Science Interviews

The Sun and Climate Change

Some people argue that climate change is not due to CO₂, but in fact due to changes in solar activity.  This was supported by the suggestion that the sun could influence cloud cover.  Professor Terry Sloan has looked into this relationship and thinks it’s more complicated than that – in fact he not convinced that the proposed link is there at all.

Helen - Thanks for joining us. What is this theory that there could be a link between solar activity and cloud cover?

Terry - A few years ago, I grew up in Denmark, observed that the satellite data on cloud cover decreased (that was about 1990). At the same time they observed cosmic ray intensity, galactic cosmic ray intensity also decreased. They saw this correlation between the two. They hypothesised that the ionisation from the cosmic rays was causing clouds.  If this is true, since the cosmic ray rate is decreased over the years (over about 100 years or so) then we have less cloud cover now than we had 100 years ago. Therefore that allows more sunlight to come to the Earth to warm the Earth and cause the global warming. That was the hypothesis. This seemed to us to be of vast significance because it means that climate change is being caused by cosmic rays and not the carbon dioxide that the RPCC say. We thought we’d better try and check this.

Helen - How did you start looking at that? How did you go into detail?

Terry - Well one of the things we did was just mentioned a few minutes ago. The solar flare of 1989 and the one in 2003. Not only did this cause huge changes in the aurora but it spewed out a whole load of particles that interacted in the atmosphere and caused a big increase in the ionisation in the air.

Chris - Crucially did you see a big change in clouds?

Terry - No.

Chris - So that was the clincher then?

Terry - That was the first nail in the coffin. Then we looked at a couple of other things. Cosmic rays occasionally go through quite a big decrease over the period of a week. They’re called Forbush decreases and we looked to see when these decreases happened that the cloud cover also decreases at the same time. Again we could find no correlation. That was the second thing we looked at.

Helen - So have you come to the point where you can say, no cloud cover isn’t affected by solar activity to the extent that it’s related to climate change or is it something we should still bear in mind when we’re thinking about what it is that’s changing the temperatures in the world around us?

Terry - We did a measurement of how much of the cloud cover could be caused by cosmic rays. Our answer came out. Then we did a statistical analysis to say how big could the effect be and we’d missed it. It came out to be about 20%. In other words, about 20% of the changes in cloud cover are caused by cosmic rays. That’s an upper limit that we deduced.

Chris - If cosmic rays are having this effect, how are they having this effect? Could that change over time?

Terry - No. Can I interrupt you there? We didn’t say cosmic rays are having this effect, we tried to check the effect that was seen by the Danish scientists. All the evidence that we looked at did not corroborate their hypothesis. It could be as high as 20% within the measurements that we made. It was compatible with zero.

Chris - So the bottom line here is that the efforts we’re making to try and cut carbon dioxide are appropriate because that’s our best contender for driving climate change at the moment.

Terry - Yes, that’s correct. The Danish group which effectively had challenged the conclusions of the IPCC had no right to challenge the IPCC. That’s the international panel on climate change.

April 2008