The Sun generates large amounts of energy that provide the Earth with its primary source of electromagnetic radiation. As the main driver for the Earth’s climate, its energy is a major factor in the generation of clouds, ocean currents, thunderstorms, and hurricanes.
The amount of incoming solar radiation is balanced by the amount of outgoing terrestrial radiation, so that the Earth does not continue to heat up nor cool down indefinitely. The Earth's climate is said to exist in equilibrium. When the climate system responds to radiative forcing, this equilibrium is temporarily upset and a discrepancy between incoming and outgoing radiation exists. In an attempt to restore equilibrium, the global climate subsequently alters by either heating up or cooling down, depending on the direction of initial forcing.
Although the climate system is in balance, that balance is dynamic, ever-changing. The system is constantly adjusting to forcing perturbations and, as it adjusts, the climate alters. A change in any one part of the climate system will have much wider consequences as the initial effect cascades through the coupled components of the system. As the effect is transferred from one sub-component of the system to another, it will be modified in character or in scale. In some cases it will be amplified (known as positive feedback) and in others it may be reduced (negative feedback).
Since the Earth has been warming particularly over the last several decades, it is reasonable to look to our nearest star and understand any variations in its output that may have an impact on the Earth’s climate. Can changes in incoming solar radiation explain all or even part of the warming we have seen in Earth’s planetary temperatures over the last several decades?
The amount of energy reaching the top of the Earth’s atmosphere is known as total solar irradiance (previously known as the solar constant) and is defined as the amount of solar radiation per unit area perpendicular to the incoming path. Since the late 1970s this value has been measured accurately by satellites and is 1,360 watts per square meter. Given the spheroidal geometry of the Earth and the fact that only half of the Earth’s surface is illuminated at a given time, the amount of sunlight arriving at the top of Earth’s atmosphere averaged over the entire planet is one-fourth of the total solar irradiance, or approximately 340 watts per square meter. Does this solar irradiance vary and if so by how much?
Ever since Galileo first observed sunspots on the Sun’s surface in 1610, sunspot observation records have been carefully kept. Sunspots are regions on the Sun’s surface that appear darker since they are cooler (although at 4500 degrees Kelvin they remain extremely hot). Sunspot regions are where the Sun’s magnetic field is modified in such a way that it prevents the hotter gases within the Sun’s interior from reaching its surface. Accurate measurements collected using radiometers on satellites over the last four decades suggest that changes in solar irradiance are a balance between darkening from sunspots and brightening from accompanying hot regions called faculae. When solar activity increases, as it does every 11 years or so, both sunspots and faculae become more numerous.
During the peak of a cycle however, the faculae brighten the sun more than sunspots dim it. The data indicate that at the maximum of an 11-year solar activity cycle, total solar irradiance is larger by about 0.1% than at the minimum. Even this seemingly small fraction in the short term variation of solar output can be important.
Furthermore, evidence from historical sunspot observations going back four centuries, tree-ring measurements going back nearly ten thousand years and other proxy climate data sources such as carbon-14 dating show that solar output has not been constant over longer timescales (hence the need to change the use of ‘solar constant’). During periods of low sunspot activity, there appears to be a greater concentration of Carbon-14. There has been recent speculation that this may be due to the increased number of highly energetic cosmic rays interacting with nitrogen in the upper atmosphere. However, this is an active area of hotly debated research. Additionally, recent studies of the effects of long-term solar irradiance changes on climate suggest that while the total solar output changes in the short term by 0.1%, changes in a specific narrow band of Extreme Ultraviolet Wavelengths (EUV) are of the order of factors of 10 or more. This specific type of solar radiation significantly changes the chemistry of the upper atmosphere, which in turn can cause changes in the lower atmosphere and regional weather patterns at the surface. Again this is an active area of research and no significant conclusions have yet been drawn due to limited data.
Let us not ignore the Sun’s nuclear core. Astronomically speaking, the Sun is an average star which over the course of its four and a half billion years has brightened by about 25%. As for the main energy flow, improved theories of the nuclear furnace deep within the Sun show stability over many millions of years. The Sun emits its huge amounts of energy throughout the entire solar system, with only a small fraction of it reaching the Earth. The other planets in the Solar System similarly receive their own specific amounts of solar radiation depending on their distance from the Sun and their own diameter. Understanding solar variations on other planets improve our understanding of the impact on our own. However the thermonuclear processes deep within the Sun’s core will continue unabated and unchanged for the next four billion years or so.
So how do the solar irradiation changes highlighted earlier affect the climate on Earth? There appears to be no evidence to support 11 year cyclical temperature differences in response to the 11 year solar cycle.
However, what changes do we have evidence for? It is important to note that any solar irradiation changes (so-called forcings), lead to “feedbacks” on the Earth, which amplify the effect of the initial forcing. For example, increased solar irradiance and any subsequent warming lead to changes in the albedo of the Earth. Albedo is an important variable that determines how much of the incoming solar energy is reflected back out into space by clouds, aerosols, ice, snow and other reflective surfaces such as deserts. The remaining energy which has not been reflected, is absorbed by the land, ocean and atmosphere and it is this solar power that drives the whole climate system.
As reflective surfaces such as ice sheets decrease, the Earth’s albedo and hence its ability to reflect the warming rays of the Sun decreases, leading to a positive feedback cycle. On the other hand, increases in aerosols, particularly sulphates, raise the Earth’s albedo by reflecting incoming rays.
Can scientists determine the significance of a 0.1% change in total solar irradiation with respect to climate change? Some studies using climate models suggest that these combined changes in solar radiation could cause surface temperature changes of the order of a few tenths of a degree Celsius. However the Earth has warmed by around 0.8’C with the most rapid warming occurring in the last 50 years, which would suggest that changes in solar irradiance are not the major cause of the temperature changes in the second half of the 20th century. According to the latest reports from NASA, the current solar cycle which started in 2008 and was expected to reach a maximum by the beginning of this year has the lowest number of sunspots since the early 1900s. This is not unusual, and sunspot prediction is not infallible. However, we are seeing evidence of a warming planet, not only with global temperatures but also with melting glaciers, sea-level rises and warmer oceans, while the Sun’s activity is temporarily diminished.
The Maunder Minimum, a cold period during the 17th century is often used as an example of a time when the Sun had very few sunspots. This corresponded to a time when the Earth’s climate was considerably colder, with frozen rivers and advancing glaciers and ice sheets around Iceland extending several miles beyond their previous boundaries, a time known as the Little Ice Age. This was also a period of increased volcanism leading to greater sulphate particles in the upper atmosphere, which as mentioned lead to an increased albedo effect. Studies have questioned whether this cooling was regional or indeed global. It cannot be said with certainty that lower sunspot activity alone led to the cooling experienced on Earth.
The Earth’s climate system is complex and interconnected. Examining a single factor such as solar variability only shows part of the story. Solar variability itself is the subject of intense research, with scientists using the entire solar spectrum and geomagnetic activity as a more accurate indicator of solar activity than sunspots alone. Furthermore, if our Earth’s climate is sensitive to almost imperceptible changes in the radiation arriving from the Sun, then we must also consider other more perceptible changes such as greenhouse gas levels in the atmosphere, agricultural changes and pollution.
The collection of enormous amounts of data, including decades of accurate daily satellite measurements, four centuries of observed sunspot data, and hundreds of thousands of years worth of climate proxy data, provide a compelling account of the Earth’s history of climate change, the Sun’s role and our role within it.
 Energy that comes to us from the sun is transported in the form of waves known as electromagnetic radiation. Examples of electromagnetic radiation include radio waves, microwave radiation, infrared radiation, visible light, ultraviolet radiation, x-rays and gamma rays
 Radiative forcing is defined as the difference between radiant energy received by the earth and energy radiated back to space and can be either positive (leading to warming) or negative (leading to cooling).