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December 2015

Can Paris Pledges Avert Severe Climate Change?

PNNL study shows that Paris emissions reduction pledges reduce risks of severe warming

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Working to improve understanding of the global implications of pledges offered by countries to reduce their emissions, researchers found they have the potential to reduce the probability of the most severe impacts, especially if followed by measures of equal or greater ambition. Image courtesy of NASA.
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Results: A study published in the journal Science shows that if implemented and followed by measures of equal or greater ambition, pledges that are made by individual countries to reduce their emissions called Intended Nationally Determined Contributions, or INDCs, have the potential to reduce the probability of the highest levels of warming and increase the probability of limiting global warming to 2 degrees Celsius. The pledges are a key part of an agreement offered by more than 190 countries meeting in Paris to address climate change. In a lead-up to the 21st Conference of the Parties in Paris, countries announced the contributions that they are willing to make to reduce greenhouse gases over time and combat global climate change.

"It's not just about 2 degrees," said Dr. Gokul Iyer, Pacific Northwest National Laboratory researcher and the study's corresponding author. "It is also important to understand what the INDCs imply for the worst levels of climate change." Iyer works at the Joint Global Change Research Institute, a collaboration between PNNL and the University of Maryland.

Why It Matters: Although many researchers have focused on the importance of the 2 degree limit, the study's authors assessed uncertainty in the climate change system from an overall risk management perspective. They analyzed the full range of temperatures the INDCs might attain, and determined the odds for achieving each of those temperatures.

Methods: In the study, the scientists compared the Paris commitments to a world in which countries don't act at all or start reducing greenhouse gas emissions only in 2030. To perform the analysis, the team incorporated the INDCs along with assumptions about future emissions reductions into a global, technologically detailed model of the world called the Global Change Assessment Model or GCAM that includes energy, economy, agriculture and other systems. The GCAM model produced numbers for global greenhouse gas emissions, which the team then fed into a climate model called Model for the Assessment of Greenhouse-gas Induced Climate Change or MAGICC. Running the simulations for each scenario 600 times resulted in a range of temperatures for the year 2100, which the team converted into probabilities.

"Long-term temperature outcomes critically hinge on emissions reduction efforts beyond 2030," said Iyer. "If countries implement their INDCs through 2030 and ramp up efforts beyond 2030, we'll have a much better chance of avoiding extreme warming and keeping temperature change below 2 degrees Celsius. It's important to know that the INDCs are a stepping stone to what we can do in the future."

The team found that if countries do nothing to reduce emissions, the Earth has almost no chance of staying under the 2 degree limit, and it is likely that the temperature increase would exceed 4 degrees. They also showed that the INDCs and the future abatement enabled by a Paris agreement introduces a chance of meeting the 2 degree target, and greatly reduces the chance that warming will exceed 4 degrees. The extent to which the odds are improved depends on how much emissions limits are tightened in future pledges after 2030.

To learn more about this study, read the PNNL news release, "Can Paris pledges avert severe climate change?"

What's Next? Researchers will next study the kinds of policies and institutional frameworks necessary to facilitate a robust process that enables progressive increases in emissions reduction efforts over time.


Sponsors: The work was supported by the Global Technology Strategy Program, the William and Flora Hewlett Foundation, the Department of State, and the Environmental Protection Agency.

Research Team: Gokul Iyer, Leon E. Clarke, James A. Edmonds, Haewon C. McJeon, Ghassem R. Asrar, Minji Jeong, Anupriya Mundra, and Wenjing Shi, PNNL; Allen A. Fawcett, Jameel Alsalam, Jared Creason, and James McFarland, U.S. Environmental Protection Agency; Nathan E. Hultman, University of Maryland; Joeri Rogelj, International Institute for Applied Systems Analysis, Austria; and Reed Schuler, U.S. Department of State.

Research Area: Climate & Earth Systems Science

Reference:  Fawcett AA, G Iyer, LE Clarke, JA Edmonds, NE Hultman, HC McJeon, J Rogelj, R Schuler, J Alsalam, GR Asrar, J Creason, M Jeong, J McFarland, A Mundra, W Shi. "Can Paris Pledges Avert Severe Climate Change?" Science, Science Express, November 26, 2015. DOI: 10.1126/science.aad5761

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In one sentence: Researchers at Pacific Northwest National Laboratory and colleagues found that if followed by measures of equal or greater ambition, individual country pledges to reduce their emissions called Intended Nationally Determined Contributions have the potential to reduce the probability of the highest levels of warming and increase the probability of limiting global warming to 2 degrees Celsius.

In 100 characters: Global emission reduction pledges may increase probability of limiting global warming to 2 degrees C