Natural factors: Variations in the Sun’s output
The Sun’s output varies over time. Scientists track these changes using observations of sunspots and (more recently) satellite measurements. Average solar energy has hardly changed in the past 50 years and may even have had a recent cooling effect. The Sun cannot account for the increase in global temperature observed since 1970.
For hundreds of years, scientists have regularly counted the number of dark patches on the face of the Sun. These ‘sunspots’ are caused by intense magnetic activity in the Sun’s interior. More sunspots mean the Sun is more active and giving off more energy, so sunspot numbers are a ‘proxy’ for solar output. By keeping track of sunspot numbers, scientists record fluctuations in the Sun’s activity over time, enabling them to calculate changes in the amount of solar energy reaching the Earth. Sunspot observations show that the Sun’s activity increased slightly in the first half of the 20th century, but has hardly changed in the past 50 years.
Since 1978 satellites orbiting our planet have been monitoring the Sun’s output with instruments called radiometers, which measure the intensity of solar energy. While telescopes on the ground have their measurements disturbed by the atmosphere, instruments in space have an unimpeded view of the Sun, enabling scientists to study its characteristics in more detail. As well as visible sunlight, the Sun gives off other frequencies of energy, including ultraviolet and infrared. Radiometers measure these different frequencies and scientists use the data to construct a record of total solar energy reaching the Earth. Satellite measurements indicate that average solar energy has hardly changed in recent decades, confirming estimates from sunspot observations.
Solar output increased slightly in the first half of the 20th century, probably causing some of the global warming observed from about 1900 to 1940. But average solar energy has hardly changed in the past 50 years. If anything, measurements suggest a slight decrease over the past few decades, so the Sun cannot account for the warming observed since 1970. Recent temperature patterns are also inconsistent with a possible solar cause. For example, an increase in solar energy would warm the stratosphere as well as the Earth’s surface. Instead, measurements show stratospheric temperatures falling, which is consistent with what scientists expect from global surface warming caused by increased greenhouse gases.
Solar output increases and decreases in cycles of about 11 years, as magnetic activity in the Sun’s interior waxes and wanes. During the increasing part of the cycle, magnetic activity intensifies and more sunspots appear on the surface, while the opposite occurs during the decreasing part of the cycle. This magnetic cycle causes slight variations in the amount of energy the Sun gives off – an increase of about 0.1% in total solar energy from the minimum to the maximum of the cycle. But because each solar maximum is followed by a solar minimum about 5–6 years later, the cycle averages out and has little effect on long-term solar energy.
Earth is constantly bombarded by subatomic particles known as cosmic rays. Some scientists have suggested that cosmic ray showers may affect global temperature by influencing cloud cover, though evidence for this is inconclusive. The Sun emits a stream of particles called ‘solar wind’, which varies in strength with the amount of sunspot activity. The Sun’s magnetic field and solar wind deflect cosmic rays, so when the Sun is more active fewer particles reach our atmosphere. If cosmic rays affect clouds, changes in solar activity might affect global temperature by changing cosmic ray intensity. But measurements show no significant change in either average solar activity or cosmic ray intensity in recent decades, so neither can account for the current period of global warming.
Some scientists have suggested that cosmic rays may influence the Earth’s cloud cover by creating tiny particles around which water droplets can condense. An increase in these particles could cause more clouds to form, which in turn might affect global temperature because clouds reflect sunlight back out into space. Measuring these effects has been difficult, both in laboratory experiments and in the atmosphere. However, recent studies comparing real observations of cloud cover with measurements of past variations in cosmic ray intensity have found no evidence of a link between the two. Calculations based on the physical mechanisms involved also suggest that the effects of cosmic rays are too small to be important in climate variability.
Prof Jo Haigh’s work looks into how solar activity influences climate. ‘How variations in the Sun’s activity affect the Earth’s climate is important as it helps us to understand the role of natural factors in global climate change,’ says Jo. By using data taken from satellites observing the Sun and feeding them into computer models, Jo has focused her work on how solar ultraviolet radiation can influence the Earth’s atmosphere. Working at the cutting edge of climate science doesn’t come without its challenges, as Jo has had to overcome time and technical constraints as well as a grilling from Jeremy Paxman on Newsnight.