Pushing the Boundaries of Life: Tropics
For many years, it was thought that tropical rainforests
were essentially unaffected by climate change Now studies are showing
that not only were they changed during past events like ice ages, but
some areas are being affected right now by warming. At Monteverde Cloud
Forest Reserve, Costa Rica, clouds are forming higher, drying out some
of the habitat and causing changes in flora and fauna.
The most celebrated peer reviewed case is the disappearance
of the golden toads, Bufo periglenes. Each year Dr. Alan Pounds and others
search for the distinctive orange amphibian in its restricted habitat
along a narrow, fog-bound ridge. About 1500 toads were sighted in 1987.
But now the breeding pools remain empty -- the toad has not been seen
since 1991 and is feared extinct.
The golden toad and other amphibians and lizards studied by Dr. Pounds
are in decline apparently due to regional temperature increases lifting
the level of clouds, effectively drying out the cloud forest moisture
on which they depend. This species, Antelopus varius, once common throughout
Costa Rica, was not found at all in a recent survey, according to Dr.
Pounds. Some of this change may be due to deforestation in lowland Costa
Rica, and a group of scientists at Monteverde is planning more study.
At the same time, research by Dr. Nalini Nadkarni shows drying will drastically
change the composition of the diverse epiphyte community that inhabits
the cloud forest canopy.
Dryer conditions in the cloud forest concern Dr. Karen
Masters, who studies tiny Pleurothallic canopy orchids. Lenghtening dry
periods could drive some into extinction. "We are now seeing 2, 3
even 5 days in a row without moisture." she reports. "This is
very challenging to these orchids." Also, recent repeat surveys of
bats by Dr. Richard LaVal, and of birds by Debra DeRosier (repeating a
1979 survey by Dr. George Powell) shows lowland, dry habitat species are
already moving higher into former cloud forest areas.
Other big changes are being monitored in the tropics, too. Sixteen years of data on tree growth, tropical air temperatures and CO2 readings indicate that a warming climate may cause the tropical forests to give off more carbon dioxide than they take up. This wouldupset the common belief that tropical forests are always a sink for carbon, taking huge amounts out of the atmosphere.
The study, by Deborah and David Clark of the La Selva biological station in Costa Rica, and Charles Keeling and Stephen Piper of the Scripps Institution, reports that rainforest trees grow much more slowly in warmer nighttime
In other parts of the tropics, even in places that have been undisturbed for more than 4500 years, the rise in atmospheric CO2 appears to be changing the composition of the forest. In a paper in the 11 March 2004 issue of Nature, William Laurance and colleagues document that many tree genera in Amazonia are growing faster than they were in the 1980s.
Other tree types are declining in vitality. The study of 13,700 trees in 18 very isolated plots in Brazil concluded that increased carbon dioxide is the most plausible explanation for the abrupt shifts in species growth. This "could also have serious ecological repercussions for the diverse Amazonian biota" wrote the scientists.
Warming Winds, Rising Tides: Oceans
Coral reefs are probably the most complex ecosystems on the planet, home to hundreds of thousands of species. They protect and support the lives of millions of people around the tropical zones, and are a font of wealth from fishing and recreation. The damage being caused to reefs by warming seas is one of the most serious effects of global warming.
Rising sea temperature coupled with the strong El Nino of 1998 was devastating to much of the world's coral reefs. High water temperatures caused coral bleaching and subsequent death or adverse change to sixteen percent of world reefs overall and up to 46 percent in parts of the Indian Ocean.
Temperatures
beyond norms causes coral to expel the microscopic symbionts, zooxanthellae,
that also give them color. If this bleaching continues for
days to weeks, the coral dies and algae takes over the reefs, changing
the ecosystem. During another bout of bleaching in 2002, the international
coral reef information network ReefBase reported 430 cases of coral bleaching,
most of them on the Great Barrier Reef, the world's largest.
As
it takes up heat, ocean water expands -- the major cause of sea level
rising at a rate now exceeding 8 inches a century. Sea level rose
about 6 inches in the 20th century, but the rise is predicted to increase
to as much as a meter by 2100 (see Coastlines and Glacier sections).
Coral, which thrives at and near the sea surface, is not expected to be
able to keep pace with this rapid increase in water depth. In addition,
seas are dissolving more and more carbon dioxide. Even though this
adds more carbon, a raw material for coral making calcium carbonate reefs,
it also acidifies the water, actually inhibiting the growth of coral.
Coupled with damage from human activities and development,
this growing danger has lead some scientists to predict the end of reefs
across much of the ocean. In reports in 1999 and 2004, Australian
Marine Biologist Ove Hoegh-Guldberg and others said high water temperatures
and bleaching will become yearly events before mid-century. Living
coral may be reduced by 95 percent on the Great Barrier Reef. Hough-Guldberg
said recently, "We are damaging a large part of the world's biodiversity"
on the reefs. "We're 'chopping them down' with global warming.
These reefs will be so changed that we'll have to find ways to re-employ
all those people," the millions who depend directly on reef fisheries
and recreation. "The implications are huge."
For a current report on a Pacific Island nation that is threatened by
higher sea levels, and other places that are being inundated, see Coastlines.
For a look at climate-driven events in the North Atlantic, see the Arctic section.
"Sez who?": References 4 and References 7
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