Green Daily: The Making of Worst-Case Climate Scenarios
In climate news today...
(Bloomberg) -- It’s time to say goodbye to a worst-case scenario that has haunted climate research for years. Cheap renewable energy and natural gas have helped speed the worldwide shift away from coal. We’ve slowed the rate of growth in carbon-dioxide emissions. That means a nightmare scenario that assumed humanity would burn virtually all available coal—warming the planet by as much as 5° Celsius this century—can be relegated to the dustbin.
A commentary published last week in Nature by Zeke Hausfather and Glen Peters calls for accepting the good news about the improbability of a coal-forever future, known to climate wonks as RCP 8.5. Actual greenhouse-gas pollution trends are now tracking far below it.
Hausfather and Peters now see the world headed towards 3°C warming by 2100, which they describe as “a catastrophic outcome” even if it’s an improvement over yesterday’s worst-case scenario. “We cannot settle for 3°C; nor should we dismiss progress.”
This all raises a big question about how scientists go about debating worst-case scenarios. If we shouldn’t continue worrying about a future that no realistic amount of coal-burning can bring about, where can we find a more useful nightmare?
One of the tools used by scientists for this purpose is climate modeling. There are dozens of software simulations maintained by different research groups that seek to recreate the entire planet. This allows for supercomputers to forecast the speed and severity of global warming under different scenarios, without waiting for the planet to fall apart.
Climate models tend to use a pretty stable range of assumptions about future emissions. Hausfather and Peters’ commentary is about which ones best describe the real-world trend. A much bigger question is the ongoing impact of changes caused by warming itself: permafrost melt, wildfires, changes to cloud cover, the release of frozen deep-sea methane. Update the science about how these feedbacks will interact and further accelerate warming, and headaches really start.
Recent updates to several climate models have been turning up surprising—and scary—new results. Models that previously agreed more or less with a 3°C long-term outlook have started putting out climate-sensitivity results higher than 5°C. It’s an emerging worst-case scenario that scientists are still unpacking.
I have a long story today about the debates raging among climate modelers over whether to trust these grim new results. The scientists who make the models are deeply unsure themselves, and there’s not likely to be consensus for months. If the results hold up, which they may not, they suggest that Earth is much, much more sensitive to CO₂ than previously thought. That would give us even less time to beat down emissions.
This is why we need to mind our worst-case scenarios. The goal of limiting warming to 1.5°C established by the 2015 Paris Agreement—and, in turn, all government and private-sector policy targets—was informed in part by the forecasts made by models. The world has already warmed by 1°C, and no one wants to learn the hard way how bad things can get.
I asked several climate researchers what political and business leaders should do if the world’s actual CO₂ trajectory has shifted away from the worst-case RCP 8.5 scenario. They advised against fine-tuning decisions just because the climate outlook may have fallen from the worst case to merely catastrophic.
“The world has started bending the curve” away from the burn-all-the-coal scenario, Hausfather told me. Climate feedbacks could still produce 4°C or more of warming if emissions stay near current levels throughout the century. A better match between the CO₂ trend and scenarios doesn’t have “much relevance to policymakers designing mitigation strategies,“ agreed Noah Kaufman, research scholar at Columbia’s Center on Global Energy Policy. After all, he said, nothing has changed “about the risks of higher warming.“
Andrew Dessler, professor of atmospheric sciences at Texas A&M University, told me that discarding too-dark scenarios can help make previously unattainable goals seem more possible: “I do now believe—and I didn’t six months ago—that two degrees is achievable with maximal effort.”
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