Two degrees of separation

One of the “magic numbers” in energy and environmental issues is 2C, as in we can only allow 2C degrees of warming over pre-industrial levels before we trigger truly nasty consequences.

I’m sure that like me, most people reading this site have seen this number quoted endlessly. But where did it come from, I wondered, and how valid is it?

I’ll save you the click-by-click replay of my online sleuthing, but I think I managed to track it down, using official or trustworthy sources.

The Worldwatch Institute’s State of the World 2009 says (page 18):

Deciding what level of climate change is dangerous and what might be safe is not a purely scientific question. It involves normative and political judgments about acceptable risks. Science has, however, a fundamental role to play in providing information and analysis relevant to this question and has contributed to policy and political debates on acceptable levels of climate change since the 1980s.

By the late 1980s the scientific community had begun to recognize that a warming of much more than 1–2 degrees Celsius over the preindustrial level could lead to rapid and adverse changes to many human and natural systems. In 1986 the U.N. Environment Programme set up an Advisory Group on Greenhouse Gases, which in 1990 reported that a 2-degree warming could be “an upper limit beyond which the risks of grave damage to ecosystems, and of non-linear responses, are expected to increase rapidly.” Also in the late 1980s the Enquete Komission, a joint committee of German parliamentarians and scientists, sought to define acceptable limits. Warming more than 0.1 degree Celsius per decade was seen as especially risky to forest ecosystems, with an overall acceptable maximum warming estimated to be 1–2 degrees Celsius. In 1995 the German government’s Global Change Advisory Council found that 2 degrees Celsius should be the upper limit of “tolerable” warming.

Efforts to define acceptable limits to warming at a political level started in the European Union and among its member states. Based on the IPCC’s Second Assessment Report at the end of 1995, the European Union’s Council of Environment Ministers in 1996 called for warming to be limited to 2 degrees Celsius above the preindustrial level. Nearly a decade later this position was confirmed by European Union Heads of Government after consideration and debate over the findings of the IPCC’s 2001 Third Assessment Report, as well as more recent scientific developments. Since 2005 other countries have joined in calling for global mean warming to be limited to 2 degrees: Chile, Iceland, Norway, Switzerland, the Least Developed Countries, and Small Island Developing States. The latter two groups of countries have argued that 2 degrees may in fact be too much warming if their safety and survival are to be guaranteed.

I also stumbled upon this presentation by Bill Hare, “The EU, the IPCC and 2C” [26 page, 6.3MB PDF] from 2008, which gives the history of the magic 2C number, picking up the trail with a 1989 UNEP report.

If you want to do some of your own detective work, one good place to start is with a Google search for “1939th European Council Meeting Luxembourg 25 June 1996″.

I think it’s reasonable to conclude from these and other sources:

• The 2C threshold is not a recent and untested finding. It’s been around for roughly 20 years.
• It’s been reexamined several times over that time span and found to be a reasonable metric.
• Despite all the science involved, as the State of the World 2009 quote above correctly points out, there’s more to it than science. We have to make some of those endlessly nasty value judgments, even before we add politics to the mix. How much should we expect China or the US to reduce their CO2 emissions if the other one is perceived as not doing “enough”? How much will or should the US or China or the EU do in response to climate impacts far from their (perceived) immediate interests? You can add your own thorny problems to the list.

But… that doesn’t feel to me like the whole story, for a fairly obvious reason: We’ve seen a steady drumbeat of “it’s worse than we thought” scientific findings over the last few years. Just the quicker melting of long-lived ice in Greenland and many glacier fields around the world, Arctic ice and permafrost should be enough to get our undivided attention, given the implications for positive feedbacks (albedo flip in the Arctic, permafrost bomb) as well as direct impacts (loss of fresh water supplies from glaciers).

So, what happened? Did the original, 20-year old assessment that 2C was the knee in the curve just happen to be right, even before two more decades of observation and study? And if so, then what did we know then that we somehow forgot in order to be surprised by these recent revelations? Or has the science community simply held onto 2C of temperature rise and 2100 as a planning horizon out of inertia?[1]

Could a lack of faith in this 2C metric be one reason why World will not meet 2C warming target, climate change experts agree:

Almost nine out of 10 climate scientists do not believe political efforts to restrict global warming to 2C will succeed, a Guardian poll reveals today. An average rise of 4-5C by the end of this century is more likely, they say, given soaring carbon emissions and political constraints.

Such a change would disrupt food and water supplies, exterminate thousands of species of plants and animals and trigger massive sea level rises that would swamp the homes of hundreds of millions of people.

The poll of those who follow global warming most closely exposes a widening gulf between political rhetoric and scientific opinions on climate change. While policymakers and campaigners focus on the 2C target, 86% of the experts told the survey they did not think it would be achieved. A continued focus on an unrealistic 2C rise, which the EU defines as dangerous, could even undermine essential efforts to adapt to inevitable higher temperature rises in the coming decades, they warned.

…

The poll asked the experts whether the 2C target could still be achieved, and whether they thought that it would be met: 60% of respondents argued that, in theory, it was still technically and economically possible to meet the target, which represents an average global warming of 2C since the industrial revolution. The world has already warmed by about 0.8C since then, and another 0.5C or so is inevitable over coming decades given past greenhouse gas emissions. But 39% said the 2C target was impossible.

I realize the scientists were not asked about the accuracy of the 2C guideline, but whether we could stay under that threshold. As asked, this is largely a question about the responsiveness of the world’s citizens and governments, not one of science. But I’d guess that many of those scientists also considered feedback mechanisms, one of the key areas where things are “worse than expected”. They complicate the situation significantly because they change the conversion rate, if you will, for translating a given amount of warming into real world human impacts. We have a pretty good handle on mapping a level of CO2 emissions to an initial amount of warming (despite what some of the deniers will tell you until your ears bleed), at least before the feedbacks get into the act. Once we take that second step and try to map warming to impacts, we’re dealing with more dynamic mechanisms, things get much more complicated. One prime example is glacial melt water draining through cracks and lubricating the underlying rocks, allowing the glaciers to move quicker and break up sooner.

Another issue here is this cognitive gap between what scientists really think and what they’re willing to stake their reputations on by saying publicly. Answering a newspaper reporter’s poll is one thing, stating something as a scientific conclusion in a journal article is something else entirely. But as I pointed out recently (see Climate chaos, indeed), we’re now seeing some on-the-record assessments from the scientific community that we have almost zero chance of staying within 2C of warming, and should instead be focused on something closer to 4C.

The bottom line is I’m still not sure what to think about 2C. Even though the official consensus of the scientific community still seems to say that it’s the critical demarcation between “acceptable” and “unacceptable” human impacts (with all the fuzziness those judgments imply), it’s feeling more and more like the next magic number that will be adjusted in a direction we won’t like, just as our assessment of the critical level of atmospheric CO2 went from 550ppm to 450ppm, with James Hansen, among others, making the case that it should be 350 [PDF].

[1] Just to be painfully clear on this point: I’m not beating up the climate scientists by suggesting they’ve fallen into orbit around a point of conventional wisdom. We all do this, and while I’m sure scientists are much less susceptible to making this particular mistake than is the average layperson, I’m equally sure they’re not immune to it.

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