David E. Steitz

Headquarters, Washington, DC

(Phone 202/358-1730)

Cynthia M. O’Carroll

Goddard Space Flight Center, Greenbelt, MD

(Phone: 301/614-5563)

RELEASE: 00-133

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 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

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-carbon dioxide 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-CO2
greenhouse gases that have been more significant in climate
change,” Hansen said.

“The good news is that the growth rate of non-carbon dioxide
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
carbon dioxide gas emissions, this could lead to a decline in the
rate of climate change.”

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

Hansen’s research looked at trends in various greenhouse
gases and noted that the growth rate of carbon dioxide 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 21st Century an
Alternate Scenario,” will appear in the August 29th version of the
Proceedings of the National Academy of Sciences.

More information on the paper can be found at:


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: