As scorching heat grips many parts of the Earth, many people are trying to put the extreme temperatures in context and ask: When was it hot before?
Globally, 2023 saw some of the hottest days by modern measurements, but what about further out, before weather stations and satellites?
Some news outlets are reporting that daily temperatures have hit a 100,000 year high.
As a paleoclimate scientist who studies the temperatures of the past, I can see where this claim is coming from, but I am shocked by the inaccurate headlines. Although this claim may be correct, there are no detailed temperature records lasting 100,000 years, so we are not sure.
Here’s what we can confidently say about when the Earth was last warm.
This is a new climate
Scientists have concluded a few years ago that the Earth has entered a new climate state that has not been seen for more than 100,000 years. As our climate scientist colleague Nick McKay recently mentioned in a scientific journal article, that conclusion is part of a climate assessment report published by the Intergovernmental Panel on Climate Change (IPCC) in 2021.
The Earth is now more than 1 degree Celsius (1.8 Fahrenheit) warmer than before industry, and the level of greenhouse gases in the atmosphere is high enough to ensure that temperatures will remain high for a long time.
Even under the most optimistic future scenarios—where people stop burning fossil fuels and reduce other greenhouse gas emissions—average global temperatures are likely to remain at least 1 C above pre-industrial temperatures, and possibly much higher, for centuries.
This recent climate situation, characterized by centuries of global warming levels of 1 C and higher, can be reliably compared to temperature changes from the very distant past.
How do we estimate past temperatures?
To reconstruct temperatures from times before thermometers, paleoclimate scientists rely on information stored in various natural archives.
The most extensive archives that go back many thousands of years are at the bottom of lakes and oceans, where various biological, chemical and physical evidences offer clues to the past. These materials build up over time and can be analyzed by taking a sediment core from the lake bed or sea floor.
These sediment-based records are a rich source of information that enables paleoclimate scientists to reconstruct past global temperatures, but they have important limitations.
For one, low currents and burrowing organisms can mix with the sediment, transmitting any short-term temperature rise. For another, the timeline for each record is not known precisely, so when multiple records are averaged to estimate past global temperatures, fine fluctuations can be canceled out.
Because of this, paleoclimate scientists are reluctant to compare the long-term record of past temperatures with short-term extremes.
Looking back ten thousand years
The world’s average temperature fluctuates between glacial and interglacial conditions in cycles lasting about 100,000 years, due to slow and predictable changes in the Earth’s orbit with changes in greenhouse gas concentrations in the atmosphere. We are now in an interglacial period that began about 12,000 years ago as the ice sheets retreated and greenhouse gases increased.
Looking at that 12,000-year interglacial period, global temperatures averaged over several centuries may have risen by about 6,000 years, but probably not more than the 1 C level of global warming at that point, according to the IPCC report. Another study found that average global temperatures continued to rise during the interglacial period. This is a subject of active research.
That means we have to look even further back to find a time that could have been as warm as today.
The last glacial period lasted nearly 100,000 years. There is no evidence that long-term global temperatures reached the preindustrial baseline at any time during that time.
If we look further back, to the last interglacial period, which lasted about 125,000 years ago, we find evidence of warmer temperatures. Evidence suggests that long-term average temperatures are unlikely to be more than 1.5 C (2.7 F) above preindustrial levels—no more than current global warming levels.
Now what?
Without rapid and sustained reductions in greenhouse gas emissions, Earth is currently on course to reach temperatures roughly 3 C (5.4 F) above preindustrial levels by the end of the century, and possibly somewhat higher.
At that point, we would have to look back millions of years to find climate conditions with very warm temperatures. That takes us back to the previous geologic era, the Pliocene, when Earth’s climate was a distant relative of one that sustained the rise of agriculture and civilization.
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Citation: Is it warmer now than at any other time in 100,000 years? (2023, July 24) retrieved 24 July 2023 from https://phys.org/news/2023-07-hotter-years.html
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