Worst climate scenario may not be most likely

The year is 2100, and the world’s 12 billion people are still burning fossil fuels with abandon. Compared to preindustrial times, the planet has warmed by 4.5 degrees. The atmosphere is filled with greenhouse gases, and parts of the Earth now sometimes experience temperatures too extreme for humans.

This bleak vision of the future has long played a significant role in scientific assessments of global warming. Sometimes called the “business as usual” scenario, it represents a worst case where countries continue to burn oil, gas and coal unabated — in contrast with a world where emissions have been dramatically reduced and global warming is more moderate.

But now, some climate scientists and energy experts say the worst-case scenario is increasingly unlikely. That’s stirred debate within the research community over whether a rare bit of good news about global warming has emerged, or if, instead, the situation is far more complicated and still quite dire.

The stakes are high, since that drastic scenario — technically dubbed “RCP 8.5” — could be used by policy makers when deciding what to do now to mitigate global warming.

In a commentary published in the journal Nature on Wednesday, Zeke Hausfather of the Breakthrough Institute and Glen Peters, an energy expert at the Norwegian science organization CICERO, argue that the scenario ought to be discarded.

“Happily — and that’s a word we climatologists rarely get to use — the world imagined in RCP 8.5 is one that, in our view, becomes increasingly implausible with every passing year,” they write. Hausfather and Peters argue that a total warming of around 5.4 degrees Fahrenheit is now where the world is likely headed. That’s severe — it would be three times the amount of change that the world has seen so far — but appreciably different from 4 to 5 degrees.

One of the most distinctive — and, some critics say, objectionable — aspects of the RCP 8.5 scenario is that it assumes heavy use of coal, the most carbon intensive fossil fuel when burned. Coal use goes up by a factor of five in the scenario, write Hausfather and Peters. But today, coal use is declining in many countries, including the U.S., being supplanted by renewable energy and natural gas.

Hausfather and Peters are not the first critics.

The levels of coal use in the high end warming scenario were always implausible given the world’s coal reserves, argues Hadi Dowlatabadi, a climate and energy expert at the University of British Columbia who has published critiques of the so-called “return to coal” hypothesis.

“The coal resource that underlies the assumptions . ... doesn’t seem to be there,” Dowlatabadi said. Yet scientists use the high warming scenario regularly in climate change studies, and especially climate model simulations, where one important goal is to push the Earth’s system to an extreme in order to see what happens. It is commonly referred to as a “baseline” or “business as usual” scenario, since it assumes nothing is done to mitigate climate change. This has given it a major prominence and has often led it to being framed as the default.

In some cases it has provided ammunition to those critical of assumptions used in climate modeling. For example, the White House dismissed a 2018 climate report produced by 13 federal agencies and outside experts in part because it included the scenario.