Climate Science - The Role of the Sun, Sunspots and Milankovitch

This seems a bit obvious really doesn't it, since the sun is essentially the ultimate driver of the Earth's climate? What I really want to talk about however is how astronomical changes can impact the climate, in particular, how much solar radiation reaches the Earth.

Sunspots appear cyclically on the surface of the sun and they can have the impact of reducing the amount of solar radiation reaching the Earth. These cycles occur every 22 years or so but the magnitude of the sunspots varies based on a longer cycle of about 100 years. Sunspots have been associated with cooler periods in the Earth's past, and to the best of our knowledge, these events can impact the amount of solar radiation reaching the sun by approximately 0.4%.

Milankovitch cycles also influence the intensity of solar radiation. There are three Milankovitch cycles; the first relates to variations in the tilt of the Earth's axis, the second relates to the eccentricity of the Earth's elliptical orbit around the sun, and the third involves the movement of the shape of this orbit.

The tilt of the Earth is responsible for the seasons, and we are currently tilted at 23.5˚ however over a period of about 40,000 years, this tilt varied between 22˚ and 24.5˚. While this particular cycle doesn't influence the overall solar radiation reaching the Earth, it does impact the intensity of seasonal weather - a shallow tilt reduces the seasonality of Earth's climate, while a stronger tilt intensifies it.

Because the Earth's orbit is elliptical, and off centre, the Earth is presently closest to the sun in January, and furthest away in July. Every 20,000 years or so, the shape of this orbit is dragged around, so that the closest point of the orbit will eventually take place in July. There is about a 6% difference in incoming solar radiation between the perihelion (closest point to the sun) and the aphelion (furthest point of the orbit). Of course, over the course of a year, the total incoming solar radiation remains the same.

Orbital eccentricity takes place on a 100,000 year cycle. This varies average annual distance between the sun and Earth and the resulting total incoming solar radiation by up to 0.2%.

These changes in solar radiation seem small, but they do make an impact on the Earth's climate system, and while these impacts are not yet fully understood, they are taken into consideration by climate scientists. This information is important because, when a conversation shifts to the topic of climate change, the cyclic nature of the sun is often discussed and regularly misinterpreted.