A Franco-chinese team including the Laboratoire des sciences du climat (LSCE, CEA/CNRS/UVSQ) has demonstrated, using a new approach, that China's "climate responsibility," is not as significant as initially estimated. However, rather paradoxically, the latter country's "responsibility" may increase rapidly over the coming years, due to future policies designed to improve the country's air quality. Researchers obtained these results using a new methodology developed to determine the impacts of greenhouse gas emissions, aerosols and compounds which are chemically active in the atmosphere on a country's climate. This research is published in Nature on the 17th February 2016.
The researchers quantified China's current contribution to global "radiative forcing" (the imbalance, of human origin, of our planet's radiation budget), by differentiating between the contributions of long-life greenhouse gases, the ozone and its precursors, as well as aerosols. To achieve this, they established the following model which combines:
· earth's major biogeochemical cycles (the carbon cycle, the atmospheric chemistry of greenhouse gases);
· a 3D reconstruction of the transporting and the chemistry of particles in the atmosphere;
· albedo reconstructions taken from satellite data.
They thereby estimated that China contributes an average of 10 % to current, global radiative forcing. Its contribution to heating forcing (caused by greenhouse gases, ground-level ozone and "black carbon") is 12% on average. Its contribution to cooling forcing (caused by aerosols, such as sulphates and nitrates, scattering solar radiation) is 15% on average.
As the world's top energy consumer, China's "climate responsibility" is below what may be expected, given its energy consumption and CO2 emissions.
There are two reasons for this; firstly, aerosols produced by China have a strong, concealing (cooling) effect and these aerosols are also a root cause of the country's poor air quality. Secondly, the method developed incorporates the effects of all emissions since 1750 (the dawn of the industrial era), thereby taking into consideration the fact that China is a relatively recent polluter.
According to the study, China's main contributions are:
· 0.16 W/m2 (watt per square metre) on average for CO2 resulting from fossil fuels;
· 0.13 W/m2 on average for methane (CH4) ;
· -- 0.11 W/m2 on average for sulphate aerosols;
· 0.09 W/m2 on average for "black carbon",produced by incomplete burning of fossil fuels and biomass.
China's stated aim of improving air quality over the coming years would change this radiative forcing, leading to a rather counter-intuitive consequence; the increase in China's contribution to global warming. Indeed, the reduction in the emission of precursors to polluting particles (sulphur dioxide) would diminish the concealing effects of Chinese aerosols, and would speed up warming,unless this effect were to be compensated elsewhere, for instance by significantly reducing long-life greenhouse gas emissions and "black carbon." This new methodology could therefore be an interesting tool which could be used to re-evaluate countries' environmental impact.
 The Laboratory of Climate Sciences
 Two other French laboratories took part : the CNRS (Centre international de recherche en environnement et développement -- The International Centre for Environmental Research and Development -- /PontsParisTech/Cirad) and the Laboratoire de météorologie dynamique -- The Laboratory of Dynamic Meteorology (CNRS/École normale supérieure/UPMC/École polytechnique)
 This research was carried out as part of an 8-year collaboration between the LSCE and Peking University.
 black carbon absorbs solar rays which can contribute to the warming of the atmosphere. It can be transported long distances and deposited on snow-covered expanses, thereby diminishing the latter's reflectivity(albedo).
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