Our atmospheric CO2 has now passed the point which in the deep past meant the melting of the Antarctic ice sheets

Ann Arbor (Educated Commentary) – So, as a historian, I’m particularly interested in what the past tells us about the present. I taught courses on climate change in history. But of course, my kind of history does not go back very far from the point of view of physicists. The academic discipline of “history” is really the history of mankind since the invention of writing. Even for the Cambridge world history textbook I attended, I doubt we’ve cited a document that’s over 4,000 years old. Writing systems first appeared in what is now Iraq around 5,200 years ago. Excitingly enough, scientists piecing together the earth’s history before humans evolved have developed tools to do so that are increasingly accurate.

Given our current situation, a rapidly warming globe, we are particularly interested in examining past periods similar to ours. One way to do this is to find surrogates for the concentration of carbon dioxide in the atmosphere.

Another way to approach the problem is to find ways to know the average surface temperature of the Earth’s oceans at various times in the past. For example, as I explained last year, scientists “examined calcareous fossils (surviving shells of microorganisms that lived near the surface of the Pacific Ocean) to determine the average temperature at which single-celled organisms called foraminifera. You see, there is a relationship in their small shells of calcium to magnesium. The warmer the water they lived in, the more magnesium their shells absorbed.

There are also ways to find out how many parts per million of carbon dioxide there were in the atmosphere during past geological eras. CO2 increased significantly before 1750 due to volcanic activity, which at times could be intense over millions of years. There are various carbon sinks like oceans and igneous rocks that remove CO2 from the atmosphere over time, so if volcanoes take hold CO2 goes down. Since carbon dioxide is an efficient heat-trapping gas, if you know the parts per million of CO2 in the atmosphere, you can get a good idea of ​​how hot it was.

A team from Victoria University in Wellington, New Zealand, and the University of Birmingham in the UK, looked at the single-celled, bacteria-like archaea. The U Birmingham Science Bulletin explains: “Archaea adjust the composition of their outer membrane lipids in response to changes in sea temperature. By studying these changes, scientists can draw conclusions about the former sea temperature that allegedly circled a particular sample when it died.

The citation is Duncan, B., McKay, R., Levy, R. et al. Climatic and tectonic factors of late Oligocene Antarctic ice volume”. Nat. Geosci. (2022). https://doi.org/10.1038/s41561-022-01025-x

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

Here in Ann Arbor, I’m 840 feet above sea level. I looked for it. So we would be OK. But lower Manhattan is only 7-13 feet above sea level, so it won’t be there after a while. (Just to be clear, I really like Lower Manhattan and I say this with huge regret).

For my purposes, here’s the silver passage: “values ​​well below 400 ppm (e.g., ~280 ppm) are required for marine ice sheet advance on the mid-continental shelf of the Ross Sea , while above 400 ppm marine-ice-based ice is absent from West Antarctica and parts of East Antarctica.

What they are saying is that when the concentration of carbon dioxide is around 280 parts per million or less, the Antarctic ice sheet has advanced onto the continental shelf of the Ross Sea. When there were 400 parts per million or more of carbon dioxide in the atmosphere, the West Antarctic Ice Sheet and part of the East Antarctic Ice Sheet disappear from the records.

The ice sheets that extend into the ocean act as barriers preventing massive glaciers, some the size of Florida, from sliding into the ocean. So if the ice sheets are melting, as they do at 400 ppm CO2 or more, the glaciers are collapsing. Even one of them can raise the global sea level by 10 feet. If enough ice melts at the poles, sea level rise increases to 70 or even 150 feet (about 50 meters). A significant percentage of humans live within 60 miles of a coast, and such a rise in levels would put them at risk.

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

You will notice that 421 ppm of CO2 is higher than 400 ppm. This is in the range at which, in past eras, the West Antarctic Ice Shelf did not exist. If our WAIS ceases to exist, as it now seems highly likely, then the mega-glaciers will have no obstacles to plunge into the sea. And, it turns out, these suckers can move quickly when conditions are warm enough. .