Is the climate chaotic

Throughout this chapter the implicit assumption has been made that climate change is predictable and that models can be used to provide predictions of climate change due to human activities. Before leaving this chapter I want to consider whether this assumption is justified.

The capability of the models themselves has been demonstrated so far as weather forecasting is concerned. They also possess some skill in seasonal forecasting. They can provide a good description of the current climate and its seasonal variations. Further, they provide predictions that on the whole are reproducible and that show substantial consistency between different models

- although it might be argued, some of this consistency could be a property of the models rather than of the climate. Further over several recent decades for which comparison with observations is possible the predictions show good agreement with observations. But is there other evidence to support the view that climate change is predictable, particularly for the longer term?

A good place to look for further evidence is in the record of climates of the past, presented in Chapter 4. Correlation between the Milankovitch cycles in the Earth's orbital parameters and the cycles of climate change over the past half million years (see Figures 4.7 and 5.19) provides strong evidence to substantiate the Earth's orbital variations as the main factor responsible for the triggering of major climate changes

- although the nature of the feedbacks which control the very different amplitudes of response to the three orbital variations still need to be understood. Some 60 ± 10% of the variance in the record of global average temperature from palaeontologi-cal sources over the past million years occurs close to frequencies identified in the Milankovitch theory. The existence of this surprising amount of regularity suggests that the climate system is not strongly chaotic so far as these large changes are concerned, but responds in a largely predictable way to the Milankovitch forcing.

This Milankovitch forcing arises from changes in the distribution of solar radiation over the Earth because of variations in the Earth's orbit. Changes in climate as a result of the increase of greenhouse gases are also driven by changes in the radiative regime at the top of the atmosphere. These changes are not dissimilar in kind (although different in distribution) from the changes that provide the Milankovitch forcing. It can be argued therefore that the increases in greenhouse gases will also result in a largely predictable response.

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Figure 5.23 Warming of the top 700 m of three of the oceans resulting from natural (solar variations and volcanic eruptions) and anthropogenic (greenhouse gases and aerosols) forcing at the surface over the period 1961-2003. Observations (red dots) of warming compared with modelled changes from both natural and anthropogenic forcing (hatched regions in (a)) and from natural forcing only (green triangles in (b)). The hatched regions in (b) represent the 90% confidence limits of natural internal variability. The ranges of model estimates in (a) are taken from 4 runs (denoted by green dots) of the HadCM3 model at the UK Hadley Centre.

Precipitation (mm day-1)

Figure 5.24 Patterns of present-day winter precipitation over Great Britain, (a) as simulated with a 300-km resolution global model, (b) with 50-km resolution regional model, (c) as observed with 10-km resolution.

Precipitation (mm day-1)

Figure 5.24 Patterns of present-day winter precipitation over Great Britain, (a) as simulated with a 300-km resolution global model, (b) with 50-km resolution regional model, (c) as observed with 10-km resolution.

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