China’s efforts to curb air pollution by cutting emissions may have caused warming in northern hemisphere: Study
The country’s policies are expected to result in more than 0.1 °C warming over the northern latitudes.
China’s success in improving its air quality by cutting polluting emissions may have a negative knock-on effect on climate change overall, according to scientists. They found that a 10-year effort by China to improve air quality and reduce pollution-related health risks has caused warming in areas across the northern hemisphere.
The research by experts from Carnegie Institution for Science, US, Chinese Academy of Environmental Planning, China, Tsinghua University, China, and the University of California Irvine, US, used modeling to analyze the effect China's success in reducing emissions such as sulfur dioxide, black carbon, and organic carbon, has had on global climate change. “From 2006 to 2017, China’s carbon dioxide emissions grew by around 54%, along with around 70% reductions in sulfur dioxide emissions, a 30% reduction in black carbon emissions, and a 40% reduction in organic carbon emissions. The decoupling of carbon dioxide and aerosol emissions is mainly caused by installing end-of-pipe control devices, which reduce aerosol emissions but not carbon dioxide. Such decoupling exacerbated the global warming effects of China’s carbon dioxide emissions,” explains co-author Professor Steven J Davis, University of California Irvine, in the study published in Environmental Research Letters, an IOP Publishing journal.
Aerosols are tiny particles that are spewed into the atmosphere by human activities, such as burning coal and wood, or by geological phenomena, like volcanos. Their negative effects on air quality can damage human health and agricultural productivity. Between 2006 and 2017, the Chinese government implemented clean-air policies to reduce the public health risks of aerosol pollutants like sulfate, a cooling agent.
“Economic growth and industrialization in China over the recent decades has been supported by increasing consumption of energy from coal, making China the world’s largest emitter of major air pollutants such as sulfur dioxide and black carbon. These pollutants have significant impacts on air quality and public health, so China put stringent measures in place to reduce them. The measures were effective, and aerosol pollution in China was substantially alleviated after 2013, with notable public health benefits,” writes lead author Dr Yixuan Zheng from the Chinese Academy of Environmental Planning.
Similar to how the aerosols emitted in a volcanic eruption can cause global temperatures to drop, some aerosols from human activity also have a cooling effect on the climate. Unlike greenhouse gases, which induce global warming by trapping heat in the atmosphere, aerosol particles can cause sunlight to be reflected away from the planet either directly or by interacting with clouds. This implies that some of the effects of global warming are being masked by aerosol pollution, explain researchers. “Changes in pollutants such as sulphur dioxide and black carbon also affect radiative forcing -- the determinant of the earth’s temperature -- with sulphate aerosol the dominant cooling agent in the atmosphere. It and other aerosols scatter and absorb incoming solar radiation and interact with clouds, affecting regional and global climate,” says the team.
Accordingly, the authors set out to investigate how these aerosol reductions have affected the global climate. A former postdoc at Carnegie Institution for Science, Dr Zheng further explains: “Anthropogenic sulfate aerosol was estimated to cool the earth on average by half a degree centigrade in 2010, equivalent to 76% of all-anthropogenic-aerosols-induced cooling. Black carbon, on the other hand, absorbs heat in the atmosphere and warms the Earth. So, understanding the effect reduction in these materials could have on warming is essential for future climate mitigation strategies.”
To understand the full range of impacts of China’s clean air actions, the team applied a sophisticated model based on atmospheric and oceanic systems over a 100-year period. They found that China’s pollution-reduction policies might have unmasked about 0.1 degrees Celsius (0.2 degrees Fahrenheit) of greenhouse-gas-induced warming throughout the northern hemisphere, not just in China itself.
“The potential climate effects of China’s air pollution control policies -- enacted between 2006 and 2017 -- were expected to result in more than 0.1°C warming over the northern hemisphere. The emission reductions in China exert warming effects not only locally but also remotely,” the findings suggest. It adds, “The success of Chinese policies to further reduce aerosol emissions may bring additional net warming, and this ‘unmasked’ warming would in turn compound the challenge and urgency of international climate mitigation efforts. In addition to the emission reductions in China from 2006 to 2017, future emission control in China could be expected, which would further impact climate.”
According to Carnegie’s professor Ken Caldeira, the health risks associated with particulate pollution are very serious and mitigation efforts are unquestionably a good thing. He, however, emphasizes that it is also important to understand how ongoing and future efforts to improve air quality will create additional challenges in the international fight against climate change. “Cleaning up aerosol emissions has tremendous health benefits, but unmasks some global warming. While this may seem like a climate setback, we need healthy people to help tackle the climate problem, and if we are to have more resources to allocate to better energy systems, we need to be spending less on the health damage caused by our aerosols,” says Caldeira, a co-author of the study.