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Global Primary Productivity and Climate Change
Volume 8, Number 47: 23 November 2005

Matthews et al. (2005) note that "coupled climate-carbon cycle model simulations have identified an important positive feedback between the terrestrial carbon cycle and climate, whereby future carbon uptake declines under anthropogenic climate warming."  Just how powerful is this positive feedback that leads to a decline in carbon uptake?  The Canadian researchers say that the first simulation to address this question, that of Cox et al. (2000), suggested that carbon uptake by the terrestrial biosphere would likely decline by over 500 GtC (gigatonnes of carbon = 1015 grams C) as a result of greenhouse gas-induced climate changes, such that the terrestrial biosphere as a whole would likely switch from being a net carbon sink to being a net carbon source around the year 2050.

As time has passed, however, and as the models have been refined, Matthews et al. report that "subsequent simulations forced by a variety of emissions scenarios" have resulted in "widely ranging feedback magnitudes."  By the year 2100, for example, the increase in atmospheric CO2 attributable to the carbon cycle-climate feedback has been projected to range from 50 ppm (Govindasamy et al., 2005) to 250 ppm (Cox et al., 2000).  The latter of these two numbers represents a CO2 concentration increase that is fully two-and-a-half times greater than what has been experienced since the beginning of the Industrial Revolution ... which is a truly whopping positive feedback at the end of a truly whopping positive feedback range.  Moreover, Matthews et al. report that "it has not been possible to reconcile the range of model results," which lingering problem represents, in their words, "one of the most important uncertainties in current simulations of future climate change."

From whence does the highly-uncertain feedback originate?  Matthews et al. say that "analysis of model results has shown that most of the additional carbon in the atmosphere comes from the soil carbon pool, suggesting that the acceleration of soil carbon decomposition under climate warming is a key component of the feedback."  Hence, as they continue, "attempts to explain the large range of model results have focused on uncertainties in the behavior of heterotrophic soil respiration under future climate change."  The genius of their contribution is that they show this assumption to be wrong, demonstrating that "the response of vegetation primary productivity to climate change may in fact be more important than the behavior of soil respiration in determining the magnitude of simulated positive carbon cycle-climate feedbacks."  It is the source of their downfall, however, that they continue to assume that the feedback in question is positive, when it more likely is negative.

In moving towards this significantly different view of ours on the matter, we still find much of Matthews et al.'s work to be helpful.  They note from the outset, for example - and quite correctly - that "the simulated feedback is highly sensitive to the temperature dependence of photosynthesis, which is currently very poorly represented in global carbon cycle models."  They also correctly report that "as temperature increases in the model, plants are increasingly required to photosynthesize at higher temperatures," but they errantly conclude that this requirement leads to "a stronger temperature suppression [our italics] of photosynthesis under climate warming," which likely is not true.

Interestingly, Matthews et al. actually mention certain important facts that should have led them to question their assumption on this point.  First, they note that "plants exhibit considerable adaptation to local temperatures," a fact we discuss in some detail in Section II A (The Adaptability of Plants to Rising Temperature) in our Major Report The Specter of Species Extinction: Will Global Warming Decimate Earth's Biosphere?  Second, they say that "the temperature response curve is further affected by CO2 concentration," a fact that we discuss in Section II B (The Extra Help Provided by Rising Atmospheric CO2 Concentrations) in the same report.  Specifically, we review the evidence for the oft-reported observation that the optimum temperature for plant growth, i.e., the temperature at which plants photosynthesize and grow best, generally rises with as the air's CO2 content rises.  In fact, we note that for a 300 ppm increase in the air's CO2 concentration, the mean increase in optimum temperature for a sizable group of plants has been determined to lie between 3.4 and 5.8C.

Simultaneously, the aerial fertilization effect provided by rising levels of atmospheric CO2 tends to increase plant photosynthetic rates at all temperatures, and progressively more so at higher temperatures, which additionally enables plants to survive at higher temperatures than they can tolerate under current atmospheric CO2 concentrations.  Acting together, these several related phenomena lead to ever-increasing levels of plant primary productivity as the air's CO2 content and temperature rise together.  In the case of the plants mentioned above, for example, the photosynthetic rates they experience at their CO2-induced higher optimum temperatures are typically much greater than those they experience at their ambient-CO2 optimum temperatures, in some cases almost twice as great (Idso, 1995).

Returning to the paper of Matthews et al. armed with this knowledge, we note they correctly state that "the parameterization of temperature constraints on photosynthesis strongly affects results from climate-carbon cycle models."  We also note that the large uncertainties related to the presumed warming-induced reductions in rates of plant photosynthesis that they discuss lead to increases in atmospheric CO2 concentration at the year 2100 that range from 50 to 250 ppm, revealing the tremendous power of modest changes in plant photosynthetic prowess to affect the whole climate-change process.  In light of these observations, it should be clear that the currently-ignored positive response of plant photosynthesis to simultaneous increases in atmospheric CO2 concentration and temperature should actually lead to a significant decrease in predicted atmospheric CO2 concentration at the year 2100, which implies the existence of a powerful negative feedback between the terrestrial carbon cycle and climate that tends to fortify the planet against the possibility of significant CO2-induced climate change.

Sherwood, Keith and Craig Idso

References
Cox, P.M., Betts, R.A., Jones, C.D., Spall, S.A. and Totterdell, I.J.  2000.  Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model.  Nature 408: 184-187.

Govindasamy, B., Thompson, S., Mirin, A., Wickett, M., Caldeira, K., Delire, C. and Duffy, P.B.  2005.  Increase of carbon cycle feedback with climate sensitivity: Results from a coupled climate and carbon cycle model.  Tellus Series B 57: 153-163.

Idso, S.B.  1995.  CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution.  Department of Soil, Water & Climate, University of Minnesota, St. Paul, Minnesota, USA.

Matthews, H.D., Eby, M., Weaver, A.J. and Hawkins, B.J.  2005.  Primary productivity control of simulated carbon cycle-climate feedbacks.  Geophysical Research Letters 32: 10.1029/2005GL022941.