What would the world be like with CO2 at 500 ppm?
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One way to get an answer, which I think makes the most sense, is to check out everything scientists have learned about Earth’s climate history. There is data available which is not not necessarily perfectly accurate, with wide variations according to who did the study, but we still want to look at it to have some references in mind. It is certain that Earth had CO2 readings of 500 and above in the past, and we want to know what things were like (mainly global temperature and sea level) at times when 500 prevailed, or any other high number for that matter. I have found some information that is clearly presented on charts and in my mind is probably the best available at this time.
1. CO2. This is the hardest trend to accurately pin down. What I am showing is a story that includes a chart based on data and charts provided in a 2013 study by Zhang et al that has been given a good reception. There is a link to the full study, which is free to read. The story posted below, originally posted by Skeptical Science in 2015, is easier to read and has a good deal of helpful background information added. The point to highlight is that you need to go back 25 million years to find CO2 readings rising above 500 in a regular way while most of the time between 5 and 25 million years they were stuck at levels close to 400, which is where we happen to be right now while heading briskly higher.
2. Temperature. I have pulled this chart from a 2013 study by Hansen et al, showing a reconstruction of average global surface temperature over 65 million years based on a widely accepted method that employs isotopes of oxygen. Today we would be spotted at 14.6C on this chart, and you can see the upside that leaves compared with the CO2 scores from earlier dates in the Zhang study.
3. Sea level. This chart is taken from the same Hansen study, which also has a full explanation of the methodology that was used for getting the numbers. If the data is correct it is not hard to picture a future rise in sea level to 15-20 meters based on just today’s CO2 score and then double that after reaching equilibrium at 500 ppm.
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Why the interest in 500 ppm? Because that is where we will be by the end of this century, possibly even sooner, and that is an optimistic number. How do we get a 100 point markup from where we stand today? About half will come from human activity, if humans can stay within their stated budget. The other half will come from soil carbon emissions, which are conveniently left out of just about all such budgets. Every scientist knows there will be emissions, substantial ones, but no one has the means of putting up a credible number, so the numbers we do get are all over the place. Still, it is not right to just enter a zero when that is a number known to be incorrect. So I am going to put in a positive number, 50 ppm, equal to just over 100 billion tons of carbon, to be emitted in this century. for you to shoot down. There is plenty more carbon lying in wait where that came from but this is enough to brings the combined total to 500. Oh, what about negative emissions? Many budgets plug in a real number for negative emissions, even though nobody has a realistic idea of how they will be accomplished. So why not soil carbon?
–Here is a man who knows a great deal about soil carbon from first hand experience, in a brief video:
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An essay from Stefan Rahmstorf about various quirks in the budget concept, with special emphasis on the recent well-publicized claim that there was some extra room in the 1.5C budget. Nothing at all is said about plugging in soil carbon, but I did notice that the person who wrote Comment #2 at the very end is also an advocate of accounting for natural feedbacks. Finally, the approach I took in the body of this letter gets rid of budget squabbling altogether. We are on the clock. The sleeping giants are now awake, and there is no place for a “safe” type of landing.
Carl