December 6, 2001
THE SUN'S CHILLY IMPACT ON EARTH
A new NASA computer climate model
reinforces the long-standing theory that
low solar activity could have changed
the atmospheric circulation in the
Northern Hemisphere from the 1400s to
the 1700s and triggered a "Little Ice
Age" in several regions including North
America and Europe. Changes in the
sun's energy was one of the biggest
factors influencing climate change
during this period, but have since been
superceded by greenhouse gases due
to the industrial revolution.
During the Little Ice Age, access to Greenland
was largely cut off by ice from 1410 to the 1720s. At the same time, canals in Holland routinely froze solid, glaciers
advanced in the Alps, and sea-ice increased so much that no open water was
present in any direction around Iceland in 1695.
Drew Shindell of NASA's Goddard
Institute for Space Studies and other
researchers have used a computer
model to reconstruct climate and
atmospheric conditions from the present
back to the Little Ice Age.
They determined that a dimmer Sun
reduced the model's westerly winds,
cooling the continents during wintertime.
Shindell's model shows large regional
climate changes, unlike other climate
models that show relatively small
temperature changes on an overall
global scale. Other models did not
assess regional changes.
During
the
coldest
part
of
the
Little
Ice
Age,
from
1645
to 1715, there is believed to have been a decrease in the total energy output
from the Sun, as indicated by little or no sunspot activity. Known as the Maunder
Minimum, astronomers of the time observed only about 50 sunspots for a 30-year
period as opposed to a more typical
40-50,000 spots. The Sun normally shows signs of variability, such as its
eleven-year sunspot cycle. Within that time, it goes from a minimum to a maximum
period of activity represented by a peak in sunspots and flare activity.
Beginning in 1611, Galileo Galilei made
drawings of lower sunspot activity
before the Maunder Minimum. Records
of sunspot activity during the Minimum
from other astronomers confirm the
lower number of sunspots over the70
year event.
During those periods of low solar
activity, levels of the Sun's ultraviolet
radiation decrease, and can significantly
impact ozone formation in the
stratosphere. "The changes in ozone
that we modeled were key in producing
the enhanced response," Shindell said.
"The changes in the upper atmosphere then feed down to the surface climate."
Between the mid-1600s and the early
1700s the Earth's surface temperatures
in the Northern Hemisphere appear to
have been at or near their lowest values
of the last millennium. European winter
temperatures over that time period were
reduced by 1.8 to 2.7 degrees
Fahrenheit (1-1.5 Celsius). This cool
down is evident through derived
temperature readings from tree rings
and ice cores, and in historical
temperature records, as gathered by
the University of
Massachusetts-Amherst and the
University of Virginia.
Shindell noted that the effects of this
period of a dimmer Sun were
concentrated more regionally than
globally. "Global average temperature
changes are small, approximately .5 to
.7 degrees Fahrenheit (0.3-0.4C), but
regional temperature changes are quite
large." Shindell said that his climate
model simulation shows the temperature
changes occurring mostly because of a
change in the Arctic Oscillation/North
Atlantic Oscillation (AO/NAO).
This oscillation is basically a hemispheric-scale see-saw of atmospheric pressure
and temperature between the mid latitudes and the Arctic which modulates the
strength of the westerly jet stream winds. These winds are reduced as the
AO/NAO shifts in response to a dimmer sun. Because the oceans are relatively
warm during the winter due to their large heat capacity, the diminished flow
creates cold land temperatures by reducing the transport of warm Pacific air to
America, and warm Atlantic air to Europe. During this shift, winter temperatures
cooling of as much as 2 to 4 degrees Fahrenheit (1-2C).
The paper, "Solar forcing of regional climate change during the Maunder
Minimum," by authors Drew Shindell, Gavin Schmidt, and David Rind, from NASA's
Goddard Institute for Space Studies and co-authors Michael Mann and Anne
Waple, from the Universities of Virginia and Massachusetts respectively, appears
in the December 7 issue of Science.
"The period of low solar activity in the middle ages led to atmospheric changes
that seem to have brought on the Little Ice Age. However, we need to keep in
mind that variations in solar output have had far less impact on the Earth's recent
climate than human actions," Shindell said. "The biggest catalyst for climate
change today are greenhouse gases," he added.
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Contact:
Cynthia O'Carroll
Goddard Space Flight Center, Greenbelt, Md.
Phone: (301) 614-5563
Rachel A. Weintraub
Associate TV Producer
Goddard Space Flight Center, Greenbelt, Md.
Phone: (301) 286-0918
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