The Three Degree World

This Hub summarizes in a table the expected and possible effects from three Celsius degrees of warming of the global average temperature above pre-Industrial values. The summary is drawn from Six Degrees, by Mark Lynas, with updates to be posted as possible. (Six Degrees was published in 2008.)

Note: The first update, incorporating information from the final draft of the Technical Summary of the IPCC Assessment Report 5, was posted 12/9/2013. It is headlined below the summary table.

The three degree world is less familiar than worlds with less extreme warming; whole nations are at risk of disappearance. It's a degree of warming which, by consensus, represents the point where damage begins to be extremely serious, and at which less and less corresponding benefit from climate change can be derived. Here's the list of Lynas's expected impacts.

The IPCC released its final draft of portion of the Fifth Assessment Report dealing with the basic physical science in the fall of 2013. Included was a "Technical Summary," summarizing in 129 dense pages the salient results.

One result is that the "Three-degree World" could well be a reality by the time today's youngsters reach what is today considered to be retirement age. The figure below gives temperature projections based upon different choices humanity could make over the next decades, in the form of four "Representative Concentration Pathways," or RCPs. Skipping the details, they range from a future where emissions keep growing--RCP 8.5--to a future in which we find ways to actively reduce atmospheric concentrations of greenhouse gases--RCP 2.6.

The resulting temperature changes are illustrated below.

As shown in the graph at lower left, the most severe RCP, RCP 8.5, would easily result in reaching a three-degree world sometime shortly after 2050. (Lest that seem unduly pessimistic, note that at this point, humanity is on track to follow RCP 8.5, rather than one of the milder scenarios.) Even the next milder scenario, RCP 6.0, would likely reach 3 C by the end of the present century.

In order to consider what this implies for the "Three Degree" impacts envisioned in Six Degrees, let's consider the impact maps below.

Here, the projected precipitation changes for 4 RCPs are given. Let's concentrate, for the moment, only on RCP 8.5. The color scale is given, but the hatching and stippling need explanation. The diagonal hash marks indicate areas where the changes are relatively small compared with natural variation--one 'standard deviation,' for the statistically-inclined. Contrariwise, the dots indicate a very large change--two standard deviations, or more.

Note that the strongest drying trends are in the Mediterranean basin, southern Africa--including Botswana--and, to a slightly lesser degree, the Caribbean, Central America, and Mexico. That is consistent Lynas's conclusions for those areas, as given in the table above.

But precipitation is only on aspect of the water cycle. With increased temperatures comes increased evaporation from the surface, be it water or soil. The Technical Summary examines this parameter as well, as well as others, in Figure 2 of their "Thematic Focus Element 1." But in the interest of brevity, I will skip to the panel 5, which gives precipitation minus evaporation (a measure of net drying of the surface) for the period 2081-2100, for RCP 8.5.

This metric shows in dramatic fashion the drying in mid-latitudes, especially in the Southern Hemisphere--although the effect in southern Africa appears more muted, and that in the Caribbean and Central America is extremely marked. And Australia actually appears to be moistening. Does that mean that Lynas's conclusions for that continent are now outdated?

Not necessarily. Consider another modeled parameter of the water cycle: soil moisture. It is affected not only by precipitation and evaporation, but by the distribution of precipitation and hence, the patterns of runoff: long-continued moderate rains saturate soils, whereas quick, violent rains may largely run off, leaving soil beneath the most superficial layer largely unmoistened.

Although the authors caution that modeling of soil moisture is subject to more uncertainty than other aspects of the water cycle, the results they present suggest that Australia may indeed suffer from seriously increased drought as a result of climate change. (Indeed, the last couple of years have seen remarkable heat waves and wildfire outbreaks there, suggestive of contemporary impacts in the "one degree world.")

Also showing considerable drying in this metric is China--a highly populated area which is suffering from serious water shortages now. (Planners there are working on a mega-project to divert water from the south to the north. As reported in The Economist, October 12, 2013, "Beijing has the sort of water scarcity usually associated with Saudi Arabia: just 100 cubic metres per person per year." The average American uses 1600 cubic metres per year.) Lynas includes this issue in the Two Degrees chapter.

Of course, Americans in the Colorado river basin may need to do with much less in the future--as Lynas mentions, there is water stress there now, and as the soil moisture map shows, this is only likely to increase in the three degree world.

Drying of the Amazon is also evident--a matter of some weight, since this could trigger a 'tipping point' in which that vast areas turns from carbon sink into carbon source as vegetation, dying on a vast scale, releases its carbon to the atmosphere--and successor vegetation regimes permanently absorb less of it back again. The current science has equivocal news on this front: such a catastrophe is quite possible, says the Technical Summary, but not assured.

There is other news of this sort: changes to the Asian monsoon and to the patterns of the El Nino/La Nina remain quite uncertain as well. They may bite us, hard--or they may not. Mildly consoling, perhaps, but scarcely reassuring.