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Why did the IPCC choose 2° C as the goal for limiting global warming?
Scientists and policymakers have long agreed that global warming beyond 2° C above the pre-industrial average would pose large and escalating risks to human life as we know it on Earth, and governments have used that number as an organizing principle.
The Intergovernmental Panel on Climate Change (IPCC) was founded in 1988 to assess science related to climate change on behalf of the United Nations. While its reports summarizing climate science from around the world are influential in international diplomacy, it was not the first body to note the importance of limiting global warming to 2° C.
In the 1970s, William Nordhaus, an economist at Yale, suggested in several papers that if global warming were to exceed 2° C on average, it would push global conditions past any point that any human civilization had experienced.1,2 At the time, Nordhaus's idea was a simple suggestion of what rise in temperature could cause extreme conditions, based on the historical record of past average temperatures, but it gained new importance a decade later.
In 1988, amid mounting evidence that the earth was warming, James Hansen, a NASA scientist, testified before Congress and became one of the first scientists to publicly link greenhouse gas emissions from humans to this warming trend. Hansen warned that if the world did not reduce emissions, it could result in catastrophic climate change, causing sea-level rise, extreme weather and damage to ecosystems and human settlements across the globe.
"At two degrees we see dramatic alterations to the ability of the Earth's system to maintain the conditions that allow for human life and indeed other species’ life," says Maria Ivanova, a professor of global governance at University of Massachusetts Boston.
After Hansen's testimony, other groups of scientists started to study what might constitute "catastrophic climate change," and many papers used 1 or 2 degrees Celsius as reference points to model what might happen to the Earth at different levels of warming. In the early 90s, governments began coming together to discuss ways to stave off climate change, and informed by these studies, world leaders often referenced either 2° or 1.5° C during their discussions.
"Concrete goals are something that people can rally around and, at the time, a two degree goal seemed both ambitious and achievable," Ivanova says. "It's a simple organizing principle to think of 2 degrees as a speed limit."
Though 2° became a commonly discussed speed limit in the 90s, it wasn't until the Cancun Agreements in 2010 that both 2 degrees and 1.5 degrees were formally written into an international accord. Even then, there wasn't a clear idea about what it would take to achieve that goal.
Finally, under the Paris Agreement in 2015, countries agreed to make plans to limit their emissions of greenhouse gasses. This agreement clearly defines 2° Celsius as the upper limit for global warming, but also lists 1.5° as a more desirable goal because it reduces the risk for the worst outcomes of climate change in most of the world. With a 2° increase the risks of extreme heat waves, droughts, water stress and extreme weather would be far greater for a larger portion of the Earth than with a 1.5° increase. The IPCC now uses 1.5° C as a target in its reports rather than 2°, including in its most recent 2018 report.3
While both 1.5° and 2° C are used as common frameworks for defining emissions goals today, the current pace of international climate action has put the world in jeopardy of exceeding either target.
"The question is collectively, will our strategies to reduce emissions reduce them sufficiently? And so far, the answer is no," Ivanova says.
1 Nordhaus, W.D. "Can We Control Carbon Dioxide?" IIASA Working Paper, International Institute for Applied Systems Analysis, Laxenburg, Austria, 1975, WP-75-063.
2 Nordhaus, W.D. "Strategies for the Control of Carbon Dioxide." Cowles Foundation Discussion Papers No. 443, Cowles Foundation for Research in Economics, Yale University, 1977.
3 "Global Warming of 1.5° C." Intergovernmental Panel on Climate Change, 2018.