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China Contributes 10% of Human Influence on Climate Change

Climate
China Contributes 10% of Human Influence on Climate Change

China accounts for a tenth of all the greenhouse gases and aerosols that have collected in the atmosphere over the industrial era, according to new research.

Boasting one of the world’s largest economies, China has overtaken the EU and the U.S. as the world’s largest emitter, with CO2 emissions from fossil fuels tripling over the past 30 years.

Boasting one of the world’s largest economies, China has overtaken the EU and the U.S. as the world’s largest emitter, with CO2 emissions from fossil fuels tripling over the past 30 years. Photo credit: Way2time / Flickr

But despite soaring CO2 emissions, China’s relative contribution to climate change has remained steady—around the 10 percent mark—over the whole industrial period, says a study published in Nature.

It’s a pretty complicated picture. But in recent times, this is because cooling from aerosols has been masking part of the warming signal. The upshot of this is that cutting some types of aerosol in a much-needed bid to improve air quality could drive faster warming in the coming decades, say the authors.

Climate Responsibility

The new study quantifies all the gases and particles currently in the atmosphere that are having an effect on global temperature and which can be traced back to human activity.

This includes factors that have a warming influence—such as greenhouse gases and black carbon—and those that induce cooling, such as sulphate and nitrate aerosols.

The authors combined this with China’s past and present emissions to estimate, for the first time, the size of the country’s total contribution to human-caused climate change.

Historical Warming

Unlike aerosols, greenhouse gases stay in the atmosphere for a very long time. Carbon dioxide emitted at the start of the industrial revolution is still warming the atmosphere today, which is why scientists take into account historical emissions as well as present day ones to work out a country’s contribution to climate change.

The graph below (main, not inset) shows the impact on global temperature—known as the radiative forcing—from different anthropogenic sources of greenhouse gases in China extending back to 1850.

Main: Historical changes in the warming attributable to greenhouse gases from different anthropogenic sources in China. Colored lines represent the model best estimates, shading gives the uncertainty range. Inset: Above, expressed as a percentage of the global total. Photo credit: Li et al

China’s single biggest source of warming is CO2 from fossil fuel burning (red line). With emissions rising steeply over the past few decades, fossil fuels overtook land-use change as the biggest source of CO2 from China in 1992 (brown line).

Warming as a result of changing land use rose throughout the 20th century, before declining in the 1980s as reforestation programs turned the land from a net emitter to a net absorber.

In a News and Views article that accompanies the study, Prof. Dominick Spracklen, associate professor of aerosols and climate at the University of Leeds, said:

“China is now planting forests on a larger scale than anywhere else on the planet. These plantations sequester CO2 from the atmosphere, so that Chinese forests are now a net sink of this gas.”

Despite this, past emissions from land-use change still account for a third of atmospheric CO2 attributable to China, with fossil-fuel burning making up the other two thirds, the paper notes.

The warming caused by emissions of nitrous oxide (yellow) and halogen compounds (pink) increased over the past two or three decades—to a far lesser extent than fossil fuel CO2, while methane-induced warming increased three-fold between 1950-2010 (orange line).

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Relative Contribution

To put the changes within China into a global context, the inset graph in the figure above shows the warming effect of Chinese greenhouse gases as a percentage of the global total.

Again, the impact of China’s recent fossil fuel expansion is clear, with the country’s contribution to total global fossil fuel CO2 almost tripling in the 30-year period between 1980-2010.

China currently accounts for 12 percent of the total greenhouse gases in the atmosphere—a figure that has remained fairly steady over the industrial period, at around 8-12 percent (blue line). This is effectively the balance of impacts from all China’s greenhouse gas emissions over time—i.e. carbon dioxide, methane, nitrous oxide and halogen compounds—and from all anthropogenic sources.

The 10 percent figure is lower than might be expected for the country topping of the list of the biggest emitters. But it reflects the fact that others, such as the U.S. and the EU, have been emitting far longer, the paper notes. The paper does not calculate the contributions from these other entities, so a direct comparison is not possible.

Relative to the rest of the world, China’s share of greenhouse gas warming fell slightly in the 1960s, 70s and 80s before starting to climb again as economic growth and fossil fuel burning accelerated.

But there’s something else going on, too. China is responsible for a disproportionate amount of the aerosols emitted around the world, some of which have a strong cooling effect.

Cooling Effect

According to the new study’s results, China is responsible for 28 percent of the sulphate aerosols currently in the atmosphere (light blue bar in the graph below), 24 percent of nitrate aerosols (dark blue) and 14 percent of black carbon (pink).

The relative contribution from China to greenhouse gases and aerosols affecting global temperature. Photo credit: Li et al.

Importantly, while the U.S. and Europe have been slashing sulphates in an effort to improve air quality, China’s aerosol emissions have been on the rise. Spracklen said:

Air pollution is a serious environmental issue in China, where 1.3m people die each year because of exposure to poor-quality air outdoors.”

As well as posing a growing public health crisis, sulphate aerosols have a strong cooling effect. Over the past few decades, this has been pulling in the opposite direction to rising CO2, methane and black carbon emissions, effectively muting China’s relative impact on global temperature.

As the graph below from Spracklen’s News and Views article shows, the balance between warming (red shading) and cooling (blue shading) have kept the country’s contribution to human-caused climate change pegged at about 10 percent in recent decades, despite soaring fossil fuel emissions.

China’s relative contribution to global radiative forcing has remained steady over the past few decades because of competing factors. Photo credit: Nature News & Views / Spracklen

The implication is that if China follows the rest of the world in cutting sulphate aerosols in a bid to reduce air pollution, global temperatures could rise even faster in the coming decades since there would be less of a cooling effect to offset the warming. But this is no reason to avoid doing it, said Spracklen:

“This means that it will be difficult to achieve rapid reductions in near-term global warming through the control of Chinese air pollutants overall—a focus on greenhouse-gas emissions, in particular, will be required.”

But given the upwards trend in fossil fuel CO2 emissions, the long lifetimes of existing infrastructure and the social and political systems in place, it is “unlikely” that China’s relative contribution to global greenhouse gas warming will come down any time soon, say the authors in the paper.

While the study goes further than previous ones in terms of pinning down China’s total contribution to climate change, the authors acknowledge a lot of remaining uncertainty, particularly about China’s methane and sulphur dioxide emissions.

There is also work to be done in completing the complex picture of influencing factors. For example, the authors acknowledge the role of aerosols in stimulating clouds to form and the darkening of snow and ice by black carbon, adding that there is still too much uncertainty to include fully in their calculations.

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