Who’s to Blame for Global Warming? Not the Sun
Humans seem to have an innate fascination with the sunspot cycle. Sunspots were known to ancient civilizations, which observed and counted them when the sun was low in the sky. After the invention of the telescope, several astronomers started regularly observing and recording the number of visible sunspots. (Warming: don’t try this at home! Looking directly at the sun, especially through a telescope, can cause blindness.) By piecing together records from various observers, we think we have good estimates of sunspot numbers dating back to 1610.
In the 19th century it was realized that the number of sunspots tends to increase and decrease with a cycle of about 11 years. Gradually, observers also noticed that these “11-year” cycles are not all the same. Some last a little longer, and some a little shorter. And some have more sunspots at their peak than others. In fact, during the late 1600s, there was a period of about 50 years when sunspots were exceedingly rare. (We’re quite sure that this was a genuine lack of sunspots, rather than a lack of observations, which would be less interesting.) Why did this happen? It’s still “a subject of active research,” which is science-speak for “we’re still working on it.”
And what do these mysterious happenings 93,000,000 miles away mean for life on Earth, in particular for our climate?
Probably not much (sorry if that is disappointing). To say that the sun has a strong influence on climate is like saying that Shakespeare has a big influence on English literature. It’s what drives everything. But the sun’s energy output is remarkably constant. So while the sun has a huge influence on our climate in general, it is much less important in driving climate change. Over the past 30 years or so, that output has been measured using satellite-based instruments, which are much more accurate than ground-based ones because they’re located above clouds, pollution, and other obscurations.
Satellite measurements show very small variations in solar energy output over time, coinciding with fluctuations in sunspot numbers. (But more sunspots actually mean more solar energy, not less.) The size of these recent variations is very small: about 0.1 percent of the total solar energy output.
Despite what you may have read, these small, cyclical variations in solar energy output are not responsible for the global warming trend over the past 50 years: greenhouse gases are almost certainly the main cause for this.
The global temperature response to the small variations in solar energy output is barely detectable, partly because the solar variations themselves are small, and partly because the climate system does not have time to fully respond to these up and down variations in the sun: by the time the ocean “gets the memo” that it’s supposed to cool off, for example, the sun is already getting brighter again.
Recently it has been suggested that the sun may be headed into an extended period with few sunspots, like what occurred during late 1600s. That time period coincided with what is now referred to as the “Maunder Minimum,” a period when parts of the world — especially Northern Europe — were colder than they are today. What would a new Maunder Minimum mean for our warming climate? Again, probably not much.
Of course we don’t know for sure how much dimmer the sun was in the late 1600s (when there were very few sunspots, but no satellites around to measure solar energy output), but estimates are that it was enough to cool global temperatures by about 0.3°C or 0.4°C. That’s certainly measurable, but it’s not much compared to increases projected for this century. Those range from about 2°C to 6°C, depending on assumptions about future rates of fossil fuel use and scientific uncertainties.
So in fact any solar dimming would be literally “in the noise” of projected future climate changes — that is, smaller than the differences among the various projections — and certainly not enough to nullify them. Furthermore, of course, such reductions in the rate or extent of warming would be temporary, and when the solar energy output picked up again, the warming would be all the more rapid.
So the bottom line is that sunspots and solar variations aren’t causing 21st century climate change, and won’t rescue us from it, either.