The good news: We can avoid multimeter sea level rise, the loss of the inland glaciers that provide water to a billion people, rapid expansion of the subtropical deserts, and mass extinctions — each of which is all-but inevitable on our current path of unrestrained greenhouse gas emissions.
The not-so-good news: We will probably need an ultimate target of 350 ppm (or lower) for atmospheric carbon dioxide — if you accept the analysis of ten leading climate scientists from around the world.
And yes, the authors of “Target Atmospheric CO2: Where Should Humanity Aim?” in The Open Atmospheric Science Journal are painfully aware we’re already at 385 ppm and rising 2 ppm a year. That is why they propose the self-described “Herculean” task of phasing out coal use that does not capture CO2 “over the next 20-25.” And that requires a global CO2 emissions profile that looks something like this:
[Note to Hansen et al: Big pet peeve -- I think you confuse the general reader by labeling your y-axis "CO2 Emissions" while expressing the units in billion metric tons of carbon. This helps foster errors in the media and elsewhere (see "The biggest source of mistakes: C vs. CO2").]
Actually, even the ultra-sharp emissions cuts depicted in the figure won’t do the trick. We would still need “reforestation of degraded land and improved agricultural practices that retain soil carbon” (aka biochar to the rescue) to “lower atmospheric CO2 by as much as 50 ppm.”
More not-so-good news: That kind of emission reduction isn’t going to happen, not even under President Obama, not even close. Heck, I doubt it would happen under a President Hansen. We just are not going to see 350 ppm this century. Unfortunately, the authors “infer from the Cenozoic data that CO2 was the dominant Cenozoic forcing, that CO2 was only ~450 ppm when Antarctica glaciated, and that glaciation is reversible.”
That is, if we stabilize at 450 ppm (or higher) we risk returning the planet to conditions when it was largely ice free, when sea levels were higher by 70 meters — more than 200 feet! Yet, “Equilibrium sea level rise for today’s 385 ppm CO2 is at least several meters, judging from paleoclimate history.” Equally worrisome,
Theory and models indicate that subtropical regions expand poleward with global warming. Data reveal a 4-degree latitudinal shift already, larger than model predictions, yielding increased aridity in southern United States, the Mediterranean region, Australia and parts of Africa. Impacts of this climate shift support the conclusion that 385 ppm CO2 is already deleterious.
In short, widespread desertification and coastal flooding, Hell and High Water, is nigh (see also “Is 450 ppm politically possible? Part 0: The alternative is humanity’s self-destruction“).
Some slightly good news: The paper does suffer from one inherent analytical weakness that makes it (a tad) less dire than it appears.