What Happened Last Time It Was as Warm as It’s Going to Get Later This Century?

Kids today will be grandparents when most climate projections end—does the past have more hints?

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.”

Read more at What Happened Last Time It Was as Warm as It’s Going to Get Later This Century?