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The State of Earth's Terrestrial Biosphere:
How is it Responding to Rising Atmospheric CO2 and Warmer Temperatures?

Continental-Scale Analyses of Terrestrial Productivity: Asia (China - Tibetan Plateau)

Taking a long temporal view on how vegetative productivity has fared in this region of China, Herzschuh et al. (2011) "critically review possible driving forces for early- to mid-Holocene vegetation shifts on the upper Tibetan Plateau (including precipitation, growing season length, radiation, human impact) with particular emphasis on changing CO2 concentrations to better predict future environmental change and impacts on the Tibetan Plateau in a rapidly changing world." In so doing, the four researchers report that "numerous pollen records from across the upper Tibetan Plateau indicate that Kobresia-dominated high-alpine meadow invaded alpine steppes during the mid- to late-Holocene." But they say that their investigation, which employed a pollen-moisture transfer function, suggested that "this marked vegetation change cannot be satisfactorily explained by climate change." In addition, they indicate that "a literature review did not reveal convincing evidence for any widespread human impact on mid-Holocene vegetation." Thus, they propose that the reconstructed vegetation changes were likely the result of "a response to Holocene CO2 concentration changes," with values rising from approximately 260 ppm in the early Holocene to near-present-day values on the order of 375 ppm.

This conclusion is based on four lines of evidence: the fact that "high-elevation vegetation is particularly sensitive to CO2 changes due to lowered CO2 partial pressure," the fact that "water conservation of steppe vegetation in response to experimental CO2 enrichment was of the same order of magnitude as inferred from mid- to late-Holocene Tibetan pollen records," the fact that "modern remote sensing-aided vegetation monitoring of the Central Tibetan Plateau yielded an increase in biomass, most probably as a response to modern CO2 increase," even in spite of "increasing land-use by herding," as well as the fact that "experimental CO2 fertilization of dry grassland and desert vegetation performed in several regions world-wide has stimulated plant growth directly through enhanced photosynthesis and indirectly through enhanced water-use efficiency (Morgan et al., 2004)."

Focusing in on the past century, Zhuang et al. (2010) used a process-based biogeochemistry model - the Terrestrial Ecosystem Model or TEM, which also employed a soil thermal model - to examine permafrost dynamics and their effects on the carbon dynamics of the Tibetan Plateau. This was done by "parameterizing and verifying" the TEM using existing real-world data for soil temperature, permafrost distribution and carbon and nitrogen distributions throughout the region, and then extrapolating the model and its parameters to the whole of the plateau.

In describing their findings, the six scientists report that "during the 20th century, the Tibetan Plateau changed from a small carbon source or neutral in the early part of the century to a sink later" (Figure 11), noting that "net primary production and soil respiration increased by 0.52 and 0.22 Tg C/year, respectively, resulting in a regional carbon sink increase of 0.3 Tg C/year," so that "by the end of the century, the regional carbon sink reached 36 Tg C/year and carbon storage in vegetation and soils is 32 and 16 Pg C, respectively."

Figure 11. Five year running average of Net Ecosystem Production (NEP) on the Tibetan Plateau over the period 1901-2002. Negative values reveal the region to be a carbon source, positive values indicate it is a carbon sink. Adapted from Zhuang et al. (2010).

Zhuang et al. say the "increasing soil temperature and deepening active layer depth enhanced soil respiration, increasing the net nitrogen mineralization rate," and that "together with the [positive] effects of warming air temperature and rising CO2 concentrations on photosynthesis, the stronger plant nitrogen uptake due to the enhanced available nitrogen stimulate[d] plant carbon uptake, thereby strengthening the regional carbon sink as the rate of increase in net primary production was faster than that of soil respiration." Thus, they say their study implies that "future warming will increase thawing of the permafrost, increase soil temperature and dry up soil moisture," and that "these physical dynamics may enhance [the] future strength of the regional carbon sink, since the rate of increase of net primary production is higher than that of soil respiration on the Tibetan Plateau."

Further evidence that the productivity of the Tibetan Plateau's vegetation has recently increased is found in satellite-based measurements taken over the past three decades. Piao et al. (2006d), for example, investigated net primary production (NPP) derived from a carbon model (Carnegie-Ames-Stanford approach, CASA) and its interannual change in the Qinghai-Xizang (Tibetan) Plateau using 1982-1999 NDVI data and paired ground-based information on vegetation, climate, soil, and solar radiation. This work revealed that over the entire study period, NPP rose at a mean annual rate of 0.7%. However, Piao et al. report that "the NPP trends in the plateau over the two decades were divided into two distinguished periods: without any clear trend from 1982 to 1990 and significant increase from 1991 to 1999."

The three researchers say their findings suggest that "vegetation growth on the plateau in the 1990s has been much enhanced compared to that in [the] 1980s, consistent with the trend in the northern latitudes indicated by Schimel et al. (2001)." In addition, they say that "previous observational and NPP modeling studies have documented substantial evidence that terrestrial photosynthetic activity has increased over the past two to three decades in the middle and high latitudes in the Northern Hemisphere," and that "satellite-based NDVI data sets for the period of 1982-1999 also indicate consistent trends of NDVI increase," citing multiple references in support of each of these statements. Piao et al.'s findings, therefore, add to the growing body of evidence that reveals that a significant Greening of the Earth is occurring in response to (1) the ongoing recovery of the planet from the growth-inhibiting chill of the Little Ice Age, which was likely the coldest period of the current interglacial, plus (2) the aerial fertilization effect of the historical and still-ongoing rise in the atmosphere's CO2 concentration, as well as (3) the growth-promoting effect of anthropogenic nitrogen deposition.

In one final study, interannual variations of Tibetan Plateau vegetative productivity were investigated by Zhou et al. (2007) using a 21-year (1982-2002) normalized difference vegetation index (NDVI) dataset to quantify the consequences of changes in temperature and precipitation for the regional ecosystem. Based on their analysis, the authors reported that "the maximum, minimum and mean temperature fluctuations all present an increasing trend over the 21 years," and that "the NDVI is comparatively large during the warm years, such as 1988, 1994, 1998 and 1999," and that "relatively small NDVI values are well coupled with the cold extreme and mean temperature in 1982, 1983 and 1997." This type of relationship, as they continue, "suggests a positive correlation between vegetation activity and surface air temperature on the plateau," and in this regard they report that "the correlation coefficient between the NDVI and the maximum, minimum and mean temperature reaches 0.674 (significant at the 99% level), 0.53 (significant at the 95% level) and 0.55 (significant at the 99% level), respectively." In contrast, they find that "the precipitation fluctuation does not show a detectable trend, and therefore its correlation with DNVI is not obvious."

Zhou et al. conclude their study by remarking that "vegetation variability on the Tibetan Plateau might be mostly driven by thermal impacts (i.e., surface air temperature), whereas precipitation impact is less clear." Overall, they say "vegetation activity demonstrates a gradual enhancement in an oscillatory manner during 1982-2002," suggesting a significant positive impact to what climate alarmists call "unprecedented" global warming over what Zhou et al. describe as "one of the most prominent features on Earth."

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