IPCC Fifth Assessment Report

• 1. Introduction

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  The IPCC Fifth Assessment Report declared, in the most unambiguous language its authors could muster, that warming of the climate system is "unequivocal." That single word, chosen carefully by scientists trained to hedge, carried enormous weight. This was 2014, and the world's most comprehensive review of climate science had just landed. It was the fifth such report produced by the Intergovernmental Panel on Climate Change, and it drew on more than 9,200 peer-reviewed studies. What had those studies found? That greenhouse gas concentrations in the atmosphere had reached levels unprecedented in at least the last 800,000 years. That the period from 1983 to 2013 was likely the warmest 30-year stretch in 1,400 years. That the people who had spent their careers studying ice sheets, ocean temperatures, and sea levels were now, collectively, 95 to 100 percent certain that human influence was the dominant cause of the warming observed since the mid-20th century. The report did not arrive quietly. Drafts were leaked months before publication. A United Nations official warned scientists to brace for a surge in negative publicity. World leaders responded from Washington to Stockholm. And behind all of it stood a question the report posed with great precision: how much warmer would the planet get, and how fast?

• 2. Building the Report

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  In March 2010, the IPCC received roughly 3,000 nominations from experts around the world. At a bureau session held in Geneva on the 19th and the 20th of May 2010, the three working groups presented their selected authors and review editors for what would become AR5. On the 23rd of June 2010, the IPCC announced the final list: 831 coordinating lead authors drawn from fields spanning meteorology, physics, oceanography, statistics, engineering, ecology, the social sciences, and economics. The contrast with the previous report was notable. For the Fourth Assessment Report, only 559 authors and review editors had been selected from 2,000 proposed nominees. AR5 received 50 percent more nominations than AR4, and about 30 percent of the authors ultimately selected came from developing countries or economies in transition. More than 60 percent of the chosen experts were new to the IPCC process entirely, bringing perspectives the organization had not previously incorporated. The report was structured around three working groups and a Synthesis Report. Working Group I, with 258 experts, handled the physical science. Working Group II, involving 302 experts, assessed impacts, adaptation, and vulnerability. Working Group III, with 271 experts, covered strategies for mitigating emissions. Each group produced its own report; a Synthesis Report then drew the threads together. The scoping process itself was inherited from past assessment cycles: governments and organizations that had participated in AR4 were invited to submit written comments, which the panel analyzed before finalizing the report's outline.

• 3. What the Science Found

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  On the 30th of September 2013, the full text of Working Group I's contribution, titled "The Physical Science Basis," was released in unedited form. It ran over 2,000 pages. The Summary for Policymakers had already appeared three days earlier, on the 27th of September, and it rated every finding on a formal confidence scale ranging from "exceptionally unlikely" (0-1 percent probability) through "virtually certain" (99-100 percent). On that scale, the finding that human influence had been the dominant cause of observed warming since 1950 landed at "extremely likely," meaning 95 to 100 percent probability. The report noted that this level of confidence had increased since the Fourth Assessment Report. The ocean data were particularly striking. It was virtually certain, the authors concluded, that the upper ocean had warmed from 1971 to 2010, and that this warming accounted for 90 percent of the energy accumulation in the climate system over that period. The Greenland and Antarctic ice sheets had been losing mass for at least two decades. Arctic sea ice and Northern Hemisphere spring snow cover had continued to decrease. Sea level rise since the middle of the 19th century had been larger, with high confidence, than the mean rate of rise over the prior two millennia. Climate models used for AR5 shifted from the emissions scenarios of previous reports to "Representative Concentration Pathways," or RCPs. These were tested against four different greenhouse gas concentration targets reaching 421, 538, 670, and 936 parts per million by 2100, allowing a wide range of future trajectories to be explored. One finding on projections stood out: global surface temperature increases by the end of the 21st century were likely to exceed 1.5 degrees Celsius relative to the 1850-1900 baseline across most scenarios, and likely to exceed 2.0 degrees under many of them.

• 4. The Mitigation Picture

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  Working Group III released its report, titled "Mitigation of Climate Change," on the 15th of April 2014. Its findings placed the current moment in stark terms. Without new policies to limit greenhouse gas emissions, projections pointed to a global mean temperature increase by 2100 of between 3.7 and 4.8 degrees Celsius above pre-industrial levels, with a wider range of 2.5 to 7.8 degrees when climate uncertainty was included. The report stated plainly that the current trajectory of global annual and cumulative greenhouse gas emissions was not consistent with widely discussed goals of limiting warming to 1.5 to 2 degrees Celsius. The Cancun Agreements, which had been pledged by nations in the years before the report, were assessed as broadly consistent with cost-effective scenarios that gave a "likely" chance, defined as 66 to 100 percent probability, of keeping warming below 3 degrees Celsius by 2100. That was considerably higher than the 2-degree threshold many governments had set as a target. Working Group III also quantified what continued inaction would cost beyond temperature alone: higher emissions meant more expensive adaptation, more species extinctions, greater food and water insecurity, larger income losses, and increased risk of conflict. The report noted that the longer the world waited to reduce emissions, the more expensive that reduction would become.

• 5. Reception and Political Fallout

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  On the 14th of December 2012, drafts of the Working Group I report were leaked and posted online, months before the official release. When the Summary for Policymakers was formally published on the 27th of September 2013, a UN official named Halldór Thorgeirsson warned that scientists should prepare for intensified attacks on their credibility. "Vested interests are paying for the discrediting of scientists all the time," he said. "We need to be ready for that." At Stockholm on the 27th of September 2013, UN Secretary General Ban Ki-moon addressed the IPCC directly. "The heat is on," he said. "We must act." Jennifer Morgan of the World Resources Institute said she hoped the report would inspire leadership "from the Mom to the business leader, to the mayor to the head of state." US Secretary of State John Kerry called the findings "yet another wakeup call," saying those who denied the science or chose excuses over action were "playing with fire." In Washington, President Obama's science adviser, John P. Holdren, cited increased scientific confidence that harms already being experienced from climate change would worsen without comprehensive global action on emissions. Ban Ki-moon also declared his intention to convene a meeting of heads of state in 2014 to work toward a treaty, noting that the previous such gathering, in Copenhagen in 2009, had ended in disarray. The Synthesis Report, released on the 2nd of November 2014, was timed specifically to lay the groundwork for negotiations at the UN Climate Change Conference in Paris, scheduled for late 2015.

• 6. Legacy in the Data

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  In August 2020, scientists reported that observed losses from the Greenland and Antarctic ice sheets were tracking the worst-case sea-level rise projections from the Fifth Assessment Report. That finding gave the AR5 an uncomfortable kind of vindication: the scenarios the report had treated as worst cases were now matching what was actually being measured. The report's framework also left a structural mark on how climate projections are communicated. The Representative Concentration Pathways introduced in AR5 replaced the older emissions scenarios and gave policymakers a clearer vocabulary for discussing the consequences of different levels of action. The Synthesis Report's release on the 2nd of November 2014 helped shape the agenda that culminated in the 2015 Paris negotiations, which the report had been designed to inform.