An Illustrated Guide to the Science of Global Warming Impacts: How We Know Inaction Is the Gravest Threat Humanity Faces | ThinkProgress

An Illustrated Guide to the Science of Global Warming Impacts: How We Know Inaction Is the Gravest Threat Humanity Faces | ThinkProgress

http://thinkprogress.org/climate/2011/06/02/234291/royal-society-7f-4c-world/
In a must-read paper that is the source of the top figure, “When could global warming reach 4°C?” Betts et al. drop this bombshell:

Using these GCM projections along with simple climate-model projections, including uncertainties in carbon-cycle feedbacks, and also comparing against other model projections from the IPCC, our best estimate is that the A1FI emissions scenario would lead to a warming of 4°C relative to pre-industrial during the 2070s. If carbon-cycle feedbacks are stronger, which appears less likely but still credible, then 4°C warming could be reached by the early 2060s in projections that are consistent with the IPCC’s ‘likely range’.

On the one hand, the A1FI is quite a high emissions scenario, and I suspect that humanity will turn off of it by 2030. On the other hand, even a much lower emissions like A2 is only a few tenths of a degree centigrade cooler. Also, while Betts et al. does a better job of incorporating carbon-cycle feedbacks into their modeling than virtually anyone else, I do not believe that they incorporate any feedback of methane emissions from the tundra or methane hydrates — and that is certainly the most worrisome of all of the carbon-cycle feedbacks (see “NSIDC bombshell: Thawing permafrost feedback will turn Arctic from carbon sink to source in the 2020s, releasing 100 billion tons of carbon by 2100” and Science: Vast East Siberian Arctic Shelf methane stores destabilizing and venting: NSF issues world a wake-up call: “Release of even a fraction of the methane stored in the shelf could trigger abrupt climate warming”).
Another important Royal Society article is the concluding piece, “The role of interactions in a world implementing adaptation and mitigation solutions to climate change,” by Rachel Warren. She makes a crucial point that is all too neglected in most discussions of adaptation — it is the interaction of impacts that is likely to overwhelm, particularly when you consider the very real risk of eco-system collapse over large parts of the Earth:

… a 4°C world would be facing enormous adaptation challenges in the agricultural sector, with large areas of cropland becoming unsuitable for cultivation, and declining agricultural yields. This world would also rapidly be losing its ecosystem services, owing to large losses in biodiversity, forests, coastal wetlands, mangroves and saltmarshes, and terrestrial carbon stores, supported by an acidified and potentially dysfunctional marine ecosystem. Drought and desertification would be widespread, with large numbers of people experiencing increased water stress, and others experiencing changes in seasonality of water supply. There would be a need to shift agricultural cropping to new areas, impinging on unmanaged ecosystems and decreasing their resilience; and large-scale adaptation to sea-level rise would be necessary. Human and natural systems would be subject to increasing levels of agricultural pests and diseases, and increases in the frequency and intensity of extreme weather events.
In such a 4°C world, the limits for human adaptation are likely to be exceeded in many parts of the world, while the limits for adaptation for natural systems would largely be exceeded throughout the world. Hence, the ecosystem services upon which human livelihoods depend would not be preserved. Even though some studies have suggested that adaptation in some areas might still be feasible for human systems, such assessments have generally not taken into account lost ecosystem services.

Precisely.
Right now, even the worst-case analyses for adaptation ignore the potential impact of ecosystem collapse (see Scientists find “net present value of climate change impacts” of $1240 TRILLION on current emissions path, making mitigation to under 450 ppm a must).
Warren also notes another area often ignored in adaptation analyses:

In the coming decades, one of the most serious impacts of climate change is projected to be the consequences of the projected increases in extreme weather events. For example, climate change-induced changes in precipitation patterns and changes in climate variability would increase the area of the globe experiencing drought at any one time from today’s 1 per cent to a future 30 per cent by the end of the twenty-first century…

Few studies examine the potential consequences of these increases in extreme weather upon individual sectors and/or regions, but these could be significant. Only a few days of high temperatures near flowering in wheat, groundnut and soybean can drastically reduce yield, while maize losses could potentially double owing to floods in the USA; and the AVOID study estimated that, in a 4°C world, 50 per cent of fluvial flood-prone people would be exposed to increased flood risk compared with approximately 25 per cent in a 2°C world.

Unfortunately, this issue was not published in time to take into account the Must-read NCAR analysis that warns we risk multiple, devastating global droughts even on moderate emissions path. And the sea level rise article adds little to the many recent scientific and media articles on the subject — see Coastal studies experts: “For coastal management purposes, a [sea level] rise of 7 feet (2 meters) should be utilized for planning major infrastructure.” Also, I would have liked to have seen an ocean acidification article that looked at what we face on the high-emissions, moderate carbon-cycle feedbacks scenario.
But there are several important articles, like “Agriculture and food systems in sub-Saharan Africa [SSA] in a 4^°C+ world,” which concludes:

The prognosis for agriculture and food security in SSA in a 4^°C+ world is bleak. Already today, the number of people at risk from hunger has never been higher: it increased from 300 million in 1990 to 700 million in 2007, and it is estimated that it may exceed 1 billion in 2010 . The cost of achieving the food security Millennium Development Goal in a +2^°C world is around $40-60 billion per year, and without this investment, serious damage from climate change will not be avoided. Currently, the prospects for such levels of sustained investment are not that bright. Croppers and livestock keepers in SSA have in the past shown themselves to be highly adaptable to short- and long-term variations in climate, but the kind of changes that would occur in a 4^°C+ world would be way beyond anything experienced in recent times. There are many options that could be effective in helping farmers adapt even to medium levels of warming, given substantial investments in technologies, institution building and infrastructural development, for example, but it is not difficult to envisage a situation where the adaptive capacity and resilience of hundreds of millions of people in SSA could simply be overwhelmed by events.

The article “Beyond ‘dangerous’ climate change: emission scenarios for a new world” concludes:

The analysis within this paper offers a stark and unremitting assessment of the climate change challenge facing the global community. There is now little to no chance of maintaining the rise in global mean surface temperature at below 2^°C, despite repeated high-level statements to the contrary. Moreover, the impacts associated with 2^°C have been revised upwards, sufficiently so that 2^°C now more appropriately represents the threshold between dangerous and extremely dangerous climate change. Consequently, and with tentative signs of global emissions returning to their earlier levels of growth, 2010 represents a political tipping point. The science of climate change allied with emission pathways for Annex 1 and non-Annex 1 nations suggests a profound departure in the scale and scope of the mitigation and adaption challenge from that detailed in many other analyses, particularly those directly informing policy.

However, this paper is not intended as a message of futility, but rather a bare and perhaps brutal assessment of where our ‘rose-tinted’ and well intentioned (though ultimately ineffective) approach to climate change has brought us. Real hope and opportunity, if it is to arise at all, will do so from a raw and dispassionate assessment of the scale of the challenge faced by the global community. This paper is intended as a small contribution to such a vision and future of hope.