Donald Trump is reported to have said that global warming is a hoax. In fact, the claim that the climate is being warmed due to human activity is not a hoax, but a hypothesis, and quite an old one.
Svante Ahrrenius, in 1896, was the first to quantify the contribution of carbon
dioxide to Earth's temperature and to speculate on whether variations in the atmospheric concentration of carbon dioxide
have contributed to long-term variations in climate. In later publications he suggested that the combustion of coal would affect global temperature.
Since 1896, world coal combustion has increased from around half a billion tons to about 16 billion tons today, while the combustion of oil and natural gas have added the equivalent of about 6.5 billion tons of coal. Thus, present day combustion of all fossil fuels is sufficient to raise atmospheric carbon dioxide concentration by several parts per million per year.
Carbon dioxide is a heat absorbing gas, which means that it absorbs some of the radiation that the Earth emits in the direction of outer space, thereby warming the atmosphere slightly. Warming of the atmosphere, in turn, warms the Earth, which then emits more radiation to outer space, until an equilibrium is reached between incoming solar radiation and the earth`s outgoing heat radiation.
Alone, the effect of a doubling of atmospheric carbon dioxide concentration from the pre-industrial concentration of around 275 parts per million (ppm) by volume, or just over 400 ppm by mass, would raise global temperature by about 1C, which would be hard to detect given the natural variation, both short- and long-term in global temperature.
However, the direct effect on global temperature due to carbon dioxide is enhanced by so-called feed-backs. For example, a rise in temperature due to an increase in atmospheric carbon dioxide concentration will increase evaporation from both land and water surfaces. This may result in an increase in atmospheric water content (though not necessarily), and if so, it will raise global temperature because water vapor is itself a heat absorbing gas, much more powerful, in fact, than carbon dioxide.
Evaluating these effects is difficult, because of many other variables. For instance, increased evaporation may affect cloud cover, or precipitation, or both. Cloud cover may enhance or reduce global temperature, depending on many factors. For example, at night, clouds tend to raise ground temperature by reflecting radiated heat back to the Earth, whereas during the daytime clouds reflect solar radiation back to space, thus cooling the Earth.
Vegetation is another factor that will complicates the picture. With more carbon dioxide in the atmosphere, vegetation cover of the earth will tend to increase, especially in dry zones, which will affect the absortance, reflectivity and emissivity of the earth`s surface, and thus its temperature.
To prove the climate warming hypothesis is thus not easy. In fact, there seems only one way to prove it, which is to show that changes in carbon dioxide can be used to predict changes in global temperature.
This, however, is extremely difficult to do, because global temperature changes due to many factors other than changes in atmospheric carbon dioxide concentration. Such factors include, changes in land use and vegetation cover, solar activity, the Earth's tilt relative to, and its distance from, the sun, emissions to the atmosphere of heat absorbing gases other than carbon dioxide, including refrigerant gases and methane, the production of aerosols of black carbon or soot, e.g., from forest fires, which heat the atmosphere, and of white sulfate particles from the combustion of high sulfur fuels, which cool the atmosphere and nucleate cloud formation.
Thus, to detect an effect of rising carbon dioxide concentration, requires that the temperature base line and its variation over time in the absence of any change in carbon dioxide concentration be determined. Only if this is achieved, can any climate model accurately predict the course of global temperature in a rising carbon dioxide world. And even then the model will only work if the effect of carbon dioxide on global temperature is correctly modeled.
Suffice it to say that many climate models have been constructed to predict global temperature in a rising carbon dioxide world, but model outputs vary greatly with no model demonstrating great reliability. Thus, the effect of human activity on climate, although undoubtedly real, may never be accurately measured.
So human caused global warming is not a hoax, it has just never been measured, and it may never be measured with any accuracy.
What then should be done?
As in any condition of uncertainty and possible danger, we should proceed with caution. That means carefully monitoring changes in the chemistry of the atmosphere and the oceans, in the climate, in the structure, function and distribution of major ecosystems, and on human performance. That means continued investment in climate research, this to be conducted out of the public eye, and free of political influence. That means shutting down the UN Panel on Climate Change, which has been a corrupting influence of longstanding on the course of climate research.
In addition, we should do what can be done most economically to cut human impacts on the environment, and in particular, emissions of heat absorbing gases and particulates. This is not hard to promote. Energy costs money, about 8% of the US GDP, thus continuously raising energy-use efficiency will pay off economically as well as environmentally, while eliminating particulate emissions will be beneficial to public health. Governments should, therefore, promote the trend in increasing energy-use efficiency through investment in research and public education.
In due course, it may become apparent that prudence requires more drastic action. However, the hope and reasonable expectation today is that alternative, carbon-free, energy sources will become increasingly cost competitive and will dominate the energy market within less than half a century, in which case no global warming crisis is likely to emerge. But should it do so, we will not be without resources to deal with it. Cloud seeding, for example, could be used to creat cloud cover to increase the reflection of solar radiation, carbon dioxide recapture could be implemented on an industrial scale, and fossil fuel use could be sharply curtailed without extinguishing civilization as we know it.
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