Although the last ice age was much colder than today, with dramatically lower sea levels, it still included some major shifts – one of which has only just been revealed. From 21,900 to 20,500 years ago, sea levels fell by 20 meters (66 feet), before recovering much more slowly. The discovery not only confirms how big the gaps in our knowledge are, but that the ice age climate was much more sensitive to small shifts than today.

We know the world was bitterly cold 30,000-20,000 years ago, even by ice age standards. Tens of millions of cubic kilometers of water was locked up in ice sheets in what’s known as the Last Glacial Maximum. However, it has proven surprisingly difficult to get finer resolution on this. In many locations, the land has shifted with time, either because of tectonic movements or because the weight of nearby ice sheets pushed the ground down. Separating true changes in sea level from local land effects is challenging.

This makes 900 ancient coral samples from two sites off what is now the Great Barrier Reef exceptionally valuable. Even during the coldest times, northern Australia wasn’t glaciated and is very tectonically stable. Dating of corals drilled at these locations has provided an unprecedentedly accurate record of Ice Age sea levels. The results, published in Nature, include the discovery that over 1,400 years, sea levels fell to around 125 meters (410 feet) below current levels. They then came back at a rate of 1.6 millimeters (0.06 inches) per year.

The findings were particularly surprising because more scattered and less reliable data from other locations had led paleoclimatologists to think sea levels were rising modestly during the period of greatest fall.

We don’t yet know what caused such a dramatic shift, nor the cause of a previously identified and even larger drop 31,000 years ago. Co-author Dr Tezer Esat of the Australian National University described it as “enigmatic and frustrating”. He also noted that the previous ice age was marked by unexplained periods of climatic change known as Heinrich events

Where exactly all this water went remains a mystery. “My gut feeling was that it went mostly to Antarctica,” Esat told IFLScience. “But gut feelings do not count as evidence: It turns out this is not possible.”

Instead, it seems there was some build-up of ice in both Antarctica and North America, with apparently little change on what was then the Eurasian Ice Sheet. It’s not clear whether the falling sea levels reflected changes in global average temperatures or a shift in winds driving moist air over the ice sheets where it fell as snow.

Inevitably, non-scientists’ responses to this discovery will include claims that, if such wild swings can happen naturally, current increases in temperature and sea level aren’t human induced and are nothing to worry about anyway. A quick check proves how wrong this is. Already sea levels are rising twice as fast as this newly discovered fall, and this is anticipated to accelerate dramatically. Even the fastest natural climate changes are slow compared to what humans are causing.

Esat confirmed to IFLScience that the findings are a sign: “Climate sensitivity was heightened during the ice age as opposed to during an interglacial,” so it took smaller shifts in atmospheric conditions to produce radical changes to the planet.

If industrial-scale burning of fossil fuels had happened at a time when the world was still in an ice age, the effects on global climate would have been much swifter, allowing people and other living things even less time to adapt to the sudden changes.

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