This Is a New Climate State - A Look at When Earth Was Last This Hot

Thursday - August 10 2023, 04:11 UTC - 1 year ago

Earth has entered a new climate state not seen in more than 100,000 years due to an increase in global temperature over 1°C since pre-industrial times. Scientists study sediment-based records from the bottom of lakes and oceans to reconstruct past global temperatures, but they are limited due to currents and burrowing organisms. Earth's average global temperature has fluctuated between glacial and interglacial conditions in cycles lasting around 100,000 years.

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.

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 one degree Celsius (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.

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 complex astronomical processes.

During the interglacial period that started about 20,000 years ago and continues today, the average global temperature has not fluctuated much.