For decades, the best estimate for climate sensitivity ranged between 1.5°C and 4.5°C. Now scientists set the range between 2.5°C and 4°C, with 3°C as the most likely value.
After honing down how the planet will respond to carbon dioxide, the next step is to figure out how much carbon dioxide will be emitted. To do this, scientists have imagined how humanity will progress from here on out. They’ve considered population growth, advances in clean energy, and an observer effect, in which alarming climate science spurs action to limit greenhouse gas emissions. Their five stories are known as shared socioeconomic pathways (SSPs), each of which makes different assumptions about shifts in policy, economics, and technology. Here are the scenarios in the latest report: 1) BEST: SSP1-1.9 — This scenario has been described as “taking the green road.” It’s the most ambitious and hardest-to-achieve storyline. It envisions a gradual but concerted shift toward clean energy, with few political barriers in adapting to and mitigating climate change. This entails a rapid drawdown of fossil fuels, widespread deployment of clean energy, increasing energy efficiency, and lower resource demands. By the middle of the century, humanity will zero out its contributions to climate change. This scenario also assumes inclusive global development that lifts all countries. It imagines improvements in education and health that would help stabilize population growth, with the total declining slightly to 7 billion people. To create this future, humans would likely need to achieve a global philosophical shift away from the pursuit of economic growth and toward improvements in human well-being. While every scenario in the new IPCC report will likely overshoot the 1.5°C target, under SSP1-1.9, global average temperatures would eventually decline below this level by 2100. It’s also worth noting that 1.5°C of warming is no picnic; that’s still warmer than the world is today, leading to effects like increasing the frequency and intensity of heat waves and extreme rainfall.
Global surface temperatures vary widely under different emissions pathways. 2. (IPCC2) SSP1-2.6 — This pathway envisions that the world will eventually get its act together on climate change, but more slowly than in the optimistic path of SSP1-1.9. It envisions an economy with net-zero emissions after 2050. SSP1-2.6 also expects the global population to reach 7 billion people. The result is a world that will warm up to 1.8°C, with a likely range between 1.3°C and 2.4°C by 2100. It may not seem like much, but this increase in warming compared to SSP1-1.9 has major ripple effects. Sea levels will have risen between 30 and 54 centimeters by the end of the century, up from the 24 cm of rise that has already occurred. That would inundate major coastal metropolises on a regular basis and put another 10 million people around the world at risk from coastal flooding. A world with 2°C of warming would double the number of people exposed to extreme heat compared to a 1.5°C scenario. And every additional bit of warming would bring more environmental declines and exposure to climate hazards.
3) SSP2-4.5 — Sometimes described as “middle of the road,” this scenario lines up with what countries have pledged to do so far about climate change. If every country actually fulfilled its existing obligations, their emissions would lead to about 2.7°C of warming by 2100, with a likely range between 2.1°C and 3.5°C. Under this scenario, the Arctic Ocean would likely be ice-free in the summer, which could have ripple effects on weather all over the world. On top of the devastating effects in the first two scenarios, scientists expect that 3°C of warming would cause a significant drop in global food production, far more extreme heat, and more devastating flooding from extreme rainfall. This storylinepresumes that future global development patterns will not radically shift from historical trends. Inequalities will still persist between countries and development will be slow, but there will be international cooperation on environmental goals. The global population in this scenario would peak at 9.6 billion people.
4) SSP3-7.0 — In this narrative, nationalism is resurgent and countries retreat from international cooperation, focusing instead on their own economic goals. “I sort of jokingly refer to this as Trumpworld,” said Zeke Hausfather, director of climate and energy at the Breakthrough Institute and a contributing author to the latest IPCC report. “It’s a reasonable storyline for what a worst-case world could look like.” This would lead to countries exploiting their own fossil fuels resources more. Investments in education and technological development would decline. Population growth would slow in industrialized countries but remain high in developing countries, with the total reaching 12.6 billion people. Solving climate change would become a low international priority. SSP3-7.0 also accounts for high levels of heat-trapping gases, other than carbon dioxide, including aerosols and methane. By the end of the century, sea levels will have risen catastrophically — between 46 cm and 74 cm in this scenario — and the world will have warmed by roughly 3.6°C, with a range between 2.8°C and 4.6°C. Fortunately, this scenario is on the fringes of what’s plausible, scientists say. “That’s not the world we’re heading toward right now, but it’s certainly a world you could see happening,” Hausfather said. 5) SSP5-8.5 — Imagine a world where humanity doesn’t just do nothing about climate change but continues to make it worse. This scenario envisions global economic growth across the board fueled by burning coal, oil, and natural gas, with the planet’s population leveling off at 7 billion people. While resources are devoted to adapting to climate change, there is little effort to mitigate emissions. The net result would be 4.4°C of warming, with a range between 3.3°C and 5.7°C. As if large-scale coastal inundation and extremely destructive weather weren’t enough, parts of the planet would become unlivable during the hottest times of the year. This odd combination of assumptions and results makes this the most dire but one of the least plausible scenarios. However, it helps scientists probe the upper limits of their models. Why the IPCC decided on these particular storylinesThe five SSPs in the new IPCC report are sorted based on the level of “radiative forcing” — how much energy the atmosphere would trap by the end of the century, in watts per square meter. So the SSP1-1.9 pathway is expected to cause 1.9 watts per square meter of radiative forcing by 2100. Radiative forcing is directly related to how much the planet will heat up on average.
These scenarios were developed in the wake of the 2015 Paris climate agreement. That’s when nearly every country in the world agreed to limit warming this century to less than 2°C above pre-industrial levels, with a high-bar target of keeping warming below 1.5°C. SSPs consider what humans need to do to meet those goals — and they also account for changes in technology, like rapidly falling prices of renewable energy, which hadn’t yet occurred when the IPCC published its last assessment report in 2013. The low-end scenarios are meant to illustrate what would happen if the world actually did meet its climate objectives. Climate models then digest the assumptions in each of these scenarios to yield estimates of how the climate will change under each one. It’s important to remember that these scenarios are not exact forecasts of the future. The scientists who created them are not making judgments about which ones are most likely to come to pass. “We do not consider the degree of realism of any one scenario,” said Amanda Maycock, an associate professor of climate dynamics at the University of Leeds and an author of the future climate scenarios chapter, in an email. SSPs are meant to illustrate the mechanisms of climate change, factoring in human decisions that will determine the scale of the problem. How should people plan for the future under climate change?In all of these potential futures, climate change will continue for years to come. Decades of past pollution, and our ongoing reliance on fossil fuels, are already baked in. “If we would bring down our CO2 emissions to zero today, there would be no further warming from CO2 — but of course that is impossible,” said Joeri Rogelj, director of research at the Grantham Institute at Imperial College London and an IPCC author. Even the most aggressive climate action would lead to a phase-out over time, not a sudden end to emissions. “There is only that steep a pathway we can follow until we get to zero emissions,” he added. But humans still have the power to determine the longer-term path. Right now, the world is somewhere between scenario 3 (SSP2-4.5) and scenario 4 (SSP3-7.0). “The five illustrative scenarios behave quite similarly over the next 20 years (2021-2040) with average global temperature anomalies differing by less than one-tenth of a degree,” said Maycock. “By mid-century (2041-2060) the scenarios start to diverge more strongly.”