Sixteen Degrees C

A paper out in Science suggests unfathomably severe warming of 16 C if Earth hits 1000 ppm atmospheric CO2.

We are presently at around 390 ppm, increasing at a rate of about 2 ppm per year. The increase is due to industrial CO2 output and other anthropogenic changes such as turning forest to fields. The actual output from human activity is more like 4 ppm, but around half of it is absorbed into the oceans. The industrial portion is growing at a few percent per year or so.

Thus, if we stay on our present emissions path, we will end up with an atmospheric concentration of around 1000 ppm by century's end.

Three main factors could change that outcome. In simple terms they are:

1. economic collapse or energy sector changes, which would suppress industrial emissions; 
2. slowing of the oceanic carbon sink, which would increase the fraction of emissions that stay in the atmosphere (warm water dissolves less gas); and
3. natural carbon reservoirs like permafrost, which already are emitting some gas (e.g., here), could pick up speed and dump their carbon stores into the atmosphere through bacterial action on stored organic material.

It appears that anything above 400 ppm or so, which is unavoidable at this point, will melt enough permafrost to cause an unstoppable, albeit slow, release of carbon sufficient to bring us to something like 1000 ppm.

In other words, it would appear that there is a very good chance that Earth will see 1000 ppm at some point in the next few hundred years regardless of what we do now.

Back to the Science paper.

The author reviews the paleoclimate record to see what happened the last time Earth had an atmospheric concentration of 1000 ppm. This was around 30 million years ago, but the date is poorly constrained (it could be 100 million years ago) because of uncertainties in ancient CO2 concentrations. He takes temperature estimates from a few places in the tropics and near the poles, makes some basic assumptions about the global temperature distribution, and adjusts for the fact that the Sun was a little bit dimmer then.

The global temperature is estimated to have been around 16 C warmer than now.

Without getting into nuts and bolts, this is very, very bad.

A while back there was a study done about the limits of human adaptation.  It focused on the fact that, when combinations of heat and humidity produce a wet bulb temperature of 35 C or more, the human body ceases to compensate.  In other words, that is a lethal condition for humans and other mammals.  This essentially does not happen now.

You start to get regional occurrences of this lethal, wet heat at global warming levels of around 7 C.  By 12 C half of the area inhabited by humans becomes subject to killing heat waves (see image below).  Add another 4 C to that and you have the world that could very well develop within a few centuries.

A separate consequence of warming is drying of soils.  The short version is, by the time you hit 4 C of warming you have a world where most of the breadbaskets turn to desert.

The above image shows what is expected with something like 560 ppm CO2 by mid century.  The scale is the Palmer Drought Severity Index.  During the Dust Bowl conditions were around -3 with brief excursions to -6 during the driest times.  This represents the complete destruction of an awful lot of agricultural land.

The only remotely comforting thing about the prospect of reaching 1000 ppm is that the full brunt of these climate changes would take longer to develop than my life will last.  Warming of 4 C is possible by mid century.  Seven C, and the beginning of lethal wet bulb temperatures, could happen around the end of the century.  The full 16 C (or more!) would take several hundred years to develop.