Carbon dioxide emissions from fossil-fuel use in China have grown dramatically in the past few decades, yet it emerges that the country's relative contribution to global climate change has remained surprisingly constant. See Letter p.357
In December 2015, world leaders agreed to limit the increase in global average temperature to less than 2 °C above pre-industrial temperatures (see Nature 528, 315–316; 2015). Meeting this aspiration will require large and rapid reductions in greenhouse-gas emissions, making it imperative to understand and account for the emissions from different countries. China has undergone rapid economic development over the past few decades and now has one of the world's largest economies — and greenhouse-gas emissions to match. On page 357 of this issue, Li et al.1 comprehensively assess China's contribution to climate change and explore how this has altered as the Chinese economy has grown.
Humans affect Earth's climate through many mechanisms by changing the abundance of greenhouse gases and air pollutants, and by altering the reflectivity of Earth's surface through changes in land use. The relative strengths of these different drivers can be compared through a metric known as radiative forcing, which quantifies the impact of each process on Earth's energy budget.
Li et al. used a model that couples biogeochemistry and climate to estimate China's contribution to global radiative forcing over the period 1980–2010. Crucially, they account for almost all anthropogenic drivers of climate change. They find that China's relative contribution to global radiative forcing from carbon dioxide emissions associated with fossil-fuel use increased almost threefold in these 30 years. This is to be expected, given the surge in China's economy over this period. More surprisingly, they find that China's relative contribution to total global radiative forcing has remained at 10% over this time.
To understand the reasons behind this remarkable result, Li and colleagues made a detailed analysis of the different drivers of radiative forcing. They found that the air pollutants that cause China's notorious pollution haze have had complex effects on climate, counteracting some of the increase in radiative forcing from greenhouse gases. Some components of air pollution, such as black-carbon particles, absorb sunlight and warm Earth's climate. By contrast, sulfate particles scatter light, resulting in climate cooling.
Over the past few decades, China's relative contribution to global radiative forcing from sulfate has increased dramatically. This is because Chinese sulfate emissions soared at the same time that Europe and the United States instigated controls that slashed their sulfate emissions. It has long been known that some air pollutants cool the climate2; what is remarkable in the present study is that the concurrent changes in different emissions have led to a stable overall contribution of China to global radiative forcing (Fig. 1).
Air pollution is a serious environmental issue in China, where 1.3 million people die each year because of exposure to poor-quality air outdoors3. Reductions in the emissions of air pollutants are urgently required to improve air quality, but this will also affect Earth's climate. Li et al. find that the current composition of Chinese air pollution causes almost no net radiative forcing — the cooling effects of sulfate aerosols balance the warming impacts of black-carbon emissions.
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. It also means that carefully managed mitigation of air pollution that focuses on reducing both black-carbon and sulfate emissions might have a minimal impact on climate, because their effects seem to counteract each other. Controlling the combustion of solid fuels for cooking and heating in the home is important in this context, because domestic solid fuel accounts for 40% of Chinese black-carbon emissions4 and causes half a million deaths annually through poor outdoor air quality3,4.
Li and co-workers went on to explore China's contributions to emissions of CO2 and methane from pre-industrial times (1750) to the present day. They find that China's relative contribution to radiative forcing from these greenhouse gases has remained remarkably constant over this much longer period as well. The extensive conversion of China's natural forests to agricultural land resulted in substantial CO2 emissions in the early part of this period. The rate of deforestation has declined in recent decades, but this has been counteracted by increasing fossil-fuel emissions. 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.
Mitigating climate change and air quality without unintended consequences will require an understanding of many complex interactions. Current models, including the one used by Li et al., do not cover many of these complexities. In particular, the authors' study does not consider the formation of secondary organic aerosols — which might dominate in the haze over China5 — from gaseous pollutants. Detailed monitoring of Chinese air pollution is urgently needed to inform the development of effective mitigation policies6.
Air pollutants also interact in complex ways with ecosystems: land-use change alters air quality7, and deposition of pollution can alter forest growth and carbon sequestration8. But these effects are not included in many models. Recent work9 has shown that fast-growing forest plantations in Europe store less biomass and absorb more sunlight than do natural forests; both of these features reduce the forests' benefit to the climate. Whether similar issues are at play across China requires investigation, but it is possible that a greater focus than at present on protecting and restoring natural forests in China might also provide greater benefits for global climate.Footnote 1
Li, B. et al. Nature 531, 357–361 (2016).
Fiore, A. M. et al. Chem. Soc. Rev. 41, 6663–6683 (2012).
Lelieveld, J., Evans, J. S., Fnais, M., Giannadaki, D. & Pozzer, A. Nature 525, 367–371 (2015).
Butt, E. W. et al. Atmos. Chem. Phys. 16, 873–905 (2016).
Huang, R.-J. et al. Nature 514, 218–222 (2014).
Kulmala, M. Nature 526, 497–499 (2015).
Heald, C. L. & Spracklen, D. V. Chem. Rev. 115, 4476–4496 (2015).
Mahowald, N. Science 334, 794–796 (2011).
Naudts, K. et al. Science 351, 597–600 (2016).
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