Climate on Earth is not immutable. For over 2.5 million years, the global climate has been swinging between strangely protracted ice ages and brief interglacial periods. We’re now in an interglacial period, by the way. Even within the eyeblink-span of modern humankind’s puny existence, the climate has fluctuated violently. The last climatic spasm was the so-called Little Ice Age in the medieval period, and people died in droves of cold and famine. But that wasn’t an ice age; that was a climate blip possibly caused by volcanism.
At the peak of the real Ice Age – called the Last Glacial Maximum, around 20,000 years ago – ice sheets covered much of the northern hemisphere. We also know that the start of the glaciers’ retreat differed from place to place; it seems Siberia’s ice began to pull back about 17,500 years ago, but extreme cold persisted for thousands of years longer in Western Europe.
But taking the Last Glacial Maximum as a fixed point, how cold was it? Now a University of Arizona-led team has nailed the answer which had been flummoxing science all this time. So: about 20,000 years ago, when the Last Glacial Maximum was at its peak and thick-furred mega-fauna trekked the snowy lands, the average global temperature was 6 degrees Celsius cooler than now, estimates the team led by associate professor Jessica E. Tierney of the University of Arizona, Tucson, in a report in Nature on Wednesday.
Note that’s global average – some places were a lot colder and some nice and balmy during the Last Glacial Maximum. Unlike the enigmatic serial Snowball Earths (or Slushball Earths, as some insist) hundreds of millions of years ago, it isn’t that the whole planet was cased in ice during the recent ice ages. If it had been, we would have been extinct. During the last one, about half of North America, Europe and South America were covered in ice and parts of Asia.
“In North America and Europe, the most northern parts were covered in ice and were extremely cold,” Tierney said – but up in the Arctic, the cooling was much more intense: about 14 degrees Celsius (25 degrees Fahrenheit) colder than now. Apropos “now,” the pace of warming in the Arctic is at least double that of the rest of the world. Temperatures in northern Siberia have surpassed those in Tel Aviv on some days this summer.
The bottom line: the average global temperature 20,000 years ago was 8 degrees Celsius (46 degrees Fahrenheit), roughly, compared with 14 degrees Celsius today, the team estimates.
If you shrug at a six-degree difference, note that global warming from the start of the industrial period is about 1 degrees Celsius on average. And look what’s been happening: weather the world wide has gone mad, storms are more violent and unpredictable, and in several regions – including parts of the Middle East – the heat index (combined heat and humidity) is already becoming unsurvivable. Yes, it’s in small areas and not for long, but the area will broaden and the durations will lengthen – and air conditioners are not part of the human condition.
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Back in the Ice Age, not only was the planet colder to a significant degree. The atmosphere was 20 to 25 times dustier than today. The dust in the air, by the way, also affected global temperatures and had to be factored into the calculation – ultimately, to estimate the role greenhouse gases such as carbon dioxide and methane had to do with the climatic changes.
A revolutionary paper published in Geoscience in 2016 discussed the modulation and rhythm of ice ages and interglacials during the late Pleistocene, because standard theory about the Earth’s long-term cycles, such as the Milankovitch-precession theory (periodic changes in the angle of the Earth vis-à-vis the Sun), can’t explain why ice ages lasted so long.
Ice ages were protracted because of a feedback system involving carbon dioxide, dust and albedo – the reflection of solar radiation from the planet surface. During the glacial periods, the northern ice sheets powerfully reflected the Sun’s rays, driving down global temperatures and also atmospheric CO2 concentrations.
During the Last Glacial Maximum, atmospheric CO2 was about 180 parts per million (ppm). Over millennia, its concentration in the air and oceans would rebound anew, eventually reaching a critical threshold of about 200 ppm, which trapped solar radiation.
Meanwhile, sea levels were low because the ice locked up the water; that and the discouraged plant life resulted in erosion on the land.
And thus, dust would fill the skies, which could theoretically block sunlight like volcanic ash does – but it also landed on the great sheets of ice. A dirty glacier absorbs more radiation and reflects less. Ultimately, these phenomena combined with the Earth’s cycles would lead to massive ice melt and there you are, in another interglacial.
When carbon dioxide doubles
Atmospheric CO2 levels during the last Ice Age were about 180 ppm, which is very low. Before the Industrial Revolution, CO2 was at 280 ppm.
Where are we now? Our CO2 is in uncharted territory at more than 415 ppm and our air is filthy. The last remnants of the great Ice Age are melting fast – the Greenland ice sheet has been declared lost forever – and we’re absorbing solar radiation like never before.
Tierney and the team note that there were no thermometers in the Ice Age, so they developed models to translate data collected from ocean plankton fossils into sea-surface temperatures. They then combined the fossil data with climate model simulations of the Last Glacial Maximum using data assimilation, as is done in weather forecasting. We all know that weather forecasting is a dubious affair, but that’s in the span of hours and days. This is much broader in scope.
So: Tierney and the team project that for every doubling of atmospheric carbon, the average global temperature will increase by 3.4 degrees Celsius (6.1 degrees Fahrenheit) – about the middle of the range predicted by the latest generation of climate models (1.8 to 5.6 degrees Celsius). Where will this hit hardest? The Arctic.
“Climate models predict that the high latitudes will get warmer faster than low latitudes,” Tierney says: future climate projections are looking at a very warm Arctic, the reverse of what happens there in ice ages.
How likely are we to halt this global warming in its tracks before even more parts of the world become intolerably hot, even for short periods? “The Paris Agreement wanted to keep global warming to no larger than 1.5 degrees Celsius over preindustrial levels, but with carbon dioxide levels increasing the way they are, it would be extremely difficult to avoid more than 2 degrees Celsius of warming,” Tierney warns. We know the planet responds violently to increases in CO2, but we’re still adding to CO2 every minute of every day. The coronavirus lockdowns were a blip, not a trend change.
Her team’s next goal? To estimate how hot the Earth got during the interglacials and how it reacted to extreme CO2. We don’t actually know that yet.