NASA: Black Carbon Contributes to Droughts and Floods in China
A new NASA climate study has found large amounts of
black carbon (soot) particles and other pollutants are
causing changes in precipitation and temperatures over China
and may be at least partially responsible for the tendency
toward increased floods and droughts in those regions over
the last several decades.
In a paper appearing in the September 27 issue of SCIENCE,
Surabi Menon of NASA and Columbia University, New York, and
her colleague, James Hansen of NASA’s Goddard Institute for
Space Studies, New York, indicate black carbon can affect
regional climate by absorbing sunlight, heating the air and
thereby altering large-scale atmospheric circulation and the
hydrologic cycle.
Using the NASA Goddard Institute for Space Studies climate
computer model and aerosol data from 46 ground stations in
China, Menon and Hansen conducted four sets of computer
simulations to monitor the effects of black carbon on the
hydrologic cycle over China and India. The aerosol data from
the Chinese ground stations were provided by Yunfeng Luo, a
co-author on the study from the Institute of Atmospheric
Physics, Chinese Academy of Sciences.
In the four numerical simulations, Menon and Hansen isolated
specific factors such as sea surface temperature, other
greenhouse gases and aerosols, and analyzed whether changes
in those factors would be responsible for hydrologic cycle
changes.
Out of the four scenarios, the effect of increased amounts of
soot (over southern China) created a clear tendency toward
the flooding scenario that has been occurring in southern
China and the increasing drought over northern China that has
persisted over the last several years
“If our interpretation is correct, then reducing the amount
of black carbon or soot may help diminish the intensity of
floods in the south and droughts in the northern areas of
China, in addition to having human health benefits,” Hansen
said. Currently research is being conducted to verify a
similar pattern over India.
Black carbon or soot is generated from industrial pollution,
traffic, outdoor fires and household burning of coal and
biomass fuels. Soot is a product of incomplete combustion
especially of coal, diesel fuels, biofuels and outdoor
biomass burning. Emissions are large in China and India
because cooking and heating are done with wood, field
residue, cow dung, and coal, at a low temperature that does
not allow for complete combustion. These resulting soot
particles absorb sunlight, just as dark pavement becomes
hotter than light pavement in the summertime.
When soot absorbs sunlight it heats the air and reduces the
amount of sunlight reaching the ground. The heated air makes
the atmosphere more unstable, creating rising air
(convection), which forms clouds and brings rainfall to
regions that are heavily polluted.
The increase of rising air in southern China is balanced by
an increase of sinking air (subsidence) and drying in
northern China. When air sinks, clouds and thus, rain cannot
form, creating dry conditions. For example, deserts are
places where subsidence occurs.
In recent years, northern China has suffered from an
increased severity of dust storms, while southern China has
had increased rainfall that is thought to be the largest
change in precipitation trends since the year 950. Menon and
Hansen believe that human-made sunlight-absorbing aerosols
may be responsible.
This research continues long-term observations of global
climate change and was funded by NASA’s Earth Science
Enterprise and the National Science Foundation. NASA’s Earth
Science Enterprise is dedicated to helping us to better
understand and protect our home planet.
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