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New Views on the Culprits of Climate Change

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 (CO2) 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-CO2 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-CO2 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 CO2 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 technology.

Hansen's research looked at trends in various greenhouse gases and noted that the growth rate of CO2 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 Aug. 29 edition of the Proceedings of the National Academy of Sciences.


Hansen, J., Mki. Sato, R. Ruedy, A. Lacis, and V. Oinas 2000. Global warming in the twenty-first century: An alternative scenario. Proc. Natl. Acad. Sci. 97, 9875-9880, doi:10.1073/pnas.170278997.

This article was derived from a NASA Goddard Space Flight Center Top Story.