The following is
a composite of the articles listed at the bottom of the post. As well as a
press release from Oregon State University about the Jung et al. study and carried by
Eureka Alert.
Earlier this
year David Bickford and colleagues summarized the impact of climate change on
the herpetofauna of Southeast Asia. They predicted the following impacts as the
temperature increases: desiccation of frog eggs laid in leaf litter and soil;
changes in tadpole behavior due to low dissolved oxygen followed by an increase
in tadpole mortality; an increase in susceptibility to diseases; loss of
freshwater turtle habitat; a decrease in mass or population size due to increased
metabolism; skewed sex rations in TSD species; elimination of males or females
in TSD species and more species of reptiles showing single sex populations;
reduced population size of fossorial species because of soil desiccation; loss
of marine turtle prey base and subsequent population declines; and reduced
diversity at low and high elevations; all of these alterations increase
competition and changes in community composition.
Additionally, Christy McCain (2010) used 25 elevational gradients of reptile
diversity from temperate, tropical and desert mountains in both hemispheres
that spanned the latitudes from 10.3° N to 46.1° N. She found reptile richness
and, specifically snake, as well as lizard, richness on mountains showed four
distinct patterns: decreasing, low-elevation plateaus, low-elevation plateaus
with mid-elevation peaks, and mid-elevation peaks. Reptile richness at various
elevations was most strongly correlated with temperature. The temperature
effect was mediated by precipitation; reptile richness was more strongly tied
to temperature on wet gradients than on arid gradients. Area was secondarily
important, while the mid-domain effect was not strongly associated with reptile
diversity on mountains. Montane reptile diversity patterns did not follow the
predicted temperature–water effect, as all diversity patterns were found on
both wet and dry mountains. However, precipitation’s effect on temperature most
likely reflects reptiles using basking opportunities that are more abundant on
arid mountains than wet mountains because of lower humidity, sparser vegetation
and less cloud cover at low and intermediate elevations.
Now,
a study lead by Martin Jung from the Max Planck Institute for
Biogeochemistry in Germany report that the soils in large areas of the Southern
Hemisphere, including major portions of Australia, Africa and South America, have
been drying up for the past decade. This is the first major study to examine
evapotranspiration on a global basis.
Most climate models have suggested
that evapotranspiration (the movement of water from the land to the atmosphere)
would increase with global warming. Jung et al’s study published online this
week in the journal Nature, found that's exactly what happened from 1982
to the late 1990s.
However, in 1998, this significant
increase in evapotranspiration – which had been seven millimeters per year –
slowed dramatically or stopped. In large portions of the world, soils are now
becoming drier than they used to be, releasing less water and offsetting some
moisture increases elsewhere.
Due to the limited number of decades
for which data are available, scientists say they can't be sure whether this is
a natural variability or part of a longer-lasting global change. But one
possibility is that on a global level, a limit to the acceleration of the
hydrological cycle on land has already been reached.
Jung et al suggest the trend could
reduced terrestrial vegetation growth, reduce carbon absorption, and reduce the
natural cooling mechanism created by evapotranspiration. The results would
produce increased heating of the land’s surface, more intense heat waves, and a
feedback loop that could intensify global warming.
Regions that show most severe drying
include southeast Africa, much of Australia, central India, large parts of
South America, and some of Indonesia. Most of these regions are historically
dry, but some are actually tropical rain forests.
The rather abrupt change from
increased global evapotranspiration to a near halt in this process coincided
with a major El Nino event in 1998, the researchers note in their report, but
they are not suggesting that is a causative mechanism for a phenomenon that has
been going on for more than a decade now.
Greater evapotranspiration was
expected with global warming, because of increased evaporation of water from
the ocean and more precipitation overall. And data indeed show that some areas
are wetter than they used to be. However, other huge areas are now drying out.
This could lead to increased drought stress on vegetation and less overall
productivity. The result could be less carbon absorbed, less cooling through
evapotranspiration, and more frequent or extreme heat waves.
Evapotranspiration returns about 60
percent of annual precipitation back to the atmosphere, in the process using
more than half of the solar energy absorbed by land surfaces. This is a key
component of the global climate system, linking the cycling of water with
energy and carbon cycles. Long term studies will be needed to determine if
these changes are part of decade trend or a longer-term shift in global
climate, the researchers said.
While changes in the evapotranspiration rate are likely to favor some species of squamates at some localities, others will be negatively impacted, thus radical changes in the species composition of communities seems likely.
Bickford, D., S, D. Howard, J. J. Ng, and J. A. Sheridan.
2010. Impacts of climate change on the amphibians and reptiles of Southeast
Asia. Biodiversity
and Conservation
9:1043-1062.
McCain, C. M.
2010. Global analysis of reptile elevational diversity. Global Ecology and Biogeography 19:541-553.
Jung, M., M. Reichstein, P. Ciais,
S.I. Seneviratne, J. Sheffield, M.L. Goulden, G. Bonan, A. Cescatti, J. Chen,
R. de Jeu, A.J. Dolman, W. Eugster, D. Gerten, D. Gianelle, N. Gobron, J.
Heinke, J. Kimball, B.E. Law, L. Montagnani, Q. Mu, B. Mueller, K. Oleson, D.
Papale, A.D. Richardson, O. Roupsard, S.W. Running, E. Tomelleri, N. Viovy, U.
Weber, C. Williams, E. Wood, S. Zaehle, K. Zhang. 2010. A recent decline in the
global land evapotranspiration trend due to limited moisture supply. Nature
xxxx: xxx-xxx. DOI 10.1038/nature09396.
