The challenge of agreeing on degrees

Andy Revkin has a nice short summary of the wisdom, or lack thereof, of pledging to avoid climate warming of 2 deg C or any other threshold beyond which climate disaster looms.

The problem is that not only is there no one firm threshold, but that even if there were, there is no reason to think we could agree on it. Yes, the uncertainty in scientific predictions is part of the problem. But the real issue is that there is no such thing as a good climate and a bad climate.

Vulnerability to climate warming varies not only between communities and ecosystems, but also between different people in any one community and different species or groups of species in any one ecosystem. Beyond that, there is an important are too often neglected difference between the perceived and actual vulnerability to climate change. Your conclusion about the limits of acceptable warming comes is determined not by how you or your community will be affected by climate warming, but how you think you or your community will be affected by climate warming, and how you think you or your community can respond.

Even for a system as vulnerable to changes in climate and ocean chemistry as coral reefs, the line is hard to draw. From my recent paper on committed warming:

The overall results of this study can provide insight into the level of atmospheric greenhouse gas concentrations required to avoid degradation of coral reef ecosystems from frequent mass coral bleaching, a proposed definition of “dangerous anthropogenic interference” in the climate system [39]. Specific recommendations about future greenhouse gas emissions pathways and/or atmospheric stabilization levels require normative judgments about the acceptable damages to coral reefs and the metrics for characterizing those damages. A comparison of the results from the key scenarios in this study does, however, present an envelope of possible climate futures for the world’s coral reefs, presuming that the models realistically represent the response of the climate system to external forcing.

Science can give guidance on the impacts of climate change and, from that, provide some general recommendations on what level of warming might be acceptable, given different assumptions. That's a key to all science. The results depend on the assumptions. When you hear that 2 deg C is the maximum "acceptable" warming, you need to ask what are the assumptions that went into defining "acceptable". And because we will not all agree on those assumptions, we will not all agree that 2 deg C, or 1 deg C, or 1.349 deg C is the threshold beyond which danger lurks.

What we're left with is a value judgment. In this case, the 2 deg C threshold is a convenient backstop, the sort of nice round number that works in policy discussions.