These considerations have led 350.org to see the 350 ppm target not only in terms of CO2, but CO2e. On a technical level, this becomes a more ambitious target, incorporating other greenhouse gases. On a practicallevel, it signifies the same priorities 350 has embodied all along. Any climate target lower than where we are right now—be it 350 CO2e, 350 CO2, or anything else—represents a transformative shift in how the world operates. Targets of 350 CO2 and 350 CO2e—both greenhouse gas concentrations significantly lower than current levels—have the same essential policy implications: we will STOP burning coal and other fossil fuel and we will START rolling out clean energy and other sustainable development strategies around the world.
Either way you slice it, in terms of CO2 or CO2-e, 350 is the mark of a completely new direction—and the movement that will get us there.
And what about all the other targets people are aiming for?
Here's Bill McKibben's response to this question:
The question of what target to aim for in the fight against global warming has always been vexed, and for one simple reason: filling the atmosphere with carbon is at base a huge experiment, one we’ve never conducted before. It’s always been tough to judge exactly where the danger lies.
At first in the late 1980s and early 1990s, the number we routinely used was 550 parts per million CO2—mostly because it was double the pre-Industrial Revolution concentrations and hence easy to model. But it became something of a red line through dint of sheer repetition—I remember writing an op-ed for the New York Times excoriating the Clinton administration for hinting that it might be okay to go past a 550 ceiling. As time went on, it became clearer that the dangerous thresholds lay somewhere lower, and we began to use—almost interchangeably—450 parts per million, or 2 degrees Celsius. Science doesn’t actually know if 450 ppm and 2 degrees are the same thing, and no one knows how much change they would produce. Again, these were guesses for the point at which catastrophic damage would begin—they were more plausible, but still not based on actual experience. They also reflected guesses of what was politically possible to achieve. They were completely defensible, given the lack of data (though the 2C target was always problematic strategically since Americans don’t use centigrade measurements and hence have no real idea what 2 degrees Celsius means.)
In the summer of 2007, though, with the rapid melt of Arctic ice, it became clear that we had already crossed serious thresholds. A number of other signs pointed in the same direction: the spike in methane emissions, likely from thawing permafrost; the melt of high-altitude glacier systems and perennial snowpack in Asia, Europe, South America and North America; the rapid and unexpected acidification of seawater. All of these implied the same thing: wherever the red line for danger was, we were already past it, even though the atmospheric concentration of CO2 was only 390 parts per million, and the temperature increase still a shade below 1 degree C. In early 2008, Jim Hansen and a team of researchers gave us a new number, verified for the first time by real-time observation (and also by reams of new paleo-climatic data).