Validation of the model

In discussing various aspects of modelling we have already indicated how some validation of the components of climate models may be carried out.17 The successful predictions of weather forecasting models provide validation of important aspects of the atmospheric component, as do the simulations mentioned earlier in the chapter of the connections between sea surface temperature

The ocean's deep circulation

For climate change over periods up to a decade, only the upper layers of the ocean have any substantial interaction with the atmosphere. For longer periods, however, links with the deep ocean circulation become important. The effects of changes in the deep circulation are of particular importance.

Experiments using chemical tracers, for instance those illustrated in Figure 5.20 (see next box), have been helpful in indicating the regions where strong coupling to the deep ocean occurs. To sink to the deep ocean, water needs to be particularly dense, in other words both cold and salty. There are two main regions where such dense water sinks down to the deep ocean, namely in the north Atlantic Ocean (in the Greenland Sea between Scandinavia and Greenland and the Labrador Sea west of Greenland) and in the region of Antarctica. Salt-laden deep water formed in this way contributes to a deep ocean circulation that involves all the oceans (Figure 5.18) and is known as the thermohaline circulation (THC).

In Chapter 4 we mentioned the link between the THC and the melting of ice. Increases in the ice melt can lead to the ocean surface water becoming less salty and therefore less dense. It will not sink so readily, the deep water formation will be inhibited and the THC is weakened. In Chapter 6, the link between the THC and the hydrological (water) cycle in the atmosphere is mentioned. Increased precipitation in the North Atlantic region, for instance, can lead to a weakening of the THC.

Figure 5.18 Deep water formation and circulation - sometimes known as the ocean 'conveyor belt' - connecting the oceans together. The deep salty current (blue) largely originates in the Nordic Seas and the Labrador Sea where northward flowing water (red) near the surface that is unusually salty becomes cooler and even more salty through evaporation, so increasing its density causing it to sink. Regions of upwelling in the southern ocean feed into the warm surface current (red).

Figure 5.18 Deep water formation and circulation - sometimes known as the ocean 'conveyor belt' - connecting the oceans together. The deep salty current (blue) largely originates in the Nordic Seas and the Labrador Sea where northward flowing water (red) near the surface that is unusually salty becomes cooler and even more salty through evaporation, so increasing its density causing it to sink. Regions of upwelling in the southern ocean feed into the warm surface current (red).

anomalies and precipitation patterns in some parts of the world. Various tests have also been carried out of the ocean component of climate models; for instance, through comparisons between the simulation and observation of the movement of chemical tracers (see box below).

Once a comprehensive climate model has been formulated it can be tested in three main ways. Firstly, it can be run for a number of years of simulated time and the climate generated by the model compared in detail to the current climate. For the model to be seen as a valid one, the average distribution and the seasonal variations of appropriate parameters such as surface pressure, temperature and rainfall have to compare well with observation. In the same way, the variability of the model's climate must be similar to the observed variability. Climate models that are currently employed for climate prediction stand up well to such comparisons.

Recent progress in model performance has been evident in improved simulations of modes of climate variability on the large scale and from intraseasonal to interdecadal timescales. This is of particular importance because of the links that are likely between variations in modes such as the northern and southern annular modes (NAM and SAM) and the ENSO (El Niño Southern Oscillation) and the growth of atmospheric greenhouse gases.18 Progress with the prediction of ENSO events and associated climate anomalies was mentioned earlier in the chapter.

Secondly, models can be compared against simulations of past climates when the distribution of key variables was substantially different from that at present; for example, the period around 9000 years ago when the configuration of the Earth's orbit around the Sun was different (see Figure 5.19). The perihelion (minimum Earth-Sun distance) was in July rather than in January as it is now; also the tilt of the Earth's axis was slightly different from its current value (24° rather than 23.5°). Resulting from these orbital differences (see Chapter 4), there were significant differences in the distribution of solar radiation throughout the year. The incoming solar energy when averaged over the northern hemisphere was about 7% greater in July and correspondingly less in January.

When these altered parameters are incorporated into a model, a different climate results. For instance, northern continents are warmer in summer and colder in winter. In summer a significantly expanded low-pressure region develops over north Africa and south Asia because of the increased land-ocean temperature contrast. The summer monsoons in these regions are strengthened and there is increased rainfall. These simulated changes are in qualitative agreement with palaeoclimate data; for example, these data provide evidence for that period (around 9000 years ago) of lakes and

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9000 years ago Present

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Figure 5.19 (a) Changes in the Earth's elliptical orbit from the present configuration to 9000 years ago and (b) changes in the average solar radiation during the year over the northern hemisphere.

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Figure 5.19 (a) Changes in the Earth's elliptical orbit from the present configuration to 9000 years ago and (b) changes in the average solar radiation during the year over the northern hemisphere.

vegetation in the southern Sahara about 1000 km north of the present limits of vegetation.

The accuracy and the coverage of data available for these past periods are limited. However, the model simulations for 9000 years ago, described above, and those for other periods in the past have demonstrated the value of such studies in the validation of climate models.19

A third way in which models can be validated is to use them to predict the effect of large perturbations on the climate such as occurs, for instance, with volcanic eruptions, the effects of which were mentioned in Chapter 1. Several climate models have been run in which the amount of incoming solar radiation has been modified to allow for the effect of the volcanic dust from Mount Pinatubo, which erupted in 1991 (Figure 5.20). Successful simulation of some of the regional anomalies of climate which followed that eruption, for instance the unusually cold winters in the Middle East and the mild winters in western Europe, has also been achieved by the models.20

In these three ways, which cover a range of timescales, confidence has been built in the ability of models to predict climate change due to human activities.

The 12 June 1991 eruption column from Mount Pinatubo taken from the east side of Clark Air Base.

Figure 5.20 The predicted and observed changes in global land and ocean surface air temperature after the eruption of Mount Pinatubo, in terms of three-month running averages from April to June 1991 to March to May 1995.

■ Observed land surface air temperature and night marine air temperature (relative to April-June 1991) Model predictions

■ Observed land surface air temperature and night marine air temperature (relative to April-June 1991) Model predictions

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Modelling of tracers in the ocean

A test that assists in validating the ocean component of the model is to compare the distribution of a chemical tracer as observed and as simulated by the model. In the 1950s radioactive tritium (an isotope of hydrogen) released in the major atomic bomb tests entered the oceans and was distributed by the ocean circulation and by mixing.

Figure 5.21 shows good agreement between the observed distribution of tritium (in tritium units) in a section of the western North Atlantic Ocean about a decade after the major bomb tests and the distribution as simulated by a 12-level ocean model. Similar comparisons have been made more recently of the measured uptake of one of the freons CFC-11, whose emissions into the atmosphere have increased rapidly since the 1950s, compared with the modelled uptake.

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Figure 5.21 The tritium distribution in a section of the western North Atlantic Ocean approximately one decade after the major atomic bomb tests, as observed in the GEOSECS programme (a) and as modelled (b).

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Figure 5.21 The tritium distribution in a section of the western North Atlantic Ocean approximately one decade after the major atomic bomb tests, as observed in the GEOSECS programme (a) and as modelled (b).

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