Solar Cycle 24 is approaching its end, which will mean the beginning of Solar Cycle 25. What does that mean for Earth’s weather—and for us?
Fig. 1 (below) is a graph from the NOAA Space Weather Prediction Center showing Cycle 24’s very low level of solar activity—the lowest in more than two centuries, even lower than the level in the early 1900s and comparable to the very low levels of solar activity that occurred in the early 1800s (the period referred to as the “Dalton Minimum,” which coincided with the “Little Ice Age”).
As shown in Fig. 2 (below), these three periods have brought the lowest solar activity levels since the Maunder Minimum, the period from about 1645 to 1715, when solar cycles apparently stopped and sunspots were exceedingly rare.
As you may know, we at The Old Farmer’s Almanac use solar activity as the primary driver of our long-range weather forecasts. We believe that changes in the Sun’s output, although relatively small, are sufficiently amplified in Earth’s upper atmosphere to strongly influence Earth’s weather patterns.
One of the most significant relationships that we have found is that periods of low solar activity are associated with colder temperatures, averaged across Earth. Our viewpoint is a controversial one, as most scientists believe that the magnitude of changes in solar activity are insufficient to have a significant effect on Earth’s weather, and they view as coincidence that past periods of exceptionally low solar activity have historically corresponded with cold periods.
However, an increasing amount of research seems to be giving credence to our theory: Although the changes in magnitude of solar activity are small, there is a mechanism in the upper atmosphere that can amplify these changes, causing larger ripples in the lower portion of Earth’s atmosphere, where weather occurs.
Historically, all of the periods in the known sunspot record that have had low activity have also had relatively cool temperatures, averaged across the globe. The Maunder Minimum coincided with an exceptionally cold period in many parts of the globe. We believe that with low solar activity continuing for at least the next 10 to 30 years, global temperatures will be cooler than they would otherwise be.
Despite the recent low solar activity, April 2018 was the third warmest April ever recorded, averaged across the globe, behind only April 2017 and April 2016. Incredibly, April was the 400th consecutive month in which temperatures averaged across the entire Earth were warmer than the month’s 20th-century average temperature.
Other Factors That Affect the Weather
So why, you might ask, have Earth’s temperatures been so consistently warm when our forecast methodology, which is based primarily on solar activity, says that they should be cool?
The answer is that solar activity is not the only factor in Earth’s weather.
For example, one factor that all atmospheric scientists believe can make Earth colder for as much as a few years is a volcanic eruption that spews ash into the middle and upper portions of the atmosphere. While this has not been a major factor in recent years, it has been at times in the past and could be again in the future.
Another factor is increased urbanization. The heat from buildings and human activities in cities makes them warmer than the surrounding countryside—something known as “the urban heat island effect.” However, most atmospheric scientists believe that this is a local effect that does not significantly raise Earth’s average temperatures.
It is important to note that although Earth, on average, has been warming for decades, not every place is or will be warmer than normal each season. Remember: Other factors are at play, including the normal variation in weather that occurs from day to day and year to year.
The most significant factor (in addition to solar activity) that has been affecting our weather in recent years has been the increase in greenhouse gases—most notably, carbon dioxide and methane—which most (but not all) atmospheric scientists believe has been making Earth progressively warmer. We have been incorporating the influence of these increases into our forecasts as a factor that will offset much of the cooling from our current period of low solar activity.
If we are correct in these factors, what this means is that the current period of low solar activity has been partially offsetting the greenhouse warming that has been occurring. This suggests that when the current period of exceptionally low solar activity ends and solar activity returns to a more normal level—perhaps in 30 years or so—we will see a rapid jump in the Earth’s average temperatures. Until that time, we would expect the general warming trend to continue, but as a slow warming in which some months set new records for global warmth but many do not.