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Global Terrestrial Productivity in the
Last Decades of the 20th Century

Volume 6, Number 24: 11 June 2003

According to the world's climate alarmists, earth experienced a number of deleterious environmental phenomena between 1980 and 2000. It weathered two of the warmest decades in the instrumental temperature record, three intense and persistent El Nio events, and the eruption of Mt. Pinatubo. Concurrently, the air's CO2 content increased by 9%, while human population grew by 37%. It was a bad time for the biosphere. Or was it?

In the latest issue of Science, Nemani et al. (2003) present the results of a study of global vegetation response to all of these dire phenomena, as well as every other supposedly bad thing that happened over this period, such as illicit logging, fires, storms, etc. Based on satellite observations of vegetative activity over the period 1982 to 1999, their findings are most revealing. Rather than depicting a terrestrial biosphere in collapse, however, they reveal just the opposite, i.e., a terrestrial biosphere that is growing ever more robust.

Globally, the group of eight scientists determined that terrestrial net primary production (NPP) increased by 6.17%, or 3.42 PgC, over the 18 years between 1982 and 1999. What is more, they observed net positive responses over all latitude bands studied: 4.2% (47.5-22.5S), 7.4% (22.5S-22.5N), 3.7% (22.5-47.5N), and 6.6% (47.5-90.0N).

The authors mention a number of likely contributing factors to these significant NPP increases: nitrogen deposition and forest regrowth in northern mid and high latitudes, wetter rainfall regimes in water-limited regions of Australia, Africa, and the Indian subcontinent, increased solar radiation reception over radiation-limited parts of Western Europe and the equatorial tropics, warming in many parts of the world, and the aerial fertilization effect of rising atmospheric CO2 concentrations everywhere.

With respect to the latter factor, which is featured prominently on our website (see our Plant Growth Data section and fully half of all of our Journal Reviews), Nemani et al. remark that "an increase in NPP of only 0.2% per 1-ppm increase in CO2 could explain all of the estimated global NPP increase of 6.17% over 18 years and is within the range of experimental evidence [our italics]." However, they report that NPP increased by more than 1% per year in Amazonia alone, noting that "this result cannot be explained solely by CO2 fertilization."

We tend to agree with them, but also note that the aerial fertilization effect of atmospheric CO2 enrichment is most pronounced at higher temperatures (see the four sub-headings under Growth Response to CO2 with Other Variables - Temperature in our Subject Index), rising from next to nothing at a mean temperature of 10C to a 0.33% NPP increase per 1-ppm increase in CO2 at a mean temperature of 36C for a mixture of plants comprised predominantly of herbaceous species (Idso and Idso, 1994). For woody plants, we could possibly expect this number to be two (Idso, 1999) or even three (Saxe et al., 1998; Idso and Kimball, 2001; Leavitt et al., 2003) times larger, yielding a 0.7% to 1% NPP increase per 1-ppm increase in CO2, which would indeed represent the lion's share of the growth stimulation observed by Nemani et al. in tropical Amazonia.

Be that as it may, the important take-home message of Nemani et al.'s new study is that satellite-derived observations reveal the planet's terrestrial biosphere to have significantly increased its productivity over the last two decades of the 20th century in the face of a host of both real and imagined environmental stresses, chief among the latter of which is what climate alarmists routinely claim to be unprecedented CO2-induced global warming, which they paint as anathema to life on earth. Once again, however, the doomsayers are shown to be 180 degrees out of phase with reality, as the greening of the earth continues.

Sherwood, Keith and Craig Idso

References
Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 year's research. Agricultural and Forest Meteorology 69: 153-203.

Idso, S.B. 1999. The long-term response of trees to atmospheric CO2 enrichment. Global Change Biology 5: 493-495.

Idso, S.B. and Kimball, B.A. 2001. CO2 enrichment of sour orange trees: 13 years and counting. Environmental and Experimental Botany 46: 147-153.

Leavitt, S.W., Idso, S.B., Kimball, B.A., Burns, J.M., Sinha, A. and Stott, L. 2003. The effect of long-term atmospheric CO2 enrichment on the intrinsic water-use efficiency of sour orange trees. Chemosphere 50: 217-222.

Nemani, R.R., Keeling, C.D., Hashimoto, H., Jolly, W.M., Piper, S.C., Tucker, C.J., Myneni, R.B. and Running. S.W. 2003. Climate-driven increases in global terrestrial net primary production from 1982 to 1999. Science 300: 1560-1563.

Saxe, H., Ellsworth, D.S. and Heath, J. 1998. Tree and forest functioning in an enriched CO2 atmosphere. New Phytologist 139: 395-436.