But Veerabhadran "Ram" Ramanathan and Yangyang Xu of the Scripps Institution of Oceanography, US, argue that such action is "significantly more formidable" than requiring 50–80% cuts in carbon dioxide emissions by 2100. Writing in PNAS, the pair detail the three-pronged approach they believe will be necessary to keep to this temperature limit.

"We still have time to avert large and abrupt climate changes, but we must act now," Ramanathan told environmentalresearchweb. "The science and observed climate changes required a comprehensive approach that will mitigate both the short and long-term effects of human impacts on the environment."

According to the researchers, a two-degree limit means that the radiant energy falling on the Earth's surface must not increase by more than 2.5 W per square metre. That corresponds to an equivalent carbon dioxide concentration of no more than 441 parts per million (ppm).

However, man's activities to date have already trapped around 3 W per square metre by surrounding Earth with a blanket of greenhouse gases. What's more, air pollution regulations in many countries are gradually cleaning up the reflective aerosol particles that have been exerting a cooling effect.

To get us back on track and keep temperature rise below 2 °C, Ramanathan and Xu recommend a three-pronged approach. Firstly, carbon dioxide concentrations must be stabilized below 441 ppm before 2100, to "decrease the rate of thickening of the greenhouse-gas blanket" and prevent the problem from getting much worse. This requires emissions reductions of about 50% by 2050 and 80% by 2100.

While such reductions may need the introduction of new technologies for renewable energy and carbon capture and storage, the researchers believe the two other prongs of their approach can be achieved using existing technologies and "stringent application of existing air pollution policies".

The second prong is to reduce the amount of warming air pollutants, such as black carbon aerosols, which adsorb solar radiation, and ozone, which is formed from air pollutants and acts as a greenhouse gas. This should act to offset the warming caused by air quality improvements; numbers of suphate, nitrate and organic aerosols, which reflect sunshine away from Earth, are decreasing.

Finally, reducing emissions of short-lived greenhouse gases, such as methane and hydrofluorocarbons, should "thin the blanket" and reduce the 3 W per square metre of energy increase that is already in the system.

The pair believe that these actions could reduce the probability of exceeding the 2 °C barrier before 2050 to less than 10%, and give a less than 50% chance of going over the limit by 2100. In the meantime, we should focus on developing revolutionary technologies to restrict warming to less than 1.5 °C, they say.

"Although there are currently many efforts at capturing carbon in CO2, we need to initiate research efforts to capture black carbon and the non-CO2 greenhouse gases, particularly methane and carbon monoxide," write the researchers in PNAS. They estimate that capturing one ton of black carbon or twenty tons of methane has the same effect on radiant energy over 50 years as capturing 3000 tons of carbon dioxide.

"First I plan to communicate these findings to policy makers in the US as well as globally," said Ramanathan. "I also plan to initiate intervention studies to find out the most effective way to battle climate change."