by Eric Grimsrud
This last (possibly last) post by me extends the discussion of future effects of increased CO2 out into the next 8 millennia – roughly the same period of time over which man has developed advanced civilizations after the last glacial period which ended about 12 millennia ago.
The essence of these predicted effects are described by the Figure 1 shown below.
Figure 1. How cumulative carbon emissions are expected to set the climate thermostat for the next 8 millennia.
These predictions are based on recent calculations by Eby et at. reported in the AMS’s Journal of Climate (abstract can be seen here) using the UVIC coupled carbon/climate model. The numbers on each curve gives the total cumulative carbon emissions (in gigatons) over the time while human activities continue to emit carbon dioxide. The results described here tell us nothing that I have not emphasized before. I am sharing them with you here, because they more quantitatively and more clearly express our planet’s expected future after the Age of Fossil Fuels.
Note in Figure 1 that it matters little to long-range temperatures whether all the CO2 is emitted in one enormous burst at the very beginning of the fossil fuel era, or if it is instead spread out over several centuries. It is only the cumulative amount of CO2 emitted that matters.
A cumulative emission of one trillion tons of carbon just might keep the Earth below a warming of 2 degrees C, a commonly accepted goal in the fight against climate change (see Figure 12 of Post 7). The extent of warming at any point over this vast time scale increases linearly with cumulative emissions. Note also that the warming you get at the peak is pretty nearly the warming you are stuck with for the next millennium, with only slight declines after that. We are currently about half of the way towards our first trillion tons of accumulative carbon emissions. Given the nature of exponential growth, we will be getting to the one trillion ton point pretty quickly if nothing changes.
If you go beyond and add 2355 gigatons of carbon into the atmosphere before stopping all emissions from fossil fuels, then the global mean temperature will still be 3 degrees C warmer than pre-industrial in the year 8000. That would give ample time for all kinds of undesirable amplifying feedbacks to occur, including the deglaciation of Greenland, the loss of the West Antarctic Ice Sheet, and/or PETM-type carbon releases from the ocean bottoms via the melting of methane clathrates.
Note further that these calculations were done with a model designed to have a climate sensitivity similar to the IPCC median (of about 3 degrees C). Therefore, even if you hold the line at one trillion tons, there is still about a 50% chance that warming will be greater than 2 degrees C.
The predictions reported in Figure 1 are not the fantasies of a few kooks working in isolation. Note that its half dozen coauthors are accomplished scientists working in top universities of the US and Canada. Given the journal this article appeared in, it is sure to have been carefully reviewed by several experts in the field of climate change. It’s results have not been seriously challenged, to date, and its basic message is entirely consistent with the common sense expectations for prolonged presence of the EXTRA CO2 in the atmosphere, which I have discussed extensively on the web site.