ABSTRACT
When the UN’s Framework Convention on Climate Change was adopted in 1994 with a mandate to ‘prevent dangerous
anthropogenic interference with the climate system ... within a time frame sufficient to allow ecosystems to adapt naturally
to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner’, the term ‘dangerous’ was left undefined.1 In its Third Assessment Report in 2001 the IPCC suggested
that a global rise in temperature of 3°C over the next century
would probably be manageable.2 Since then, however, there has been considerable convergence of opinion among scientists, economists and policymakers that the global temperature
increase should be contained to 2°C above pre-industrial levels
– a ‘guardrail’ beyond which ‘the possibilities for adaptation
of society and ecosystems rapidly decline, with an increasing risk of social disruption through health impacts, water shortages and food insecurity’.3 The choice of 2°C was based on the
IPCC’s synthesis of five ‘reasons for concern’ – risks to unique
and threatened systems, risks from extreme climate events,
distribution of impacts, aggregate impacts, and risks from
future large-scale discontinuities – in the 2001 report. However,
an update of this analysis using the same methodology and the
most recent research suggests that the 2°C guardrail is insufficient to avoid dangerous impacts in the first two areas, and
that the risk of large-scale discontinuities at the 2°C level is
moderate rather than very low, as was previously reported.4 It is still possible to have a reasonable chance of keeping warming below the guardrail, but even the drastic emissions reductions
necessary to achieve this will have little effect on temperature
increases over the next few decades. These will approach if not
exceed a 1.5°C increase above pre-industrial levels, a target
advocated by most of the world’s most vulnerable countries in
negotiations at the 2009 climate conference in Copenhagen.
