Deep Time Perspectives on Climate Change

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Deep Time Perspectives on Climate Change: How to Use the Paleoclimatic Record to Find Analogues and Solutions

In April of 2018, the concentration of carbon dioxide in the Earth’s atmosphere reached 410 ppm. This is the highest in over 3 million years, and a 45% increase over the 280 ppm recent average, which held from the end of the last major glaciation until the start of the industrial revolution. The last time CO2 levels were this high, in the middle Pliocene epoch, the average global temperature was 5C greater than today, and sea levels were 25 meters higher. In the early Miocene, 20 mya, atmospheric CO2 again exceeded 400 ppm, at which time sea level exceeded today’s by nearly 100 meters.

A temperature rise of 4-5C is on the high end of estimates of warming by the end of the century under a “business as usual” scenario, where we are for whatever reason unable to take meaningful action to address CO2 buildup. Atmospheric CO2 concentrations in these models range between 600 and 800 ppm by 2100. In our most optimistic projections, emissions quickly peak and begin to decline within the next 20 years. This results in concentrations remaining steady at roughly 400 ppm throughout the 21st century, giving a projected temperature increase of 2℃. The Paris Agreement considers this to represent a limiting value before truly catastrophic consequences take hold (although the Intergovernmental Panel on Climate Change has since revised this downward to 1.5℃). But this rise is less than half of that observed in the paleoclimatic record for the CO2 concentration already in the atmosphere.

Most models assume that, under a best case scenario, we will avoid this degree of warming due to a steady decline in atmospheric CO2 after 2100, due to a residence time of CO2 in the atmosphere of roughly a century.

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