5.7 What is a ‘safe’ limit of climate change?
One answer to this (not as glib as it sounds) is that ‘there isn't one’ – at least if we mean by that a safe limit of the enhanced greenhouse effect. Defining an acceptable temperature increase is a political issue, although it may be informed by science. But the relationship between stabilisation levels for CO2 concentration, emission reductions, carbon budgets and temperature rise is mainly based on our scientific understanding of how the climate works.
The question raises a number of serious equity issues, some of which are outlined below.
The IPCC Third Assessment Report (2001) identified five reasons for concern in which to consider climate change risks with the intensity of impacts increasing from 1 to 6°C. These are still valid, although recent work suggests that the temperature thresholds for impacts may need to be lowered, and the upper range extended to 7°C.
The five reasons are:
risks to unique and threatened systems – from ‘risks to some (0.6°C) to ‘risks to many’ (6°C)
risk of extreme weather events – from ‘increase’ (0.6°C) to ‘large increase’ (6°C)
distribution of impacts – from ‘negative for some regions, positive for others’ (0.6°C) to ‘negative for most regions’ (6°C)
aggregate economic impacts – from ‘positive or negative market impacts; majority of people adversely affected’ (0.6°C) to ‘net negative in all metrics’ (6°C)
risks of large-scale discontinuities – from ‘very low’ (0.6°C) to ‘higher’ (6°C)
The notion of a ‘safe’ level of climate change goes to the heart of current policy debates. Current focus remains on the desire to ‘avoid dangerous climate change’ – the phrase at the core of the UN Framework Convention on Climate Change. That limit is commonly expressed as a rise in global average mean surface temperature of 2°C above pre-industrial levels.
However, we can already see that the impacts of climate change are not evenly distributed. Current impacts associated with an increase of 0.74°C over the last 100 years are already having serious impacts on a number of ecosystems and countries in different parts of the world. To the people affected by recent climate change, even this apparently small increase due to human activity looks pretty ‘dangerous’.
As authors on RealClimate put it
even a ‘moderate’ warming of 2°C stands a strong chance of provoking drought and storm responses that could challenge civilized society, leading potentially to the conflict and suffering that go with failed states and mass migrations. Global warming of 2°C would leave the Earth warmer than it has been in millions of years, a disruption of climate conditions that have been stable for longer than the history of human agriculture. Given the drought that already afflicts Australia, the crumbling of the sea ice in the Arctic, and the increasing storm damage after only 0.8°C of warming so far, calling 2°C a danger limit seems to us pretty cavalier.
The increasing diversity and intensity of impacts associated with increases in global average mean surface temperature above 2°C is the basis for that interpretation of ‘dangerous’, especially as above that level the effects are likely to become more and more widespread and so noticeable in the developed countries of temperate latitudes.
Furthermore, there are a number of technical and scientific problems in relating 2°C to a particular concentration of atmospheric greenhouse gases (a ‘stabilisation target’), as explored in the next question.
The IPCC Fourth Assessment Report – ‘Impacts, Adaptation and Vulnerability: Summary for Policy Makers discusses diversity and intensity of impacts with increasing temperature.
RealClimate on ‘Hit the brakes hard’.
5.6 So how much warming is likely?