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Carbon Sequestration by a Subarctic Sedge Fen
Northern peatlands figure prominently in nearly all scenarios of CO2-induced global change for two major reasons: (1) they cover a vast area of land, estimated to be close to 350 million hectares, and (2) they contain an enormous amount of carbon, estimated to be on the order of 450 Pg (Gorham, 1991). Consequently, peatlands possess the potential to return great quantities of carbon to the atmosphere, as is typically emphasized by those who say global warming could easily trigger such a release. Having removed so much carbon from the air in the past, however, earth's peatlands could well do more of the same in the future. So which way are the scales more likely to tip if the planet were to warm and experience more precipitation, as predicted by most climate models?

This question drives much current research and analysis. In one of the most realistic and comprehensive of such endeavors, Griffis and Rouse (2001) draw upon the findings of a number of experiments conducted over the past quarter-century at a subarctic sedge fen near Churchill, Manitoba, Canada to develop an empirically-based model of net ecosystem CO2 exchange there. Grounded in real-world observations, as opposed to purely theoretical considerations, this model teaches us much about what we could expect from northern peatlands in the way of carbon sequestration or release if the air's CO2 content were ever to double (which may well be a real possibility) and produce serious global warming (which is not so likely, in our opinion, but nevertheless worthy of consideration).

The most fundamental of Griffis and Rouse's findings is that "carbon acquisition is greatest during wet and warm conditions," both of which climate characteristics are what is generally predicted for the world as a whole by today's most advanced climate models (as well as most of the not-so-advanced ones). However, since regional climate change predictions are not yet very dependable, the scientists investigated the consequences of a 4C increase in temperature accompanied by both a 30% increase and decrease in precipitation. And "in all cases," as they put it, "the equilibrium response showed substantial increases in carbon acquisition."

Dryness, however, is definitely not a plus and may well be responsible for the observation that the contemporary annual CO2 balance of the Churchill fen is probably slightly negative. Over the past 2200 years, however, the net long-term balance has been decidedly positive, likely due, in Griffis and Rouses' opinion, to "a progression toward drier conditions than in the past." What they are talking about here is that palaeoclimate reconstructions from the Eastern Arctic indicate that "climate in the region was warmer and more humid during the late Holocene with cooling and increased occurrence of fire during the last millennium." Hence, we once again have a demonstration of the biological superiority of Medieval Warm Period conditions as compared to Little Ice Age conditions, from which we are currently transitioning as we approach desired Modern Warm Period conditions.

But isn't warming supposed to increase ecosystem respiration and return more carbon to the atmosphere? Sometimes it does; but in the case of the subarctic sedge fen and other peatland ecosystems studied by Griffis and Rouse and many others, the data suggest, in the words of the former, "that arctic ecosystems photosynthesize below their temperature optimum over the majority of the growing season," so that increasing temperatures enhance plant growth rates considerably more than they increase plant decay rates.

In summing up their findings, Griffis and Rouse reiterate the fact that "warm surface temperatures combined with wet soil conditions in the early growing season increase above ground biomass and carbon acquisition throughout the summer season." Indeed, they note that "wet spring conditions can lead to greater CO2 acquisition through much of the growing period even when drier conditions persist [our italics]." They thus conclude that if climate change plays out as described by current climate models, i.e., if the world becomes warmer and wetter - as is also suggested by historical climate analogue - "northern wetlands should therefore become larger sinks for atmospheric CO2."

We definitely agree, for as we indicated in an earlier essay in this series of commentaries - Another Global Warming Horror Story Bites the Dust, featuring the work of Camill et al. (2001) - a number of studies have observed significant increases in peatland carbon sequestration in response to regional warming. Hence, rather than adding to the air's burden of CO2, further warming would actually tend to reduce it, even in peatlands that already contain a tremendous reservoir of sequestered carbon.

Dr. Sherwood B. Idso Dr. Keith E. Idso

Camill, P., Lynch, J.A., Clark, J.S., Adams, J.B. and Jordan, B. 2001. Changes in biomass, aboveground net primary production, and peat accumulation following permafrost thaw in the boreal peatlands of Manitoba, Canada. Ecosystems 4: 461-478.

Gorham, E. 1991. Northern peatlands role in the carbon cycle and probable responses to climatic warming. Ecological Applications 1: 182-195.

Griffis, T.J. and Rouse, W.R. 2001. Modelling the interannual variability of net ecosystem CO2 exchange at a subarctic sedge fen. Global Change Biology 7: 511-530.