UC San Diego climate researchers have outlined three steps countries should take in order to avoid the global warming threshold.
Major greenhouse gas-emitting countries agreed in December climate talks held in Copenhagen that substantial action is required to limit the increase of global average temperature to less than 2 degrees Celsius or 3.6 degrees Fahrenheit.
Scripps Institution of Oceanography climate scientists Veerabhadran Ramanathan and Yangyang Xu, have recommended steps to reduce carbon dioxide (CO2) emissions with reductions in other warming agents.
Ramanathan said development of new technologies to reduce emissions will be needed.
But he said diesel particulate filters are one example of what can be done now.
"The actions have to be taken now but for many of these compounds, the current technologies would be adequate to bring down the emissions," said Ramanathan.
The paper was published May 3 in the Proceedings of the National Academy of Sciences.
In a news release from the Scripps Institution of Oceanography about the research, Ramanathan and Xu describe three steps that must be taken simultaneously to avoid the threshold, stressing that carbon dioxide control alone is not sufficient.
Recommended steps include stabilizing CO2 concentrations in the atmosphere, and fashioning warming-neutral pollution laws that will balance the removal of aerosols that have an atmospheric cooling effect with the removal of warming agents such as soot and ozone.
Finally, the authors advocate achieving reductions in methane, hydrofluorocarbons and other greenhouse gases that remain in the atmosphere for short periods of time.
The authors say aggressive and simultaneous pursuit of these strategies could reduce the probability of reaching the temperature threshold to less than 10 percent before the year 2050.
"By taking a comprehensive look at human induced climate change, this paper clearly separates the global actions which must be undertaken simultaneously and how quickly these actions must be taken," said Larry Smarr, founding Director of the California Institute for Telecommunications and Information Technology (Calit2) and a collaborator with Ramanathan on CO2 reduction strategies. "This paper should be required reading for all policy makers."
Ramanathan and Xu note that even if greenhouse gas emissions stop increasing in the next five years, human activities will probably create almost double that much radiant energy, which is compensated partially by the masking effect of certain kinds of aerosols that are produced in large part by pollution.
Tiny particles of sulfates and other pollutants serve to cool the atmosphere by reflecting sunlight rather than absorbing it, directing heat away from the earth's surface. Therefore, the authors argue, pollution control measures must take into account and counterbalance the warming that will happen when certain types of pollutants are removed from the skies.
Ramanathan and Xu acknowledge that there are uncertainties about the nature of aerosols and the sensitivity of climate to mitigation actions that make the effects of their suggested course of action hard to determine with precision.
They propose demonstration projects to clarify and reduce the uncertainties and verify the efficacies of the various mitigation avenues proposed in the study.
The authors add that trends in energy added to the oceans would respond to mitigation actions even before 2050, making them an important diagnostic tool that can gauge the success of mitigation within 20 years.
Supporters of the so-called Copenhagen Accord agreed that the 2-degree Celsius threshold must not be crossed, but the United Nations-sponsored conference did not produce an agreement for greenhouse gas emissions reductions.
Scientists have suggested that exceeding that temperature threshold would trigger irreversible phenomena such as widespread release of methane from melting permafrost and large-scale glacial melt, both of which scenarios would exacerbate climate change-related problems such as sea-level rise and acceleration of global warming.
Avoiding the threshold requires holding carbon dioxide levels to less than 441 parts per million, according to the authors, only slightly higher than today's value of 389 ppm.
This equates to a 50-percent reduction in greenhouse gas emissions by 2050 and an 80-percent reduction by 2100.
Ramanathan and Xu acknowledge that such drastic reduction will require a "portfolio of actions in the energy, industrial, agricultural and forestry sections."
Some of these actions will require development of new technologies.
"A massive decarbonization of the energy sector is necessary to accomplish this "Herculean task," the authors write.
But the strategies not focused on CO2 reduction can largely take advantage of existing technologies and more aggressive enforcement of existing regulations. Actions that can be taken immediately include replacement of biomass-fueled stoves with cleaner alternatives in developing countries and retrofitting of diesel filters on vehicles throughout the world.
The authors also point out that the world has already succeeded before in removing dangerous warming agents.
The 1987 Montreal Protocol regulated the use of chlorofluorocarbons and the damaging effect of the chemicals on the planet's ozone layer was diminished.
