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Reducing short-lived pollutants

By Drew Shindell (China Daily)
Updated: 2010-12-09 08:01

Rapid growth in carbon dioxide (CO2) emissions must stop to avoid a disastrous future for our planet. As the greatest contributor to global warming, CO2 is the natural focus of current climate negotiations. Unfortunately, one of the very properties that makes CO2 so problematic, the long time it stays in the atmosphere, creates seemingly insurmountable barriers to international agreements.

First, benefits from limiting CO2 emissions mostly occur only after many decades, which is well beyond the focus of most politicians or corporations. Second, nations disagree over whose emissions are most important.

However, CO2 is not the only driver of climate change. Other pollutants with shorter life spans also contribute. Black carbon (BC) is emitted during incomplete burning in sources such as brick kilns, coke ovens, diesel engines, biomass cookstoves and outdoor fires. These sooty particles absorb sunlight, warming the Earth.

Ozone in the lower atmosphere is a potent greenhouse gas, but is not emitted directly. It is produced by chemical reactions usually involving carbon monoxide (CO) or methane.

Thus emissions of these "precursors" contribute to warming. The same sources that generate BC produce carbon monoxide. Methane is emitted by sources including coal mines, gas flaring, venting and transport, landfills, rice agriculture and manure. Methane is a powerful greenhouse gas in addition to leading to ozone formation.

Though long-lived compared with ozone or BC, which last for only days to weeks in the atmosphere, methane's lifetime of about a decade means it is still short-lived relative to CO2.

Together, it is estimated that emissions of methane, CO and BC have contributed approximately the same amount to global warming as CO2. Though uncertainties for BC are larger than for the others, it likely contributes substantially to warming and regional climate shifts. So these lesser-known pollutants are not just a small part of the problem. Furthermore, ozone and BC also degrade air quality.

These properties could lead to reduced barriers to international cooperation on emissions mitigation compared with CO2. Since they are short-lived, benefits would be felt very rapidly if emissions were reduced.

Governments and corporate boards may find it easier to take actions when the dividends will be enjoyed on their watch. Furthermore, the effect on the climate of short-lived pollutants is primarily a result of their current emissions. Thus disputes about historical responsibility versus present-day influence are largely irrelevant.

For CO2, those reducing emissions pay the costs while the whole planet shares the benefits; the familiar recipe for a "tragedy of the commons".

While the situation is similar for methane, it is starkly different for BC and CO. Reduced emissions of those compounds will provide the largest benefits locally. This is especially true of air quality. Local crop yields increase and premature deaths due to air pollution decrease for areas reducing short-lived emissions.

Additionally, most methane, BC and CO emissions come from relatively small-scale activities. These are unlikely to generate the same fierce opposition that the fossil fuel industry has put up to reducing CO2 emissions, at least in the US, where it has effectively used weak campaign finance laws to encourage Congress to do what's best for the industry rather than what's best for society.

Though barriers to action are lower than for CO2, reducing emissions of short-lived warmers still requires concerted effort. Methane financing could help small stakeholders implement emissions reduction technologies. These sometimes pay for themselves, as the captured methane can be sold, but up-front costs can be prohibitive.

Local efforts to reduce BC and CO could be greatly boosted by increased international cooperation, technology transfer, and acknowledgement of the climate benefits produced. Such efforts are in each nation's own interest, as they would produce large air quality benefits, as well as serving the global community.

A forthcoming UN Environment Program assessment of BC and ozone will quantify the climate and air quality benefits of achievable BC, CO and methane emissions reductions, but it is already clear that aggressive action would save millions of lives and increase crop yields by millions of tons annually (worth billions of dollars) in addition to substantially reducing near-term warming. A push towards reducing these emissions now could also help build confidence among the negotiating partners.

We are not yet on a path to avoiding the worst effects of climate change. Additional actions are clearly needed. Reducing emissions of methane, BC and CO are not enough and cannot substitute or "buy time" for CO2 reductions, which need to begin quickly due to CO2's long lifetime. However, together with CO2 reductions, controls on short-lived pollutants could greatly improve our chances of keeping global warming to manageable levels while simultaneously saving millions of lives and helping feed the world's growing population.

The author is a senior scientist at NASA's Goddard Institute for Space Studies and Chair of the forthcoming United Nations Environment Program BC and ozone assessment.