As scorching heat grips large swathes of the Earth, many people try to put extreme temperatures in context and ask: When was it ever hot?
Globally, 2023 saw some of the hottest days in recent measurements, but what about further afield, ahead of weather stations and satellites?
Some news outlets are reporting that daily temperatures are at their highest level in 100,000 years.
As a paleoclimatologist who studies past temperatures, I see where this claim comes from, but I cringe at the inaccurate headlines. While this claim may be true, there are no detailed records of temperature extending back 100,000 years ago, so we don’t know for sure.
Here’s what we can say with confidence when the Earth was so hot.
This is a new weather condition
Scientists concluded a few years ago that the Earth has entered a new climate state not seen in more than 100,000 years. As fellow climate scientist Nick McKay and I discussed recently in a science journal article, this conclusion was part of the Climate Assessment Report published by the Intergovernmental Panel on Climate Change (IPCC) in 2021.
The Earth was already about 1°C (1.8°F) warmer than in pre-industrial times, and levels of greenhouse gases in the atmosphere were high enough to ensure that temperatures would remain high for an extended period.
Even under the most optimistic future scenario – where humans stop burning fossil fuels and reduce emissions of other greenhouse gases – the global average temperature is very likely to remain 1°C above pre-industrial temperatures, and possibly much higher, for centuries.
This new climate state, characterized by a level of global warming for centuries of 1°C and above, can be reliably compared with temperature reconstructions from the very distant past.
How to estimate past temperature
To reconstruct temperatures from times before thermometers, paleoclimatologists draw on information stored in a variety of natural archives.
The most extensive archives dating back thousands of years are at the bottom of lakes and oceans, where a variety of biological, chemical and physical clues provide clues to the past. This material accumulates continuously over time and can be analyzed by extracting sediment cores from the bottom of the lake or ocean floor.
These sediment-based records are rich sources of information that have enabled paleoclimatologists to reconstruct past global temperatures, but they have important limitations.
For example, bottom currents and burrowing organisms can mix sediments, obliterating any short-term warming. On the other hand, the timescale for each record is not precisely known, so when multiple records are averaged together to estimate the previous global temperature, fine scale fluctuations can be canceled out.
For this reason, paleoclimatologists 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 temperature has fluctuated between glacial and interglacial conditions in cycles lasting about 100,000 years, driven largely by slow and predictable changes in Earth’s orbit with concomitant changes in greenhouse gas concentrations in the atmosphere. We are currently in an interglacial period that began about 12,000 years ago in which ice sheets retreated and greenhouse gases rose.
Given that interglacial period of 12,000 years, the average global temperature over several centuries may have peaked around 6,000 years ago, but the level of warming may not have exceeded 1°C at that point, according to the IPCC report. Another study found that average global temperatures continued to increase across the interglacial period. This is an active research topic.
This means we have to look further back to find a time that might have been as warm as today.
The last glacial episode lasted for nearly 100,000 years. There is no evidence that long-term global temperatures reached a pre-industrial baseline at any time during that period.
Looking further afield, to the earlier interglacial period, which peaked around 125,000 years ago, we find evidence of warming. Evidence suggests that the long-term average temperature may have been no more than 1.5°C (2.7°F) above pre-industrial levels – not much more than the current level of global warming.
Without rapid and sustained reductions in greenhouse gas emissions, the Earth is currently on track to reach temperatures around 3 degrees Celsius (5.4 Fahrenheit) above pre-industrial levels by the end of the century, and possibly a little higher.
At this point, we would need to look back millions of years to find a climate condition in which temperatures are high. This will take us back to the previous geological era, the Pliocene, when the Earth’s climate was close to the climate that continued the rise of agriculture and civilization.
Darrell Kaufman is Professor of Earth and Environmental Sciences at Northern Arizona University.
This article is republished from The Conversation under a Creative Commons license. Read the original article.