Comparison with observations

More than 20 centres in the world located in more than ten countries are currently running climate models of the kind I have described in which the circulations of the atmosphere and the ocean are fully coupled together. Some of these models have been employed to simulate the climate of the last 150 years allowing for variations in aspects of natural forcing (e.g. solar variations and volcanoes) and anthropogenic forcings (i.e. increases in the concentrations of greenhouse gases and aerosols).

Year Year

Figure 5.22 Global mean temperature anomalies relative to the period 1901-50 as observed (black) and from an ensemble of 58 simulations by 14 models with individual simulations shown by very thin lines. Simulations are shown with both anthropogenic and natural forcings (a) and with natural forcings only (b). The multimodel ensemble means are shown in red (a) and blue (b). Vertical lines indicate the timing of major volcanic events.

Year Year

Figure 5.22 Global mean temperature anomalies relative to the period 1901-50 as observed (black) and from an ensemble of 58 simulations by 14 models with individual simulations shown by very thin lines. Simulations are shown with both anthropogenic and natural forcings (a) and with natural forcings only (b). The multimodel ensemble means are shown in red (a) and blue (b). Vertical lines indicate the timing of major volcanic events.

Examples of such simulations are shown in Figure 5.22, where the observed record of global average surface air temperature is compared with model simulations taking into account the combination of natural and anthropogenic forcings and natural forcings on their own.

Three interesting features of Figure 5.22 can be noted. Firstly, that the inclusion of both natural and anthropogenic forcings provides a plausible explanation for a large part of the observed temperature changes over the last century, and that the inclusion of anthropogenic factors is essential to explain the rapid increase in temperature over the last 40 years. Further, it is likely that changes in solar output and the comparative absence of volcanic activity were the most important variations in natural forcing factors during the first part of the twentieth century. Secondly, the model simulations show variability up to a tenth of a degree Celsius or more over periods of a few years up to decades. This variability is due to internal exchanges in the model between different parts of the climate system, and is not dissimilar to that which appears in the observed record. Thirdly, due to the slowing effect of the oceans on climate change, the warming observed or modelled so far is less than would be expected if the climate system were in equilibrium under the amount of radiative forcing due to the current increase in greenhouse gases and aerosols.

Because of the large amount of natural variability in both the observations and the simulations, much debate has taken place over the last two decades about the strength of the evidence that global warming due to the increase in greenhouse gases has actually been observed in the climate record. In other words has the 'signal' attributed to global warming risen sufficiently above the 'noise' of natural variability? The Intergovernmental Panel on Climate Change (IPCC) has been much involved in this debate.

The IPCC's first Report published in 199021 made a carefully worded statement to the effect that, although the size of the observed warming is broadly consistent with the predictions of climate models, it is also of similar magnitude to natural climate variability. An unequivocal statement that anthropogenic climate change had been detected could not therefore be made at that time. By 1995 more evidence was available and the IPCC 1995 Report22 reached the cautious conclusion as follows.

Our ability to quantify the human influence on global climate is currently limited because the expected signal is still emerging from the noise of natural climate variability, and because there are uncertainties in key factors. These include the magnitude and patterns of long term natural variability and the time-evolving pattern of forcing by, and response to, changes in the concentrations of greenhouse gases and aerosols, and land surface changes. Nevertheless, the balance of evidence suggests a discernible influence on global climate.

Since 1995 a large number of studies have addressed the problems of detection and attribution23 of climate change. Better estimates of natural variability have been made, especially using models, and the conclusion reached that the warming over the last 100 years is very unlikely to be due to natural variability alone. In addition to studies using globally averaged parameters, there have been detailed statistical studies using pattern correlations based on optimum detection techniques applied to both model results and observations. The conclusion reached in the IPCC 2001 Report:24

In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.

was strengthened in the IPCC 2007 Report25 which summarised its conclusion as follows:

It is very likely that anthropogenic greenhouse gas increases caused most of the observed increase in globally averaged temperatures since the mid-20th century. Discernible human influences now extend to other aspects of climate, including continental-average temperatures, atmospheric circulation patterns and some types of extremes.

The 2007 Report made four further summary points:

• It is likely that greenhouse gases alone would have caused more warming than observed because volcanic and anthropogenic aerosols have offset some warming that would otherwise have taken place.

• The observed widespread warming of the atmosphere and ocean, together with ice mass loss, support the conclusion that it is extremely unlikely that global climate change of the past 50 years was caused by unforced variability alone.

• Warming of the climate system has been detected and attributed to anthropogenic forcing in surface and free atmosphere temperatures, in temperatures of the upper several hundred metres of the ocean and in contributions to sea-level rise. The observed pattern of tropospheric warming and stratospheric cooling can be largely attributed to the combined influences of greenhouse gas increases and stratospheric ozone depletion.

• It is likely that there has been significant anthropogenic warming over the past 50 years averaged over each continent except Antarctica. The observed patterns of warming, including greater warming over land than over the ocean and their changes over time, are simulated by models that include anthropogenic forcing.

Confidence having been established in climate models in the ways we have outlined in the last two sections, these models can now be used to generate projections of the likely climate change in the future due to human activities. Details of such projections will be presented in the next chapter.

Before leaving comparison with observations, I should mention observations of the warming of the ocean that add further confirmation to the picture that has been presented. In Chapter 2, the effect of an increase of greenhouse gases was expressed in terms of radiative forcing or, in other words, a net input of heat energy into the earth-atmosphere system. Most of this extra energy is stored in the ocean. From large numbers of measurements of the temperature increase in the ocean at different locations and depths down to 3 km, it has been estimated that, over the period 1961-2003, the ocean has been absorbing energy at a rate of 0.21 ± 0.04 W m- globally averaged over the Earth's surface.26 Two-thirds of this energy is stored in the upper 700 m of the ocean. Within the limits of uncertainty, it agrees well with model estimates of ocean heat uptake.27

More detail from observations and models of heat penetration into the oceans is shown in Figure 5.2328 that demonstrates that natural forcing due to solar variations and volcanic eruptions cannot explain the observed warming but that the addition of human induced greenhouse gas forcings brings observations and model simulations into good agreement for all three oceans.

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