Large volcanic eruptions inject sulfur gases into the stratosphere, which
convert to sulfate aerosols with an e-folding residence time of about one
year. The radiative and chemical effects of this aerosol cloud produce
responses in the climate system. Using examples from major eruptions of
the past and results from experiments with numerical models of the climate
system, this talk illustrates the major impacts. Volcanic eruptions
produce global cooling, and are an important natural cause of interdecadal
and interannual climate change. One of the most interesting volcanic
effects is the “winter warming” of Northern Hemisphere continents
following major tropical eruptions. During the winter in the Northern
Hemisphere following every large tropical eruption of the past century,
surface air temperatures over North America, Europe, and East Asia were
warmer than normal, while they were colder over Greenland and the Middle
East. This pattern and the coincident atmospheric circulation correspond
to the positive phase of the Arctic Oscillation. High latitude eruptions
in the Northern Hemisphere, while also producing global cooling, do not
have the same impact on atmospheric dynamics. They weaken the Indian and
African summer monsoon, and the effects can be seen in past records of
flow in the Nile and Niger Rivers. In fact we can use records of the Nile
River flow to provide an improved date for the Eldgjá eruption in Iceland,
which we now date at 939 A.D. Very large, but rare, eruptions, such as
that of Toba 74,000 years ago, may have caused very large climate changes.
However, their effects could not have lasted long enough to produce ice
ages in the current climate. Since the Mt. Pinatubo eruption in the
Philippines in 1991, there have been no large eruptions that affected
climate, but the cumulative effects of small eruptions over the past
decade had a small effect on global temperature trends. Volcanic
eruptions serve as an important analog for suggested human control of
climate (geoengineering), and teach us that while an artificial aerosol
cloud in the stratosphere would cool the climate, it would also destroy
ozone, enhancing ultraviolet radiation at the surface, weaken the Asian
summer monsoon, reduce solar power for systems using direct solar
radiation, and impact plants by changing the ratio of direct to diffuse
radiation. The volcanic analog also supports climate model simulations of
the climatic effect of nuclear war, giving more confidence to fears of
nuclear winter.
Visit Coordinator: Doug Rickman (NASA/UAH)