Ecosystems: Other
From ClimateWiki
Shifting from forests to other ecosystems, we turn to the study of Tian et al. (2010), who like others before them note terrestrial ecosystems provide food, fiber, and various amenities for man and nature alike, and who once again state climate change is typically forecast to negatively affect ecosystem productivity, with potentially disruptive consequences.
Not convinced of the validity of these dire forecasts, Tian et al. used a mechanistic ecosystem model—employing historical (real-world) data for land use, atmospheric CO2 concentration, nitrogen deposition, fertilization, ozone, and climate—to simulate plant growth responses for multiple biome types (grasslands, forests, wetlands, and agriculture) of the Southern United States at an 8x8-km grid resolution from 1895 to 2007. They found that from Texas through Virginia, net primary productivity rose by 27 percent over the study period (most of it occurring after 1950), with increases in grassland and shrubland (mainly in Texas) and with large increases in cropland. These gains came in spite of increased tropospheric ozone damage.The ten researchers note all biomes showed increases in water use efficiency, contradicting assertions that negative effects of climate change are not only sure to occur but are already apparent.
Contemporaneously, Anderson et al. (2010) studied various root responses of a C3-C4 grassland community at Temple, Texas over a CO2 concentration gradient stretching from 230 to 550 ppm, which they created in two CO2-gradient above-ground “tunnels” of clear polyethylene film. One of the 60-meter-long and 1.5-meter-wide chambers had ambient air pumped into one end of it, and by the time the air exited the chamber at its other end, its CO2 concentration was reduced by the photosynthetic activity of the plants within the chamber to a value of approximately 230 ppm. At the same time, the other chamber had air enriched to a CO2 concentration of 550 ppm pumped into one end of it, and as this air exited the other end of the chamber, its CO2 concentration was reduced to a value approximately equivalent to that of the ambient air (~380 ppm). Community in-growth root biomass was assessed along the lengths of the tunnels every two to four months from May 1997 through November 1999, with the help of two in-growth cores in each five-meter chamber section, and root biomass response was calculated as the ratio of each measurement date’s result to that prevailing at the start of the experiment in May 1997.
Anderson et al. report that based on the linear relationship they derived from all 20 of the in-growth biomass assessments they conducted, there was a 40 percent increase in the in-growth root biomass ratio going from 380 to 480 ppm CO2, and a 36 percent increase going from 280 to 380 ppm. However, excluding one extremely variable data point and using a power function they fit to the data, the researchers found “the contrast is even greater: a 50% increase from 380 to 480 ppm vs. a 41% increase from 280 to 380 ppm.” And in going from the linear relationship to the power function, the r2 value of the relationship jumped from 0.10 to 0.50, and P dropped from 0.095 to less than 0.001.
Thus the six scientists state their data “suggest that root biomass in grasslands may have changed markedly as atmospheric CO2 increased since the last glacial period, but more substantial changes are ahead if the air’s CO2 content doubles by the end of this century as predicted.”
Moving upward in scale and in latitude, Qian et al. (2010) note it has been hypothesized that if or when the frozen soils of Earth’s Northern High Latitudes (NHLs, poleward of 60°N) begin to thaw in response to any new global warming that might occur, the metabolism of soil microbes will be enhanced and the decomposition of soil organic matter will accelerate, and that this, in turn, will lead to an increase in soil organic carbon release to the atmosphere that will amplify global warming. Satellite and phenology studies, however, have shown that during the past several decades the planet’s boreal forests have experienced greening and an increase in photosynthetic activity, which extracts carbon (in the form of CO2) from the atmosphere.
Consequently, and in order to get some indication as to which of these two outcomes might likely predominate over the course of the twenty-first century, Qian et al. explored the potential magnitudes of the two sets of competing processes by analyzing 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 the 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.”
According to the three scientists, the ten models predicted a mean warming of 5.6°C from 1901 to 2100 in the NHLs, and they state “the NHLs will be a carbon sink of 0.3 ± 0.3 PgCyr-1 by 2100.” They also state “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 percent increase] and 21 ± 16 PgC from the soil [an 8 percent increase],” noting “both CO2 fertilization and warming enhance vegetation growth in the NHLs.” Thus over the course of the current century, even the severe warming predicted by current climate models would likely not be a detriment to plant growth and productivity in the NHLs. In fact, it would likely prove a benefit, enhancing plant growth and soil organic carbon storage, which in addition to their own intrinsic virtues would provide a significant negative feedback to global warming.
In a similar study, 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) projected concurrent changes in climate and atmospheric CO2 concentration. Friend worked with 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, the net primary production (NPP) of the planet as a whole was found to be reduced by 2.5 percent, with the largest negative impacts occurring over southern Africa, central Australia, northern Mexico, and the Mediterranean region, where reductions of more than 20 percent were common. At the other extreme, climatic impacts were modestly positive throughout most of the world’s boreal forests, as might be expected when these colder regions receive an influx of 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 percent, driven by mean increases of 43.9–52.9 percent among C3 plants and 5.9 percent 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.
In conclusion, it would appear—at least from climate models—that we can probably expect the historical “greening of the earth” phenomenon to continue.
References
Anderson, L.J., Derner, J.D., Polley, H.W., Gordon, W.S., Eissenstat, D.M., and Jackson, R.B. 2010. Root responses along a subambient to elevated CO2 gradient in a C3-C4 grassland. Global Change Biology 16: 454–468.
Dilley, M., Chen, R.S., Deichmann, U., Lerner-Lam, A.L., and Arnold, M. 2005. Natural Disaster Hotspots: A Global Risk Analysis. Washington, DC: The World Bank and Columbia University.
Friend, A.D. 2010. Terrestrial plant production and climate change. Journal of Experimental Botany 61: 1293–1309.
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. Oak Ridge, TN: Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy. Available at http://cdiac.ornl.gov/trends/emis/overview.html.
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.
Tian, H., Chen, G., Liu, M., Zhang, C., Sun, G., Lu, C., Xu, X., Ren, W., Pan, S., and Chappelka, A. 2010. Model estimates of net primary productivity, evapotranspiration, and water use efficiency in the terrestrial ecosystems of the southern United States during 1895–2007. Forest Ecology and Management 259: 1311–1327.
