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Finally, an overview on climate change for the intelligent layperson

28 Nov 2012 by Jim Fickett.

The World Bank recently released a superb 106-page overview of climate change. If you think, as I do, that climate change is happening fast enough to influence your current decisions on where to live, who to vote for, and what to invest in, then you should read this report.

There are three things that make this report quite different from anything else I've read on the subject:

  • Pragmatic estimates of outcomes. On each topic – global policy initiatives, likely rise in sea level, effect on crop yields, etc – a helpful summary of the most likely outcomes and the possible range of outcomes is given.
  • Evidence. The report gives straightforward explanations of the evidence for each conclusion, including realistic appraisals of what we really know and what we don't know.
  • Excellent science writing. The authors have not been afraid to use probabilities, limits on precision, and other relevant scientific concepts. At the same time, everything is explained in a way that the non-specialist can grasp.

I'll give a few examples. But first let me say that my purpose in giving the examples is different from my usual goal. Ordinarily I try to find the most significant highlights to pass on. Here, instead, I am trying to give a flavor of the style of the report, hoping to convince you to at least read the executive summary (6 pages), and the first three chapters, on changes that have occurred to date and changes likely to happen by 2100 (28 pages).

The core prediction, of course, is how much the world will warm in the next few decades. The report makes it quite clear that, assuming politics as usual (a depressing but very safe assumption), we are headed for warming of 4°C:

Without further commitments and action to reduce greenhouse gas emissions, the world is likely to warm by more than 3°C above the preindustrial climate. Even with the current mitigation commitments and pledges fully implemented, there is roughly a 20 percent likelihood of exceeding 4°C by 2100. If they are not met, a warming of 4°C could occur as early as the 2060s. Such a warming level and associated sea-level rise of 0.5 to 1 meter, or more, by 2100 would not be the end point: a further warming to levels over 6°C, with several meters of sea-level rise, would likely occur over the following centuries.

One reason voters have not been up in arms for carbon policy changes is that climate scientists have repeatedly emphasized 2°C as a limit we must not exceed, without explaining very well why such a change, which seems very small, is a very big deal. This report tackles that communication failure:

Global mean temperature … is now about 0.8°C above preindustrial levels.

A global warming of 0.8°C may not seem large, but many climate change impacts have already started to emerge, and the shift from 0.8°C to 2°C warming or beyond will pose even greater challenges. It is also useful to recall that a global mean temperature increase of 4°C approaches the difference between temperatures today and those of the last ice age, when much of central Europe and the northern United States were covered with kilometers of ice and global mean temperatures were about 4.5°C to 7°C lower.

Now an example of explaining the mechanics behind important changes, in this case, ocean acidification and its consequences for sea life:

The increase of carbon dioxide concentration to the present-day value of 390 ppm has caused the pH to drop by 0.1 since preindustrial conditions. This has increased ocean acidity, which because of the logarithmic scale of pH is equivalent to a 30 percent increase in ocean acidity (concentration of hydrogen ions). The scenarios of 4°C warming or more by 2100 correspond to a carbon dioxide concentration of above 800 ppm and lead to a further decrease of pH by another 0.3, equivalent to a 150 percent acidity increase since preindustrial levels.

and

Surface waters are typically supersaturated with aragonite (a mineral form of CaCO3), favoring the formation of shells and skeletons. If saturation levels are below a value of 1.0, the water is corrosive to pure aragonite and unprotected aragonite shells (Feely, Sabine, Hernandez-Ayon, Ianson, and Hales 2008). Because of anthropogenic CO2 emissions, the levels at which waters become undersaturated with respect to aragonite have become shallower when compared to preindustrial levels. Aragonite saturation depths have been calculated to be 100 to 200 m shallower in the Arabian Sea and Bay of Bengal, while in the Pacific they are between 30 and 80 m shallower south of 38°S and between 30 and 100 m north of 3°N (Feely et al. 2004). In upwelling areas, which are often biologically highly productive, undersaturation levels have been observed to be shallow enough for corrosive waters to be upwelled intermittently to the surface.

The recent videos of the New York Subway flooding during Hurricane Sandy should bring home that sea-level rise must be a core concern. Much of the world's current investment in infrastructure is concentrated in the region of major ports. If sea-level rise threatens those areas it will be costly and disruptive. Hence the report has considerable material on the likely rise in sea levels. The next example explains what we know and don't know in this area, and why estimates of sea-level rises so often differ.

Future sea-level rise can be described as the sum of global mean change (as if the ocean surface as a whole were to undergo a uniform vertical displacement, because of heating or addition of mass) and local deviations from this mean value (readjustment of the ocean surface resulting from gravity forces, winds, and currents). The components of both global and regional sea-level rise are known with varying levels of confidence. global mean thermal expansion is relatively well simulated by climate models, as it depends on the total amount of atmospheric warming and the rate of downward mixing of heat in the oceans. The spread in existing climate model projections is, therefore, well understood and probably gives an adequate estimate of the uncertainty. Projected melt in mountain glaciers and ice caps is also considered reliable, or at least its potential contribution to sea-level rise is limited by their moderate total volume, equal to 0.60 ±0.07 m sea-level equivalent, of which a third is located at the margin of the large Greenland and Antarctic ice sheets (Radić and Hock 2010).

The Greenland and Antarctic ice sheets themselves constitute a markedly different problem. Their potential contributions to future global mean sea-level rise is very large, namely 7 m and 57 m, respectively, for complete melting. While a recent study (Robinson et al. 2012) suggests that a critical threshold for complete disintegration of the Greenland ice sheet might be 1.6°C, it should not be forgotten that this applies to an ice sheet that can reach its equilibrium state in a world where temperature stays at levels above that threshold for a long time. The time frame for such a complete disintegration, is of the order of at least several centuries or even millennia, even though it is not precisely known. This means, that a world that crosses that threshold but returns to lower levels thereafter, is not necessarily doomed to lose the Greenland ice sheet. Although the question of committed sea-level rise is important, currently projections of the nearer future are needed. however, the physics of the large ice sheets is poorly understood. There are indications that current physical models do not capture these fast timescales: model simulations are so far not able to reproduce their presently observed contribution to current sea-level rise (Rahmstorf et al. 2007). This casts doubt on their ability to project changes into the future …

Regional variations of future sea-level also have uncertainties, but—concerning ocean dynamics—they remain within reach of the current generation of coupled ocean-atmosphere models, in the sense that an ensemble of model projections may be a good approach to estimate future changes and their associated uncertainties. Concerning changes in gravitational patterns, however, they are inherently linked to ice- sheet projections. Nevertheless, several attempts have been made to project regional sea-level changes (Katsman et al. 2008, 2011; Perrette, Landerer, Riva, Frieler, and meinshausen 2012; Slangen, Katsman, Wal, vermeersen, and Riva 2011).

Past sea-level records indicate that it has varied by about 120 m between glacial periods and warmer interglacials (Figure 27), most of which is due to ice-sheet melt and regrowth. The most recent deglaciation has been accompanied by very rapid rates of rise (~40 mm/year) (Deschamps et al. 2012). however, that is not directly applicable to anthropogenic climate change because present-day ice sheets are much smaller than they were during the last ice age, and less numerous (the Laurentide and Fenno-scandinavian ice sheets do not exist anymore). A more relevant period to look at is the last warm, or interglacial, period (120,000 years ago). The global mean temperature was then likely 1–2°C above current values, and sea level was 6.6–9.4 m above the present level (Kopp, Simons, Mitrovica, Maloof, and Oppenheimer 2009), as revealed by a compilation of various proxy data around the world. Important caveats in the study of paleo-climate as analog for future climate change are the nature of the forcing, which leads to sea-level rise (Ganopolski and Robinson 2011), and the rate of sea-level rise. The latter is often very poorly known due to a lack of temporal resolution in the data. Despite the various caveats associated with the use of paleo-climatic data, a lesson from the past is that ice sheets may have been very sensitive to changes in climate conditions and did collapse in the past. That is a strong motivation to better understand what leads to these changes and to pursue the efforts to assess the risk of large ice-sheet contributions to sea-level rise in the future.

It is helpful to have an educated guess from experts, even if we know the uncertainty is large. The report does give an educated guess in such cases, in particular for the likely rise in sea level:

Warming of 4°C will likely lead to a sea-level rise of 0.5 to 1 meter, and possibly more, by 2100 …

Sea-level rise will vary regionally: for a number of geophysically determined reasons, it is projected to be up to 20 percent higher in the tropics and below average at higher latitudes. In particular, the melting of the ice sheets will reduce the gravitational pull on the ocean toward the ice sheets and, as a consequence, ocean water will tend to gravitate toward the Equator.

Coverage is not perfect. There is no mention of the possible feedback loop in methane released from permafrost, which I am not alone in thinking might become a serious problem. But the report does cover a great deal, and includes some areas that were completely new to me, like recent work on the non-linear response of agricultural crops to warming:

As global warming approaches and exceeds 2°C, the risk of crossing thresholds of nonlinear tipping elements in the Earth system, with abrupt climate change impacts and unprecedented high-temperature climate regimes, increases. Examples include the disintegration of the West Antarctic ice sheet leading to more rapid sea-level rise than projected in this analysis or large-scale Amazon dieback drastically affecting ecosystems, rivers, agriculture, energy production, and livelihoods in an almost continental scale region and potentially adding substantially to 21st-century global warming. …

nonlinear temperature effects on crops are likely to be extremely relevant as the world warms to 2°C and above. However, most of our current crop models do not yet fully account for this effect, or for the potential increased ranges of variability (for example, extreme temperatures, new invading pests and diseases, abrupt shifts in critical climate factors that have large impacts on yields and/or quality of grains). …

Based on a large number of maize trials (covering varieties that are already used or intended to be used by African farmers) and associated daily weather data in Africa, Lobell et al. (2011) have found a particularly high sensitivity of yields to temperatures exceeding 30°C within the growing season. Overall, they found that each “growing degree day” spent at a temperature above 30°C decreases yields by 1 percent under optimal (drought-free) rainfed conditions. A test experiment where daily temperatures were artificially increased by 1°C showed that—based on the statistical model the researchers fitted to the data—65 percent of the currently maize growing areas in Africa would be affected by yield losses under optimal rainfed conditions. The trial conditions the researchers analyzed were usually not as nutrient limited as many agricultural areas in Africa. Therefore, the situation is not directly comparable to “real world” conditions, but the study underlines the nonlinear relationship between warming and yields. …

In the United State, significant nonlinear effects are observed above local temperatures of 29°C for maize, 30°C for soybeans, and 32°C for cotton (Schlenker and Roberts 2009).