How do we determine a dangerous level of climate change?

As you can probably guess, this question is too big to really answer fully in one blog post (in fact you might struggle to cover every aspect in an entire series). So to avoid the risk of turning this blog into a textbook, I thought it would be best if I just cover the key interesting points:

A danger threshold

The UN’s definition of dangerous climate change really frames the issue around a kind of “acceptability threshold”. It promotes the idea that there is a distinct and quantifiable level of climate change, beyond which the impacts cross over into being dangerous. And in some contexts, this approach makes sense.

Thresholds are a major part of ecological research, especially when considering extinctions. The idea of Minimum Viable Population, where species populations can essentially be pushed to a threshold point beyond which they are doomed to go extinct without intervention, is a really fascinating one. At the same time, a lot of research is going into biome thresholds. Marengo et al., 2011 (as well as many others) have discussed the possibility of the Amazonian rain forest experiencing a tipping point into a savanna like state in response to continued warming (save the rain forest campaigns don’t seems quite as melodramatic knowing that).

Threshold thinking can also be useful when considering climate change impacts on the human world. Gitay et al., 2001 discuss the idea that global agricultural production may actually benefit from small levels of global warming, but transition into dangerous impacts at greater levels.

The table below (taken from the IPCC AR4 report) showcases a number of key impact thresholds and how it is possible to therefore to think about these issues from the “acceptability threshold” approach.

The issue is that this is not the case for all climate change impacts (Schellnhuber, 2006).Climate change is already believed to have contributed to the ‘global burden of disease and premature death’. Similarly, there is growing evidence of a ‘climate change signal’ in 20^th Century extinctions (Parmesan and Yohe, 2003). The point I’m trying to make then is that, in many respects, climate change impacts are already dangerous by the UN’s definition and appear to have already crossed an acceptability threshold.

Considering this, the issue of dangerous climate change transitions from one of “at what level do the impacts become dangerous” to “what level of danger is acceptable”. And as I’m sure you can see, this completely changes the issue at hand from an objective and generally quantifiable consideration, into a largely subjective and ethics heavy debate.

Take, for example, the issue of human health and climate change:

“World health experts have concluded with ‘‘very high confidence’’ that climate change already contributes to the global burden of disease and premature death.” (IPCC, 2007)

Put yourself in the shoes of a policy-maker informed of this. How would you go about establishing a level of unacceptable climate change? There are proposed solutions to this dilemma, (I’ve put a list of some interesting stuff below) but ultimately the point I want to get across is that:

• For many climate change impacts, a level of climate change considered dangerous becomes strongly dependent upon ethics and the opinions of ‘particular policy-makers’ (Schneider et al., 2006)

This Burning Ember’s diagram really expresses the issue, especially when compared to the more quantitative table above. While certain aspects of climate change conform to clear-cut threshold thinking (as proposed by the UN) the vast majority of impacts involve an element of subjectivity ( In fact this diagram was controversially excluded from the IPCC AR4 report because of this factor).The result is a difficulty in creating an agreeable and clear-cut target of dangerous climate change which we should avoid.

Hopefully you have now reached the point where you are rightfully asking "why have I heard so much about avoiding 2 degrees of warming then?". Well in my next post I'll go over that question, drawing everything I talked about back to the issue of a stabilisation target.  

Secret Link!

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Climate Change and Health:

An interesting paper on the differing regional effects on climate change on health. I raises more questions of whether it is useful to consider a dangerous level of CC on a global level, when the impacts are so closely tied to individual regions.  http://www.nature.com/nature/journal/v438/n7066/abs/nature04188.html

This chapter from the book “Coping with Global Environmental Change, Disasters and Security” is a good summary

 of some of the ethics of risk management in the context of Climate Change. It is long, but has been written with policy-makers in mind so is definitely worth a read.

http://www.springer.com/cda/content/document/cda_downloaddocument/9783642177750-c1.pdf?SGWID=0-0-45-1068963-p174086661