The IPCC's AR5 is grim in what it observes and projects, yet it represents an overly cautious projection of climate change, with a mean increase of only 1.0°C to 3.7°C by 2100. Conservatism is built into the organization's consensus structure, which tends to produce a lowest common denominator on which a large number of scientists can agree.
And, this will not be the first time the IPCC has erred on the side of caution. The last major UN assessment, AR4 in 2007, projected a controversial low-ball sea level rise. However, AR4 predicted runaway temperature rises of 6°C or more by the end of the century.
That is now thought unlikely by AR5 scientists. They project that average land and sea temperatures may possibly reach 4°C above present levels – enough to devastate crops and make life in many cities unbearably hot.
A main focus of AR5 is convincing policymakers to take actions that will limit global surface temperature increase to around 2°C. The IPCC scientists found that to hold warming to 2°C, total emissions cannot exceed 1,000 gigatonne (2,204 trillion lb) of carbon. Yet by 2011, more than half of that total allowance, 531 gigatonne (1,170 trillion lb), had already been emitted.i And, global industrial emissions since 1751 may already be as high as 1,450 gigatonne (3,196 trillion lb).ii
The AR5 authors themselves claim that with business-as-usual emissions, global 2°C warming threshold will be breached within 30 years, by 2043. Furthermore, the slow progress of global climate talks makes the IPCC's 'carbon budget', and so-called safe limit of a 2°C rise, impossible to keep.
After a slight dip in GHG emissions caused by an economic recession, emissions are rising again. Moreover, the impacts associated with 2°C have been revised upwards so that 2°C now represents the threshold of extremely dangerous climate change.iii
In choosing the conservative approach of trying to stay under 2°C, the IPCC report fails to explore the catastrophic impacts and vulnerability in the business as usual case where the Earth warms up 4°C to 5°C (7°F to 9°F) more. And, AR5 has less to say about even higher warming, which the latest science suggests we are headed toward.
iFiona Harvey. 2013. "IPCC: 30 years to climate calamity if we carry on blowing the carbon budget." The Guardian. September 23.