Causes: the sun
Solar flare image: NASA
Solar energy
The Sun is the primary* source of energy (heat) being delivered to Earth. The Sun’s energy output has increased by about 30% since the Earth was formed 4.5 billion years ago. The increasing heat output was and remains very slow, around 1% every hundred million years. This is because the Sun burns hydrogen into helium in its core. As it does, the core becomes denser and contract. This increases the pressure, making the core hotter. That in turn increases the sun’s brightness and the amount of heat emits. Earth would have frozen, but the atmosphere was composed of hydrogen sulphide and the greenhouse gases methane and carbon dioxide, but no oxygen.
The sun also has a rhythmic cycle. About every 11 years the number of sunspots increases to a high, and then declines (Fig. 1). These sunspots signify increasing activity on the surface of the sun, but the changes in the amount of energy it delivers to Earth is a relatively minor change in the climate forcing.
* Heat energy also comes from Earth’s interior.
In the late 1600s, people noticed significantly fewer sunspots over a long period called the Maunder Minimum, which corresponded with slightly lower global temperatures. A second slightly cooler period called the Dalton Minimum also corresponded with the so-called ‘Little Ice Age‘ in Europe (Fig. 2). However most of this temporary cooling had little impact on the rest of the planet. The primary cause of regional cooling has been attributed to several large volcanic eruptions.
Sunspot activity 1880 – 1970 increased slightly, corresponding to a slight increase in temperature. However, sunspot activity subsequently declined while temperatures continued to rise significantly. In fact, slightly lower activity in Sun from 1950-2020 may have slightly helped offset global warming (Fig. 3).
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Higher levels of solar activity often results in more intense auroras