William Happer

CO2 is the control knob.

Don't confuse me with other possibilities.

But nobody is quite sure about the sun.

We have not got good records of the sun for a long time, so we're stuck with proxies of how bright was the sun 500 years ago or 5,000 years ago.

And one of the proxies is when the sun activity changes, it changes the amount of radioactive isotopes that it makes in the atmosphere, things like carbon-14 or beryllium-10.

These stick around for long, you know, thousands of years or longer.

And you can from that infer how many of them were made 500 years ago or 5,000 years ago.

They don't give any support to the idea that the sun has been constant.

It's very clear, for example, that the amount of carbon-14, you know, this radioactivity that's produced changes from year to year.

If you don't take that into account, you get all the dates wrong from carbon-14 dating, you know, where you...

Take an Egyptian mummy and you burn up the cloth and you measure the carbon-14 in it and you get the wrong answer unless you assume that the rate of production then was different from what it is today.

Because you know what the right answer is from the Egyptian mummies.

There's a pretty good historical record of that.

So it's clear the sun is always changing.

And over the last 10,000 years, since the last glacial maximum, there have been many warmings and coolings, very large warmings and coolings, and that's particularly noticeable near the Arctic, you know, in high latitudes in the north.

For example...

My father's home in Scotland, I was a kid.

I would walk up into the hills south of Edinburgh, and you could see these farms from the year 1000 where people were able to make a crop at altitudes where you can't farm today.

It's too cold today, but it was clearly warming up in the year 1000, which was the time when the Norse farmed Greenland.

So what caused those?