How does the terrestrial vegetation of earth's natural ecosystems respond to increases in atmospheric temperature and CO2 concentration? We here consider this question as it applies to Eurasia.
In a paper entitled "Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999," Zhou et al. (2001) determined that the satellite-derived normalized difference vegetation index (NDVI) rose by an average of 12.41% throughout the whole of Eurasia over this period. Noting that the NDVI "is well correlated with the fraction of photosynthetically active radiation absorbed by plant canopies and thus leaf area, leaf biomass, and potential photosynthesis," they went on to suggest that the increases in plant growth and vitality implied by their NDVI data were primarily driven by concurrent increases in near-surface air temperature. Subsequently, Ahlbeck (2002) argued that the concomitant increase in atmospheric CO2 concentration was likely even more important in orchestrating the productivity increase, as we discuss in our Editorial of 18 Sep 2002. Be that as it may, there is absolutely no question that in the face of documented increases in both of these environmental parameters, Eurasia's terrestrial vegetation has responded extremely positively.
In a much more spatially restricted study, Allen et al. (1999) analyzed sediment cores extracted from a lake in southern Italy and from the Mediterranean Sea, reconstructing a high-resolution climate and vegetation history for the region that spans the last 102,000 years. Over this period, rapid changes in vegetation were found to correlate with rapid changes in climate, such that complete shifts in natural ecosystems would sometimes occur over periods of less than 200 years. Over the warmest portion of the record (the Holocene), the total organic carbon content of the vegetation reached its highest level, more than doubling values experienced over the glacial portion of the record. Other proxy indicators revealed that during the more productive woody-plant period of the Holocene, the increased vegetative cover also led to less soil erosion. The results of this study thus demonstrate that the biosphere can - and does! - successfully respond to rapid increases in temperature. In fact, the 15 scientists associated with the study pointedly state that "the biosphere was a full participant in these rapid fluctuations, contrary to widely held views that vegetation is unable to change with such rapidity." Furthermore, warmer times were always found to be better times in terms of plant productivity. Hence, it may safely be anticipated that any future warming of the region would likely lead to more productive vegetative conditions.
In another study from this part of the world, Osborne et al. (2000) used an empirically-based mechanistic model of Mediterranean shrub vegetation to address two important questions: Has recent climate change, especially increased drought, negatively impacted Mediterranean shrublands? and Has the historical increase in the air's CO2 concentration modified this impact? The data-based model they employed suggests that the warming and reduced precipitation experienced in the Mediterranean area over the past century should have had negative impacts on net primary production and leaf area index. When the measured increase in atmospheric CO2 concentration experienced over the period was factored into the calculation, however, these negative influences were overpowered, with the net effect that both measures of vegetative prowess increased, net primary productivity by 25% and leaf area index by 7%. These results, in the scientists' words, "indicate that the recent rise in atmospheric CO2 may already have had significant impacts on productivity, structure and water relations of sclerophyllous shrub vegetation, which tended to offset the detrimental effects of climate change in the region."
How can we relate this observation to climate change predictions for the earth as a whole? For a nominal doubling of the air's CO2 concentration from 300 to 600 ppm, earth's mean surface air temperature is predicted by current climate models to rise by approximately 3°C, which equates to a temperature rise of 0.01°C per ppm CO2. In the case of the Mediterranean region here described, the temperature rise over the past century was quoted by Osborne et al. as being 0.75°C, over which period of time the air's CO2 concentration rose by approximately 75 ppm, for an analogous climate response of exactly the same value: 0.01°C per ppm CO2.
With respect to model-predicted changes in earth's precipitation regime, a doubling of the air's CO2 content is projected to lead to a modest intensification of the planet's hydrologic cycle. In the case of the Mediterranean region over the last century, however, there has been a recent tendency toward drier conditions. Hence, the specific case investigated by Osborne et al. represents a much-worse-case scenario than what is predicted by current climate models for the earth as a whole. Nevertheless, the area's vegetation has done even better than it did before the climatic change, thanks to the over-powering beneficial effects of the concurrent rise in the air's CO2 content. By analogy, therefore, we would expect the entire biosphere to do likewise, as all data analyzed to date suggest it is doing [see the other sub-categories of Biospheric Productivity in our Subject Index: Global, Terrestrial (Worldwide), Terrestrial (Regional - North America), and Oceanic].
Ahlbeck, J.R. 2002. Comment on "Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999" by L. Zhou et al. Journal of Geophysical Research 107: 10.1029/2001389.
Allen, J.R.M., Brandt, U., Brauer, A., Hubberten, H.-W., Huntley, B., Keller, J., Kraml, M., Mackensen, A., Mingram, J., Negendank, J.F.W., Nowaczyk, N.R., Oberhansli, H., Watts, W.A., Wulf, S. and Zolitschka, B. 1999. Rapid environmental changes in southern Europe during the last glacial period. Nature 400: 740-743.
Osborne, C.P., Mitchell, P.L., Sheehy, J.E. and Woodward, F.I. 2000. Modellng the recent historical impacts of atmospheric CO2 and climate change on Mediterranean vegetation. Global Change Biology 6: 445-458.
Zhou, L., Tucker, C.J., Kaufmann, R.K., Slayback, D., Shabanov, N.V. and Myneni, R.B. 2001. Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999. Journal of Geophysical Research 106: 20,069-20,083.