It is abundantly clear that the target of a 2-degree Celsius limit to climate change was mostly derived from what seemed convenient and doable without any reference to what it really means environmentally. Two degrees is actually too much for ecosystems. Tropical coral reefs are extremely vulnerable to even brief periods of warming. The elevated atmospheric CO2 also has raised the acidity of the oceans, which affects the ability of coral and mollusks such as oysters to build shells and skeletons. A 2-degree world will be one without coral reefs (on which millions of human beings depend for their well-being).

At current global warming of 0.8-0.9 degrees, the fingerprints of climate change can be seen virtually everywhere in nature. The coniferous forests of western North America are currently experiencing massive tree mortality because climate change has tipped the balance in favor of native bark beetles. The Amazon seems to be edging close to dieback in the southern and southeastern portions of the great forest.

At essentially double that current temperature increase, there undoubtedly will be massive extinctions and widespread ecosystem collapse. The difficulty of trying to buffer and manage change will increase exponentially with only small increments of warming.

In addition, the last time the planet was 2 degrees warmer, the oceans were four to six (perhaps eight) meters higher. We may not know how fast that will happen (although it is already occurring more rapidly than initially estimated), but the end point in sea-level rise is not in question. A major portion of humanity lives in coastal areas and small island states that will go under water. The site of the Earth Summit and Rio+20 will disappear under water fairly early on.

More than a 2-degree increase should be unimaginable. Yet to stop at 2 degrees, global emissions have to peak in 2016. The Carbon Tracker organization has examined fossil-fuel investments around the world (including 1,200 new coal plants) and determined that they would lead to a 6-degree world. A recent World Bank report indicates the bank cannot fulfill its development mission in a 4-degree world. Given what we know about planetary biology, 2 degrees seems nightmarish as it is.