All of this data are fed into massive supercomputers that simulate how the climate system—the interaction between land, oceans, and atmosphere—behaves after we dump billions of tons of GHG into it.

What was missing from these climate models, however, was the biggest variable of all: our decisions about what kind of world we’re building.

Politics can profoundly affect how these models play out, since they largely dictate how much carbon dioxide and methane we pump into the atmosphere. Socioeconomic factors from childhood education to trade policies become an ever more important part of the climate equation over time.

Scientists made previous attempts to account for these influences. In the 1990s, the “SRES” scenarios considered factors like population and economic growth in climate projections. But the “Representative Concentration Pathways” (RCPs) adopted as the basis for later the IPCC’s Fifth Assessment Report in 2014 excluded those factors entirely.

In parallel, researchers began developing five “Shared Socioeconomic Pathways” to see how the world might evolve. By combining these with the RCP climate-only models, the broader scenarios could tell us just how the world might evolve under different political circumstances, and how hard it will be to achieve emission reductions under them. Scientists could then better understand the interplay between the climate system and humanity’s future development.

Those researchers began publishing their initial findings in 2016, after the publication of the IPCC’s Fifth Assessment Report, so their work was pushed into this latest report. While scientists have been running and refining these models for years, they’re now part of the official narrative around our climate future.

From The Green Road to The Highway

Years of supercomputer time have now been devoted to giving us the ability to test very different assumptions about population, technology, economics, and politics that lead to very different worlds. Five baseline worlds laid out by the Shared Socioeconomic Pathway reveal “alternative pathways for future society,” says Hausfather.”We were missing some of the arrows in our quiver the last time around.”

This time the Shared Socioeconomic Pathway models illustrate, in a future without climate policy, just how different those worlds can be. The scenarios, as summarized from the scientific literature, cover five baselines offering alternative pathways for humanity.

The differences are stark. Under the green road (SSP1), emissions may peak as early as 2040—or not at all under SSP5, where temperatures soar well past the 2ºC increase that scientists say are likely to acidify oceans, intensify storms and coastal flooding, and fuel roasting heat waves and droughts. You can see how emissions and global mean temperature fare under different scenarios below in the absence of climate action.

A diverse range of “what if” scenarios

Of course, all of these outcomes are approximations at best. And some of the underlying assumptions are tenuous (SSP3 asserts that most world leaders simultaneously share nationalist governing philosophies) if not altogether implausible (for instance, the assumption that solar power will be more expensive in 2050 than today). But the scenarios aren’t meant to be precise mirrors of future reality. They’re designed to explore a diverse range of “what if” scenarios revealing just how hard it will be to tackle climate change under different conditions. Our future will be a fluid mix of these scenarios over time.

“These are very idealized pathways,” says climatologist Glen Peters, a senior researcher at the Center for International Climate Research in Norway. “In reality, we have a world that is amidst all these different approaches. Our world is a unique world.”

So while none of these scenarios describes our world perfectly, we seem to be moving between them regularly. The “Trump World” of SSP3 might have seemed plausible just a few years ago. But Trump has since lost re-election, Europe’s right-wing swing is stalled, and Brazil’s far-right president, Jair Bolsonaro, is battling for political survival.

Indeed, there are already signs that political momentum might shift behind more ambitious climate targets. “In climate change, there aren’t too many silver linings,” says Hausfather, “but we are finally starting to see commitments from countries that are mainly in line with what’s needed.”

Today, a concerted climate effort, or a breakthrough technology such as cheap energy storage, could change baseline scenarios in a matter of years.

A multiverse of potential realities

Policymakers now face a multiverse of possible worlds developed by climate modelers. And their decisions, in part, will decide which scenario plays out. The good news, says Peters, is that the chances for the worst-case scenario, a catastrophic world with a global mean temperature rise above 5ºC, now seem lower. But then so do the odds of a base-case 1.5ºC world where emissions peak and then drop to zero by around 2050.

In this week’s IPCC report, researchers say we’re now “very likely” to exceed 1.°C to 1.8°C of warming above pre-industrial levels by 2100, even in a very low GHG emissions scenario (SSP1-1.9). This suggests a “middle of the road” scenario best describes our emissions trajectory. “We’re not following a 1.5ºC or a 5ºC emissions trajectory,” says Peters. “We’re going somewhere in between.” Hausfather agrees. The most likely emissions scenario, he estimates, puts us on track for about 3ºC of warming by the end of the century, to an average temperature the Earth hasn’t experienced for more than 3 million years. That would severely test many countries’ ability to adapt, and for the impacts for some, such as islands nations, would be catastrophic.

And after that? We don’t know. The IPCC’s models only extend to about the end of the century. In the next IPCC report, researchers can be expected to start looking out toward 2150 and beyond, well within the lifetime for the children of someone born today. ”The world doesn’t end in 2100,” says Hausfather, “even though our models do.”