Most people believe that carbon dioxide is a serious threat to the future of the planet. I happen to share this belief, but for very different reasons than those which are predominate in the media.
I believe that carbon dioxide is not a direct thermal threat planet wide, the reason for this is that the predominate absorption line of carbon dioxide is at approximately 13-15 microns and the gas concentrations are already at the point where 99.99% of the radiation in this band is absorbed within ten meters at atmospheric pressure. Increases in carbon dioxide levels won’t change this appreciably but they will broaden the absorption line. The net result is that increased CO2 will warm the Earth but nowhere near at the rate suggested by many.
The Earth’s blackbody temperature is around 285°K but the absorption lines of carbon dioxide that are relevant peak between 193-220°K. The amount of radiation from Earth absorbed by carbon dioxide is thus going to be more significant in parts of the world that are very cold, and we do see significant warming in Alaska, but Antarctica is actually getting colder. But to the degree with carbon dioxide affects Earth’s temperature directly those are the places that are going to be affected directly.
I believe a larger concern are the chemical effects of carbon dioxide most notably on the worlds oceans. If you take a can of pop or beer, put it in the freezer, let it cool below freezing, and then pull it out and open it, initially it won’t be frozen but it will rapidly, in just seconds, freeze.
The reason for this is that carbon dioxide dissolved in water forms carbolic acid. This depresses the freezing point of water. That is, it allows water to be cooled below 32F and remain liquid. Now, just as it depresses the freezing point in soda or beer, it also depresses the freezing point of ocean water. That is, water will become liquid at a lower temperature. Presently, there is about 50 times as much carbon dioxide dissolved in the ocean as present in the air, so there is already significant carbon dioxide in the oceans.
At many locations on the ocean floor, particularly along continental shelves, there are methane hydride formations, this is basically methane molecules trapped in ice. The amount of these hydrides far exceeds the carbon that we’ve burned in our history. Methane is a far more potent greenhouse gas than carbon dioxide, perhaps two hundred times as potent, both because the absorption lines of methane aren’t yet saturated, and because they lie nearer the peak of the blackbody radiation from the Earth. All of that methane being released into the atmosphere would be a very bad thing.
A second issue is that increased carbon dioxide levels reduce the amount of oxygen that can be dissolved in the water. Most of the oxygen that is dissolved into the oceans is dissolved at the poles, because oxygen can dissolve more easily in cold water than warm, and then moved via the ocean currents. Those currents depend upon a salinity imbalance between high latitude and low latitude ocean water and as more fresh water enters the ocean diluting the salinity, those ocean currents are slowing. This is reducing the oxygen levels in the ocean water.
There are many forms of sea life that have carbonate shells that dissolve readily in carbolic acid; the raising of the acid levels in the ocean has the potential to kill coral reefs as well as all sorts of shell fish. As those shell-fish die-off they consume oxygen and again deplete the oxygen from the oceans.
And then we have the effect of nutrients entering the ocean, fertilizer run-off, sewage, animal waste. These things cause algae blooms near the surface which then blocks light from getting to deeper levels depriving deeper levels of oxygen. Further, the dying organisms near the surface sink, and then consume any remaining oxygen below. This is creating vast dead-zones in the ocean.
So we’ve got four big things driving lower oxygen levels lower in the oceans, three of which are completely carbon dioxide related, one of which is indirectly related. As the world demand for oil exceeds supply, and biofuels have been one place people have turned to resolve this; the increased use of fertilizers to grow these biofuels is contributing to the problem.
Now, there are a couple of reasons that oxygen levels in the ocean are very important. First, the worlds oceans make up 71% of the surface area of the planet. They supply 70% of our protein needs. If the oceans die, so does 70% of our food supply. So if you like to eat; healthy oceans are essential.
Where there is sufficient oxygen, bacteria in the ocean predominantly make their living by breaking down organic substances and oxidizing those substances. But where there is insufficient oxygen, near ocean thermal vents for example, bacteria have adapted to use sulfur instead of oxygen. Where as normal bacteria produce water and carbon dioxide, these sulfur loving bacteria produce hydrogen sulfide, which is deadly to humans and most life forms in concentrations of about 200 parts per million.
The largest extinction in the Earth’s history, the Permian extinction, may well have been caused by a build-up of hydrogen sulfide when the worlds oceans went through a period of low circulation and oxygenation. During this same time frame there was also a large release of methane.
In my opinion, these issues are far more threatening than carbon dioxide build-up in the atmosphere.