Scientists Retire the Worst-Case Climate Scenario, But Warn the Danger Is Not Over

Climate scientists have released a fresh set of global emission scenarios that will guide climate research for the coming years, including the next (seventh) assessment report of the United Nations Intergovernmental Panel on Climate Change (IPCC). The headline change is that the long-feared worst-case scenario, which had the world heading towards warming well above 4 degrees Celsius by the year 2100, has been officially dropped after about 15 years of use. This high-emission pathway was known as RCP8.5, and it has now been judged unrealistic.

To understand the news, it helps to know what these scenarios are. Emission scenarios are not predictions of the future. They are carefully built storylines about how much greenhouse gas the world might release, based on assumptions about population, economic growth, technology and government policy. Scientists feed these storylines into computer climate models to estimate outcomes such as temperature rise, changing rainfall, sea-level rise and ice-sheet melting. The scenarios are revised roughly every six to eight years. RCP8.5 was designed only as an extreme upper edge, a world with no climate policy and runaway fossil-fuel use, but over the years many studies and media reports treated it loosely as the likely 'business as usual' future, which exaggerated the sense of doom.

The new set has seven scenarios, published in April. Its highest-emission scenario now points to about 3.5 degrees Celsius of warming by 2100, down from the roughly 5 degrees linked to the old RCP8.5. Scientists say RCP8.5 was retired mainly because of the rapid spread of renewable energy and the climate policies countries have actually adopted. Emissions have peaked and are falling in the United States and Europe and are levelling off in China, which is currently the world's largest emitter. Avoiding the 4-to-5-degree range is genuinely good news, because the most extreme damages tied to that level of heating can now be considered very unlikely.

However, the same experts stress this is not a reason to relax. Under current policies the world is still on course for serious warming, close to the middle of the new scenarios, with up to about 2.8 degrees Celsius of warming projected by the end of the century. Even 2 degrees of warming, studies warn, can bring severe droughts in major food-growing regions, extreme rainfall over crowded cities, and worse fire weather in forests. More troubling, the targets of the Paris Agreement, especially holding warming to 1.5 degrees, are now seen as reachable only through 'overshoot', meaning the world would temporarily cross the 1.5-degree limit before bringing temperatures back down by 2100. In some ways the situation is harder than in 2010, because emissions kept rising in the years since, making the safest low-emission paths much tougher to achieve.

For India, the message is mixed and important. India already faces severe heatwaves, erratic monsoons and flooding, so avoiding the catastrophic 4-to-5-degree future is welcome. But a world warming towards 2.8 degrees would still hit Indian agriculture, water supply and coastal areas hard. For aspirants, the key takeaways are the difference between a scenario and a prediction, the meaning of terms like RCP, overshoot and the IPCC's role, and the fact that the credit for retiring the worst case goes largely to the global shift to renewable energy and stronger climate policy. It is a reminder that climate science constantly updates itself by discarding unrealistic assumptions and adding new evidence.