The Four Degree World

The 4 degree world is still more strange (and uncomfortable) than its 3-degree counterpart--but worst is the possibility that it may not be stable. Do 'tipping points' lurk at or near this level of warming?

Note: Update from the AR5 Technical Summary posted 12/11/13. (Below the summary table.) It presents likely temperature and sea level projections in relation to the points made in this chapter of Six Degrees.

In many ways, the answer to the 'tipping points' question depends upon water--rising oceans, melting ice, and changing precipitation are all important in the 4 degree world. Here's the summary table of Lynas's view.

In the fall of 2013 the International Panel on Climate Change released the final draft of the Working Group 1 Fifth Assessment Report ('AR5.') The sea level section of the 129-page Technical Summary states, among other things, that:

During the warm intervals of the middle Pliocene (3.3 to 3.0 million years ago), when there is medium confidence that global mean surface temperatures were 2°C to 3.5°C warmer than for pre-industrial climate and CO2 levels were between 250 and 450 ppm, sedimentary records suggest periodic deglaciation of West Antarctica and parts of East Antarctica. Ice-sheet models suggest near-complete deglaciation of Greenland, West Antarctica and partial deglaciation of East Antarctica. Together, the evidence suggests that Global Mean Sea Level (GMSL) was above present levels at that time, but did not exceed 20 m above present (high confidence).

In other words, CO2 levels in the mid-Pliocene were comparable with our own, the climate was much warmer, the planet was largely deglaciated, and sea level was much higher than at present. Since our high CO2 levels are geologically very sudden event, the implication is that our temperatures, sea level, and cryosphere would probably tend to approach Pliocene conditions over time--if we were to stop emitting greenhouse gases, and simply hold atmospheric levels constant. (Sadly, we are not on track to do this at present; emissions are continuing to rise as of this writing.)

The Technical Summary continues on to consider less extreme conditions:

During the Last Interglacial Period (about 129,000 to 116,000 years ago) when peak global warmth was not more than 2°C above pre-industrial temperatures, and peak global annual sea surface temperatures were 0.7 [0.1 to 1.3] °C warmer (medium confidence), maximum GMSL was at least 5 m higher than at present (very high confidence), but did not exceed 10 m (high confidence). Based on ice-sheet model simulations consistent with elevation changes derived from a new Greenland ice core, the Greenland ice sheet very likely contributed between 1.4 m and 4.3 m sea level equivalent, implying with medium confidence a contribution from the Antarctic ice sheet to the global mean sea level during the last interglacial period.

In other words, the LIP--often referred to as the "Eemian Period"--saw very significant deglaciation, and sea levels "at least 5 m higher than at present", with warming less than three times the current .7 C.

Moreover, warming of 2 C has now become quite likely, barring either prompt and drastic reductions in emissions, or deployment of some as-yet unproven technology to actively draw CO² out of the atmosphere. The projected 2081-2100 range of even Representative Concentration Pathway (RCP) 4.5--an emissions scenario which we are currently exceeding greatly, and the mildest one barring the sort active drawdown just mentioned--is 1.1 to 2.6 C, with a mean of 1.8 C.

Here is the relevant table summarizing likely temperature and sea level changes.

In terms of Four Degrees, the message is mixed. There is good news: there is very little chance that global temperatures will hit 4 C by 2050--the worst outcome for mid-century currently considered 'likely' is 2.6 C. That is bad enough, to be sure--see the preceding Hub, The Three Degree World.

But better bad than worse. It would seem that Alexandria--and the other coastal cities--have a few more decades to prepare their defenses--and their routes of retreat. Some cities are planning now (see sidebar links.)

On the other hand, the eventual outcome seems unchanged--and perhaps more likely than before, given that five years after the publication of Six Degrees, no meaningful emissions reductions have taken place on a global level, and there is no sign that such reductions will be forthcoming soon. Returning to the summary table, we see that the likely sea level rise for mid-century is roughly half of the 50 centimeters discussed in "Death on the Nile." That will be enough to cause large impacts around the world. And for the end of the century, the range is likely 26 to 81 centimeters--dependent, or course, on the choices we make in the interim.

It's worth noting that that is not actually the worst case for sea level rise. The Panel concluded that:

Based on current understanding, only the collapse of marine-based sectors of the Antarctic Ice Sheet, if initiated, could cause global mean sea level to rise substantially above the likely range during the 21st century. There is medium confidence that this additional contribution would not exceed several tenths of a meter of sea level rise during the 21st century.

The likelihood of such a collapse is not presently able to be estimated, but seems not to be considered highly likely by most. Certainly, it can't be excluded as a possibility.

A new study by Sherwood et al finds that models providing lower climate sensitivity estimates match observations less well than others.

One of the implications is that 'business as usual' would be very likely to lead to the 4-degree world by the end of the present century.