Greening of the Earth: Miscellaneous
From ClimateWiki
Logan et al. (2010) describe and discuss what was revealed at a Tropospheric Ozone Changes Workshop in Boulder, Colorado (USA) on 14–16 October 2009, where “long-term ozone records from regionally representative surface and mountain sites, ozonesondes, and aircraft were reviewed by region.” They report, “in the Southern Hemisphere, surface measurements from South Africa and Tasmania and sonde data from New Zealand show a significant increase over the past 25 years.” North of the equator, on the other hand, the story is somewhat different. In western Europe, for example, “several time series of ~15-40 years ... show a rise in ozone into the middle to late 1990s and a leveling off, or in some cases declines, in the 2000s, in general agreement with precursor emission changes.” Similarly, they state “surface measurements within North America show a pattern of mostly unchanged or declining ozone over the past two decades that seems broadly consistent with decreases in precursor emissions,” while noting “the Japanese sonde record suggests rising ozone into the 1980s and small changes thereafter.”
The spatial and temporal distributions of these observations would seem to suggest: Whereas increasing industrialization originally tends to increase the emissions of precursor substances that lead to the creation of greater tropospheric ozone pollution, subsequent technological advances tend to ameliorate that phenomenon as they gradually lead to (1) a leveling off of the magnitude of precursor emissions and (2) an ultimately decreasing trend in tropospheric ozone pollution. This suggests that when atmospheric ozone and CO2 concentrations both rise together, the plant-growth-enhancing effect of atmospheric CO2 enrichment is significantly muted by the plant-growth-retarding effect of contemporaneous increases in ozone pollution, but as the troposphere’s ozone concentration gradually levels off and declines—as it appears to be doing with the development of new and better anti-pollution technology in the planet’s more economically advanced countries—we should begin to see more-rapid-than-usual increases in Earth’s vegetative productivity, which should promote an acceleration of the greening of the Earth.
Contemporaneously, Lazzarotto et al. (2010) note “white clover (Trifolium repens L.) is the most important pasture legume grown in temperate climates in association with a variety of grasses, notably perennial ryegrass (Lolium perenne L.).” They explain “white clover improves the nutritional quality and digestibility of the herbage” and “contributes substantially to the nitrogen status of the sward through biological nitrogen fixation.” They state, however, that there is some concern future drought predicted to occur in tandem with CO2-induced global warming will hurt clover more than the grass with which it is intermingled, thereby degrading the nutritional quality and digestibility of pasture swards.
To test this mix of facts and presumptions, Lazzarotto et al. planned and conducted a study in which “mechanisms controlling transient responses to elevated CO2 concentration and climate change in an unfertilized grassland on the Swiss Plateau were examined in light of simulations with PROGRASS,” a process-based model of grass-clover interactions developed by Lazzarotto et al. (2009) in which “daily weather for a series of transient climate scenarios spanning the 21st century were developed for the study site with the help of the LARS-WG weather generator,” which is described by Semenov and Barrow (1997) and Semenov et al. (1998). In addition, “changes in the length of dry and wet spells, temperature, precipitation and solar radiation defining the scenarios were obtained from regional climate simulations carried out in the framework of the PRUDENCE project,” which is described by Christensen and Christensen (2007).
“Compared to 1961–1990,” the Swiss and UK scientists write, the climate scenarios they developed for a CO2 increase from 370 to 860 ppm “indicated that for 2071–2100 there would be a noticeable increase in temperature (roughly 3°C in winter and 5°C in summer), a significant drop in summer precipitation (of the order of -30%), and a nearly 2-fold increase in the length of dry spells.” The four researchers report these significant climate changes had no projected negative effect on the grass-clover swards: “clover abundance did not decline even in the absence of CO2 stimulation.” And when the atmospheric CO2 concentration was programmed to gradually rise from an initial value of 370 ppm to a final value of 860 ppm, they found “clover development benefited from the overall positive effects of CO2 on nitrogen acquisition,” which they report was also “the reason for increasing productivity of the [entire] sward.”
For Swiss grass-clover swards, therefore, the rather large predicted increases in temperature and decreases in precipitation predicted for the remainder of the twenty-first century will not have much of an effect, but the concomitant increase in the air’s CO2 content will be of considerable benefit. In addition, Lazzarotto et al. state it is likely “technical progress in the management of grasslands and pastures,” which will surely occur, will help such pastures even more.
In a similar type of study, Friend (2010) used an advanced mechanistic physiological model (Hybrid6.5) of leaf and whole-plant canopy response to climate for basically the same purposes as Lazzarotto et al. This model considers light extinction within the leaf and through the canopy, the gradient of nitrogen content through the canopy, and other factors. It also distinguishes between C3 and C4 plants, broadleaf and conifer trees, and other life forms; and it has been verified by close matches to local, regional, and global net primary production (NPP) data. The model was run with current global vegetation distributions and the GISS-AOM climate model using the IPCC A1B scenario, with CO2 rising to 720 ppm by AD 2100. When it was run with only climate change (CO2 fixed at current levels), it showed a 2.5 percent reduction in global NPP, but when CO2 change was added to the model, global NPP increased 37.3 percent to 80.7 Pg C/year.
This rise was most evident in absolute terms in tropical rainforests. In percentage terms, temperate and boreal forests and tundra showed the largest increases, along with C3 grasslands and agricultural lands. C4 grasses and crops showed only a 5.9 percent increase in NPP, because of the less-responsive photosynthetic pathway in C4 plants. Only very small areas of the globe showed any decrease in NPP with this model. The results therefore suggest elevated CO2 will help plants cope with the modest changes in climate that might otherwise be slightly harmful to their growth. The study also confirms past work showing that as models of plant growth become more realistic and mechanistic, they tend to predict positive responses to CO2 and climate changes over the next 100 years in most regions and ecosystems.
References
Christensen, J.H. and Christensen, O.B. 2007. A summary of the PRUDENCE model projections of changes in European climate by the end of this century. Climatic Change 81: 7–30.
Friend, A.D. 2010. Terrestrial plant production and climate change. Journal of Experimental Botany 61: 10.1093/jxb/erq019.
Lazzarotto, P., Calanca, P., and Fuhrer, J. 2009. Dynamics of grass-clover mixtures—an analysis of the response to management with the PROductive GRASsland Simulator (PROGRASS). Ecological Modelling 220: 703–724.
Lazzarotto, P., Calanca, P., Semenov, M., and Fuhrer, J. 2010. Transient responses to increasing CO2 and climate change in an unfertilized grass-clover sward. Climate Research 41: 221–232.
Logan, J., Schultz, M., and Oltmans, S. 2010. Observing and understanding tropospheric ozone changes. EOS, Transactions, American Geophysical Union 91: 119.
Semenov, M.A. and Barrow, E.M. 1997. Use of a stochastic weather generator in the development of climate change scenarios. Climatic Change 35: 397–414.
Semenov, M.A., Books, R.J., Barrow, E.M., and Richardson, C.W. 1998. Comparison of the WGEN and LARS-WG stochastic weather generators for diverse climates. Climate Research 10: 95–107.
