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Global Terrestrial Net Primary Productivity: 2000-2009 and Beyond
Volume 13, Number 36: 8 September 2010

In a study recently published in Science, Zhao and Running write that "previous studies have shown that climate constraints were relaxing with increasing temperature and solar radiation, allowing an upward trend in NPP [net primary production] from 1982-1999," but that over the past decade (2000-2009), satellite data "suggest a reduction in the global NPP," which finding has caused some alarm among the climate-alarmist community. But is there much cause for such concern?

Not really. Zhao and Running state, for example, that their work suggests "a reduction in the global NPP of 0.55 petagrams of carbon" over the period 2000-2009, which is indeed correct. But in viewing a graphical representation of their results (see Figure 1 below), it can be seen that apart from the starting point of the initial year (2000) of their study, there is only one other year (2004) in which the global NPP was greater than what it was at the end of the study (2009). And since global NPP was on the rise from 1982-1999, what the more recent data show would more accurately be described as a leveling off of that prior upward trend.

Figure 1. Interannual variations from the mean of global NPP over the past ten years. Adapted from Zhao and Running (2010).

But what about the future? Zhao and Running correctly report that the leveling off of global NPP over the past decade was induced by drought; and more frequent and more intense droughts have long been predicted by climate alarmists to accompany global warming. In this regard, therefore, it is of more than passing interest that the two researchers found that "NPP in the tropics explains 93% of variations in the global NPP," and that "tropical rainforests explain 61% of global NPP variations". These findings are especially important because of the recent work of Coelho and Goddard (2009), who studied El Niņo-induced tropical droughts as portrayed in climate models.

In introducing their study, the latter two researchers write that "the majority of drought-related hazards and the attendant economic losses and mortality risks reside in the tropics," citing Dilley et al. (2005); and they further note that "changes in climate variability, including more frequent and damaging extreme events such as drought, is one of many anticipated impacts of climate change." More specifically (and germane to the subject at hand), they write that "El Niņo brings widespread drought (i.e., precipitation deficit) to the tropics," and that "stronger or more frequent El Niņo events in the future" would "exacerbate drought risk in highly vulnerable tropical areas."

As a result of these observations, the two researchers evaluated "the patterns, magnitude, and spatial extent of El Niņo-induced tropical droughts during a control period in the twentieth century in climate simulations, which have realistic evolution of greenhouse gases," after which they examined "the projected changes in the characteristics of El Niņo and in the strength of the indentified patterns of El Niņo-induced tropical drought in the twenty-first century," which then allowed them to examine patterns of mean precipitation changes in order to "assess whether those changes exacerbate or ameliorate the risk of El Niņo-induced drought conditions in the twenty-first century." So what did they find?

In the first instance, Coelho and Goddard found that the models they studied "exhibit realistic patterns, magnitude, and spatial extent of El Niņo-induced drought patterns in the twentieth century," and they further observed that "the teleconnections are not projected to change in the twenty-first century," although they add that "a possible slight reduction in the spatial extent of droughts is indicated over the tropics as a whole." And they report that "all model groups investigated show similar changes in mean precipitation for the end of the twenty-first century, with increased precipitation projected between 10°S and 10°N."

In conclusion, it would appear -- at least from a climate modeling perspective -- that we can probably expect tropical drought to decrease throughout the remainder of the twenty-first century, which should enable the historical "greening of the earth" to continue, although there may well be decadal periods of drought-induced "leveling off" of global NPP that give rise to periodic concerns. Applying the precautionary principle within this context, it may thus be wise to not embark upon a course of action designed to promote biofuels, for as Zhao and Running warn us, a significant decline in NPP would "intensify future competition between food demand and proposed biofuel production." And with ever more people expected to populate the planet in the ensuing decades, even a leveling off of global NPP could pose such a problem.

Sherwood, Keith and Craig Idso

Coelho, C.A.S. and Goddard, L. 2009. El Niņo-induced tropical droughts in climate change projections. Journal of Climate 22: 6456-6476.

Dilley, M., Chen, R.S., Deichmann, U., Lerner-Lam, A.L. and Arnold, M. 2005. Natural Disaster Hotspots: A Global Risk Analysis. World Bank, 145 pp.

Zhao, M. and Running, S.W. 2010. Drought-induced reduction in global terrestrial net primary production from 2000 through 2009. Science 329: 940-943.