by Javier Vinos
Over the past two decades, solar activity has been characterized by an extended solar minimum spanning two solar cycles, known as the Clilverd Minimum. This phenomenon is currently affecting the climate, but before we can understand its impact, we must address the significant discrepancy between the solar effects observed in paleoclimate proxy records and modern observations.
The relationship between solar signals and climate response is complex and not fully understood. However, there is substantial evidence from models and reanalyses that the relationship exists. A recent hypothesis is that the solar signal modulates heat and moisture transport to the Arctic, which explains its relatively small effect during a single solar cycle. However, when an anomaly in solar activity persists over several cycles, as it did during the 70-year modern solar maximum, its effect accumulates and has a large impact on the planet’s energy budget. Understanding this mechanism is critical to understanding the overall impact of solar activity on our climate.
Current Solar Activity
The monthly sunspot number for June 2023 reached 163.4. While this figure may be revised slightly, it’s likely to stand as the highest number seen in over two decades, since September 2002. Solar Cycle 25 is relatively young, only three and a half years old, which means there are ample opportunities over the next three years to surpass this month’s 20-year record. Based on recent data, it seems very likely that Solar Cycle 25 will surpass Solar Cycle 24 in terms of activity.
Both solar cycles 24 and 25 show significantly low activity compared to the average of the last 300 years. Together they represent an extended solar minimum, recently proposed to be known as the Clilverd Minimum. This name proposal is due to a paper published in 2006 by Mark Clilverd and colleagues, in which they successfully predicted the occurrence of this event.
Contrary to earlier speculations, the likelihood of a solar grand minimum in the 21st century is becoming increasingly remote. Similarly, predictions that the current extended solar minimum would lead to a marked decrease in temperature are incorrect. However, this doesn’t mean that the Clilverd minimum has no effect at all. Changes in solar activity indirectly affect surface temperatures in a complex way. Understanding how these solar variations affect the climate is crucial to identifying their effects.
Figure 2. Projected solar activity based on my 2018 model, which relies on long-period solar cycles. The model uses the total number of sunspots in a cycle, rather than peak activity, and assumes regular 11-year cycles. At each point, it estimates the effect of five different long cycles, considering their historical impact on sunspots or 14C records. Four Feynman (100-year) solar cycle periods are indicated at the bottom.
The solar effect on climate (I). Modern observations
There exists a great discrepancy between the solar effects observed in paleoclimatic proxy records and modern observations. According to satellite instruments, the change observed over the solar cycle amounts to a mere 1.1 W m–2, and the variability observed over the past 9,000 years doesn’t appear to be much higher, approximately 1.5 W m–2. This presents another challenge because the change is so minuscule that its impact should be indiscernible amidst the noise of climate data. However, numerous studies consistently identify a climate influence of approximately 0.1°C attributed to the solar cycle, which is about four times larger than expected from the slight radiative change. Consequently, there arises a necessity for an amplifying mechanism to account for this second discrepancy.
Adding to the complexity, the effect of the solar cycle on surface temperatures is not what would be expected from a marginal increase in total irradiance over the entire surface. Rather, it reveals a highly dynamic pattern characterized by certain regions experiencing warming of more than 1°C, while others show cooling trends (Figure 3). Interestingly, this pattern is similar to the warming observed between 1976 and 2000. During this period, the Northern Hemisphere experienced more warming than the Southern Hemisphere, land surfaces warmed more than the oceans, and the mid-latitudes of the Northern Hemisphere experienced the most pronounced warming effects.