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Climate reflections
Reflections on recent global surface air temperature changes Recently there has been much
attention drawn to the global temperature record since 1998 (IPCC
2007, Fourth Assessment Report of the Intergovernmental Panel on Climate Change).
No less than five years since 1998 has been the warmest recorded since the
latter part of the Little Ice Age (1850). The years listed by IPCC in their 2007
report as being especially warm are 1998, 2002, 2003, 2004 and 2005 (Trenberth
et al. 2007, p.237). The El Niņo effect on the year 1998 is mentioned by Trenbert
et al. (2007), but the 1998 high global annual temperature remain listed
among the top five annual temperatures. In the 2007 IPCC Summary for Policymakers
the series of record high temperatures is highlighted, but unfortunately without
calling attention to the special character of the year 1998.
There is reason, therefore, to consider the whole temperature record since 1998
in some detail. Estimates of the recent global temperature changes suggests that the planet may now have entered a period which deviates from the previous period since 1980, where increasing temperatures have prevailed. In contrast to this previous development, net changes since 1998 appear to be small. The year 1998 was especially warm due to the 1997-1998 El Niņo in the Pacific Ocean, and was followed by a cooler period 1999-2000. From 2001 surface air temperatures remained essentially stable until 2007, where temperatures again began to decrease. Considering the whole 10-year period 1998-2007, however, net changes have been small. It is impossible to know if this lack of warming will continue, but these observations are inconsistent with the predictions of the long-term global climate predictions, such as reported in the 2007 IPCC report. Three possibilities remain for the future development of the average global surface air temperature:
None of the existing global climate models have forecasted the apparent discontinuation of rising air temperatures following the transition period 1998-2001. Climate models forecast increasing temperatures along with the ongoing increase of atmospheric CO2. For the next two decades a warming of about 0.2°C per decade is projected for a range of SRES emission scenarios according to the 2007 IPCC Summary for Policymakers (p.6 and Fig.SMM.5). It is therefore of considerable interest to monitor the future surface air temperature development, especially in the polar regions, where changes are expected to be more rapid than elsewhere. The cooling interval of the atmosphere since early 2007 is interesting, but may represent a short interval of low temperatures, only. This cooling does not demonstrate that global warming has stopped for many years to come, and it would be premature to attempt drawing conclusions before more data are available. Surface air temperatures might well bounce back to the previous level during 2008. Presumably the La Niņa event in the Pacific Ocean since August 2007 contributes to the present interval of cooling, affecting large surface areas near the Equator, just like another La Niņa episode probably contributed to the 1999-2000 cool episode (see diagram above). What appear noteworthy is the fact that several other ocean areas also are developing relatively low sea surface temperatures. Generally speaking, only the North Atlantic remains relatively warm. Click here to see a map of the present global sea surface temperature anomaly. Also other factors might be relevant to consider. As one example, it is interesting that the 1998 termination of global warming (since 1978) apparently is synchronous with a change in the global cloud cover: Satellite observations since 1983 show a decreasing trend from 1987 until 1999, from when the global cloud cover again has increased somewhat. As the global cloud cover have a clear net cooling effect on the global climate, the variations of the global cloud cover presumably is well worth to follow in the years to come. Click here to see a graph showing the variations of the global cloud cover since 1983. Click here to jump back to list of contents.
Reflections on the significance of recent surface air temperature changes 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. 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. Click here to jump back to list of contents.
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