John Burn-Murdoch How the IPCC’s projections match up

This week scientists, policymakers and leaders meet in Stockholm to finalise The Physical Science Basis – the first part of the long-awaited fifth assessment report (known as AR5) by the UN’s Intergovernmental Panel on Climate Change

This report – the fifth of its kind in 25 years – sets out the latest state of thinking on the impact of climate change. The report’s projections, like those of its predecessors, will be heavily scrutinised in keeping with the controversies that surround this field of science.

Here, we show how the projections for temperature and sea level rise set out in the previous version of this report in 2007 compare with actual measurements. The chart shows that global average temperatures have not accelerated at the rate predicted in some scenarios but sea levels have risen at the upper end of predicted ranges.

The graphic also shows what happened to ice cover on the North and South Poles over the same period. No numerical projections were made for ice coverage, owing to a still developing understanding of ice sheet flow, but the report did state that “sea ice is projected to shrink in both the Arctic and Antarctic” over the course of the 21st century.


Emissions scenarios

The Special Report of Emissions Scenarios (2000) set out the emissions scenario ‘families’ used in the third and fourth IPCC reports. Each scenario is based on assumptions about global economic and technological development, and does not take into account proposed measures to limit emissions. No individual scenario was stated as being any more likely to occur than another. The scenarios used in the graphic were chosen to give a broad spread of conditions and for reasons of data availability.

They can be summarised as follows:
A1 – Rapid economic growth, population peaking in 2050 at 9bn, rapid adoption of new and efficient technologies, global convergence of incomes and lifestyles.
A2 – A fragmented world with regional differences in economic development, global population rising throughout the 21st Century.
B1 – Economic growth and population as in A1, but global economy moving to a focus on services and information, resulting in a lower material intensity and rise in clean technologies.
B2 – Continuous population rise but slower than in A2, emphasis on regional solutions to economic, social and environmental stability. Slower and more fragmented technological change than A1 or B1.
A1F1 – Variation on A1, where the global energy mix is dominated by fossil fuels.
A1T – Variation on A1, where the global energy mix is dominated by non-fossil energy sources.
A1B – Variation on A1, where the global energy mix is not dominated by any single energy source, and all energy supply and end-use technologies see similar rates of improvement.
Commit – Idealised scenario where the atmospheric burdens of long-lived greenhouse gasses stay at 2000 concentrations.

Data sources

Links to the specific datasets used in the graphic are as follows:
The Met Office Hadley Centre
Church and White (2011)
University of Colorado Boulder
CSIRO Marine and Atmospheric Research

Church, J. A. and N.J. White (2011), Sea-level rise from the late 19th to the early 21st Century. Surveys in Geophysics, doi:10.1007/s10712-011-9119-1