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Sea level rise and 550 ppm CO2

Yesterday, while COP-15 seemed to be spinning off the tracks, I spent the day at the American Geophysical Union’s fall meeting. Don’t let the title mislead: this is likely the largest meeting of climate scientists each year. The meeting is truly enormous – this year more than 15,000 are attending. As a former member of the climate science community, I always try to go for a few of the days, to see old friends and learn about the latest science.

One study discussed yesterday in the aisles of the poster sessions struck me as especially relevant given the brouhaha over a leaked UN report showing that even if there is agreement in Copenhagen, pCO2 levels in the atmosphere will still top 550 ppm. The report argues that this concentration will lead to a 3°C rise in global average temperatures – much more than the 1.5 or 2° that many would like to achieve.

Back to the science: the last time that scientists have fairly solid evidence that the Earth was this warm was about 3 million years ago, during a period referred to as the Mid-Pliocene climate optimum. One longstanding question about this interval has been, how high was the concentration of CO2 in the atmosphere. The best guesses have been somewhere between 400 and 600 ppm. But these estimates have been derived from single sites using proxies for CO2 concentration of very rough accuracy and precision. So uncertainty has remained about the relationship between pCO2 and temperature during this interval.

Yesterday at the meeting, I had the chance to talk to Mark Pagani of Yale and Christina Ravelo of UC Santa Cruz (full disclosure – Christina was my doctoral advisor) who have new work, to appear shortly in Nature Geosciences, that will apparently show, using more accurate and precise proxies, and a wide variety of sites distributed all over the globe, that pCO2 concentrations were approximately 400 ppm during this interval – the low end of previous estimates. This is of not that different than they are likely to be around 2020 (current levels are in the mid-380’s).

Why is this important? One reason is that associated with the Pliocene warmth was a pronounced increase in global sea levels – of between 5 and 30 meters. A warming of 3°C was apparently sufficient to melt most of the grounded ice on Greenland and a big piece of the West Antarctic Ice Sheet. The high end estimate requires substantial reduction in the size of the East Antarctic Ice Sheet as well. But let’s just take the low-end estimate. A 5 meter or 15 foot rise in sea level would fundamentally alter the distribution of human societies on the globe. It would also profoundly disrupting coastal ecosystems, both terrestrial and marine.

If it were to happen, this meltback of the world’s major ice sheets wouldn’t occur in lockstep with the temperature changes – ice sheets have enormous thermal inertia that slows their response to higher temperature. But it will likely happen. What the new work probably shows is that we, or more precisely our grand- and great grandchildren, are already committed to this level of disruption. 5 meters if we are likely – more if we aren’t. Sobering stuff as the other big meeting on climate change, happening in Denmark, may produce an insubstantial outcome.

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