Scientists Dispute 2-Degree Model Guiding Climate Talks (via wsj.com)

Even if we stopped all CO2 emissions today, the world is very likely to hit significant tipping points already, unless you're the sort of person who doesn't consider the disintegration of the Larsen B ice shelf, the massive Greenland glacier melts, and the increase of Siberian methane emissions (that lovely, pants-shitting and realistic close-range extinction scenario) to be "tipping points".

The only way this plays out is that the less damage you do, the easier it will be future generations to stabilize the situation through proper technology and policy (or you think people will just idly accept 500 years from now that the actions of today will lead to their demise?).

So models have curves where depending on the different scenarios, the amount of damage varies. Since this is a continuum of environmental effects, not discrete points, picking a single point as a goal is a mix of what you can do realistically and the amount of political buy-in we can get about how much guilt we can take for fucking things up, plus/minus the inherent uncertainty in any model.

So yeah, you can say that it's arbitrary about as much as a recommended dietary intake is arbitrary, or a recommended maximum radiation dosage is arbitrary, or the number of minutes of exercise per day is arbitrary.

This is a great example on how to turn a one-paragraph response into a two-page fluff article.

When your options are nuance or action, sometimes action.

Remember your audience.

Whatever your views on global warming, it is simply a fact that people in the developing world want to enjoy the standard of living experienced by the developed world and that means much increased energy consumption. The latter will not be met in any very significant way by windmills or solar panels.