Antarctic Ice Shelf

Iceberg A-74 calved from Antarctica's Brunt Ice Shelf in February 2021. (Photo: Gallo Images/Orbital Horizon/Copernicus Sentinel Data 2021)

The Terrifying Future in Which We Return to a Past Too Warm for Antarctica's Ice Shelves

There is bad news for the planet when we consider what it means to have atmospheric carbon levels of 421 ppm.

So as a historian, I am particularly interested in what the past tells us about the present. I've taught courses on climate change in history. But of course my kind of history doesn't go back very far from the point of view of physical scientists. The academic discipline of "history" is really the history of humanity since the invention of writing. Even for the world history textbook at Cambridge in which I was involved I doubt we cited any document older than 4,000 years. Writing systems emerged in what is now Iraq around 5,200 years ago. Excitingly enough, scientists reconstructing the history of the earth before humans evolved have developed tools to do so that are increasingly precise.

Given our current predicament, of a rapidly heating globe, we are especially interested in looking at past periods similar to our own. One way to do so is to find proxies for the concentration of carbon dioxide in the atmosphere.

Another way to approach the problem is to find ways of knowing the average surface temperature of the earth's oceans at various points in the past. For instance, as I explained last year, scientists have "examined calcareous fossils (surviving shells of microorganisms that lived close to the surface of the Pacific Ocean) to determine the average temperature in which the single-cell organisms called foraminifers, lived. You see, there is a ratio in their little shells of calcium to magnesium. The hotter the water in which they lived, the more magnesium their shells absorbed."

There are also ways of knowing how many parts per million of carbon dioxide there were in the atmosphere during past geologic eras. CO2 largely increased before 1750 because of volcanic activity, which could in some eras be intense over millions of years. There are various carbon sinks like the oceans and igneous rocks that scrub the CO2 out of the atmosphere over time, so if the volcanoes settle down, the CO2 decreases. Since carbon dioxide is an efficient heat-trapping gas, if you know the parts per million of CO2 in the atmosphere you can have a fair idea of how hot it was.

A team from Victoria University in Wellington, NZ and Birmingham University in the UK looked at the single-celled, bacteria-like archea. U Birmingham's science newsletter explains, "The archaea adjust the composition of their outer membrane lipids in response to changing sea temperatures. By studying these changes, scientists can draw conclusions about the ancient sea temperature which would have surrounded a particular sample as it died."

The cite is Duncan, B., McKay, R., Levy, R. et al. Climatic and tectonic drivers of late Oligocene Antarctic ice volume,". Nat. Geosci. (2022).

So this ambitious study was able to use the archea molecular fossils to examine changes in surface sea temperatures over the past 45 million years, and to match them up with what ice cores tell us about the extent of glaciation at the poles. They found that with one minor exception, there was an exact line-up between hot surface temperatures in the oceans and glacial minimums, periods when the ice at the poles melted, causing as much as 150 feet of sea level rise.

Here in Ann Arbor, I am 840 feet above sea level. I looked it up. So we'd be OK. But lower Manhattan is only 7-13 feet above sea level, so it just isn't going to be there after a while. (Just to be clear, I really like lower Manhattan and say this with enormous regret).

For my purposes, here is the money passage: "values much lower than 400 ppm (for example, ~280 ppm) are required for marine ice-sheet advance onto the mid-continental shelf of the Ross Sea, while above 400 ppm marine-based ice is absent from West Antarctica and sectors of East Antarctica."

What they are saying is that when the concentration of carbon dioxide is on the order of 280 parts per million or less, the Antarctic ice sheet advanced into the Ross Sea's continental shelf. When there were 400 parts per million or more of carbon dioxide in the atmosphere, the West Antarctic ice sheet and some of that of East Antarctica disappears from the record.

The ice sheets that extend into the ocean act as barriers keeping massive glaciers, some the size of Florida, from slipping into the ocean. So if the ice sheets melt, as they do at 400 ppm of CO2 or more, the glaciers plop in. Even one of them can raise global sea levels 10 feet. If enough ice melts at the poles, sea level rise goes to 70 or even 150 feet (roughly 50 meters). A significant percentage of human beings live within 60 miles of a sea-coast, and such a rise in levels would endanger them.

So here is the bad news. In June, NOAA announced that parts per million of carbon dioxide in our atmosphere had hit a new high, of 421 ppm.

You will note that 421 ppm of CO2 is higher than 400 ppm. It is in the range at which, in past epochs, the West Antarctic Ice Shelf did not exist. If our WAIS stops existing, as now seems highly likely, then the mega-glaciers will have no obstacle to diving into the sea. And, it turns out that those suckers can move fast when the conditions are warm enough.

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© 2023 Juan Cole