After years of resisting efforts to define a dangerous level of warming in international climate discussions, the United States joined with the rest of the world’s major industrial powers on Wednesday in a (non-binding) pledge to avoid warming the planet beyond a threshold long favored by European governments and many climate campaigners as a no-go zone.
The chosen danger zone, derived from a host of scientific studies over the last two decades, lies 2 degrees Celsius (3.6 degrees Fahrenheit) beyond the planet’s average temperature in 1850 or so. (That translates to about a 2 degree Fahrenheit warming from today’s global average, by some ways of measuring, of about 59 degrees.)
But, given the persistent lack of clarity on how much the world will warm from a certain buildup of greenhouse gases and the divergent views around the world on what an ideal climate is in any case, is this threshold meaningful or useful? (And of course there’s the question of whether it’s an utterly wishful goal given emissions trends and energy options, as Richard Black explores on his BBC blog.)
The Intergovernmental Panel on Climate Change, proscribed from making judgments about how much warming is too much by its charter, has focused on laying out what could happen as warming proceeds, degree by degree, demarcating a list of “reasons for concern” that build rapidly after such a warming. The purpose of the panel’s “burning embers” diagram was to convey how dangers build without telling governments which point was excessive.
It’s not hard to find climate scientists and policy experts who strongly feel the world needs to move rapidly to curb emissions, but who say there are no such clean lines. Some go further, saying that setting such thresholds can be counterproductive. I touched on this issue a few months ago in my article and Dot Earth post examining “tipping points” in the climate system.
Back in 2006, Gavin Schmidt, the NASA scientist and Realclimate.org blogger, critiqued thresholds in a piece with a great title, “Runaway Tipping Points of No Return”:
This can lead to two seemingly opposite, and erroneous, conclusions – that nothing will happen until we reach the ‘point’ and conversely, that once we’ve reached it, there will be nothing that can be done about it. i.e. it promotes both a cavalier and fatalistic outlook. However, it seems more appropriate to view the system as having multiple tipping points and thresholds that range in importance and scale from the smallest ecosystem to the size of the planet. As the system is forced into new configurations more and more of those points are likely to be passed, but some of those points are more globally serious than others.
Stephen H. Schneider, the Stanford climatologist I’ve been interviewing since 1988 on this issue, has long favored pursuing climate policies that reflect the overall reality that the risk of bad outcomes rises with gas concentrations:
I like to use the analogy to a kids skateboard park, where the ramp starts up slowly, and gets non-linearly steeper until it is vertical at the top and the kid jumps and the parents hide their eyes! In other words, there are many such threshold tipping points in the bio-geophysical-social system, but the problem is we don’t know precisely where they are — ergo the need to frame it probabilistically and my skateboard ramp is an analogy to the steepening threats as we add warming. The warmer we get the more systems there are at risk and the deeper the impacts.
Kenneth Caldeira of the Carnegie Institution and Stanford University says that implying, as many do, that the 2-degree threshold is defined by science and not politics and values is unwise at best:
This presents a value judgment as if it’s a scientific conclusion. I would personally be in favor of a ban on new CO2-emitting devices, but I wouldn’t think of presenting that as a scientific result. We still don’t know for a CO2 doubling whether Earth will warm 2 degrees, 4 degrees or whatever. And we don’t know exactly how the Earth is going to respond to that 2 or 4 degrees of warming. The situation is that we’re entering risky territory and every emission pushes us deeper into that territory faster.
Here’s more from Realclimate.org on the merits and drawbacks of 2 degrees:
We feel compelled to note that even a “moderate” warming of 2°C stands a strong chance of provoking drought and storm responses that could challenge civilized society, leading potentially to the conflict and suffering that go with failed states and mass migrations. Global warming of 2°C would leave the Earth warmer than it has been in millions of years, a disruption of climate conditions that have been stable for longer than the history of human agriculture. Given the drought that already afflicts Australia, the crumbling of the sea ice in the Arctic, and the increasing storm damage after only 0.8°C of warming so far, calling 2°C a danger limit seems to us pretty cavalier.
Realclimate.org authors also said (in 2005):
Is there a “point of no return” or “critical threshold” that will be crossed when the forcings exceed this level, as reported in some media? We don’t believe there is scientific evidence for this. However, as was pointed out at an international symposium on this topic last year in Beijing by Carlo Jaeger, setting a limit is a sensible way to collectively deal with a risk. A speed limit is a prime example. When we set a speed limit at 60 mph, there is no “critical threshold” there – nothing terrible happens if you go to 65 or 70 mph, say. But perhaps at 90 mph the fatalities would clearly exceed acceptable levels. Setting a limit to global warming at 2ºC above pre-industrial temperature is the official policy target of the European Union, and may seem a sensible limit in this sense. But, just like speed limits, it may be difficult to adhere to.