IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group III: Mitigation of Climate Change

1.2.2 What is dangerous interference with the climate system?

Defining what is dangerous interference with the climate system is a complex task that can only be partially supported by science, as it inherently involves normative judgements. There are different approaches to defining danger, and an interpretation of Article 2 is likely to rely on scientific, ethical, cultural, political and/or legal judgements. As such, the agreement(s) reached among the Parties in terms of what may constitute unacceptable impacts on the climate system, food production, ecosystems or sustainable economic development will represent a synthesis of these different perspectives.

Over the past two decades several expert groups have sought to define levels of climate change that could be tolerable or intolerable, or which could be characterized by different levels of risk. In the late 1980s, the World Meteorological Organization (WMO)/International Council of Scientific Unions (ICSU)/UN Environment Programme (UNEP) Advisory Group on Greenhouse Gases (AGGG) identified two main temperature indicators or thresholds with different levels of risk (Rijsberman and Swart, 1990). Based on the available knowledge at the time a 2ºC increase was determined to be ‘an upper limit beyond which the risks of grave damage to ecosystems, and of non-linear responses, are expected to increase rapidly’. This early work also identified the rate of change to be of importance to determining the level of risk, a conclusion that has subsequently been confirmed qualitatively (IPCC, 2007b, Chapters 4 and 19). More recently, others in the scientific community have reached conclusions that point in a similar direction ‘that global warming of more than 1°C, relative to 2000, will constitute “dangerous” climate change as judged from likely effects on sea level and extermination of species’ (Hansen et al., 2006). Probabilistic assessments have also been made that demonstrate how scientific uncertainties, different normative judgments on acceptable risks to different systems (Mastrandrea and Schneider, 2004) and/or interference with the climate system (Harvey, 2007) affect the levels of change or interference set as goals for policy (IPCC, 2007b, Chapter 19). From an economic perspective, the Stern Review (Stern, 2006) found that in order to minimise the most harmful consequences of climate change, concentrations would need to be stabilized below 550 ppm CO2-eq. The Review further argues that any delay in reducing emissions would be ‘would be costly and dangerous’. This latter conclusion is at variance with the conclusions drawn from earlier economic analyses which support a slow ‘ramp up’ of climate policy action (Nordhaus, 2006) and, it has been argued, is a consequence of the approach taken by the Stern Review to intergenerational equity (Dasgupta, 2006).

The IPCC Third Assessment Report (TAR) identified five broad categories of reasons for concern that are relevant to Article 2: (1) Risks to unique and threatened systems, (2) risks from extreme climatic events, (3) regional distribution of impacts, (4) aggregate impacts and (5) risks from large-scale discontinuities. The Fourth Assessment Report (AR4) focuses on Key Vulnerabilities relevant to Article 2, which are broadly categorized into biological systems, social systems, geophysical systems, extreme events and regional systems (IPCC, 2007b, Chapter 19). The implications of different interpretations of dangerous anthropogenic interference for future emission pathways are reviewed in IPCC (2007b), Chapter 9 and also in Chapter 3 of this report. The literature confirms that climate policy can substantially reduce the risk of crossing thresholds deemed dangerous (IPCC, 2007b, SPM and Chapter 19; Chapter 3, Section 3.5.2 of this report).

While the works cited above are principally scientific (expert-led) assessments, there is also an example of governments seeking to define acceptable levels of climate change based on interpretations of scientific findings. In 2005, the EU Council (25 Heads of Government of the European Union) agreed that – with a view to achieving the ultimate objective of the Convention – the global annual mean surface temperature increase should not exceed 2ºC above pre-industrial levels (CEU, 2005).