4.4 Predicting Climate Change

In short, the projections made by climate models have generally been proved correct by real data. Depending on the scenario on which the prediction is based (see below) the outlook is not very good…

The Executive Summary, part of the Introduction to the IPCC Fifth Assessment Report provides a very concise roundup to the predictions, observed recordings and the modelling behind the report. It’s definitely worth a read.

An example of an article in the general Press is here, from the Guardian last July. It discusses the accuracy of climate model predictions and the possible reasons to explain differences in predicted climate and actual climate, when these have been present. It is based upon a paper led by James Risbey in Nature Climate Change, which “takes a clever approach to evaluating how accurate climate model temperature predictions have been while getting around the noise caused by natural cycles”. 

Projections are run for different scenarios for the climate. Each can have a very different outcome. Here are some examples of different scenarios from the IPCC and their predictions for effects on temperature and sea level rise. These are taken from the IPCC Fourth Report in 2007. Each report is based on slightly different data which alters as the technology gets better and more accurate.

Table SPM.3. Projected global average surface warming and sea level rise at the end of the 21st century. {10.5, 10.6, Table 10.7}

	Temperature Change		Sea Level Rise)
	(°C at 2090-2099 relative to 1980-1999)a		(m at 2090-2099 relative to 1980-1999)
Case	Best estimate	Likely range	Model-based range excluding future
			rapid dynamical changes in ice flow
Constant Year 2000 concentrationsb	0.6	0.3 – 0.9	NA
B1 scenario	1.8	1.1 – 2.9	0.18 – 0.38
A1T scenario	2.4	1.4 – 3.8	0.20 – 0.45
B2 scenario	2.4	1.4 – 3.8	0.20 – 0.43
A1B scenario	2.8	1.7 – 4.4	0.21 – 0.48
A2 scenario	3.4	2.0 – 5.4	0.23 – 0.51
A1FI scenario	4.0	2.4 – 6.4	0.26 – 0.59

MULTI-MODEL AVERAGES AND ASSESSED RANGES FOR SURFACE WARMING

Figure SPM.5. Solid lines are multi-model global averages of surface warming (relative to 1980–1999) for the scenarios A2, A1B and B1, shown as continuations of the 20th century simulations. Shading denotes the ±1 standard deviation range of individual model annual averages. The orange line is for the experiment where concentrations were held constant at year 2000 values. The grey bars at right indicate the best estimate (solid line within each bar) and the likely range assessed for the six SRES marker scenarios. The assessment of the best estimate and likely ranges in the grey bars includes the AOGCMs in the left part of the figure, as well as results from a hierarchy of independent models and observational constraints. {Figures 10.4 and 10.29}