As scorching heat grips large swaths of the Earth, a lot of people are trying to put the extreme temperatures into context and asking: When was it ever this hot before?
Globally, 2023 has seen some of the hottest days in modern measurements, but what about farther back, before weather stations and satellites?
Some news outlets have reported that daily temperatures hit a 100,000-year high.
As a paleoclimate scientist who studies temperatures of the past, I see where this claim comes from, but I cringe at the inexact headlines. While this claim may well be correct, there are no detailed temperature records extending back 100,000 years, so we don’t know for sure.
Here’s what we can confidently say about when Earth was last this hot.
This is a new climate state
Scientists concluded a few years ago that Earth had entered a new climate state not seen in more than 100,000 years. As fellow climate scientist Nick McKay and I recently discussed in a scientific journal article, that conclusion was part of a climate assessment report published by the Intergovernmental Panel on Climate Change (IPCC) in 2021.
Earth was already more than 1 degree Celsius (1.8 Fahrenheit) warmer than preindustrial times, and the levels of greenhouse gases in the atmosphere were high enough to assure temperatures would stay elevated for a long time.
Earth’s average temperature has exceeded 1 degree Celsius (1.8 F) above the preindustrial baseline. This new climate state will very likely persist for centuries as the warmest period in more than 100,000 years. The chart shows different reconstructions of temperature over time, with measured temperatures since 1850 and a projection to 2300 based on an intermediate emissions scenario.
D.S. Kaufman and N.P. McKay, 2022, and published datasets, Author provided
Even under the most optimistic scenarios of the future – in which humans stop burning fossil fuels and reduce other greenhouse gas emissions – average global temperature will very likely remain at least 1 C above preindustrial temperatures, and possibly much higher, for multiple centuries.
This new climate state, characterized by a multi-century global warming level of 1 C and higher, can be reliably compared with temperature reconstructions from the very distant past.
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.
