Bad boy carbon dioxide does its bit to beat heat

Is the bad boy of global warming — carbon dioxide (CO₂) — safeguarding India by preventing summer temperatures from rising at the same rate as the rest of the world? A research paper from the Beijing-based Chinese Academy of Sciences suggests that increasing atmospheric CO₂ concentrations are inducing summertime cooling over India (bit.ly/Summer-Cooling).

The country has a "warming hole", which means temperatures haven't risen here at the same rate as elsewhere in the world under climate change. The anomaly has puzzled scientists, and numerous theories offer no satisfactory answers.

The Chinese theory is that the direct radiative effect of CO₂ enhances low-level moisture transport and vertical motion during the summer months in India, when the Eurasian plains are warming faster than the surrounding seas. "Combined with abundant summer-monsoon moisture and the topographic blocking effects of the Himalayas… these circulation changes increase cloud cover," the paper explains. The enhanced cloud cover reduces incoming solar radiation, producing regional cooling. "These results reveal a previously underappreciated mechanism whereby greenhouse gas forcing can paradoxically induce regional cooling through atmospheric dynamical pathways," the paper says. Additionally, the rise in CO₂ intensifies the flow of cooler air from the Arabian Sea into India.

The researchers arrived at their conclusion after preparing computer-simulated models. East Asia, which lacks an orographic barrier, heats up more, it says. India's projections for reducing CO₂ emissions might exacerbate seasonal warming, the paper says.

"This paper is yet another piece in the puzzle, but it does not entirely solve the puzzle," notes Raghu Murtugudde, Earth System Scientist and Visiting Professor at the Indian Institute of Technology, Kanpur. "It explains a seasonal phenomenon, and does not account for the entire year." He believes that changing wind patterns caused by increased heating of the Arabian Peninsula play a bigger role.

The corresponding author, Xia Qu, agrees that radiative forcing only cools during the summer months. "We do not deny the role of aerosols, changing wind patterns and even agriculture. The CO₂ radiative forcing is one of the several contributors," he says.