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Has CO2-Induced Global Warming Been Wrongly Accused of Decimating Cloud-Forest Ecosystems?
Volume 4, Number 47: 21 November 2001

In the 15 April 1999 issue of Nature, two groups of scientists - Still et al. (1999) and Pounds et al. (1999) - published a pair of papers dealing with an extremely complex subject: the cause of major decreases in frog and toad populations in the highland forests of Monteverde, Costa Rica.  These diebacks (in which 20 of 50 local species totally disappeared) had occurred over the preceding two decades, decades that climate alarmists describe as having experienced "unprecedented warming."

The frog and toad declines had also been accompanied by changes in bird and lizard populations that made the composition of the cloud-forest fauna look a lot more like that of forests further downslope; and the ecological mystery surrounding these changes captured the attention of a large sector of a public already conditioned to hearing all sorts of bad things attributed to the rising CO2 content of earth's atmosphere.  Thus it was perhaps only to be expected that in a popular article describing the mystery's putative solution, Holmes (1999) noted that the authors of the Science reports made "a convincing case blaming global climate change for these ecological events," which, of course, they truly did.

Here's how the theory developed.  Still et al. ran a global climate model simulation for a doubled atmospheric CO2 concentration, finding - after what Holmes says "might seem like a lot of hand waving" - that the absolute humidity required to create and maintain the clouds that periodically shroud the Monteverde mountain tops shifted upwards in response to this perturbation (CO2-induced global warming, which was supposedly manifest in increasing sea surface temperatures), especially during the winter dry season when the forests there rely most heavily on the moisture they receive directly from the clouds.  At the same time, the climate modelers noted an increase in a parameter they termed the "warmth index," which change implied a greater concurrent demand for evapotranspiration; and it was the combination of these two changes, i.e., an implied reduction in the amount of cloud contact with the mountain-top forest and the forest's increased need for water, that led the modelers to believe that (presumed) CO2-induced global warming was indeed the culprit behind the observed change in environmental conditions (essentially more dry days) that were believed to be responsible for the changes in animal life documented by Pounds et al.

At the time of the publication of the two Nature papers, and for a year or more thereafter, the explanation put forth by the two groups of scientists looked pretty strong.  In fact, to many it was compelling.  Now, however, comes the study of Lawton et al. (2001) that suggests something quite different, in which the authors present what they call "an alternative mechanism - upwind deforestation of lowlands - that may increase convective and orographic cloud bases even more than changes in sea surface temperature do."

Lawton et al. begin by noting that the trade winds that reach the Monteverde cloud-forest ecosystem flow across approximately 100 km of the lowlands of the Rio San Juan basin, and that deforestation proceeded rapidly in the Costa Rican part of this basin over the past century.  By 1992, in fact, only 18% of the original lowland forest remained.  The authors note that this conversion of forest to pasture and farm land significantly alters the properties of the air flowing across the landscape.  The reduced evapotranspiration that follows deforestation, for example, decreases the moisture content of the air mass; and regional atmospheric model simulations suggest (quite logically) that there should be reduced cloud formation and higher cloud bases over such deforested areas, which would also cause there to be fewer and higher-based clouds than there would otherwise be when the surface-modified air moves into the higher Monteverde region.

At this point, we thus have two theories from which to choose a candidate mechanism for the environmental changes that have altered the Monteverde cloud-forest ecosystem: one that is global (CO2-induced warming) and one that is local (upwind lowland deforestation).  So how does one pick the winner?

Lawton et al. chose an approach that pretty much proves their case.  Noting that the lowland forests north of the San Juan River in southeastern Nicaragua remain largely intact - providing a striking contrast to the mostly-deforested lands in neighboring Costa Rica - they used Landsat and Geostationary Operational Environmental Satellite imagery to show that "deforested areas of Costa Rica's Caribbean lowlands remain relatively cloud-free when forested regions have well-developed dry season cumulus cloud fields," noting further that the prominent zone of reduced cumulus cloudiness in Costa Rica "lies directly upwind of the Monteverde tropical montane cloud forest."  Hence, they demonstrated by direct observation that the effects predicted by the theory they developed did indeed occur in the real world, and that they occurred right alongside a "control" area that was identical in all respects but for the perturbation (deforestation) that produced the effects.

What is the take-home message of this intriguing story?  First of all, CO2-induced global warming does not appear to be the cause of the disruptions observed to be occurring in the Monteverde cloud-forest ecosystem by Pounds et al.  Second, the reality of the climate alarmists' "unprecedented" global warming of the past two decades is called into question; for where there is no upwind lowland deforestation, there are no corresponding changes in cloud properties of the type predicted by Still et al. to result from rising temperatures.

Most important of all, perhaps, is the demonstration of how dangerous it can sometimes be to follow the environmentalist dictum to "think globally but act locally."  In the case of the Monteverde cloud-forest ecosystem, for example, global thinking likely identified the wrong cause of the observed problem.  Man was to blame for the ecosystem perturbations, all right, but not in the way suggested by the climate alarmists; and if the original analysis had stood, effective ameliorative actions would likely never have been identified.

Similar global thinking may also be stalling effective local actions that could be taken to solve a number of other important environmental problems.  One that stands out in our minds is the preservation of the planet's threatened coral reefs, which are suffering from a whole host of vexing anthropogenic stresses (see our Editorials of 12 and 19 September 2001).  Proponents of the Kyoto Protocol, however, have got everyone so fixated on the deleterious consequences that global warming is predicted to have on these species-rich ecosystems that we are slow to fund and implement well-defined site-specific programs that could dramatically improve their health.  And while these and many other forms of life - both aquatic and terrestrial - enter upon what could well be preventable pathways to extinction (knowing we have the means to help them avoid that end), we contemplate the spending of untold amounts of money to fight an unnecessary and unwinable battle against a friend (rising atmospheric CO2 concentrations) we will likely sorely need to provide the food and water both we and the rest of the biosphere will require in the years ahead (see our Editorials of 15 November 2000 and 21 February, 2 May and 13 June 2001).

Where, oh where, has reason fled?

Dr. Sherwood B. Idso
Dr. Keith E. Idso
Vice President

Holmes, R.  1999.  Heads in the clouds.  New Scientist (8 May): 32-36.

Lawton, R.O., Nair, U.S., Pielke Sr., R.A. and Welch, R.M.  2001.  Climatic impact of tropical lowland deforestation on nearby montane cloud forests.  Science 294: 584-587.

Pounds, J.A., Fogden, M.P.L. and Campbell, J.H.  1999.  Biological response to climate change on a tropical mountain.  Nature 398: 611-615.

Still, C.J., Foster, P.N. and Schneider, S.H.  1999.  Simulating the effects of climate change on tropical montane cloud forests.  Nature 398: 608-610.