What causes climate change?
(Image: NASA Earth Observatory)
Sunspots and solar activity
- 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.
It’s impossible to predict with certainty what the new solar cycle—No. 25, which began April 2020—will 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).
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.
References and further reading
- NASA: Is the sun causing global warming?
- NASA: Solar Cycle 25 Forecast
- Space weather: Solar Cycle 25 coming to life
- Space Weather: Historic and daily solar and sunspot activity
- Carbon Brief: Why scientists think 100% of global warming is due to humans
- Science Direct: The Law of Conservation of Energy
- IPPC Chapter 8: Anthropogenic and Natural Radiative Forcing in: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change