What Counts as Geologically Rapid Warming?

I frequently hear people downplay the claim that the 1.3°C warming above the 1850-1900 mean is geologically significant. I hear words like "slight" or "modest" or "insignificant" thrown around a lot. And certainly global temperatures have increased by a lot more than 1.3°C on geologic time scales. My first response is typically to point out that on geologic time scales, global temperatures do change a lot, but at much slower rates. Current warming rates are exceptional, even on geologic time scales. And I think this point can be relatively easily demonstrated with the evidence we have, even taking into consideration the fact that proxies do not preserve a high degree of temporal resolution, and actual warming rates may exceed what we can detect with proxy evidence. I think we need a two part response to this.

Defining Geologically "Rapid" Warming

Since terms like "rapid" are relative terms, I think we need to come up with a standard for what should count as "rapid" on geologic time scales. I think most everyone agrees that warming rates coming out of glacial maxima count as rapid, so I propose we use that as a definition of what counts as rapid. That is, at the very least, if global temperatures warm more rapidly than deglaciation warming rates, it's safe to say that warming rate counts as rapid. So we need a metric for warming rates of global temperatures coming out of glacial maxima. For this I'm going to use Friedrich et al 2016.[1]


The graph above shows a successful reconstruction of global temperatures for the last 800,000 years. The glacial cycles last about 100,000 years each (synced with Earth's eccentricity), but the magnitude of warming varies between 4°C and 7°C. The length time between glacial maxima and thermal maxima of warming is also variable, ranging between 10,000 and 20,000 years. Since we're interested in defining geologically "rapid" here, let's use 7°C for the amount of warming and 10,000 years for duration of warming. This implies a rate of 0.7°C/millennium or 0.007°C/decade. This would be on the high end of mean warming rates, but certainly there were time intervals in which warming rates that exceeded the mean warming rate, so let's increase this to 1°C/millennium or 0.01°C/decade. That is, a sustained increase of 0.01°C/decade should count as "geologically rapid," since we can agree that global warming rates coming out of glacial maxima are "rapid." A couple caveats are in order here. 
  1. Locally, sudden climate changes can and do happen that can produce much more rapid warming. But these events typically involve a sudden redistribution of heat around the globe, and rapid warming in one area is compensated by cooling in another. An example of this would be the Younger Dryas event, where local Greenland temperatures suddenly cooled by perhaps as much as 10°C on decadal time scales, but Antarctica warmed, and the global mean temperature change wasn't nearly that rapid. 
  2. Global temperatures fluctuate by much more than 0.01°C annually; annual variability can range by 0.25°C or more. But here we aren't interested in annual variability; even when stationarity can be assumed internal variability happens. We're interested in sustained changes in global temperatures on decadal time scales or longer.
When climate forcings cause a sustained change in global temperatures, geologically speaking, 1°C/millennium (0.01°C/decade) is geologically "rapid."
With this in mind, I decided to plot how HadCRUT5 global warming rates have changed through 2024. The graph below shows linear global warming rates beginning in 1850-2024 and ending in 2005-2024. That is, data points for each year show the linear warming trends beginning in that year and ending in 2024, or beginning with 175 years in 1850 and ending with 20 years in 2005. The slowest warming trend was 1850-2024 at 0.065°C/decade or 6.5x faster than "rapid" warming during deglaciation events. We crossed 0.2°C/decade in the 1960s, or 20x faster than deglaciation rates. And warming rates have only increased from there. So warming rates from 1850 have accelerated from 6.5x what counts as rapid to rates exceeding 20x what counts as geologically rapid.

This also undermines claims that warming in the UAH satellite dataset shows warming isn't actually rapid. Aside from the fact that UAH does not measure surface temperature, their data still shows warming at 0.15°C/decade since 1979. It turns out that's still 15x faster than what counts as geologically "rapid" warming. GMST warming rates exceeded 10x what counts as geologically rapid in 1900 and they have not looked back since.

How Exceptional is Recent Warming?

The follow up question we have to ask is, how much variability in warming rates do paleoclimate reconstructions miss because of the resolution of proxy evidence? Could it be that warming rates regularly match or exceed warming rates since 1850? In 2015, David Kemp and his colleagues published a paper[2] designed to answer that question. They point out that even the most rapid warming events show much less warming than current rates. Even the PETM, shows "a global surface ocean warming of ∼6 °C over 5–20 kyr has been linked to a pronounced perturbation of the global carbon cycle, and suggests a warming rate at least six times slower than modern." If recent warming currently exceeds 6x the most rapid rates detected in geologic history, then modern warming is not only exceptional, it's unprecedented. But on decadal to century time scales, warming rates may have exceeded the mean established from proxy evidence (and certainly did), but by how much? 

Kemp et al argue that, if we consider the maximum rates that likely occurred on decadal or centennial time scales given the resolution of proxy evidence, geologists have systematically underestimated these maximum warming rates in the geologic record. Rates of climate change detected from proxies exhibit scaling that can alias variability. Even during some of the most rapid warming events detected in geologic history (like the end-Permian event and the PETM), the maximum rate of warming from natural variability may exceed what has been detected by geologists. The maximum warming rates over short periods of time, during these the geologic events with the rapid warming may be closer to warming currently observed.
Taking into account timespan-dependent scaling, warming rates through intervals such as the Permian–Triassic boundary and the PETM likely exceeded current rates on decadal timescales, at least intermittently. Warming across the Permian–Triassic boundary stands out as the most significant temperature change of the past ∼0.5 billion years.
In other words, there are just a few events in geologic history where warming happened rapidly enough that, on decadal time scales undetectable by proxy evidence, warming occurred more rapidly than current warming. So it's true that proxy evidence can't preserve evidence of the maximum rates of climate change, and we can infer that warming on decadal time scales occurred more rapidly than proxies detect. However, the two candidates that likely exceeded current warming are found in exceedingly extreme events like the end-Permian extinction and PETM, which were associated with mass extinctions of nearly all life (end-Permian) and benthic foraminifera (PETM). So if current warming rates are not geologically unprecedented (yet), current warming is still exceptionally rapid geologically peaking and ranks among the most rapid warming events in the last half billion years, events also associated with mass extinctions. We also know that the warming during both of these extinction events were linked to long-term increases in GHG concentrations from persistent eruptions of large igneous provinces, which is not currently occurring. So we can rule out the possibility that recent warming has the same natural cause as the end Permian warming or the PETM. The cause of recent warming is due to human emissions of greenhouse gases.

But it's the rate as well as the amount of warming that is significant. We can consider the difference between the LGM and the HTM an "ice age unit" (IAU); the temperature change from a glacial maximum to thermal maximum in the Quaternary is ~6°C. We've seen ~1.33°C warming since 1850, and that's about 22% of an IAU. That's significant, but it's mostly significant because it only took 175 years see that amount of warming, rather than the ~1,300 years it takes a deglaciation event to accomplish the same thing.


References:

[1] Friedrich et al, "Nonlinear climate sensitivity and its implications for future greenhouse warming," Sci. Adv. 2.11 (2016): e1501923.
https://www.researchgate.net/publication/309791338_Nonlinear_climate_sensitivity_and_its_implications_for_future_greenhouse_warming

[2] Kemp, D., Eichenseer, K. & Kiessling, W. Maximum rates of climate change are systematically underestimated in the geological record. Nat Commun 6, 8890 (2015). https://doi.org/10.1038/ncomms9890

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