Guesstimating and extrapolating the new “underlying rate of rise” farther out into the future at a conservative 2.5 ppm/year to a still realistic 3.3 ppm/year, we reach 420 ppm in 2020 or 2021. Why is this significant? It marks the point at which atmospheric carbon reaches 50% above its pre-industrial value of about 280 ppm. Most estimates of the warming effect of atmospheric carbon base themselves on a doubling, with somewhere between 2 and 2.8 degrees C warming as a direct effect of the carbon, and between 4 and 5 degrees C (delayed) due to related increases in other GHGs (greenhouse gases) and additional warming due to changes in the Earth’s albedo. The best estimates are based on previous episodes of large global warming 55 and 250 million years ago, which according to Hansen et al. resulted in about 4 degrees C increase in global temperature eventually per doubling of atmospheric carbon. [Note that this may underestimate the effect of today’s atmospheric carbon increases over a 100s of times shorter period compared to previous global warming episodes] If we follow Hansen et al., then, we will have 2.67 degrees C of warming already “baked in” as of 2121. Somewhere between 1.33 and 1.75 degrees C will happen pretty quickly, apparently, by 2050; and while we don’t know how fast the rest will happen, much of it will happen in the period 2050-2100.
We may also note a recent Hansen et al. article that revises the estimated sea level rise due to this warming to between 5 and 9 feet by 2100 – and, again, the model that projects this, as noted by the authors, may still underestimate the effects of “business as usual”. Finally, scientists have now noted several new catastrophes that would happen even with a 2 degrees C rise.
We continue to underestimate what the planet will do to us on our present path.
That is all.