In exploring the issue further, Matthews et al. say that the first simulation to address this concern, 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. reported at the time that "it has not been possible to reconcile the range of model results," which 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." 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 (see, for example, the many papers supporting this latter interpretation of the situation at http://www.co2science.org/subject/b/bioproductivity.php).

In moving towards this significantly different view on the matter, there is much of Matthews et al.'s work that is 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 [italics added] of photosynthesis under climate warming," which likely is also not true (see http://www.co2science.org/subject/f/foresttemp.php).

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." Second, they say that "the temperature response curve is further affected by CO2 concentration," and there is much 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 as the air's CO2 content rises. In fact, 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 5.8°C (see the four sub-headings under Growth Response to CO2 with Other Variables - Temperature in our Subject Index).

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 with this knowledge to the paper of Matthews et al., they correctly state that "the parameterization of temperature constraints on photosynthesis strongly affects results from climate-carbon cycle models." It should be noted 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 clarifications, 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. And there is ample real-world evidence that this is indeed how the biosphere is responding to the "twin evils" of the climate-alarmist movement, i.e., rising atmospheric CO2 concentrations and rising atmospheric temperatures (see the many sub-headings under Biospheric Productivity in our Subject Index). Nevertheless, we continue with our analysis of other model-based studies to learn what others have established in this regard.

In an earlier analysis incorporating the best approximations available at the time, Xiao et al. (1998) used a process-based ecosystem model that made monthly calculations of carbon and nitrogen fluxes and their pool sizes in various terrestrial ecosystems to compute global net ecosystem production (NEP) from 1990 to 2100 based on three different scenarios of atmospheric CO2 and temperature change. In doing so, they found that for the scenario with lower-than-typically-assumed increases in CO2 emissions and air temperature, the NEP of the globe increased from a positive (carbon-sequestering) value of 0.8 Pg C yr^-1 in 1990 to one of 1.3 Pg C yr^-1 in 2100, for a rate increase of 62% over the 110-year period. For typically-assumed increases in CO2 and temperature, corresponding NEP values were 0.8 and 2.6 Pg C yr^-1, yielding a rate increase of 225%. And for higher-than-typically-assumed increases in CO2 and temperature, NEP values of 0.8 and 3.4 Pg C yr^-1 were obtained, for a rate increase of 325%, which suggests a pretty dramatic increase in biospheric prowess in the future.

Similar findings were reported by Qian et al. (2010), who focused their analysis on the frozen soils of Earth's Northern High Latitudes (NHLs, land poleward of 60°N). Specifically, they analyzed the outputs of ten different models that took part in the Coupled Carbon Cycle Climate Model Intercomparison Project (C4MIP) of the International Geosphere-Biosphere Program and the World Climate Research Program, all of which models, in their words, "used the same anthropogenic fossil fuel emissions from Marland et al. (2005) from the beginning of the industrial period until 2000 and the IPCC SRES A2 scenario for the 2000-2100 period." Accordingly, the ten C4MIP models predicted a mean warming of 5.6°C from 1901 to 2100 in the NHL, yet the three researchers confirm that "the NHL will be a carbon sink of 0.3 ± 0.3 PgCyr^-1 by 2100." They also state that "the cumulative land organic carbon storage is modeled to increase by 38 ± 20 PgC over 1901 levels, of which 17 ± 8 PgC comes from vegetation [a 43% increase] and 21 ± 16 PgC from the soil [an 8% increase]," noting that "both CO2 fertilization and warming enhance vegetation growth in the NHL."

In another take on this issue, Friend (2010) calculated the percentage changes in terrestrial plant production that would occur throughout the world in response to (1) projected climate changes alone, and (2) the projected concurrent changes in climate and atmospheric CO2 concentration. This was accomplished using the Hybrid6.5 model of terrestrial primary production, which "simulates the carbon, nitrogen, phosphorus, water, and energy fluxes and structural changes in terrestrial ecosystems at hourly to decadal timescales, and at spatial scales ranging from the individual plant to the whole Earth," while employing "the climate change anomalies predicted by the GISS-AOM GCM under the A1B emissions scenario for the 2090s [relative] to observed modern climate, and with atmospheric CO2 increased from 375.7 ppm to 720 ppm."

In response to projected climate changes between 2001-2010 and 2091-2100, Friend reports that net primary production (NPP) of the planet as a whole was reduced by 2.5%, with the largest negative impacts occurring over southern Africa, central Australia, northern Mexico, and the Mediterranean region, where reductions of over 20% were common. At the other extreme, climatic impacts were modestly positive throughout most of the world's boreal forests, as might have been expected when these colder regions received an influx of welcome heat. When both climate and atmospheric CO2 concentration were changed concurrently, however, the story was vastly different, with a mean increase in global NPP of 37.3%, driven by mean increases of 43.9-52.9% among C3 plants and 5.9% among C4 species. And in this case of concurrent increases in the globe's air temperature and CO2 concentration, the largest increases occurred in tropical rainforests and C3 grass and croplands.

Additional optimism for the future of the biosphere is seen in the fascinating review article of Sage and Coleman (2001), who discuss the idea that modern bioengineering techniques might enable us to make plants even more responsive to increases in the air's CO2 content and thereby portend an even greater future for the terrestrial biosphere.

Their thinking runs this way. During the peak of the last ice age - and throughout the bulk of all prior ice ages of the past two million years - atmospheric CO2 concentrations have tended to hover at approximately 180 ppm. This value, say Sage and Coleman, might not be much above the "critical CO2 threshold at which catastrophic interactions occur." Thus, they reasonably speculate that plants of the late Pleistocene "might have been adapted to lower CO2 concentrations than currently exist."

In light of the short period of evolutionary time (a mere 15,000 years) since these low-CO2 conditions predominated, the scientists advance the logical thought that "many if not most plants might still be adapted to CO2 levels much lower than those that exist today," even though thousands of experiments have demonstrated that Earth's vegetation responds in dramatic positive fashion to atmospheric CO2 enrichment far above what is characteristic of the elevated CO2 conditions of the present. In considering such, the innovative plant scientists therefore conclude that - as good as things currently are, and as significantly better as they are expected to become as the air's CO2 content continues to rise - there may well be additional and what they call "substantial room for natural selection and bioengineering to remove the constraints [of low CO2 adaptation], thereby creating novel genotypes able to exploit high CO2 conditions to best advantage."

How important are these ideas? Sage and Coleman state that the low CO2 levels of the past "could have had significant consequences for much of the Earth's biota." In fact, they suggest that the origin of agriculture itself "might have been impeded by reduced ecosystem productivity during low CO2 episodes of the late Pleistocene." Since that time, however, the increase in the air's CO2 content has essentially doubled the biological prowess of the planet's vegetation; and projected increases in the air's CO2 content could readily lead to a tripling of the paltry productivity of Earth's ice-age past. On top of these phenomenal benefits, Sage and Coleman suggest there may be still other opportunities to improve plant performance even more, by using modern bioengineering techniques to overcome genetic constraints linked to adaptations to low levels of CO2 that may persist in many of Earth's plants. Indeed, they note that, for agriculture, "this could be a major opportunity to improve crop productivity and the efficiency of fertilizer and water use."

In spite of rocky beginnings, it thus appears that the picture being painted for Earth's future biosphere is one that is bright and optimistic, as more and more improvements are made to the models that are predicting that future. Throughout the course of the current century, even the severe warming predicted by current climate models will not likely be detrimental to plant growth and productivity. Rather, it will likely be a major benefit, enhancing plant growth and soil organic carbon storage, which (in addition to their own virtues) will provide a significant negative feedback to global warming as the Greening of the Earth continues!

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.

Friend, A.D. 2010. Terrestrial plant production and climate change. Journal of Experimental Botany 61: 1293-1309.

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.

Marland, G., Boden, T.A. and Andres, R.J. 2005. Global, regional, and national CO2 emissions. In: Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, USA. Available at http://cdiac.ornl.gov/trends/emis/overview.html.

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.

Qian, H., Joseph, R. and Zeng, N. 2010. Enhanced terrestrial carbon uptake in the Northern High Latitudes in the 21st century from the Coupled Carbon Cycle Climate Model Intercomparison Project model projections. Global Change Biology 16: 641-656.

Sage, R.F. and Coleman, J.R. 2001. Effects of low atmospheric CO2 on plants: more than a thing of the past. TRENDS in Plant Science 6: 18-24.

Xiao, X., Melillo, J.M., Kicklighter, D.W., McGuire, A.D., Prinn, R.G., Wang, C., Stone, P.H. and Sokolov, A. 1998. Transient climate change and net ecosystem production of the terrestrial biosphere. Global Biogeochemical Cycles 12: 345-360.