LAI indicates overall denser, more vigorous, vegetation.


Figure 1. Increase in Leaf Area Index (LAI) in the doubled CO2 scenario. (from Bounoua et al., 2010).

The changes in LAI influence the earth’s carbon cycle and its climate. The authors describe the impacts this way:

[I]ncreases in LAI not only affect the carbon uptake, transpiration and interception rates, but also alter surface albedo and roughness and so affect the exchanges of carbon, energy, water and momentum at the land-atmosphere interface. Furthermore, the model’s vegetation physiological growing season is controlled by low-temperature stress levels below which photosynthesis is inhibited. As temperatures increase with CO2, these stress levels become less severe earlier during the onset of vegetation greening and later during the dormancy phase, increasing thus the length of the growing season.

Clearly a wide range of influence.

And taken together, the net result of the modeled vegetation influences is an overall cooling which offsets some of the modeled CO2-induced warming.

The mechanism is somewhat complex, but basically, the increased LAI intercepts a greater percentage of the incident precipitation and redirects it for use of the vegetation. This has a two-fold impact, 1) it increases evapotranspiration and 2) it decreases the volume of run-off. The former produces a cooling effect on the climate and the latter lessens flash flooding.

While the authors don’t discuss the flooding (or reduction thereof) aspect of their findings, they do look into the temperature effects. Using their climate model (which they admit has a fairly low climate sensitivity), they find that the effect of increasing LAI has a pretty large effect on lessening the CO2-induced warming. Compared with a global average temperature rise of 1.94°C (for a CO2 doubling), they find that incorporating climate/vegetation interactions into the model produces a rise of 1.68°C—or that increased LAI offsets about 13% of the projected warming on a global average. As you may imagine, the cooling influence is greater when only averaged over land areas. There the non-interactive model produces 2.80°C of temperature rise, while the interactive model produces only 2.23°C, or some 20% less. The impact can be even larger on regional scales. For instance, over the eastern U.S., Bounoua et al. find that LAI increases from the effects of increasing CO2 act to offset a whopping 52% of the projected warming—a large, noteworthy, and significant (in more ways than one) impact.

The authors sum up their findings this way:

When we combine these interactions in climate simulations with 2 × CO2, the associated increase in precipitation contributes primarily to increase evapotranspiration rather than surface runoff, consistent with observations, and results in an additional cooling effect not fully accounted for in previous simulations with elevated CO2. By accelerating the water cycle, this feedback slows but does not alleviate the projected warming, reducing the land surface warming by 0.6°C. Compared to previous studies, these results imply that long term negative feedback from CO2-induced increases in vegetation density could reduce temperature following a stabilization of CO2 concentration.

True, these results are produced from a modeling study—but they clearly show that current climate models (which do not incorporate interactive vegetation models) fail to include an important climate feedback.

As eminent climatologist Dr. Roger Pielke Sr. is fond of pointing out time and time again over on his excellent blog, the earth’s climate system and its interactions with the rest of the world’s environment are far more complex than current climate models come close to handling—and what’s worse, is that organizations like the IPCC and the EPA, rather than embracing this fact, try to hide it under the rug.

Papers like the new Bounoua et al. study make this plain as day and should be required reading for all folks considering action based on climate model projections.

Reference:

Bounoua, L. et al., 2010. Quantifying the negative feedback of vegetation to greenhouse warming: A modeling approach. Geophysical Research Letters, 37, L23701, doi+10.1029/2010GL045338.




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