How much of a difference will the Paris Agreement make?

MIT study projects end-of-century climate under different scenarios

Mark Dwortzan MIT News

Signed in December by climate negotiators from around the globe, the Paris Agreement centers on pledges from 188 countries to reduce their human-made greenhouse gas emissions, with the ultimate goal of capping the rise in global mean surface air temperature (SAT) since preindustrial times at 2 degrees Celsius. Toward that end, these pledges, which cover the years 2020-2030, are expected to be reviewed and strengthened periodically, but do not commit nations to any course of action after 2030. As a result, projections of the long-term climate impact of the Paris Agreement vary widely.

A useful way to assess that impact is to simulate the effects of policies that extend the Agreement’s 188 pledges (known as Nationally Determined Contributions, or NDCs) to the end of the century. In a new study that takes this approach, a team of climate scientists and economists from the MIT Joint Program on the Science and Policy of Global Change led by research scientist Andrei Sokolov finds that by 2100, the Paris Agreement reduces the SAT considerably, but still exceeds the 2 C goal by about 1 C.

One of the study’s co-authors, Joint Program Principal Research Scientist Erwan Monier, discussed the team’s results at the General Assembly of the European Geosciences Union on April 21 in a panel/press conference, “Historical Responsibilities and Climate Impacts of the Paris Agreement.”

Using the MIT Integrated Global System Modeling (IGSM) framework, which combines a human activity model with a climate model of intermediate complexity, the researchers project the climate impact of a “no climate policy” case and three scenarios that effectively extend the NDCs to 2100. The scenarios considered range from a pessimistic world where no further action is specified past 2030 to a world where the same level of commitment as in the Paris agreement is extended until the end of the century.

Assuming a climate system response to anthropogenic greenhouse gas emissions that’s of median strength, the three scenarios reduce the SAT in 2100 between 0.6 and 1.1 C relative to the “no climate policy” case. But because the climate system takes many years to respond to emissions reductions, in 2050 the SAT falls by only about 0.1 C in all three cases. Meanwhile, the rise in SAT since preindustrial times exceeds 2 C in 2053, and in 2100, reaches between 2.7 and 3.6 C — far exceeding the 2 C goal.

“The Paris agreement is certainly a step in the right direction, but it is only a step,” said Monier. “It puts us on the right path to keep warming under 3 C, but even under the same level of commitment of the Paris agreement after 2030, our study indicates a 95 percent probability that the world will warm by more than 2 C by 2100.”

The Joint Program on the Science and Policy of Global Change is MIT’s response to the research, analysis and communication challenges of global environmental change. The program combines scientific research with policy analysis to provide an independent, integrative assessment of the impacts of global change and how best to respond.

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