Thanks to unbridled greenhouse gas emissions, our planet is stitching together a climate version of Dr. Frankenstein’s monster. We still have ice from the warmer parts of the Pleistocene even as our temperature approaches the warmer Pliocene levels of 3 million years ago. Meanwhile, our CO2 level is between the Pliocene and the Miocene of 10 million years ago, and we risk an Eocene hothouse not seen in 40 million years.
At some point, this unnatural fusion of incongruous climate parts must resolve into a new equilibrium—but at what point? And what does that equilibrium look like? Much of that is up to us, based on how fast we reach net-zero greenhouse gas emissions. But it’s also up to our planet—how “sensitive” it is to greenhouse gases and how quickly it reacts to changes.
Discovering our planet’s sensitivity to greenhouse gases has been a “holy grail” for scientists since the 1970s, but it has stubbornly resisted attempts to constrain it. The best we can do is a wide range: 1.5° to 4.5° C of warming if CO2 levels double. That’s a huge temperature range, and we’re likely to double preindustrial CO2 levels this century even as we want to avoid warming above 2° C. Narrowing this range will be key to understanding what our Frankenstein-like climate will look like when it settles into a new equilibrium.
We have left the Holocene behind
Part of the uncertainty is because our instrument records only capture a short bit of the cool climate we’ve left behind. By 2100, we’re on track for global temperatures between ~2.7° C and 3.6° C warmer than the preindustrial era. This is warmer than the entire Holocene, the geological period since the last glaciation in which human society with agriculture, cities, and industry flourished.
“We’re talking about a warming that is on par with the Pliocene,” Dr. Jessica Tierney of University of Arizona told me, “but at CO2 levels that… would take us back farther in time to… probably [the] Miocene warm period.”
In the Pliocene, our ancestors were Australopiths not yet fully committed to life out of trees. Most of the ice currently on Greenland and West Antarctica was not there, and sea levels were between five and 25 meters higher than in 1900. In the Miocene, our ancestors were still apes, sea levels were perhaps 48 meters higher, and parts of Antarctica were lushly forested.
“Ancient climates are our only context for what a warm world looks like,” said Tierney. “We ask our models to simulate it for us, but if we want to know what actually happens in a high-CO2 world, we have to look to the past for the examples.”
Dipping a toe in ancient seas
To do that, you first have to measure those ancient temperatures and CO2 levels.
Since there were no thermometers or infrared spectroscopes back then, scientists use indirect measurements, or “proxies,” items that change their chemical, isotopic, or physical makeup in proportion to changes in temperature, CO2 levels, or even rainfall. There’s a cornucopia of such proxies to choose from, ranging from the banal (various element and isotope ratios) to the bizarre (packrat urine, leaf wax, moth scales, and leaf pores).
Each proxy is calibrated to enable conversion into the relevant climate value, like temperature, CO2, rainfall regime, and so on. That calibration is often far from straightforward, so proxies were generally considered too uncertain and noisy to constrain climate sensitivity numbers much. But that has changed.
To give one example, temperature estimates for the Eocene used to vary wildly. It was clearly one of the hotter periods of the planet’s history, with conditions thought to match our extreme and unlikely worst-case, high-emissions future. Yet estimates for exactly how much hotter it was than the preindustrial era varied hugely, from 9° to 23° C warmer. Inconsistencies between proxies, methods, and timeframes all added to the noise.
Several projects set out to fix those issues. In one example, Dr. Gordon Inglis of the University of Southampton, with help from colleagues, carefully curated and analyzed a large collection of proxies with reasonable global coverage. “We have looked at multiple methods (for the first time) and applied this to the same data set (for the first time),” Inglis told me via email last year. “This enables an apples-for-apples comparison between methods.”
The team’s estimate of early Eocene temperatures suggests the era was 10° to 16° C warmer than the preindustrial era. That estimate is more robust and precise than before, enabling scientists to estimate a climate sensitivity for our time of 3.1° C of warming per CO2 doubling, validating the value in the latest IPCC report. But the team’s rigorous approach highlighted some problems. A marine temperature proxy based on microbe fat called “TEX86,” for example, tends to give warmer temperatures than other proxies, and some land-based proxies (leaf fossils, pollen, and chemicals called “branched GDGTs” from soil and peat bacteria) max out when their environmental temperature is around 25° to 30° C.
“If we exclude the terrestrial data … we get much higher temperature estimates,” Inglis explained.
Marine proxy data is often measured from tiny plankton corpses that are only .01 millimeter (.004 inch) wide. Oxygen isotopes in the plankton remains tell us the temperature of the seawater the plankton lived in, but those isotopes can change in their sedimentary tombs over the millions of years before they are sampled.
This problem afflicted early work that suggested past warm climates had tropics that were about as cool as today. It turned out that the isotopes of those tiny skeletons had been reset after death by cold groundwater, so now scientists use only pristine corpses that were quickly buried in clay, sealing out the water.
At other times, inconsistencies between different labs have swamped the climate signal, so scientists made standard reference chemicals to ensure that labs have a consistent baseline. This reduced uncertainty in ancient CO2 measurements based on Boron isotopes by an order of magnitude. Meanwhile, increasingly sensitive instruments have made it possible to get better results from smaller samples, opening the door to new, more robust techniques.
These efforts have earned ancient climates (“paleoclimates”) an equal footing with other lines of evidence as scientists seek to narrow climate uncertainty for our future. “These paleo time periods are potentially a very powerful tool to allow us to tune our models,” Professor Dan Lunt of the University of Bristol told me.