What causes climate change?

What causes climate change?

Menu: The causes

(Image: NASA Earth Observatory)

Sunspots and solar activity

Summary

  • Solar activity is a natural climate forcing. When the Sun has fewer sunspots, it’s an indicator that it gives off vert slightly less solar energy, that is, less heat.
  • Generally speaking, about every 11 years the number of sunspots reaches a high and then declines again. This is the ‘solar cycle’. Sometimes these cycles last for longer periods. In the late 1600s, people noticed significantly fewer sunspots over a long period called the Maunder Minimum, which corresponded with slightly lower global temperatures (Fig. 1).
  • A second slightly cooler period called the Dalton Minimum also corresponded with the ‘Year without Summer‘ in Europe (Fig. 1).
  • Sunspot activity 1880 – 1970 increased slightly, matching a slight increase in temperature. However, it has since been declining while temperatures continue to rise significantly (Fig. 2). This indicates that any activity associated with the Sun in the past 50 years may be very slightly helping to offset global warming.

Future sunspots?

It’s impossible to predict with certainty what the new solar cycleNo. 25, which began April 2020will bring, but NASA has forecast it will likely be much the same as the last 11-year solar cycle.

Overall, changes in solar activity do not appear to be a significant forcing. Although at the moment, it’s helping a small amount to offset anthropogenic warming (Fig. 2).

Fig. 1: Changes in sunspot activity over the past 400 years. Maunder Minimum and Dalton Minimum (Image: Wikipedia)
Fig. 2: The above graph compares global surface temperature changes (red line) and the Sun’s energy received by the Earth (yellow line) in watts (units of energy) per square metre since 1880. The lighter/thinner lines show the yearly levels while the heavier/thicker lines show the 11-year average trends. Eleven-year averages are used to reduce the year-to-year natural noise in the data, making the underlying trends more obvious. The amount of solar energy received by the Earth has followed the Sun’s natural 11-year cycle of small ups and downs with no net increase since the 1950s. Over the same period, global temperature has risen markedly. (Image: NASA)

Explainer

Forcing:

The term ‘climate forcing’ comes from ‘radiative forcing’ or RF, which is the difference between the amount of solar energy reaching Earth’s atmosphere and the amount that escapes. If more solar energy escapes than arrives, the planet cools. Conversely, if less energy escapes than gets in, the planet warms. This is due to the Law of Conservation of Energy, a basic law of thermodynamics, which states that: ‘Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.’

Different climate forcings each determine how much solar energy arrives and escapes.

  • Natural Forcings are those that happen through natural changes, including the Milankovich cycles the position of Earth’s continents, and volcanoes
  • Anthropogenic Forcings are those due to human activities.

Click here to learn about the main forcings and how they work (links to page on this site).

References and further reading