How we estimate past temperature
To reconstruct temperatures from times before thermometers, paleoclimate scientists rely on information stored in a variety of natural archives.
The most widespread archive going back many thousands of years is at the bottom of lakes and oceans, where an assortment of biological, chemical and physical evidence offers clues to the past. These materials build up continuously over time and can be analyzed by extracting a sediment core from the lake bed or ocean floor.
These sediment-based records are rich sources of information that have enabled paleoclimate scientists to reconstruct past global temperatures, but they have important limitations.
For one, bottom currents and burrowing organisms can mix the sediment, blurring any short-term temperature spikes. For another, the timeline for each record is not known precisely, so when multiple records are averaged together to estimate past global temperature, fine-scale fluctuations can be canceled out.
Because of this, paleoclimate scientists are reluctant to compare the long-term record of past temperature with short-term extremes.
Looking back tens of thousands of years
Earth’s average global temperature has fluctuated between glacial and interglacial conditions in cycles lasting around 100,000 years, driven largely by slow and predictable changes in Earth’s orbit with attendant changes in greenhouse gas concentrations in the atmosphere. We are currently in an interglacial period that began around 12,000 years ago as ice sheets retreated and greenhouse gases rose.
Looking at that 12,000-year interglacial period, global temperature averaged over multiple centuries might have peaked roughly around 6,000 years ago, but probably did not exceed the 1 C global warming level at that point, according to the IPCC report. Another study found that global average temperatures continued to increase across the interglacial period. This is a topic of active research.
That means we have to look farther back to find a time that might have been as warm as today.
The last glacial episode lasted nearly 100,000 years. There is no evidence that long-term global temperatures reached the preindustrial baseline anytime during that period.
If we look even farther back, to the previous interglacial period, which peaked around 125,000 years ago, we do find evidence of warmer temperatures. The evidence suggests the long-term average temperature was probably no more than 1.5 C (2.7 F) above preindustrial levels – not much more than the current global warming level.
Without rapid and sustained reductions in greenhouse gas emissions, the Earth is currently on course to reach temperatures of roughly 3 C (5.4 F) above preindustrial levels by the end of the century, and possibly quite a bit higher.
At that point, we would need to look back millions of years to find a climate state with temperatures as hot. That would take us back to the previous geologic epoch, the Pliocene, when the Earth’s climate was a distant relative of the one that sustained the rise of agriculture and civilization.