David E. Steitz

Headquarters, Washington, DC August 29, 2000

(Phone 202/358-1730)

Cynthia M. O\'Carroll

Goddard Space Flight Center, Greenbelt, MD

(Phone: 301/614-5563)

RELEASE: 00-133

**NEW VIEW ON THE CULPRITS OF CLIMATE CHANGE PUBLISHED**

Since climate change affects everyone on Earth, scientists have been
trying to pinpoint its causes. For many years, researchers agreed that
climate change was triggered by what they called \"greenhouse gases,\"
with carbon dioxide (CO~2~) from burning of fossil fuels such as coal,
oil, and gas, playing the biggest role. However, new research suggests
fossil fuel burning may not be as important in the mechanics of climate
change as previously thought.

NASA funded research by Dr. James Hansen of the Goddard Institute for
Space Studies, New York, NY, and his colleagues, suggests that climate
change in recent decades has been mainly caused by air pollution
containing non-CO~2~ greenhouse gases, particularly tropospheric ozone,
methane, chlorofluorocarbons (CFCs), and black carbon (soot) particles.

Since 1975, global surface temperatures have increased by about 0.9
degrees Fahrenheit, a trend that has taken global temperatures to their
highest level in the past millennium. \"Our estimates of global climate
forcings, or factors that promote warming, indicate that it is the
processes producing non-CO~2~ greenhouse gases that have been more
significant in climate change,\" Hansen said.

\"The good news is that the growth rate of non-CO~2~ greenhouse gases
has declined in the past decade, and if sources of methane and
tropospheric ozone were reduced in the future, further changes in
climate due to these gases in the next 50 years could be near zero,\"
Hansen explained. \"If these reductions were coupled with a reduction in
both particles of black carbon and CO~2~ gas emissions, this could lead
to a decline in the rate of climate change.\"

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Black carbon particles are generated by burning coal and diesel fuel and
cause a semi-direct reduction of cloud cover. This reduction in cloud
cover is an important factor in Earth\'s radiation balance, because
clouds reflect 40 percent to 90 percent of the Sun\'s radiation
depending on their type and thickness. Black carbon emission is not an
essential element of energy production and it can be reduced or
eliminated with improved technology.

Hansen\'s research looked at trends in various greenhouse gases and
noted that the growth rate of CO~2~ in the atmosphere doubled between
1950 and 1970, but leveled off from the late 1970s to the late 1990s.

The other critical piece of information this research is based on, in
addition to greenhouse gas levels, is observed heat storage, or warmer
ocean temperatures, over the last century. Heat storage in the ocean
provides a consistency check on climate change. The ocean is the only
place that energy forms an imbalance. In this case a warming can
accumulate, and global ocean data reveals that ocean heat content has
increased between the mid-1950s and the mid-1990s.

Hansen\'s paper, \"Global Warming in the 21^st^ Century an Alternate
Scenario,\" will appear in the August 29^th^ version of the Proceedings
of the National Academy of Sciences. More information on the paper can
be found at:

[**http://www.pnas.org/papbyrecent.shtml**](http://www.pnas.org/papbyrecent.shtml)

NASA\'s Office of Earth Sciences, Headquarters, Washington, DC, sponsor
research that studies how human-induced and natural changes affect our
global environment. For more information about the Earth Sciences
Enterprise, please see:

[**http://www.earth.nasa.gov**](http://www.earth.nasa.gov/)

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