There is much controversy about the realities of global warming and how they may or may not relate to anthropogenic increases in atmospheric greenhouse gases. But there are four things that should not be disputed:
- Scientists don't know everything.
- The fact that we don't know everything doesn't mean that we don't know some things.
- It is virtually impossible to predict the future with certainty.
- Global warming, per se, should not be the only issue.
What We Do Know
Billions of years ago much of the carbon that now exists under the surface of the earth as fossil fuels was actually in Earth's atmosphere in the form of carbon dioxide – CO2 – and methane.
Scientists believe that Earth's atmosphere initially consisted of hydrogen and helium, but this "first" atmosphere was dissipated by heat from Earth's still-molten crust, the sun, and solar winds. Then, as the crust cooled, gases released by numerous volcanoes on the surface produced a "second" atmosphere – about 100 times as large as the one we have now – consisting primarily of CO2, water vapor, hydrogen, and nitrogen in the form of ammonia. As Earth cooled further a large portion of the water condensed on the surface and much of the CO2 dissolved in the resulting oceans, combined with minerals, and precipitated out as carbonates. This left a residual "late" second atmosphere consisting of CO2, water vapor, ammonia, and methane probably formed by reaction of hydrogen and CO2. These gases exerted a "greenhouse" effect, trapping radiant energy from the surface as heat, which prevented the surface from cooling further and becoming a frozen waste. There was little or no free oxygen, and oxygen-dependent life forms, as we know them, would not have been able to exist at that time.
However, sometime between three and four billion years ago, simple life forms known as cyanobacteria did develop. These were photosynthetic, i.e., they had the ability to capture radiant energy from the sun and to grow by using that energy to take CO2 and ammonia and synthesize from them, in combination with water, the larger, more complex carbon-, nitrogen-, and oxygen-containing molecules – nucleic acids, proteins, and carbohydrates – that made up their structures and living material. In doing this, they also released oxygen, which gradually accumulated in the atmosphere and made it more like the air we now breathe. Then, during the subsequent billions of years, more complex organisms evolved – some photosynthetic, but some also developing the ability to respire and derive energy by combining a portion of the free oxygen released by photosynthesis with other carbon sources, and some, once the required oxygen became readily available, losing the ability to derive energy from photosynthesis and living by respiration alone. The actions of all these living organisms, together with other chemical processes including removal of oxygen by combination with other chemicals in the crust and photolytic reaction with methane and with ammonia to produce CO2, free nitrogen, and water – produced the "third" atmosphere containing roughly the composition of 78% nitrogen, 21% oxygen, and trace levels of CO2 and other gases we see today. Meanwhile, a portion of the living material of these organisms became captured under Earth's crust and lost oxygen to form the fossil fuels – coal, oil, and natural gas – that are so useful to our society.
Earth's atmosphere exists in its present chemical composition largely as a balance between the photosynthetic plants and bacteria that consume carbon dioxide and release free oxygen and oxygen-respiring life forms that reverse the process. In all likelihood, this balance is not perfect, and the chemical composition of the atmosphere – CO2, oxygen, nitrogen, and whatever – would still be changing over time even in the absence of any human consumption of fossil fuels. However, the changes that created our current atmosphere took place over billions of years. It was an incredibly slow process, almost unimaginably slow by human experience – a billion years being a million millennia, or slightly longer than ten million human life spans – so that changes that might occur in a period of one year, or ten, or a hundred, or a millennium could be infinitesimal and very difficult to measure. The same applies to the rate at which fossil fuels were created, and no doubt continue to be created, over a time span of billions of years.
The concern, of course, is that human use of fossil fuels, which began in earnest only about two hundred years ago and has rapidly accelerated in the past fifty is removing carbon from subterranean deposits at a much faster rate than it is being replaced – much faster by many orders of magnitude. As a result of this liberation and consumption of fossil fuels, carbon dioxide is being returned to Earth's atmosphere at a much faster rate than it would be by the balance of natural systems alone. In addition, other human activities result in release or accelerated release of methane, ammonia, carbon monoxide and nitrogen oxides as well as synthetic greenhouse gases such as fluorocarbons and chlorocarbons, all of which tends to push atmospheric composition away from the balance in which current life forms exist and are viable. We don't know how fast these changes will occur. There are moderating effects, such as dissolution of CO2 in the oceans and sequestration, and there are magnifying effects such as increase of water vapor in the atmosphere and release of methane as glaciers melt, both as a result of warming. The combined effect of these complex factors is virtually impossible to predict. But we do know that it is potentially threatening to living things, such as humans, that are dependent on atmospheric composition. Global warming may be just a side effect. Regardless, if we continue to consume fossil fuels at a high rate we may be pushing Earth toward a time in the future when conditions become less comfortable and a lot less livable for our descendants.
There are good arguments on both sides of the debate over whether atmospheric CO2 and human activities are responsible for global warming or whether the average temperature of Earth's surface varies mainly as a result of other, possibly cyclic, phenomena [Videos: Pro and Con]. It is not clear that this issue will be resolved soon, although many proponents of the anthropogenic theory are firmly convinced that it already has been and may successfully influence government policy on that basis.
Economic Factors May Supercede
A much more important factor for consideration may be that we are already rapidly consuming the fossil fuels that are easily recoverable. The difficulty and expense of recovering additional fuels that are more deeply embedded or less chemically convenient will inevitably increase, and the economic effects of this are likely to make our society with its current high level of dependence on fossil fuels unsustainable. This will probably happen much faster than atmospheric change and is the best argument, by far, for increasing energy efficiency and simultaneously replacing our dependence on fossil fuels by developing sustainable alternative energy sources – solar, wind, hydro, and nuclear.