So let me make a final point about CO2. And let me make it from an astronomical point of view.

Please look at this graph:

[Linked Image]

This graph shows how stars of different masses will evolve over time when it comes to luminosity and temperature. Please note that luminosity has to do with total energy output. The more luminous a star is, the more energy it releases.

The graph shows the evolutionary tracks of stars of different masses. The lowest track shows the evolution of a star like the Sun.

The solid diagonal line represent the zero age main sequence. The zero age main sequence is when the hydrogen in the core of the very young star has become compressed and hot enough to spontaneously start hydrogen fusion. To put it simpler, that is when the star gets access to its internal fuel and gets its stellar engine going.

The slashed diagonal line represents what happens when the star has used up the hydrogen in its core. At this point, the star has no more hydrogen fuel in its "core tank". After that, the star's luminosity and temperature will evolve dramatically, as you can see from the evolutionary tracks to the right of the dashed diagonal line.

I want you to note three things about the evolution of the Sun. First, of course, that the Sun is going to grow dramatically brighter when it has used up the hydrogen in its core. It is going to swell to a red giant, and it might possibly grow large enough to engulf the Earth. As you can understand, life on Earth will not be able to survive that.

The second thing I want you to note is that the main sequence - the distance between the solid and the dashed diagonal lines - seems so incredibly short for a star like the Sun. That, however, is not true at all. The Sun is going to spend about 90% of its lifetime on the main sequence and only about 10% as a red giant. In other words, it is going to remain more or less the way it is now for hundreds of millions of years, maybe for a few billion more
years.

Note that I said more or less the same.

Look at the graph again. You can see that the Sun's is going to be brighter when it leaves the main sequence than it was when it first entered it.

In short, the Sun is going to grow slowly but steadily brighter during 90% of its lifetime. It has grown slowly and steadily brighter while humanity wasn't around, and it will grow slowly and steadily even brighter from this point onwards.

I'm not saying that the Sun may not have periods of lower activity, the way it seemingly does right now, when it has no sunspots at all. But the overall trend is toward a higher luminosity and a greater energy output. Consider this graph, which, I must point out, has nothing to do with the Sun at all:

[Linked Image]

In this graph there are short-term peaks and dips, as well as longer-term hills and depressions. The overall trend, however, is that this graph is pointing upwards. And so it is with the Sun. We know that the Sun is mildly variable and cyclical, but there is an overall trend of an ever-increasing energy output.

This is an image of a famous young cluster, the Pleiades:

[Linked Image]

All the stars in this cluster were born together about a hundred million years ago. Compare that with the age of the Sun, which is 4.5 billion years. The Sun is 45 times older than the Pleiades.

Astronomers have found many stars which resemble our own Sun in the Pleiades. They look like faint dots in the picture above, insignificant compared with the short-lived bright blue giants which dominate the cluster.

The stars which resemble the Sun can be easily identified by their "spectral lines" (I won't even try to explain that here), which show that their temperature is the same as the Sun's. However, given the distance to the Pleiades, 400 lightyears, astronomers can see that the sunlike stars in the Pleiades are 20-30% fainter than the Sun.

The Sun is 45 times older than the Pleiades. The sunlike stars in the Pleaides are 20-30% fainter than the Sun. But it is probable that the Sun was about as bright, or faint, as its counterparts in the Pleiades when the Sun itself was only a hundred million years old.

The Sun is growing brighter. Slowly but inexorably.

Now maybe you are saying that if the Sun had been that faint in its youth, then the Earth would have been frozen solid and there could have been no life on Earth for a very long time. But in fact, biologists and geologists claim to have found evidence for microbial life on Earth three billion years ago.

The reason for that appears to be that the Earth itself was hotter in its youth than it is today. Back then it had more internal heat and more heat-generating radioactive elements than it does today. The low energy levels from the young Sun were made up for by the heat leaking out of the young Earth's red-hot core. In those days the Earth needed as much internal heat and atmospheric insulation as it could get. Today the situation is different.

Imagine that you lived not too far from a big fire, which for some reason couldn't be put out (and you wouldn't want it to be put it out) and also it couldn't be really contained. It was bound to spread, slowly but inexorably. And for whatever reason, you couldn't move farther away from it.

Would you tell your kids that it was okay to play with gasoline around your house?

Would you tell your neighbours that it was okay to release flammable gases into the air close to your house?

Would you tell them it was okay to release flammable gases into the air close to the fire?

The Sun is like that fire that can't be put out and can't be contained. The CO2 that humanity releases into the atmosphere is like the flammable gases that those people released close to the fire close to their house.

I agree that there is no simple solution to the problems of global pollution, global deforestation, increasing global drought, increasingly severe hurricanes, and the ever-increasingly rapid extinction of huge numbers of plant and animal species.

You can take three approaches to this situation. You can say that there is no real problem, of if there is, it is too costly to do anything about it. Therefore we must go on releasing flammable gases into the air close to a fire just like before.

Or you can say that those who talk about the problem are the real problem, and people like Al Gore should be jettisoned into the Sun.

Or you can decide that you are going to get together and pool your resources and do whatever you can to deal with the problem.

As you can imagine, I'm for the idea of getting together and trying to do something real and serious about the problem.

There is only so much we are going to be able to do. We can't stop the Sun from getting inexorably brighter.

But we can buy ourselves and other lifeforms on this Earth some time.

Ann