We’ve all heard that it is essential to keep the Earth from warming more than 2 degrees Celsius above pre-industrial levels. This target was formally accepted by the Conference of Parties to the UNFCCC in 2010, but it had traction in international venues as early as 1996. As we begin to put Paris in our scope and hope for international agreements to reduce CO2 emissions at the COP meeting this December, we ought also reconsider the 2 degree target, for three reasons.
First, the number, 2 degrees, was assigned without any awareness of tipping points. We simply do not know whether gradual increases in surface temperature will produce graduate changes in weather and climate. The worst case scenario is that there is a tipping point somewhere below 2 degrees.
Second, 2 degrees was selected because it would produce manageable impacts, not negligible impacts. At 2 degrees, many Pacific Island nations are likely underwater, many coral reefs are gone, most the glaciers are gone, and there are dozens of other impacts that we can only project may happen. In summary, we have only a partial idea of what a world that is 2 degrees warmer looks like.
Finally, we ought to remember that 2 degrees is presumed to be a limit of our tolerance for harm. A world that is 2 degrees warmer is filled with damage, risk, and sorrow. It is neither a condition we want to reach, nor one we want to remain in. However, we have no idea whether the warming of the planet can be effectively reversed by humans. Perhaps we could tolerate a 2 degrees warmer world for one year or two years, but what about ten or twenty?
As climate change negotiations again come into the public agenda, we ought to resist the temptation to fall into the 2 degree trap. It is not a world we want to create, nor is it likely one we will enjoy. Instead, our efforts ought to be oriented toward reducing greenhouse emissions as quickly as possible. Likewise, the idea of a carbon budget is valuable, but we need to remember that our goal is to underspend our budget, not to push right up against its limits. Given our lack of knowledge of tipping points, being precautionary is warranted. Of course, my economist friends will point to the inefficiencies of not using the entire budget. But when I weigh precaution again efficiency, in this instance, I would argue that we ought to choose precaution.
Neither a 2 degree surface warming target nor a 1,000 GT carbon budget are sufficient indicators or targets to steer such an important global negotiation such as this. Instead, a multitude of indicators and measures ought to be used to make sure we are on the right path. Fortunately, the USGS along with NOAA has been leading an effort among scientists to select the best indicators for climate health. A report on the indicators this project recommended is here: https://www.ncdc.noaa.gov/indicators/
directs research at the Energy Trans Lab