Wednesday, November 26, 2014

World at 2 degrees warming, 3 degrees, etc

(All temps in Celsius of course.)

Long ago Nordhaus set a plausible goal for the human forced temperature increase of 2 degrees C.

That become an internationally recognized, or at least mentioned, standard…the target.

Well, by now, the failure to ratchet up or even meet standards, 3 degrees would pretty much be on target if all countries actually met voluntary targets for themselves they have already set.  And they've blown past all previous proposed targets already, so there's good chance they won't keep remaining commitments either.

Well, this can't be so bad, how about 5 or 6 degrees which many climatologists believe is plausible over 100 years.

What these would mean to climate, looking back at earth history, is discussed here.

2 degrees, which goes back in the record to when CO2 was 360-400ppm, btw (from before the development of Agriculture to when the Industrial Revolution started, we were at 290; we're at 400 now, and that will climb another 100, 200, or 300 before 2100, if not more, and continuing up after that).  Like major deserts in the center of most continents, no annual ice in the northern hemisphere, sea level 25 meters higher.

That's 2 degrees, the target which we might meet if we stopped all CO2 production in 5 years…or maybe not.

3 degrees, 5 degrees, etc., sound like very different Planets.  It gets far worse rapidly at some points.  The level of change is not linear but more like exponential over all.  Plus there are "tipping points" which have automatic positive feedback.  If we reach a tipping point, which might be between 2-3 degrees, we cannot prevent another rise.  For example if we reach a point where the amazon rainforest is deforested, methane clathrates all bubble out, then microbes eat all the dead venation in the soil everywhere, we might not be able to prevent another 2-3 degree rise (or more, as tipping points lead to crossing other tipping points).

Back over at the "Planet saved due to US/China Agreement" blog at Crooked Timber, comment 29 features ZM commenting about the decreases in CO2 production which would have to occur to keep us to only 2 degrees.  Basically it would have to plummet radically, not the sort of weak tea people talk about.  We have to get down to 1GtC per year globally by 2050, from 36GtC per year now.  (Or we could just stop emitting altogether (net zero) in 5, or 10 years, depending on climate model assumptions, and I suspect all those models leave out the huge hard-to-model feedbacks like methane clathrates we may be facing.)  Here's what ZM says is the IPCC scenario for 198GtC total emissions by 2050:

2015-2016 = 36GtC p.a. = 36GtC over the period
2016-2020 = 15GtC p.a. = 60GtC over the period
2020-2030 = 5GtC p.a. = 50GtC over the period
2030-2040 = 3GtC p.a. = 30GtC over the period
2040-2045 = 2GtC p.a. = 10GtC over the period
2045-2049 = 1GtC p.a. = 4GtC over the period
2049-2050 = 1GtC p.a. = 1GtC over the period
That's pretty radical, all right.  We'd need to drop emissions more to less than 50% of current levels by 2020, for example.  And even that's putting off the heavy lifting as much as possible.

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