Although there has been much political strife over the concept of global warming and the greenhouse effect, the scientific community often is portrayed as divided when in actuality, there is a great degree of consensus. It is beyond reasonable dispute that the increase in greenhouse gases produced by industrial expansion has contributed to increased surface and ocean temperatures through trapping heat in the form of radiation; the degree and significance of that contribution, however, is still a matter of debate.
A recent study from the Scripps Institute of Oceanography by Barnett et al. described an analysis of 7 million data points of temperature, salinity and other factors over 40 years in the ocean. The Institute used several different models, including independent models, representing the greenhouse effect and natural variability due to factors like volcanic activity and solar variation.
The model representing the greenhouse effect matched best with the data for oceanic warming, exceeding statistical confidence levels of 95 percent. This is seen as a significant improvement upon a previous analysis by the Scripps Institute including heat deep into ocean basins, improved data in data impoverished regions and work of a second group from Livermore National Laboratory.
The best explanation for the warming trend is anthropogenic warming through the greenhouse effect. It is important, however, to evaluate possible challenges to the mechanism of such an explanation to ensure that it is indeed the best explanation. It is important to note that although natural variation has resulted in wider ranges of temperatures on earth throughout its natural history, the known mechanisms behind these variations did not fit the data as well as anthropogenic warming.
Furthermore, two new studies published recently in Science by Meehl et al. and Tom Wigley suggest that the greenhouse effect will have a delayed impact upon the ocean as heat is transferred from the air to the ocean, slowly resulting in a time lag for effects like thermal expansion and polar icecap melting. This suggests that many of the effects on the ocean of increasing temperatures will not be realized fully until many years after the start of the greenhouse effect with long lasting effects due to a form of thermal inertia.
Much of the criticism for anthropogenic warming has been focused on the climate modeling, but one residual objection remains. Although surface warming has been observed, similar warming in the troposphere, the lowermost portion of the atmosphere, has not been observed by the National Oceanic and Atmospheric Administration.
As was pointed out by Fu et al. in Nature in 2004, however, the method of collecting data through microwave radiation emitted by oxygen molecules to calculate temperature is flawed because it analyzes the troposphere as well as a significant part (~20%) of the stratosphere. Once properly corrected, a warming trend is observed in the troposphere of roughly .2 degrees Celsius per decade.
The main thrust of the argument against significant anthropogenic warming rests on the inaccuracy of climate modeling, especially with regard to feedback mechanisms. An example of a negative feedback mechanism would be increasing dust particles caused by desertification resulting in increased atmospheric reflectivity. Skeptics state that there is not sufficient modeling for such phenomena and thus predictions are overestimated.
There are also other important positive feedback mechanisms, such as the evaporation of water, which would trap heat and consequently increase the greenhouse effect. It is important to recognize that these climate models are not perfect, but only improvement in understanding will aide with possible discrepancies in cloud physics, feedback mechanisms and predictive inaccuracy. That being said, the available models are quite accurate for large-scale phenomena based upon testing like the reproduction of temperature data from the last century.
It is clear that anthropogenic warming is the major factor in the abrupt temperature increase that has been observed in the last 30 years and the last century, so the focus of debate should be the consequences of global warming and how we can cope with or attempt to slow the rate of global warming, if that is feasible given thermal inertia in oceans.
Michael McDuffie can be reached at mm9kn@virginia.edu.
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