Developed nations have dominated global surface temperature change historically, and according to a new study, they will continue to do so until 2030, when developing nations, particularly the two population giants—China and India—will overtake them.
“This is significant,” said lead author Daniel Ward, a postdoctoral researcher from Cornell University, because most human-induced “climate change will come from developed countries until about the year 2030, whereas previous estimates put this ‘crossover’ year at 2020 or earlier.”
Unlike previous studies, which have focused primarily on quantifying long-lived greenhouse gases such as carbon dioxide and methane, this study includes the effects of aerosols, ozone, short-lived gases, and land cover changes to give a more complete picture of the role of each country in climate change.
By incorporating these factors, the study estimates contributions of developed and developing countries to average global surface temperature change, from 1850 (a preindustrial era used as a baseline) to 2010. In addition, it predicts the contribution of each country to global surface temperature change through 2100 by simulating both a high and a low emissions scenario.
With the rise in greenhouse gas emissions from developing countries, the debate continues over which countries—developed or developing—should do more to mitigate their emissions.
“Attributing climate change to past contributions from individual countries improves the fairness of decision-making about where future emission reductions should come from if we are to meet global mitigation goals,” said Dr Ward.
Developed countries comprise 58% of global surface temperature change from 1850 to 2010, a greater proportion when compared to only carbon-equivalent emissions (52%). Among these countries, the US is responsible for nearly one quarter—the largest historical contribution compared with any other country. In comparison, China’s contribution—the highest among the developing countries—is less than half of the US.
Compared to emissions of carbon-equivalent gases, the US’s contribution towards global surface temperature change is around 4% higher. Interestingly, the authors note, both China and Africa together released the same amount of carbon dioxide-equivalent gases as the US during this period, yet they accounted for only 14% of global surface temperature change.
Some of this difference lies in the emissions of methane, which the authors explain, play a greater role for developing countries, but because of its relatively short lifetime compared with carbon dioxide, it reduces their contribution toward global surface temperature change.
Aerosols are tiny airborne particles released into the atmosphere by burning of fossil fuels. They can exert a temporary cooling effect on the Earth, either directly, by reflecting sunlight back into space, or indirectly, by interacting with clouds, making them reflect sunlight. But the indirect effects remain highly uncertain because their interactions with clouds are poorly understood.
They also have opposing effects: While some aerosols, such as sulfates emitted from burning of coal, exhibit a cooling effect by reflecting incoming solar radiation, others such as non-sulfate aerosols, like black aerosols released from diesel exhausts, lead to warming.
As it happens, developing countries tend to emit more cooling aerosols, which mask their contribution toward global surface temperature change, explains Dr Ward. In contrast, developed countries have lowered their aerosol emissions, making their contribution larger.
The uncertainty of indirect aerosol effects impact the contributions of the US and China the most because they depend largely on the magnitude of the aerosol effects, he said.
China, in particular, benefits from aerosol emissions, which comprise a large proportion, thereby cutting down its contribution to historical climate change. The US, in contrast, shows rising contributions to climate change.
Aerosols, however, pollute the air, causing various respiratory diseases, which China has already witnessed. And it is implementing various strategies—with some success—to curb aerosols emitted from cars, coal, and industry. In fact, China is set to ban coal use, and other polluting fuels, in six major districts by 2020, according to a very recent report by Xinhua News Agency.
This may result in an increased contribution toward global temperatures, but it may not be faster than anticipated, said Dr Ward, because the study includes such air quality policies in predicting future emissions.
The contribution of both China and India to global surface temperature change is projected to escalate dramatically.
In 2030, the roles are expected to reverse, when developing countries—dominated by China, India, and Indonesia—will account for the majority (60%) of global surface temperature change in both projection scenarios, eclipsing the contributions of developed countries.
This 10-year-later crossover year is mainly a consequence of cooling aerosols emitted by developing countries. With global aerosol emissions set to drop by 2050, China’s contribution to global surface temperature change will rise, as it will lose the cooling benefit of aerosol indirect effects.
A crucial question the study raises, according to Dr Ward, is whether China, and other developing countries, should receive “climate credit” for emitting cooling aerosols. He suggests that “one solution may be to give countries credit for aerosol emissions before the current day, but then to not extend that credit to future aerosol emissions.”
Both the US and China combined are projected to account for 35 to 40% of global surface temperature change by 2100.
And, irrespective of aerosol effects, the onus is on four culprits that account for more than 50% of global surface temperature change—the US, EU, China, and India—at present and in the future. “So substantial progress could be made if these few, highly populous nations were to agree on significant mitigation policy,” said Dr Ward.
But this “study is only one piece of the puzzle,” he cautions, emphasizing that there are other factors that need to be taken into consideration to designate responsibilities for climate change. These include per capita GDP, outsourcing of emissions, the effect of international trade, emissions required to meet basic societal needs, and development of technology with global benefits.
Nevertheless, the findings of this study will help inform decision-makers such as the United Nations Framework Convention of Climate Change (UNFCCC) when assigning responsibilities for mitigation strategies.
Ward, D.S., Mahowald, N.M. (2014). Contributions of developed and developing countries to global climate forcing and surface temperature change. Environ. Res. Lett, 9, 074008 (10pp).