Geoscience Reference
In-Depth Information
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Past Climate Change in the Tropics
Tropical rainforests have expanded and contracted
over the last 10 million years due to changes in
climate. Pollen records show several episodes of
montane and savanna expansion, while rainforest dis-
appeared from marginal areas. Fossil evidence indi-
cates much drier and cooler conditions than present
occurred over most tropical and equatorial areas.
During this time, the rainforests of Africa and the Neo-
tropics continued to contract into smaller fragments
now called refugia. During interglacial cycles rainfor-
ests expanded, and species that survived in these refu-
gia were able to radiate to fill newly available niches.
These cycles of cooling and warming continue to
affect the distribution of rainforest worldwide.
Geomorphic research on rising and falling lake lev-
els in tropical areas confirm what fossils and pollen
suggest. Lake levels decreased significantly during
the glacial maximums. They rose during the intergla-
cials. Fallen lake levels and drier climate contracted
the rainforests of Africa, the Neotropics, and Australia,
but had different effects on Southeast Asia. Lower sea
levels during these times exposed extensive land
bridges throughout the islands of the China Sea. This
allowed for the widespread dispersal of plants and
animals throughout the region.
The last glacial cycle occurred 18,000
warmer temperatures correlate with greater
water vapor content, and consequently
higher rainfall occurs with warmer climates.
Changes in vegetation relate to these cli-
matic shifts. Current and future climate
change, caused by high levels of greenhouse
gases emitted into the atmosphere, will con-
tinue to cause changes within the Tropical
Rainforest Biome. Climate change scien-
tists predict various scenarios for tropical
rainforests, from decreased precipitation,
prolonged drought, increasing temperatures,
and seasonality leading to severe desertifica-
tion in the Amazon and African rainforests
to increased rainfall—by as much as 50
percent—in the tropical forests of Asia. Any
of these changes could affect the fragile bal-
ance of the Tropical Rainforest Biome and
lead to its destruction. Add to these potential
changes the direct changes caused by defor-
estation, fire, agriculture, mining, popula-
tion pressure, and other exploitive events,
and the future of the tropical rainforest may
be filled with profound ecological changes.
As the Tropical Rainforest Biome has great
influence on the world's climate, its potential
demise could bring with it catastrophic effects
throughout the globe. Further research, con-
servation, and a decrease in the factors caus-
ing global climate change are necessary to
ensure the future survival of this biome and
the stability of the biosphere as a whole.
12,000 years
ago, and a significant decrease in rainforest species is
evident during that time. After this last glacial cycle,
warmer and wetter conditions characterized the early
Holocene (10,000
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5,000 years ago) and caused the
rainforest to expand slightly. By about 5,000 years
ago, humans had begun to alter the landscape signifi-
cantly by clearing forests for firewood, charcoal, and
agriculture. These activities may have affected rainfor-
est distribution even more than climate change.
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Soils
Soil is a major component in the creation
and maintenance of vegetation in tropical
biomes; and similarly, vegetation, along
with climate, parent material, topography,
and time, influence soil types and characteristics. Soil is formed by the weathering
of rock from parent material (the geologic formation in the region) and the addition
of dead and decaying organic matter from plants and animals. Weathering is the
