Trace Gases Are Key To Halting Global Warming

In the current edition of Proceedings of the National Academy of Sciences, Drs. James Hansen and Makiko Sato of NASA's Goddard Institute for Space Studies (GISS) at the Earth Institute at Columbia University suggest that avoidance of large climate change requires the global community to consider aggressive reductions in the emissions of both carbon dioxide and non-carbon dioxide gases called trace gases. Humans have already increased the amount of carbon dioxide in the air from 280 parts per million (ppm) to 380 ppm. If the world continues on its current trajectory of increasing carbon dioxide, methane and ozone, the likely result will be large climate change, with sea level rise of a few meters or more.

Hansen and Sato point out that if methane and other trace gases are reduced, climate could be stabilized, with warming less than 1°C, at carbon dioxide levels of 520 ppm. However, if the trace gases continue to increase, carbon dioxide would have to be kept beneath 440 ppm. A cap of 440 ppm seems practically impossible to stay under due to existing energy infrastructure. However, Hansen and Sato suggest that, with the possibility of new technologies by mid-century, it is feasible to keep carbon dioxide levels from exceeding approximately 520 ppm.

The co-authors suggest that the non-CO2 gases could be addressed via a Montreal Protocol-like process, or by adding additional gases to the Montreal Protocol itself. The Montreal Protocol has been very effective in reducing emissions of gases that destroy stratospheric ozone. Developed and developing countries have worked together harmoniously in this process, with the World Bank providing support for participation of developing countries.

"Carbon dioxide is the main greenhouse gas (GHG), and slowdown of its emissions must have priority. It will be a growing issue in international relations for decades, if not longer," says Dr. Hansen. "However, that does not necessarily mean that 'Kyoto' is the best way to address the trace gases. 'Kyoto' gives too little or no weight to gases such as methane, the trace gas HFC-134a, ozone and the precursor gases that form ozone. We could get moving now on non-carbon dioxide gases with benefits such as improved human health, in addition to a slowing of global warming. The resulting international good will might also make discussions about carbon dioxide more productive."

"The slowdown in the growth rate of the GHGs contribution to global warming from the peak in the 1980s is due mainly to the phase out of CFCs as dictated by the Montreal Protocol. This success could be diminished by increases of other trace gases not controlled by the Montreal Protocol. We argue that it is well worth extending the Montreal Protocol machinery to phase out many of these trace gases," added Sato.

Additionally, the co-authors warn that trace gases also influence the rate at which major atmospheric GHGs are sequestered, a primary strategy for curbing global warming from carbon dioxide emissions. As global warming proceeds, the Earth naturally releases carbon dioxide, methane and nitrous oxide. Therefore, another benefit of reducing trace gases and their warming effect is a reduction of the induced 'natural' releases of these gases. Other bonuses of reducing warming agents such as ozone and soot are improvements in human health and agricultural productivity.

NASA provided funding for this study, and the NOAA Climate Monitoring and Diagnostics Laboratory provided access to current measurements of GHGs.