Study finds we’re already committed to more global warming—sort of

Someday, humans will get it together and dramatically reduce greenhouse gas emissions. But—not to fact check your daydreams too strictly here—how exactly will global temperatures respond to that day? This is a question climate science has long worked to answer, although devils in the details have led to some confusion.

A new study led by Nanjing University’s Chen Zhou tracks down another devil and puts it on display. Research has increasingly shown that it’s not just the planet’s average surface temperature that matters for tracking warming, but the spatial pattern of those temperatures. That can be important for calculating things like the climate’s sensitivity to greenhouse gases, but it hasn’t been accounted for in some methods of estimating how emissions cuts affect warming.

Seeing a pattern

This “pattern effect” of warming in different areas of the globe influences the way the planet sheds heat back to space. For example, if warming is a little stronger in the western equatorial Pacific Ocean—which it has been—that region is better at producing sunlight-reflecting cloud cover and releasing heat upward. If you assume the warming is occurring evenly around the world, you will miss that slightly offsetting behavior.

Building on previous work, the researchers calculated the influence of the pattern effect on today’s world by comparing historical observations to climate model simulations of a pre-Industrial-Revolution climate. They then tested the pattern effect numbers using satellite measurements of Earth’s overall energy balance for the last few decades. With no pattern effect, the estimated accumulation of energy in Earth’s climate runs a little higher than the satellite measurements. But mixing in their pattern effect numbers results in predictions that match the measurements nicely, including year-to-year wiggles.

What does that mean for a low-emissions future? One way to calculate this has been to use the observed human-caused strengthening of the greenhouse effect and past temperature change. Based on this calculation of the Earth’s climate sensitivity, you could then ask how much warming should occur once greenhouse gases stop increasing. Because the climate (primarily the oceans) cannot instantaneously equilibrate to a stronger greenhouse effect, temperatures take a while to fully catch up.

But where will it be once it does catch up? If the pattern effect has been dampening Earth’s past response, there could be more warming in the pipeline—and a warmer final outcome.

Fear of commitment

Calculations of the warming that we’re already committed to also depend critically on assumptions about what our future emissions will look like—a major source of confusion. The scenario used in this paper is one where we reduce emissions enough to simply maintain current greenhouse gas concentrations. They aren’t going up anymore, but they aren’t going down, either. In this simple scenario, the climate system gets the chance to catch up and reach a new equilibrium. However, this is not a zero emissions scenario, where we halt all emissions and greenhouse gas concentrations slowly begin to decline as the Earth soaks them up.

With that in mind, the results show that accounting for the pattern effect should increase committed warming. For concentrations stabilizing at 2020 levels, if we wait centuries for temperatures to equilibrate, total warming since pre-Industrial times grows from about 1.3°C to 2.3°C. (We have so far experienced about 1.1°C warming.)

An alternate version of this scenario allows short-lived gases and particulate matter to fade out; here, the ultimate warming grows from 1.6°C to 2.8°C. Restricting this very long-term view to just the year 2100, warming grows from 1.3°C to 1.8°C when accounting for the pattern effect.

The exact numbers aren’t really the point here—the researchers note that using a different dataset for past ocean temperatures causes the differences to shrink. It’s the general finding—the existence of the pattern effect implies more committed warming—that’s potentially important. It could mean that if you really want to permanently limit warming to a certain goal, like 1.5°C or 2°C, you need to err on the side of even lower emissions (or plan on actively removing more CO₂ later on).

But this shouldn’t trigger a fear that a lot more global warming is suddenly inevitable. A constant concentration future is different from a zero emissions future, and the final temperatures don’t arrive until after some pretty long time horizons. The study’s real contribution is addressing a shortcoming in some methods of calculating our committed warming. And we first have to show a commitment to halting warming if we want any of these scenarios—or better ones—to become futures we can choose between.

Nature Climate Change, 2020. DOI: 10.1038/s41558-020-00955-x (About DOIs).

https://arstechnica.com/?p=1733198