Throughout most of the succeeding years, this optimistic view of the ongoing rise in the air's CO2 content -- and the great good it can do for humanity and nature alike -- was largely ignored, as the world's climate alarmists took center stage with headline-grabbing predictions of catastrophic CO2-induced global warming.  Now, however, it appears that enough has finally been learned to take the positive view more seriously, in support of which statement we note the following titles of some science stories that have appeared of late in the popular press.

"Greenhouse Gas Might Green Up the Desert" declares a ScienceDaily headline.  "Missing Carbon Dioxide Greens Up the Desert" chimes in the Israel National News.  "Greenhouse Gas Soaked Up by Forests Expanding into Deserts" proclaims The Independent.  And in a grudging acknowledgement of the hard-to-ignore good news, the World News reports that "Deserts Bloom in Bad Air."

What are the sources of this spate of positive stories?  One that cannot be ignored is the study of Grunzweig et al. (2003), wherein the authors tell the tale of the Yatir forest -- a 2800-hectare stand of primarily Aleppo pine (Pinus halepensis Mill.) containing smaller amounts of Cupressus sempervirens and other pine trees (mostly P. brutia) -- which was planted some 35 years ago at the edge of the Negev Desert in Israel.

An intriguing aspect of this particular forest -- which Grunzweig et al. characterize as growing in poor soil of only 0.2- to 1.0-meter depth above chalk and limestone -- is that although it is located in an arid region that receives less annual precipitation than all of the other scores of global FluxNet stations that measure exchanges of CO2 between terrestrial ecosystems and the atmosphere (Baldocchi et al., 2001), its annual net ecosystem CO2 exchange is just as high as that of many high-latitude boreal forests and actually higher than that of most temperate forests.

How can this possibly be?  Grunzweig et al. note that the increase in atmospheric CO2 concentration that has occurred since pre-industrial times should have improved the water use efficiency (WUE) of most of earth's plants by increasing the ratio of CO2 fixed by photosynthesis to water lost via evapotranspiration.  That this hypothesis is indeed correct has been demonstrated under controlled experimental conditions by Leavitt et al. (2003) within the context of the still-ongoing long-term atmospheric CO2 enrichment study of Idso and Kimball (2001) on sour orange (Citrus aurantium L.) trees.  It has also been confirmed in nature by Feng (1999), who obtained identical CO2-induced WUE responses for 23 groups of naturally-occurring trees (scattered across western North America) that were caused by the rise in the air's CO2 content that occurred between 1800 and 1985.  In commenting on his remarkable findings, Feng says this phenomenon "would have caused natural trees in arid environments to grow more rapidly, acting as a carbon sink for anthropogenic CO2," which is exactly what Grunzweig et al. have demonstrated to be happening in the Yatir forest on the edge of the Negev Desert.  In addition, they report that "reducing water loss in arid regions improves soil moisture conditions, decreases water stress and extends water availability," which "can indirectly increase carbon sequestration by influencing plant distribution, survival and expansion into water-limited environments."

Much the same conclusions may be derived from the study of Grunzweig and Korner (2001), who constructed model grasslands representative of the Negev of Israel and placed them in growth chambers maintained at atmospheric CO2 concentrations of 280, 440 and 600 ppm for a period of five months.  They found that the elevated CO2 treatments reduced rates of evapotranspiration and increased soil moisture contents in the communities exposed to elevated CO2.  Between two periods of imposed drought, for example, soil moisture was 22 and 27% higher in communities exposed to 440 and 600 ppm CO2, respectively, than it was in control communities exposed to pre-industrial levels of atmospheric CO2.  These increases in soil moisture content likely contributed to peak ecosystem CO2 uptake rates that were 21 and 31% greater at 400 and 600 ppm CO2 than they were at 280 ppm CO2.  In addition, atmospheric CO2 enrichment had no effect on nighttime respiratory carbon losses from the ecosystems.  Thus, these model semi-arid grasslands were clearly acting as carbon sinks under CO2-enriched conditions.  In fact, the elevated CO2 increased total community biomass by 14% over that produced by the communities exposed to the subambient CO2 concentration.  Also, when the total biomass produced was related to the total amount of water lost via evapotranspiration, the communities grown at atmospheric CO2 concentrations of 440 and 600 ppm exhibited CO2-induced increases in water-use efficiency that were 17 and 28% higher, respectively, than those displayed by the control communities exposed to air of 280 ppm CO2.

That these phenomena are indeed widespread and operative in the real world is suggested by a number of observational studies, beginning with that of Nicholson et al. (1998), who used satellite images of the Central and Western Sahel from 1980 to 1995 to determine the extent of purported desertification in this region.  In addition, rain-use efficiency (RUE), which relates plant productivity to rainfall, was calculated to determine if the biological productivity of the area was affected by factors other than drought.  The scientists reported finding no overall expansion of deserts during their 16-year study, and no decrease in RUE, although vegetation did expand and contract somewhat in response to periods of relatively more or less rainfall.  Hence, neither human activities nor climatic changes in this huge arid region caused massive desertification of the type that was highly hyped by the United Nations in the 1970s.

In a second such study, Prince et al. (1998) also used satellite images and RUE to map the occurrence and severity of desertification, but they did so for the entire Sahel from 1982 to 1990.  They too could find no evidence of widespread desertification, and they determined that RUE did not decline during their 9-year investigation.  In fact, they discovered a small but steady rise in RUE for the Sahel as a whole, suggesting that plant productivity there had increased over the time of their study.

A third study of note is that of Eklundh and Olsson (2003), who analyzed Normalized Difference Vegetation Index (NDVI) data from the NOAA Advanced Very High Resolution Radiometer that were obtained over the African Sahel for the period 1982-2000.  As they describe their findings, "strong positive change in NDVI occurred in about 22% of the area, and weak positive change in 60% of the area," while "weak negative change occurred in 17% of the area, and strong negative change in 0.6% of the area."  They also report that "integrated NDVI has increased by about 80% in the areas with strong positive change," while in areas with weak negative change, "integrated NDVI has decreased on average by 13%."  The primary story told by these data, therefore, is one of strong positive trends in NDVI for large areas of the African Sahel over the last two decades of the 20th century; and Eklundh and Olsson conclude that the "increased vegetation, as suggested by the observed NDVI trend, could be part of the proposed tropical sink of carbon."

Finally, with respect to the climate-alarmist claim that desertification will intensify as a consequence of CO2-induced global warming, we refer to the study of Nicholson (2001), who reviews what is known about precipitation changes in Africa over the past two centuries, much of which work she herself was instrumental in conducting.  "The most significant climatic change that has occurred," in her words, "has been a long-term reduction in rainfall in the semi-arid regions of West Africa," which has been "on the order of 20 to 40% in parts of the Sahel."  There have been, she says, "three decades of protracted aridity," and "nearly all of Africa has been affected ... particularly since the 1980s."  However, she goes on to note that "the rainfall conditions over Africa during the last 2 to 3 decades are not unprecedented," and that "a similar dry episode prevailed during most of the first half [our italics] of the 19th century."

Continuing, Nicholson says "the 3 decades of dry conditions evidenced in the Sahel are not in themselves evidence of irreversible global change."  And especially, we would add, they are certainly not evidence of global warming-induced change.  Why not (to both points)?  Because a longer historical perspective of the type we are constantly striving to obtain clearly indicates, in the first instance, that an even longer period of similar dry conditions occurred between 1800 and 1850.  And in the second instance, this remarkable dry period occurred when the earth was still in the icy grip of the Little Ice Age, a period of cold that is without precedent in at least the last 6500 years ... even in Africa [see our Journal Review of the work of Lee-Thorp et al. (2001)].  Hence, there is no reason to think that the past two- to three-decade Sahelian drought is in any way unusual or that it was caused by the putative higher temperatures of that period.  Simply put, like many other things, droughts happen.

As ever more data are thus obtained from various parts of the world [see Greening of the Earth (Summary) for observations from many non-arid regions of the planet], it is becoming ever more evident that the CO2-induced reverse desertification theory of Idso (1982, 1986) is receiving ever more support in the way of real-world observations.  So what can we expect to see in the future?

One example of likely change is provided by Cheddadi et al. (2001), who apply what is known about these matters to lands bordering the Mediterranean Sea.  Specifically, they employ a standard biogeochemical model (BIOME3) - which uses monthly temperature and precipitation data, certain soil characteristics, cloudiness and atmospheric CO2 concentration as inputs - to simulate the responses of the various biomes of the region to changes in both climate (temperature and precipitation) and the air's CO2 content.

Cheddadi et al.'s first step was to validate the model for two test periods: the present and 6000 years before present (BP).  Recent instrumental records provided actual atmospheric CO2, temperature and precipitation data for the present period; while pollen data were used to reconstruct monthly temperature and precipitation values for 6000 years BP, and ice core records were used to determine the atmospheric CO2 concentration of that earlier epoch.  These efforts suggested that winter temperatures 6000 years ago were about 2°C cooler than they are now, that annual rainfall was approximately 200 mm less than today, and that the air's CO2 concentration averaged 280 ppm, which is considerably less than the value of 345 ppm the authors used to represent the present, i.e., the mid-point of the period used for calculating 30-year climate normals at the time they wrote their paper.  Applying the model to these two sets of conditions, they demonstrated that "BIOME3 can be used to simulate ... the vegetation distribution under ... different climate and [CO2] conditions than today," where [CO2] is the abbreviation they use to represent "atmospheric CO2 concentration."

Cheddadi et al.'s next step was to use their validated model to explore the vegetative consequences of an increase in anthropogenic CO2 emissions that pushes the air's CO2 concentration to a value of 500 ppm and its mean annual temperature to a value 2°C higher than today's mean value.  The basic response of the vegetation to this change in environmental conditions was "a substantial southward shift of Mediterranean vegetation and a spread of evergreen and conifer forests in the northern Mediterranean."

More specifically, in the words of the authors, "when precipitation is maintained at its present-day level, an evergreen forest spreads in the eastern Mediterranean and a conifer forest in Turkey."  Current xerophytic woodlands in this scenario become "restricted to southern Spain and southern Italy and they no longer occur in southern France."  In northwest Africa, on the other hand, "Mediterranean xerophytic vegetation occupies a more extensive territory than today and the arid steppe/desert boundary shifts southward," as each vegetation zone becomes significantly more verdant than it is currently.

What is the basis for these positive developments?  The authors say "the replacement of xerophytic woodlands by evergreen and conifer forests could be explained by the enhancement of photosynthesis due to the increase of [CO2]."  Likewise, they note that "under a high [CO2] stomata will be much less open which will lead to a reduced evapotranspiration and lower water loss, both for C3 and C4 plants," adding that "such mechanisms may help plants to resist long-lasting drought periods that characterize the Mediterranean climate."

Contrary to what is often predicted for much of the world's moisture-challenged lands, therefore, Cheddadi et al. were able to report that "an increase of [CO2], jointly with an increase of ca. 2°C in annual temperature would not lead to desertification on any part of the Mediterranean unless annual precipitation decreased drastically," where they define a drastic decrease as a decline of 30% or more.  Equally important in this context is the fact that Hennessy et al. (1997) have indicated that a doubling of the air's CO2 content would in all likelihood lead to a 5 to 10% increase in annual precipitation at Mediterranean latitudes, which is also what is predicted for most of the rest of the world.  Hence, the results of Cheddadi et al.'s study are likely very conservative, with the true vegetative response being even better than the good-news results they report, even when utilizing what we believe to be erroneously-inflated global warming predictions.

So how good could things get?  For perhaps the ultimate positive response, see our Editorial of 6 Feb 2002.

References
Baldocchi, D., Falge, E., Gu, L.H., Olson, R., Hollinger, D., Running, S., Anthoni, P., Bernhofer, C., Davis, K., Evans, R., Fuentes, J., Goldstein, A., Katul, G., Law B., Lee, X.H., Malhi, Y., Meyers, T., Munger, W., Oechel, W., Paw U, K.T., Pilegaard, K., Schmid, H.P., Valentini, R., Verma, S., Vesala, T., Wilson, K. and Wofsy, S.  2001.  FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor, and energy flux densities.  Bulletin of the American Meteorological Society 82: 2415-2434.

Cheddadi, R., Guiot, J. and Jolly, D.  2001.  The Mediterranean vegetation: what if the atmospheric CO2 increased?  Landscape Ecology 16: 667-675.

Eklundh, L. and Olssson, L.  2003.  Vegetation index trends for the African Sahel 1982-1999.  Geophysical Research Letters 30: 10.1029/2002GL016772.

Feng, X.  1999.  Trends in intrinsic water-use efficiency of natural trees for the past 100-200 years: A response to atmospheric CO2 concentration.  Geochimica et Cosmochimica Acta 63: 1891-1903.

Grunzweig, J.M. and Korner, C.  2001.  Growth, water and nitrogen relations in grassland model ecosystems of the semi-arid Negev of Israel exposed to elevated CO2.  Oecologia 128: 251-262.

Grunzweig, J.M., Lin, T., Rotenberg, E., Schwartz, A. and Yakir, D.  2003.  Carbon sequestration in arid-land forest.  Global Change Biology 9: 791-799.

Hennessy, K.J., Gregory, J.M. and Mitchell, J.F.B.  1997.  Changes in daily precipitation under enhanced greenhouse conditions.  Climate Dynamics 13: 667-680.

Idso, S.B.  1982.  Carbon Dioxide: Friend or Foe?  IBR Press, Tempe, Arizona, USA.

Idso, S.B.  1986.  Industrial age leading to the greening of the Earth?  Nature 320: 22.

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.

Nicholson, S.E.  2001.  Climatic and environmental change in Africa during the last two centuries.  Climate Research 17: 123-144.

Nicholson, S.E., Tucker, C.J. and Ba, M.B.  1998.  Desertification, drought, and surface vegetation: An example from the West African Sahel.  Bulletin of the American Meteorological Society 79: 815-829.

Prince, S.D., Brown De Colstoun, E. and Kravitz, L.L.  1998.  Evidence from rain-use efficiencies does not indicate extensive Sahelian desertification.  Global Change Biology 4: 359-374.