In the late '70s there were several cold winters, and there was talk, on the talk shows anyhow, of a new ice age. By the end of the '80s, several warm summers presented us with a new worry--global warming. Then, we heard horror stories about the earth turning into a desert and about all the ice melting, and the oceans rising up 30 feet or so, and places like New Orleans, Charleston, Miami, and even London being in big trouble because of it. But we can't believe the horror stories all that much. We need to take them with a grain of salt. Of course, the potential effects from an ice age or from global warming are rather tremendous and certainly possible. But the bottom line is that scientists don't know if we're going to burn or freeze.
What we do know, however, is that there are a number of different processes at work on the earth and in the earth's atmosphere that affect global temperature. One process that stabilizes the temperature is called negative feedback. It starts when the temperature goes up and more moisture evaporates from the warm lands--the same way clothes hung outside to dry on a warm day dry faster than on a cold day. The evaporated moisture turns into clouds, which prevent the sun from reaching us. So we cool back down again, and the clouds go away. It's self-regulating, like the thermostat in your house, which turns the heater on when the house is cold and off when the house is hot.
But there is another process, called positive feedback, that directly competes with the negative. Positive feedback causes a runaway change in temperature. If the earth warms up, you may get more clouds, but these clouds bring more rain and snow, and the oceanic basins become more full of water. The larger the oceans are, the more coast we have. Because water is a big reservoir of heat, coastal climates are milder than continental climates, so the earth warms perceptibly. This gives you more clouds, rain, snow, oceans, and so on. In reverse, if the earth cools off, there's less melting of continental ice sheets. If the ice sheets get bigger, there's less ocean and less coastal climate, so the earth gets cooler, and so on. With positive feedback, if you start off with a little bit of warm, you get a lot more warm, and if you start with a little bit of cold, you get a lot more cold. Negative feedback stabilizes the temperature, and positive feedback tends to run it away, and we can't tell exactly which of these is going to win. When scientists see a little warming up, some say that nothing's going to happen, and others say it's going to run away, and we're in for trouble.
Other effects in the atmosphere and solar system also give us some warming up or cooling down. I've already mentioned water vapor in the form of clouds, which tends to keep the sun's energy from getting to the earth, and cools us down. There's also carbon dioxide in the atmosphere, coming from factories, automobiles, and even our own breathing. Carbon dioxide leads to the famous greenhouse effect because it lets visible light through but absorbs infrared light, a form of heat. The sun's energy comes to the earth as visible light and radiates out into space as infrared light, but the carbon dioxide blocks the infrared light. The atmosphere absorbs the light and heats up, heating up the whole earth. Carbon dioxide in the atmosphere lets visible light in but doesn't let heat out, just as a greenhouse does.
And then, of course, there's the concept of nuclear winter, which was being talked about when nuclear bombs were more threatening than they are today. The fear was that if enough nuclear bombs were dropped they would throw tons of dust into the air, the dust would block the sun, and the earth would cool off so much that we would literally have winter all year. Even without nuclear bombs we have dust in the air. For instance, when the volcano Pinatubo erupted in the Philippines a couple of years ago, the dust it let out actually cooled the earth more than one degree. It completely threw off the people who were charting temperatures to study global warming. Factories also put out dust along with carbon dioxide. Again, we don't know which will win--the warming effect of the carbon dioxide or the cooling effect of the dust.
Lurking behind all these processes is the motion of the earth around the sun. The earth doesn't stay on a perfect orbit, and its axis doesn't always point in the same direction. It tilts and wobbles, and its orbit goes from an ellipse to a circle and back to an ellipse. When the orbit is an ellipse, the earth gets farther and closer to the sun than when the orbit's a circle--farther from the sun it's cooler, closer it's warmer. If you do the proper supercomputer experiments, it turns out that the motion of the earth isn't enough to cause large temperature differences. It's been assumed that the temperature difference from the earth's motion is a trigger for warming or cooling. The earth's motion is in the background, behind the other factors.
All these effects, monitored by large system satellites and computers, are rather imperceptible, but they can certainly add up. Assuming we don't have nuclear winter or a terribly large volcanic event, we probably will not freeze. But whether or not we're going to roast, I don't know. Carbon dioxide is the biggest problem right now. We know that it does warm up the air, and we certainly do produce a lot of it. We wonder if this warming will continue and how rapidly it will happen. If we put out no carbon dioxide, we wouldn't have any global warming from it. But to do that we couldn't have any factories, any decomposition of compost heaps, or any breathing. It's like the debate about the speed limit--the ultimate conclusion is that we'd all be safe if we drove around in military tanks at five mph. This is why people are concerned about countries clearing their rain forests, because then less carbon dioxide gets naturally absorbed through photosynthesis. We need to know where the excess carbon dioxide is going before it's too late, before Miami drowns or something like that happens. Scientists would love to do the experiment of releasing five million tons of carbon dioxide into the atmosphere, waiting 50 years, and seeing what happens to it. But we can't do that. Physicists can put their experiments into a vacuum chamber, put electromagnetic shielding around them, and isolate them from everything, but you can't do that in the earth sciences. The earth has to go on.
Many scientists are looking to the past to try and determine what temperatures were like then in order to study global warming and cooling today. It seems that the earth is definitely warmer than it was 200 years ago, back when George Washington and his troops were freezing at Valley Forge. But George was sitting at the end of a little ice age that affected the United States and Europe. Remember Hans Brinker and his silver skates, skating on the canals in Holland? The last time those canals froze was a long, long time ago. So Europe has been gradually getting warmer. But it all depends on where you start.
Some people at the Weather Bureau decided to check their records for the past 100 years. It turns out that there are ups and downs in the yearly record, but no overall tilts are discernible. But 100 years seems pretty short considering the last glacier disappeared around 10,000 years ago. Scientists are now trying to go all the way back to the last glaciers. They're looking at the kinds of plant pollen buried in various lakes across the United States. Pollen is very indicative of climate, since forests and grasslands grow in different kinds of climate. If the scientists find grass pollen in the lake mud, then the area was once a plain. If they find pine pollen, then the area was once a forest. These pollen records are good for 100,000 years. In Yellowstone National Park, they've discovered through the pollen that the place was a forest some of the time and a grassland some of the time. They should eventually be able to date these changes. If we can do this research in enough places, we may get a real picture of temperature changes over thousands of years in a corner of North America, or maybe in all of North America. If you start taking measurements and making charts from way back then, rather than simply saying that the '90s are warmer than the '80s or the '90s are warmer than the '50s, then you may have a more accurate perspective on where global temperature has been and where it may be going.