Isn’t the Sun going into a quiet period? Wouldn't a "grand minimum" cool Earth down?

The Sun’s energy output rises and falls in a regular cycle, with peaks every 11 to 12 years. Data since the 1950s show that the difference in output from each peak to valley is about 0.1%, which has little effect on Earth’s temperature. The last solar cycle ended with a minimum in 2008–09, when sunspot activity dropped to its lowest level since the 1910s.

The Sun also goes through longer, more irregular periods of greater or lesser activity. These include the Maunder Minimum, when sunspots nearly vanished from 1645 to 1715. Parts of the Northern Hemisphere were significantly cooler during this time, which occurred within a longer period called the Little Ice Age. Other factors were also involved in the cooling, including powerful volcanic eruptions.

Since 2010 , the Sun has been building toward the next peak in its 11-year cycle, which typically boosts sunspot activity and space-weather events. However, the peak may have occurred already, judging from monthly plots of sunspot activity published by NOAA. Some data now indicate that the Sun may be entering a longer period of relatively low activity. If this were to extend over several decades, creating what’s known as a “grand minimum,” then the usual peaks in the Sun’s 11-year cycle could weaken or even disappear. The amount of solar energy reaching Earth—the total solar irradiance (TSI)—could drop to the levels seen in a typical 11-year minimum, or perhaps further, and remain there for decades.

Because many solar variables have been measured only for a few decades, and because the Sun includes both predictable and naturally chaotic behavior, it is not possible to pin down the likelihood of a grand minimum. Its effect on Earth’s temperature would depend on exactly how much the TSI dropped and for how long. The TSI has only been measured directly for about 30 years, but proxy data from lake sediments, ice cores, tree rings, and other sources suggest that the total solar energy reaching Earth dropped by as little as 0.1% during the Maunder Minimum.

So could a lengthy drop in solar output be enough to counteract human-caused climate change? Recent studies at NCAR and elsewhere have estimated that the total global cooling effect to be expected from reduced TSI during a grand minimum such as Maunder might be in the range of 0.1° to 0.3° Celsius (0.18° to 0.54° Fahrenheit). A 2013 study confirms the findings. This compares to an expected warming effect of 3.0°C (5.4°F) or more by 2100 due to greenhouse gas emissions. In other words, even a grand solar minimum might only be enough to offset one decade of global warming. Moreover, since greenhouse gases linger in the atmosphere, the impacts of those added gases would continue after the end of any grand minimum.

During the solar cycle, the Sun's ultraviolet (UV) output varies much more than TSI does, including sharp sudden increases associated with solar storms. UV variations affect Earth’s upper atmosphere and may also influence weather and climate, particularly by way of the stratosphere, through their effects on ozone and related processes.

Related links

Don't changes in the Sun, like sunspots, explain global warming? (Climate FAQ)

Turning down the heat (AtmosNews - Research Brief)

Could a weaker Sun avert global warming? (AtmosNews - Perspective)

Learn more about the Sun (NCAR)