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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?


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 COlasts 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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Comments 1 to 3:

  1. While many metres of sea level rise over millenia is very concerning, its rapid sea level rise that should be concerning us most. We are loading the dice to triggering this, due to the rapidity of emissions and the consequent rapid warming trend.

    Consider that Meltwater pulse 1a was more recent than the miocene, but lead to about 4 metres of sea level rise per century, ongoing for several centuries, and was consistent with 5 degrees of warming. About 2 metres of this has been related to destabilisation of the antractic, and we are currently seeing signs that the antarctic is destabilising. If this was to spread to the eastern antarctic this century, imho it could lead to 2 metres of sea level rise this century as a very distinct possibility. Our rapid CO2 emissions are making this a real possibility.

    J Hansen has raised the possibilty that sea level rise could be rapid, and even up to 5 metres per century. His scenario b temperature predictions have been vindicated by reality, so we better pay attention to his sea level rise views.

    Regarding the issue of the miocene being wetter and greener and climate models not predicting this. According to this article, large parts of the world were actually pretty arid during the miocene, with only some parts of the world wetter. The reasons for the aridity relates to mountain building and continental drift. I'm not sure why this account differs from the article, others may know? So climate models may not be so far off.

    In any event climate models would probably struggle to incorporate such complex geological changes, and this would explain any discrepancy if one exists.

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  2. >J Hansen has raised the possibilty that sea level rise could be rapid,

    He's not the only one, Harold Wanless does as well

    You know, 20 years ago I never thought I would end up seeing the rise because everything, all the projections at that time, really didn’t ramp up until well into the 21st century. But then I started going out to Cape Sable.” Cape Sable is the southernmost part of the mainland; it reaches into the Florida Bay like a swollen hook. “Out there the beaches were disappearing, mangroves were moving in, tiny channels turned into huge rivers in a matter of years. Even the roseate spoonbills started abandoning their nesting grounds. I had never, in my life of studying the geology of the coast of Florida, seen anything like it. That is when I knew in my gut that the early predictions were wrong and that sea level rise was unfolding a lot faster than any of us ever imagined.”


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  3. We should be going into the next glacial period of this 2.75m (so far) ice age but have put it off.  For the far future, it is sad that we are wasting our ability to put off another glacier advance so prfligately.  Our far distant decendants will need this carbon to hold off a slide into glaciation until the Milankovitch cycle puts us natually into the next interglacial.  Now we have no choice but to prepare by retreating from flood areas both by the sea and by rivers, working hard on developing new agriculture for areas that are continually warming and so forth.  We seem to do best under pressure and this will certainly test us out.  We would have a much better chance if vested interests didn't finance our politicians.  Who Pays the Piper Calls the Tune.

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