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In a normal scientific context the present period with essentially stable global air temperatures represents a classic example of what is known as empirical falsification, disproving the hypothesis on the dominant influence of CO2 and alleged associated effects on the global temperature. The atmospheric content of CO2 continues to increase, but the global surface air temperature is essentially stable, without large volcanic eruptions or other specific phenomena to explain the lack of increasing air temperature.
Clearly other factors not fully represented by global climate models must recently have had a net cooling effect of the same magnitude as the alleged warming effect of greenhouse gasses. Consequently, global climate models apparently are still not perfect, and may require modification and presumably also the inclusion of additional processes.
This observation does not disprove the general notion that atmospheric CO2 has influence on the global temperature. There are good reasons to expect that all other things being equal, an increasing amount of atmospheric CO2 may lead to higher global temperature.
What the falsification do demonstrate, however, is that the alleged dominance of CO2 apparently is wrong, and that CO2 with associated warming effects have a smaller relative importance than previously believed. The real relative importance of CO2 for the global temperature and other drivers will be demonstrated by the future global temperature development as well as by various future research initiatives. Once again, the attention is called to the change in global cloud cover and total atmospheric water vapour content, both changes being almost synchronous with the 1998 termination of the period of recent global warming since 1978.
In addition, the amount of atmospheric CO2, like most other phenomena in nature, is likely to undergo natural variations. For CO2 this has the additional implication, that the ocean temperature presumably represents an important control on the amount of atmospheric CO2.
Notwithstanding these considerations, the global average surface temperature since 1998 is still high and presumably higher than at any time since the end of the Little Ice Age (Trenberth et al. 2007). This observation, however, does not represent a valid argument for ignoring the significance of the recent temperature development. The CO2 hypothesis forecasts the global temperature to increase whenever the atmospheric CO2 concentration goes up, if not counteracted by any other known climatic phenomena, such as, e.g., volcanic eruptions. Whenever a situation like the present occurs, with increasing atmospheric CO2 and essentially stable (or decreasing) global temperatures over several years without known counter effect, the hypothesis is falsified. The general level of the global temperature, high or low, when seen in a longer time perspective does not matter in this context, as this may well be the result of variations in a number of other climatic drivers.
There has in the recent past been other periods where the global temperature stopped increasing, e.g. 1988-1989 and 1991-1993 (click here for diagram). These periods are, however, shorter than the present period, and some of these may be related directly to the cooling effects of volcanic eruptions (e.g., the 1991 Mt. Pinatubo eruption). On this background, the present period with essentially stable global temperatures over several years differs and is remarkable. It is useful to remember that the modern global warming concept refers to a 20-year period only, from 1978 to 1998, which is not a long period in a climatic context. The present period since 1998 without increasing temperatures is already half as long.
Click here to read a statisticians evaluation of the recent global temperature changes (since 1998) compared to the IPCC-temperature forecasts (located on the Public Policy Forecasting website).
In addition to the above considerations, the interested reader might also find it valuable to read a critique on the whole concept of calculating an average global temperature (Essex et al. 2006). In addition to this, surface air temperatures is a poor indicator of global climate heat changes, as air has relatively little mass associated with it. Ocean heat changes are the dominant factor for global heat changes. Apparently, climate models still have difficulties to model the dynamics of the oceans correctly.
The fact remains, however, that the estimated global surface air temperature continues to attract widespread interest, and many agree that this number at least may be considered a useful proxy for the present state of the global climate system. Transferring information on global temperature changes to a regional scale, however, gives only very little useful information. Climate change is essentially a phenomenon which must be understood on a regional scale. Click here to see an illustration of this for Europe during the 20th century.