Inside Science >> When a 2°C rise isn't a 2°C rise

Limiting global temperature increases to below plus 2°C compared with the pre-industrial average is a long-standing goal for international climate-change mitigation efforts.

A change in global average temperatures of plus 2°C above pre-industrial levels does not sound dramatic to most people. It is misguided , however, as changes in global average annual temperatures can mean much larger changes regionally, with greater extremes.

The UK summer heatwave in 2003 led to more than 2,000 premature deaths in the UK, with the highest ever recorded maximum temperature of 38.5°C occurring in Faversham; the average summer temperature increase across the UK was just plus 2°C.

Research by the Met Office shows that for a 2°C global rise, the hottest day in a UK summer could increase by up to 8°C. In general, a global average of 2°C warming comprises greater than average warming over land and lower than average over oceans, but there is much local variation.

The Intergovernmental Panel on Climate Change in its fourth assessment report (released in 2007) suggested that 20%–30% of species could ultimately face extinction with temperature increases of 1.5–2.5°C.

One of the striking things about the report was the shift in linking significant impacts to relatively smaller degrees of average temperature change compared with earlier assessments.

In other words, as more evidence of climate impacts accumulates, it is becoming increasingly clear that a 2°C warming raises significant issues in a number of areas, and the assessment of “dangerous” levels of change has been shifted downwards, towards lower temperature increases.

In polar regions, for example, a global average temperature rise of 2°C could result in temperature increases of as much as 6°C, which includes feedback where the normal “albedo effect” – ice reflecting much incoming solar radiation – is reduced due to ice melt.

There are large uncertainties over the amount of sea level rise that may occur as a result of melting ice caps in Antarctica and Greenland. Different studies suggest ranges of sea level rise between 0.5–2 metres by 2100 if temperatures rise by less than 2°C.

Given the current lack of sufficient commitments globally to reduce greenhouse-gas emissions at the required rate to have even a 50% chance of limiting warming to no more than 2°C, much research effort now is focused on the risks and consequences of warmer scenarios, and the challenges in adapting to these.