Amber waves bring cooler temps
Researchers in the Lab’s Atmospheric Science Division have demonstrated
a cooling of up to 2-degrees Fahrenheit over land between 1000 and 1900
AD as a result of changes from natural vegetation, such as forests, to
agriculture.
Through climate model simulations, the research team made up of Bala Govindasamy,
Ken Caldeira and Philip Duffy determined that a previously recognized
cooling trend up to the last century could, in part, be attributed to
the land-use change.
Previous studies had attributed cooling to natural climate variations.
The Livermore research, however, suggests that much of this cooling could
have been the result of human activity.
Forests tend to look dark from the sky, but agricultural lands, with their
amber waves of grain, tend to look much lighter. Dark colors tend to absorb
sunlight and light colors tend to reflect sunlight back out to space.
Changing from forests to crops results in more sunlight reflected back
to space. This reflection of solar energy to space tends to cool the earth,
especially in regions such as the Eastern and Midwestern United States,
where huge tracts of land have been converted to crops. In the 20th century,
some of this cropland has been reverting back to forest, especially in
the eastern United States.
Greenhouse gas emissions in the 20th century likely overcame any cooling
trends that took place up to that time. Growing more trees has been suggested
as a way to soak up carbon dioxide, a greenhouse gas, from the atmosphere.
However, earlier studies demonstrate that growing dark forests could actually
heat the earth’s surface more because dark colors tend to absorb
more sunlight, despite the uptake of carbon dioxide.
"The Earth’s land surface has cooled by about 0.41 K (equal
by about 3/4 of a degree Fahrenheit) due to the replacement of dark forests
by lighter farms growing wheat, corn, etc.," said Caldeira, a climate
model researcher who also is co-director for the Department of Energy’s
Center for Research on ocean carbon sequestration. "This is an example
of inadvertent geoengineering — we changed the reflectivity of the
earth and have probably caused a global cooling in the past. This is now
probably being overwhelmed by our greenhouse gas emissions."
The research, published in the Geophysical Research Letters, also shows
a slight increase in the annual means of global and Northern Hemisphere
sea ice volumes in association with the cooling. The simulated annual
average cooling due to land-use change during this period is almost a
half a degree Fahrenheit globally, 0.66 degrees F for the Northern Hemisphere
and .74 degrees F over land.
In the simulations, land use data for 1000 AD uses potential natural vegetation,
made up mainly of forests, while data for the 1900 AD period uses standard
current vegetation data, which is a mix of forest and croplands, taken
from the Community Climate Model developed at the National Center for
Atmospheric Research. The greenhouse gas levels in both simulations are
in concentrations taken at pre-industrial levels.
"The estimated temperature change in the continental United States
as a result of change from forests to agriculture is up to a 2-degree
Fahrenheit cooling," Caldeira said. "So, when we talk about
global warming, we can no longer take for granted that this global warming
is starting from some natural climate state, undisturbed by human activities."