CLIMATE WATCH - Potential Futures for the Earth's Climate

1) 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.

2) 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 storyline presumes 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.

The graph and descriptions come from

https://www.vox.com/22620706/climate-change-ipcc-report-2021-ssp-scenario-future-warming 

IPCC AR6 WG1 Headline Statements: 

https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Headline_Statements.pdf