...speaking of Mars, there's global warming there, too. Must be all our polluting space craft ruining the neighborhood...
Indeed, Pam, there
is global warming on Mars. And it isn't caused by our space craft, no. The overwhelmingly likely explanation is variations in the activity of the Sun. For all of this, Mars is still very, very cold.
I am one of those who firmly believe in the majority view of scientists that the Earth is about four and a half billion years old (give or take a couple of hundred million years). I also believe that life on Earth is around three billion years old (this, too, is a consensus belief among scientists). Furthermore, I believe that life has kept on existing here more or less continuously since it first appeared here. If that is true, then life has existed continuously on the Earth for three billion years.
Now, practically
all scientists believe that
all life on Earth requires liquid water. People, do you realize what that means? If life has existed on the Earth
continuously for three billion years, and this life has continuously required liquid water, that means that the temperature on the Earth has varied by less than a hundred degrees Celsius for three billion years! It has varied by less than the temperature span between which water freezes and water boils!
Doesn't this sound incredible to you? Okay. Then think of this. Astronomers study millions of stars in the sky, and they are particularly interested in stars in clusters. The overall properties of a cluster give a very good indication of that cluster's age. By studying many clusters of different ages, astronomers have reached a rather firm conclusion that stars steadily grow brighter as they grow older (up until a certain point where they die, of course). The fact that the stars grow brighter shows that they put out steadily more energy into space.
Now, because of this observation that stars grow brighter as they age, astronomers have concluded that the Sun is 20-30% brighter now than it was when life first appeared on the Earth, three billion years ago. In other words, the Sun puts out 20-30% more energy now than it did when life first appeared on the Earth. For all of that, the temperature of the Earth
has remained virtually unchanged for these three billion years!!!
To make you see the situation more clearly, let's talk about the temperature of the Earth in degrees Kelvin, since the Kelvin temperature scale starts at absolute zero (that is, at the coldest possible temperature in the universe). Now, ice melts (or water freezes, if you want) at 273 degrees Kelvin. When life first appeared on the Earth, the overall temperature of the Earth can't have been much lower than that. During the twentieth century, when the Sun had grown 20-30% brighter, the overall temperature of the Earth was about 288 degrees Kelvin (15 degrees Celsius). This means that while the Sun grew 20-30% brighter, the temperature of the Earth rose by, at most, a little more than 5%. And that is assuming that the overall temperature of the Earth really
was very close to the freezing point of water, 273 degrees Kelvin, three billion years ago. If the temperature back then was higher, the temperature increase since then has been even smaller.
So, people, this appears to be the situation. The temperature of the Earth has remained absolutely
incredibly constant for three billion years. There are various reasons for this, the most important one being that when the Earth first formed, it was actually molten. It has slowly cooled ever since it formed. And for three billion years, the gradual cooling of the Earth has been almost perfectly balanced by the increased energy output of the Sun.
A colleague of mine, a man with relatively conservative political views, was recently quite shaken by a TV program he had seen about global warming. In this program, a scientist claimed that during the latest ice age, Earth had apparently only been
two degrees colder than it is now. Two degrees! And this was enough to plunge the Earth into an ice age! According to other scientists in the same program, if the current global warming trend continues unchecked for a hundred years, the Earth will become
four degrees warmer than it is now. Four degrees! If a temperature drop of two degrees could plunge the Earth into an ice age, what will a temperature increase of four degrees do to this Earth?
My point is that the Earth has been dancing on a knife-edge of a temperature balance for three billion years. Sooner or later we are going to fall down from this wonderful thin line of perfection. Unfortunately we can't look into the future, and we don't know all the facts about the Earth's total energy and temperature budget. But personally, I believe in two things. Greenhouse gases in the atmosphere increase the temperature of a planet. And the Sun is growing progressively brighter, putting out more and more energy into space. The gradual changing of the Sun is enough to
guarantee that we are going to see global warming in the future. If we burn fossil fuels at an unprecedented rate and give the Earth an entirely new source of greenhouse gases - that which is coming out of our man-made chimneys and exhaust pipes - then the global warming which is inevitable in the long run anyway could conceivably come that much sooner.
Ann
