Can the Sun affect Earth weather?

Earlier we heard about the effects that storms on the Sun's surface can have on spacecraft and power distribution grids. But does variability in the Sun also affect the weather?

Dominic Ford spoke to Joanna Haigh from Imperial College, London.

Dominic - Joanna, we've been hearing a lot about the sun's activity in the form of spewing out plasma in the solar system. When we're talking about the sun affecting climate, is that the activity that we're talking about?

Joanna - No, it's not. So, those solar storms affect a very, very high atmosphere of the Earth. But when we're talking about climates, we're more interested in the sun's electromagnetic radiation. So, just the visible and perhaps a bit of the UV light, near infrared light that gets into near the Earth's surface and is warming the planet.

Dominic - So, it's changes in the amount of light the sun is putting out?

Joanna - That's right. So, we've heard about the 11-year cycle on the programme already and the amount of energy that comes out of the sun varies by about 1/10^th of 1% over the 11-year cycle. So, that influences the amount of energy received into the climate system.

Dominic - So, with growing evidence for climate change in recent decades, do you think change in the sun's behaviour could be contributing to that?

Joanna - It's probably contributing a little bit, but we know enough about the energy coming out of the sun and about the factors that are causing climate change to be able to say that the sun can't be causing recent global warming. It might contribute a small fraction, but it can't be responsible for the whole thing.

Dominic - So, in terms of your work looking at connections between solar activity that the light the sun is putting out and the Earth's climate, does that mean you're having to look back in history to a time when we didn't have the industrial revolution and carbon dioxide levels rising in the atmosphere?

Joanna - Of course, that's very interesting. So, if we look back over very long periods, so sort of hundreds of thousands of years, you can see there's a cycle on ice ages and interglacials that are almost certainly related to the amount of radiation received from the sun. Now however, that's not due to solar activity. That's just due to changes in the Earth's orbit around the sun. So, we can see changes in climate due to changes in solar radiation. If we go to more recent history and then you look at the number of sunspots on the sun and you can see changes over hundreds years as Lucie has described, there are small changes in the global average temperature that perhaps are responding to those changes in the solar energy coming out.

Dominic - I suppose you must have a problem that the telescope was invented only 400 years ago. So, the best observations of the sun are only relatively recent. How do you know what the sun was doing more than 400 years ago?

Joanna - Indeed. So, you have to rely on what are referred to as proxy measures. So, measures of things that sort of you can interpret as being unrelated to solar activity. The most useful one is in cosmogenic isotopes. So, if we look in tree rings or ice cores, you can see that there's changes in the amount of ratios of beryllium atoms or carbon atoms. The ratios of those isotopes is depending on the amount of cosmic rays that are coming in from outer space. Now the amount of cosmic rays are modulated by solar activity. So, when the sun is more active, there's less cosmic rays and there's less of these cosmogenic isotopes. Those records go back very, very long time, hundreds of thousands of years.

Dominic - Now something that surprised me, reading up on this, was that people seem to be talking not just about the sun's light affecting global climate, but also on region weather systems. I would've thought the Earth was so far from the sun that it wouldn't affect one region differently from another.

Joanna - Exactly, that's very interesting. So, I think what we think is happening is that we've talked about the spectrum of radiation coming out of the sun and some of that radiation is in the ultraviolet. The ultraviolet radiation varies by more than the 1/10^th of 1% I've already mentioned over the solar cycle. That radiation gets absorbed in the stratosphere where the ozone is. We can see signals of the solar cycle in the stratosphere and temperatures and ozone. Recent studies are suggesting that when that is going on that somehow couples with the lower atmosphere and you can get changes in circulation which then impact for example the temperatures in the North Atlantic. So, you can get quite much larger changes in Western Europe in response to the solar cycle than you do if you just look at the global average temperature.