What happened last time it was as warm as it’s going to get later this century?

Posted on 28 June 2018 by howardlee

This is a re-post from Ars Technica

"What's past is prologue"- Shakespeare’s The Tempest.

The year 2100 stands like a line of checkered flags at the climate change finish line, as if all our goals expire then. But like the warning etched on a car mirror: it’s closer than it appears. Kids born today will be grandparents when most climate projections end.

And yet, the climate won’t stop changing in 2100. Even if we succeed in limiting warming this century to 2ºC, we’ll have CO2 at around 500 parts per million. That’s a level not seen on this planet since the Middle Miocene, 16 million years ago, when our ancestors were apes. Temperatures then were about 5 to 8ºC warmer not 2º, and sea levels were some 40 meters (130 feet) or more higher, not the 1.5 feet (half a meter) anticipated at the end of this century by the 2013 IPCC report.

Why is there a yawning gap between end-century projections and what happened in Earth’s past? Are past climates telling us we’re missing something?

Time

One big reason for the gap is simple: time.

Earth takes time to respond to changes in greenhouse gases. Some changes happen within years, while others take generations to reach a new equilibrium. Ice sheets melting, permafrost thawing, deep ocean warming, peat formation, and reorganizations of vegetation take centuries to millennia.

These slow responses are typically not included in climate models. That’s partly because of the computing time they would take to calculate, partly because we’re naturally focused on what we can expect over the next few decades, and partly because those processes are uncertain. And even though climate models have been successful at predicting climate change observed so far, uncertainties remain for even some fast responses, like clouds or the amplification of warming at the poles.

Earth’s past, on the other hand, shows us how its climate actually changed, integrating the full spectrum of our planet’s fast and slow responses. During past climate changes when Earth had ice sheets (like today) it typically warmed by around 5ºC to 6ºC for each doubling of CO2 levels, with the process taking about a millennium. That’s roughly double the “Equilibrium Climate Sensitivity” (ECS) values used in climate model projections for 2100, which are calculated mainly from historical observations.

“We do expect the Earth System Sensitivity (change CO2 and have all the systems react—including ice sheets, vegetation, methane, aerosols etc.) to be larger than ECS. Work we did on the Pliocenesuggested about 50 percent bigger, but it could be larger than that,” Gavin Schmidt, director of the NASA Goddard Institute for Space Studies in New York, told me.

Or, as Dana Royer of Wesleyan University put it, “In short, climate models tend to under-predict the magnitude of climate change relative to geologic evidence.”

Part of that greater magnitude is simply down to Earth’s slow responses, which produce a net warming. Even if greenhouse gas emissions were to cease completely tomorrow, sea levels are committed to keep rising for centuries from thermal expansion and melting glaciers; ice sheets in Antarctica and Greenland are also committed to keep melting from the heat already built into the climate over recent decades. And because CO2 lasts a long time in the atmosphere, in the absence of geoengineering to remove it, the world will overshoot any of our end-century temperature targets and stay elevated for centuries.

But those don’t explain the entire gap, which suggests we’re missing some other amplifying feedbacks. As the 2017 US National Climate Assessment put it: “model-data mismatch for past warm climates suggests that climate models are omitting at least one, and probably more, processes crucial to future warming, especially in polar regions.”

Can the Miocene tell our future?

The Mid-Miocene Climate Optimum (MMCO) was an ancient global warming episode when CO2levels surged from less than 400ppm to around 500ppm. (Ancient CO2 is measured in a variety of indirect ways like isotopes of boron or carbon in fossils and ancient soils, or from the pores on fossil leaves.) The cause of that surge was a rare volcanic phenomenon called a “Large Igneous Province” that erupted vast quantities of basalt in the Western USA 16.6 million years ago. Yvette Eley and Michael Hren of the University of Connecticut have been investigating how that changed the climate.

The tool? Fat molecules left in sediments by plants and microbes that lived at the time. Eley and Hren exhumed the chemical remains of microbes from Miocene muds in Maryland and then converted ratios of different fat molecules into soil temperature, using calibrations based on more than a decade of study of microbe fats in modern soils all over the planet. “Certainly, the timing of those flood basalts and the timing of when we see the shifts are pretty, pretty tight,” said Eley. “Our biomarkers definitely track what CO2 was doing. Whatever is happening in the terrestrial system in terms of what’s driving this event, it’s definitely following pCO2.”

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