Nobel laureate Mario Molina of the University of California San Diego (UCSD), Veerabhadran Ramanathan, of Scripps Institution of Oceanography at UCSD and other leading climate scientists said in a new paper that black carbon soot, tropospheric ozone, and hydrofluorocarbons (HFCs), as well as expanding bio-sequestration, can forestall fast approaching abrupt climate changes.

The paper was published Tuesday in the Proceedings of the National Academy of Sciences (PNAS).

"Cutting HFCs, black carbon, tropospheric ozone, and methane can buy us about 40 years before we approach the dangerous threshold of two degrees Celsius," said Ramanathan.

"By targeting these short-term climate forcers, we can make a down payment on climate," said co-author Durwood Zaelke, co-director of the Program on Governance for Sustainable Development at the Bren School of Environmental Science & Management, UC Santa Barbara and president of the Institute for Governance & Sustainable Development.

Black carbon soot is an aerosol produced largely from the incomplete combustion of diesel fuels and biofuels, and from biomass burning. It is now considered to be the second or third largest contributor to climate change.

Black carbon is responsible for almost 50 percent of the 1.9 degrees Celsius increase in warming of the Arctic since 1890 as well as significant melting of the Himalaya-Tibetan glaciers that feed the major rivers of Asia, providing fresh water to billions of people.

Researchers consider black carbon an ideal target for achieving quick mitigation because it remains in the atmosphere a few days to a few weeks and can be reduced by expanding the use of diesel particulate filters for vehicles and clean-burning or solar stoves to replace those burning dung and wood.

With indoor air pollution killing 1.6 million people a year, global action to cut soot emissions would reap major benefits for both public health and climate, the authors said.

"If we reduce black carbon emissions worldwide by 50 percent by fully deploying all available emissions-control technologies, we could delay the warming effects of carbon dioxide by one to two decades and at the same time greatly improve the health of those living in heavily polluted regions," said Ramanathan.

Like black carbon, ground level or tropospheric ozone doubles as a major warming agent and health hazard. It also lowers crop yields. A recent study reported that ozone's damage to crop yields in 2000 resulted in an economic loss of up to $26 billion annually.

It is formed by "ozone precursor" gases such as carbon monoxide, nitrogen oxides, methane and other hydrocarbons, many of which can be reduced by improving the efficiency of industrial combustion processes. Reducing tropospheric ozone by 50 percent could buy another decade's worth of time for countries to start making substantial cuts in carbon dioxide emissions.

HFCs are powerful greenhouse gases originally developed as substitutes for ozone-depleting chemicals. They are poised to become a larger part of the climate problem over the next few decades. HFCs are used primarily as refrigerants and in making insulating foam, and emissions are expected to grow dramatically due to increased demand for air conditioning in developing countries.

By 2050, HFC emissions could equal up to 19 percent of the effect of global carbon dioxide emissions under business-as-usual scenarios. The good news, the paper points out, is that a binding legal agreement exists that can cut HFCs now: the Montreal Protocol treaty that began the phaseout of ozone layer-damaging chemicals in 1989.

"The Montreal Protocol has already delayed climate change by seven to 12 years, and put the ozone layer on the path to recovery later this century," said Molina, a professor of chemistry and biochemistry at UCSD and a recipient of the 1995 Nobel Prize in chemistry for his groundbreaking work in 1974 that sounded the alarm on ozone-depleting CFCs.