The Carbon Dioxide Balance - why the relatively low amount of human-caused greenhouse gas emissions is doing so much damage
Although the amount of human-caused CO2 emissions (~30 billion tons/yr) is small compared to naturally occurring emissions (750 billion tons/yr), the natural CO2 emissions were, for thousands of years, in balance with the Earth's ability to absorb them in land and water. Once the balance was upset (primarily) by the burning of fossil fuels starting in the second half of the 18th century, the carbon dioxide that cannot be absorbed naturally enters the atmosphere and stays for a very long time. It is estimated that about 40% of human-induced CO2 emissions are absorbed - so we are adding 18 billion tons of CO2 to the atmosphere annually.
Other Effects and Complications - why things could be even worse than you thought
There are other more immediate, shorter term effects of global warming besides the potential destruction caused by rising sea levels. In particular, more intense storms, greater incidences of flooding and the destruction of wildlife habitats are even now becoming evident.
Before turning to possible solutions, we unfortuantely need to add some more bad news to the mix. Key to this discussion is the boot strapping effect that occurs as the global temperature increases.
The first complication is that the ability of water to absorb carbon dioxide (CO2) decreases as temperature increases. A five degree Fahrenheit increase in water temperature would result roughly in a 10% decrease in the solubility of CO2 in water. This will decrease the capacity of the oceans to absorb CO2 from the atmosphere and more CO2 will remain in the atmosphere, increasing the greenhouse effect and global warming.
The second complication comes from the release of methane currently trapped in ice and ocean sediment in arctic regions. Geologist John Atcheson, among others, have warned of this effect. A temperature increase of a few degrees in the Earth's annual temperature would cause some methane to volatilize and get into the atmosphere. Methane is a strong greenhouse gas and this increased concentration in the atmosphere would further raise temperatures, leading to a runaway effect. There are 400 billion tons of methane locked in the frozen arctic tundra - more than enough to start this chain reaction.
Alternate Worst Case Scenario - why you really need to hope this doesn't happen
Finally, there is an alternate worst-case scenario to the increased atmospheric concentration of greenhouse gasses. This involves the shutdown of the ocean conveyor belt that distributes heat through the ocean currents. The Gulf Stream, which warms Western Europe, for example, is part of this system.
The scientific term for the ocean conveyor belt is thermohaline circulation. Temperature (thermo) and salt (haline) content determine the density of ocean water. Differences in ocean water density drive this (literally) world-scale conveyor belt. If this conveyor belt receives a big enough shock - for example, large amounts of fresh water from melting glaciers or increased precipitation - the northern oceans will experience a decrease in salt content and density. This will interfere with or shutdown the ocean conveyor belt. The result would be somewhere between the Younger Dryas event (10-20 degree F. temperature drop) and a return to a full-blown glacial period. If this catastrophe does occur, the change will be very quick - not quite as quick as in the movie The Day After - but on the order of a decade. In any case, it will be difficult to do anything about it once it begins.
To be continued
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