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W R Johns © 2007
Carbon Dioxide releases are the most likely cause of the current period of global warming. The science is strong enough to justify major reductions in the use of non-renewable energy. However, the science is not strong enough to justify the risk of installing nuclear capacity to reduce carbon dioxide releases.
Here, we summarize the evidence for believing that carbon dioxide is the main cause of the current global warming (the “Carbon Hypothesis”). We then consider the science behind each of the arguments for supporting the Carbon Hypothesis. In each case, we find that the evidence is weaker than appears at first sight. We do not disprove the Carbon Hypothesis, but find that there are other plausible causes of global warming that are consistent with the data.
Geological data, such as the Vostok Ice-core data show a correlation between global carbon dioxide levels and global temperature. For hundreds of thousands of years, whenever carbon dioxide levels increase, temperatures increase, and vice-versa.
Globally, temperatures seem to be rising faster than at any time since records began. This temperature rise is associated with an unprecedented rise in global carbon dioxide concentrations. At least 70% of the increased atmospheric carbon dioxide results from human activity in burning fossil fuels.
Carbon dioxide is about 20 times as transparent to energy radiated from the Sun as it is to energy radiated from the Earth. Consequently, it tends to trap heat and raise atmospheric temperatures. For this reason, carbon dioxide is known as a “greenhouse gas”. It is certain that increased carbon dioxide levels must have some effect in increasing global temperatures.
Our “twin planet”, Venus, has a much higher atmospheric carbon dioxide concentration and is hellishly hot. It shows over 400 C of global warming.
Much effort has been invested in Global Circulation models of the climate. These show an excellent correlation between increased carbon dioxide levels over the last century and increased temperature levels.
The above observations have been extensively checked. In this presentation, we critically review the extent to which each of these observations supports the hypothesis that carbon dioxide is causing the current global warming.
We consider the observations in a logically consistent sequence. We consider first the carbon dioxide/temperature correlation from ice-core data. We then review the current temperature rise data. The current data is considered in two parts. The first considers whether the current rate of temperature rise is exceptionally rapid, and the second whether the correlation with increasing carbon dioxide levels is significant. These observations may be considered “circumstantial evidence”. Thus, the cause/effect relationship is a statistical one and not based on scientific understanding. The science centres on the observation that carbon dioxide is a greenhouse gas and that models can link its atmospheric concentration to observed global warming. Thus, we next consider climate modelling. We show that, for determining whether carbon dioxide causes global warming, simple models are preferable to complex global circulation models. (The latter are preferred for predicting the geographical impact of global warming, once the cause is known). We show that it is important that climate models should be tested against data other than the highly correlated climate data from Earth. Hence, we present models that should be equally applicable to other planets.
We conclude that:
The geological correlation between carbon dioxide concentration and temperature results from equilibrium between carbon dioxide dissolved in the oceans and carbon dioxide free in the atmosphere. As temperatures rise, carbon dioxide is released from the oceans and causes increased atmospheric concentrations. Similarly, when temperatures fall, the oceans dissolve more carbon dioxide and the atmospheric concentration falls. Ocean temperatures lag air temperatures. Hence, periods of global warming are always associated with lower ocean temperatures and lower atmospheric carbon dioxide concentrations than periods of global cooling. (The mean difference is 10-ppm carbon dioxide). Thus, the often-quoted ice-core correlation between carbon dioxide and temperature does not prove that increased carbon dioxide causes global warming. The correlation is not relevant to the current situation in which increased carbon dioxide levels are caused by mankind’s activities.
Ice-core data can be used to derive relationships for the natural variability of climate. These relationships show that there is approximately a 50% chance of the current rate of global warming arising from natural climate variation. The data also show that the current temperature levels are not exceptional compared to those that would be expected as a result of natural climate variability.
There is no statistical significance to correlations between recent increased levels of atmospheric carbon dioxide and global warming.
The case for carbon dioxide causing global warming depends entirely on mathematical models relating the two factors. It is impossible to test these models convincingly. (Unlike mathematical models of aircraft performance or of a manufacturing process, it is impossible to build prototypes or pilot plants. Hence, no controlled tests are possible). Furthermore, the results from such models depend on a fine balance between convection of heat up to a greenhouse gas layer and radiation of heat back from that layer to the surface of the Earth. Thus, by making plausible minor adjustments to the models, they can be tuned to show either that greenhouse gases result in no global warming or in substantial global warming. On balance, the consistency of model results by a number of independent authors suggests that man-made carbon dioxide probably is causing the current global warming.
The science behind the current climate models is sufficiently sound to justify substantial efforts to reduce (or eliminate) man-made carbon dioxide emissions. However, the models are not sufficiently proven to justify replacing carbon-based fuels with nuclear power. Proliferation of nuclear power increases the risk of nuclear conflict. Early climate models showed that even a minor nuclear conflict could inflict more climate damage in days than could be caused by hundreds of years of carbon releases. These models have never been refuted. If global warming continues, despite eliminating carbon dioxide releases, the pressure for major population movements is certain to stimulate conflict.
In summary, we recommend substantial reductions in the use of both carbon-based and nuclear fuels.
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