Hi Adam,

The heat output from the Sun that we feel on Earth actually changes very little, not only from winter to summer but over many years. Some people have thought that such variations might be causing global warming but that doesn't seem to be the case. The Sun does have things like solar flares and coronal mass ejections (where basically the Sun spits out a big chunk of gas) but the density of the particles in these events by the time they reach Earth is so low that it doesn't add much to the heating of the Earth.

The Earth is actually closer to the Sun in the (northern hemisphere) winter time and farther from the Sun in the summer. What makes summer hot and winter cold is the tilt of the Earth rather than the change in its distance to the Sun. As you know the difference between summer and winter temperatures is most noticible near away from the Earth's equator.

Best,

Christina

Hello Adam, This is a very important question. There is a lot of evidence over the history of the Earth that the Sun can have major effects on the climate. The Sun is thought to be about 30% brighter today than it was when the Earth first formed and this changes how much heating the Earth receives. Changes in the Earth's orbit and tilt of its axis are thought to be the cause of the major ice ages in the past and not chnages in the brightness. however, there is some more recent evidence that changes in solar brightness can have some effect. In the years 1645 - 1715 there were hardly any sunspots detected on the surface of the Sun. The number of sunspots are related to solar activity and the overall brightness of the Sun. The lack of sunspots meant that the average radiation from the Sun was lower than normal for longer periods of time (70 years is about 7 soalr cycles). This extended period of no sunspots coincide with deeper winters in northern Europe and America, and so this period has been called The Little Ice Age.

While there is clearly a collection, modern studies have shown that the changes in the Sun over the last 100 - 200 years only account for about 25% of the currently measured global warming. The rest, I am afraid to say, is due to human activity, deforestation, farming and burning of fossil fules for cars and energy production. You can see the current ideas in a recently published report at http://www.ipcc.ch/

Cheers - David