Report in science journal Nature finds nation's contribution to global warming was overstated

China is responsible for a far smaller share of global warming than previously thought, according to new evidence published in the science journal Nature on Thursday.

The study, the first comprehensive assessment of the country's total climate change contribution since the preindustrial era, was done by a group of researchers from Peking University.

It is generally accepted that China now contributes more than 26 percent of the world's total CO2, the study said, yet the country's relative contribution to global warming has remained surprisingly stable - at 8-12 percent - over the past 150 years.

"If we want to know how much a country's total emissions would influence the climate, we have to take all known climate forces into account, which includes well mixed greenhouse gases, short-lived atmospheric climate forces and land-use change," said Li Bengang, a professor at the College of Urban and Environmental Sciences of Peking University and one of the authors of the study.

To understand China's total contribution to climate change, the research team established complex mathematical models to calculate the country's global radiative force-which refers to the difference between solar energy absorbed and that which is radiated back into space - between 1750 and 2010.

They found that despite a recent abrupt increase in China's contribution to world CO2 emissions, the country only contributed to 10 percent of global warming.

"Pollutant emissions and climate change do not have a linear relationship, because there are complex, dynamic processes between the emitting and radiation effect. Some pollutants have a warming effect while others have a cooling effect, and they interact with each other to generate a combined effect to the atmosphere," said LI.

"As a result, we have to use radiative forcing to evaluate a country's relative contribution to global climate change." The study also suggested that China's goal of improving air quality could actually increase its contribution to global warming.

"Air pollution is a serious problem damaging people's health in China. But measures to reduce emissions of air pollutants may, at the same time, affect Earth's climate by weakening the cooling effect," Li said.

Radiative forcing is defined by the Intergovernmental Panel on Climate Change as referring to the extent that a factor alters the balance of incoming and outgoing energy in the Earth-atmosphere system, relative to preindustrial conditions before 1750. Positive radiative forcing leads to surface warming, whereas negative radiative forcing leads to surface cooling.

Examples of factors causing positive radiative forcing include well-mixed greenhouse gases, tropospheric ozone and black carbon aerosols, while negative radiative forcing comes from land-use change that increases its reflectivity, stratospheric ozone; the effect of ozone precursors on methane; and sulfate, nitrate and particulate organic matter aerosols.

An anonymous peer reviewer for Nature described the researchers' conclusions as "scientifically very interesting and also very important politically".

Another said the study was relevant to the wider community because "knowledge of a country's contribution to pollutant emissions, concentrations and resulting global climate forcing is important for both scientific and policy applications".

chengyingqi@chinadaily.com.cn

(China Daily 03/18/2016 page5)