Scientist models Earth's climate, vegetation patterns at last glacial peak
October 21, 1996
Boston buried under ice? Idaho sun-dried into desert?
No, this isn't science fiction.
Benjamin Felzer, a climatologist and geologist at the National
Center for Atmospheric Research in Boulder, has used NCAR computer
models of climate and vegetation to find which plant types our
ancestors would have wandered among during the last glacial
maximum (LGM) 21,000 years ago. Felzer presented his work on
Thursday, Oct. 30, at the annual meeting of the Geological
Society of America in Denver.
These models present a world blanketed by massive ice sheets
over Canada and Eurasia, with tundra covering much of Europe and
North America as the Sahara Desert crept southward towards central
Africa. Rain forests still existed in South America and Africa,
but there were fewer trees globally. The whole world was colder
in both summer and winter, with a global average temperature four
degrees Celsius lower than now and atmospheric carbon dioxide (a
greenhouse gas) at just over half of today's levels.
Felzer's study checks the NCAR model's reliability by
simulating past climates and comparing its results to geological
data, such as pollen deposited in lake sediments at the time,
fossilized, and recently retrieved from lake cores. Once
verified, the model can be used to estimate what will happen to
today's plants in the next century as increasing greenhouse gases
warm the climate by several degrees.
"The plants we see around us today had 21,000 years to adapt to
a several-degree warming. Now these same plant types may have a
hundred years or less to make the same transition," explained
Felzer. Will they have time to migrate and adapt or will they
die? The model may eventually shed light on such questions as his
NCAR colleague Jon Bergengren shifts the model's gaze out of the
distant past and into the next century.
In the real world, the growth and melt of the ice sheets over
the past two million years has resulted from long-term cyclical
changes in the earth's axial tilt, its precession (or motion
around the tilted axis), and its elliptical orbit around the sun.
The cold temperatures of the last glacial maximum resulted from
the existence of the continental ice sheets and were intensified
by reduced amounts of atmospheric carbon dioxide, which insulates
the earth by trapping heat reflected from the surface toward
space.
While 4 degrees might not sound like much, it translates into
regional average temperatures that were colder than today's
temperatures by 2 to 20 degrees in various parts of North America.
Above the ice sheets, average temperatures may have plummeted to
40 degrees colder for those regions. Besides temperature, the
model calculates all other climate indicators, including moisture
levels (precipitation and evaporation).
To the climate model Felzer added a vegetation model that
included 110 different plant types divided into 12 categories,
including needleleaf evergreen and broadleaf deciduous forests,
savanna, shrub land, and desert. The model ranked the vegetation
by which plant type was best adapted to which regional climate.
It computed how much surface area each type occupied based on
competition for light (related to canopy cover) and disturbances
such as tree fall and lightning-sparked fire. The result was a
global picture of vegetation 21,000 years ago.
The model shows fragile, treeless tundra covering most of
Europe. Desert spread into the northern Rocky Mountains. A
wetter Southwest still bore mostly desert plants, while the
Pacific Northwest dried slightly. Forests gave way to tundra and
polar desert in Alaska. Worldwide, the biggest vegetation changes
occurred in central Asia, where needle-leaf evergreen spruces were
replaced by needle-leaf deciduous larches. The Sahara expanded
southward as the Asian monsoon weakened and drier conditions
prevailed across Africa and Southeast Asia.
Felzer checked his results against earlier LGM vegetation
scenarios based on data from fossilized pollen retrieved from lake
cores. His simulated vegetation matched up fairly well with these
geological reconstructions of plant life. Where differences
exist, a colder simulation might help produce the correct
vegetation, since there is evidence in the data for a slightly
colder world than the model shows, especially in the tropics.
According to Felzer, the model is accurate enough in showing past
climates and vegetation that it can be a useful tool in simulating
the future.
The two NCAR models used in the study were GENESIS (Global
Environmental and Ecological Simulation of Interactive Systems)
created by Starley Thompson and Dave Pollard with funding from the
Environmental Protection Agency, and EVE (Equilibrium Vegetation
Ecology Model) developed by Bergengren and also funded by the EPA.
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