Why are global sea levels rising and why is it worrying?
Yes, sea level is rising due to global warming. The current total rise is small, but there are indications that the rate will increase dramatically in the future. The cause of this warming is a matter of continuing and emotional debate. Many scientists feel that there may be natural fluctuations in average surface temperatures which are having an effect. This is certainly true, and may account for a portion of the temperature changes we are recording. However, many more scientists feel that human actions are contributing to the warming. We're doing this by the burning of fossil fuels: coal, oil, and gas.
Think of it this way: the earth's atmosphere currently contains approximately 0.04% carbon dioxide. But this has not always been the case. Carbon dioxide levels were certainly much higher in the geologic past. For the past 3 billion years or so, plants have been using the carbon from carbon dioxide to make their cells, and have been releasing the oxygen as a waste product. This is a real good deal for the animals, and we return the favor by using the oxygen and returning the carbon dioxide as our waste product - another example of how the earth is real good at maintaining a balance which works.
Most land plants that die (or drop their leaves) return their carbon to the atmosphere when they decompose, but the vast majority of plants live in the ocean (this was especially true in the geologic past). When these plants die their cells fall to the seafloor and get mixed in with, and covered by, sediments. This effectively removes their carbon from the atmosphere. Over the course of time they are converted to fossil fuels (a very appropriate term), and hide this carbon deep within the earth.
But here come humans (and the problem). This hidden carbon is a vast energy reserve which we have learned to find, recover, and use. When we burn fossil fuels, we re-combine the contained carbon with oxygen and release it back into the atmosphere as carbon dioxide. In actuality, we are currently taking a hidden carbon source, which the earth has taken 3 billion years to clean from the atmosphere, and are returning it to the atmosphere over the course of several generations. Unfortunately, as our population and the demand for energy both increase, this process will continue to accelerate.
This is not a good idea. Carbon dioxide is what is called a "greenhouse gas." The wavelengths of energy we get from the sun can still penetrate through the atmosphere as usual, but the resulting heat which is produced at the surface of the earth cannot escape back into space. The wavelength of heat energy is blocked by the carbon dioxide molecules. Therefore, as the percentage of carbon dioxide in the atmosphere increases, more of the heat is trapped near the surface. This is causing the average temperature of our planet to also slowly increase.
Why is this of concern? Well, there are several reasons... all of them good ones. We like the atmosphere the way it is, and any change will affect the lifeforms currently living on earth. Climatic patterns are based on the current atmospheric and temperature conditions on earth. A change in either (we're changing both) will begin to cause changes in the earth's climate. With regard to your original question, the increase in global heat is beginning to melt the polar ice caps, resulting in the rise of sea level you are asking about. Some scientists predict that sea level could rise by 300 feet or more if the ice caps would completely melt. Unfortunately, many (most) humans live within the possible flooded areas. How long can YOU tread water?
Sorry if this sounds negative, but this is an emotional issue, and one which needs to be addressed IMMEDIATELY. If humans as a species (and the developed nations in particular) fail to make the hard decisions now, we are in for real problems in the future. What can we do? STOP DRIVING NOW. Raise the price of gasoline to $10.00 per gallon and maybe your grandchildren will have a chance.
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