But how did this specific value emerge? Why was it selected? Is it the outcome of a political negotiation process? A scientific fact? What if we overshoot this amount of change?
On May 20, we have started a series of seminars looking into this issue. During this meeting a presentation on the history of the concept was given followed by some discussion on how to proceed.
This idea of using past climate range, especially the relatively stable Holocene interglacial epoch, as a “safe operating space for humanity” has stuck. Indeed Johan Rockström and colleagues in their 2009 Nature paper and 2015 Science update extended the idea to 9 distinct planetary boundaries (climate change; ocean acidification; ozone; nutrients; fresh water; land use; biodiversity; aerosols and chemical pollution).
While simply using the Holocene as a template for a 'safe' climate could be disregarded as fear of the unknown, a more evidence-based approach is too look at the effects of climate change, and the risks involved. This approach was pioneered in a study carried out by a predecessor of the IPCC in the early 1990's, in which it was argued that major threats associated with climate change are a rapid sea level rise and the inability of ecosystems to migrate along with global temperature rise which is either too rapid or too strong (Rijsberman and Swart, 1990).
The arguments that climatic excursions into non-Holocene territory are associated with unacceptable risks approach landed in fertile soil: in 1995 the German Advisory Council on Global Change (WBGU) warned that “if this [Quaternary] temperature range is exceeded in either direction, dramatic changes in the composition and function of today’s eco-systems can be expected” (WBGU, 1995). Based on an acceptable (Quaternary) temperature window of 9.9 to 16.6 °C, a 1995 global average temperature of 15.3 °C and an amount of warming of 0.7 °C that already occurred since the pre-industrial age, this resulted in the familiar 2 °C target.
Subsequently, this warning by the WBGU was picked up by Angela Merkel and other EU leaders, resulting in the formal adoption of the 2 °C norm by the EU Council: “Given the serious risk of such an increase and particularly the very high rate of change, the Council believes that global average temperatures should not exceed 2 degrees above pre-industrial level”.
Meanwhile, a different approach was explored also, based on economical rather than ecological arguments. Cost-benefit models were used to assess how expensive climate change policy would be, taking into account current investments and future damage. However, the concept linking these two time frames, the so-called discount rate, proved to be a continuing source of fierce debate, culminating in the critique on the well-known 2006 Stern Review on the Economics of Climate Change focussing on this type of analysis. Ironically, though, one of the major opposing view on which discount rate to use is due to the same Nordhaus that `invented' the 2 °C target some 30 years before.
Nevertheless, the 2 °C target was established firmly in political reality and acts as the main guiding principle for all global change policy, despite all possible critique on both the ecological 'catastrophic' or the economical 'cost-benefit' approach. Jaeger and Jaeger, in their 2011 review of the 2 °C target concept, acknowledge the arbitrariness of a fixed threshold value separating `safe' from 'dangerous' but hightlight it's use a a so-called “focal point” that can motivate and structure practical steps forward.
In conclusion, the 2 °C target is neither a scientific fact (although many policy makers treat it like that) nor a political choice (despite many scientists think it is) but the result of a rather contingent interplay between science and policy. Despite the arbitrariness involved, “working towards a 2°C target provides as good a focus as is currently needed” (Jaeger and Jaeger, 2011).
How to proceed
Despite the usefulness of the current 2°C target as a pivot in climate change policy, it is, from a scientific point of view, still interesting to understand more of the reasoning underlying climate change effect assessments. How reliable are the methodologies? Are there knowledge gaps? What are the opportunities for climate adaptation? How do we align adaptation and mitigation measures? Over the next few weeks we will continue this debate.
To be continued...