10 March 2017 – ‘Way back in the 1970s, when I was an astophysics graduate student, I was hot on the trail of why solar prominences had the shapes we observe them to have. Being a good little budding scientist, I spent most of my waking hours in the library poring over old research notes from the (at that time barely existing) current solar research, back to the beginning of time. Or, at least to the invention of the telescope.

The fact that solar prominences are closely associated with sunspots led me to studying historical measurements of sunspots. Of course, I quickly ran across two well-known anomalies known as the Maunder and Sporer minima. These were periods in the middle ages when sunspots practically disappeared for decades at a time. Astronomers of the time commented on it, but hadn’t a clue as to why.

The idea that sunspots could disappear for extended periods is not really surprising. The Sun is well known to be a variable star whose surface activity varies on a more-or-less regular 11-year cycle (22 years if you count the fact that the magnetic polarity reverses after every minimum). The idea that any such oscillator can drop out once in a while isn’t hard to swallow.

Besides, when Mommy Nature presents you with an observable fact, it’s best not to doubt the fact, but to ask “Why?” That leads to much more fun research and interesting insights.

More surprising (at the time) was the observed correlation between the Maunder and Sporer minima and a period of anomalously cold temperatures throughout Europe known as the “Little Ice Age.” Interesting effects of the Little Ice Age included the invention of buttons to make winter garments more effective, advances of glaciers in the mountains, ice skating on rivers that previously never froze at all, and the abandonment of Viking settlements in Greenland.

And, crop failures. Can’t forget crop failures! Marie Antoinette’s famous “Let ’em eat cake” faux pas was triggered by consistent failures of the French wheat harvest.

The moral of the Little Ice Age story is:

Global Cooling = BAD

The converse conclusion:

Global Warming = GOOD

seems less well documented. A Medieval Warm Period from about 950-1250 did correlate with fairly active times for European culture. Similarly, the Roman Warm Period (250 BCE – 400 CE) saw the rise of the Roman civilization. So, we can tentatively conclude that global warming is generally NOT bad.

Sunspots as Markers

The reason seeing sunspot minima coincide with cool temperatures was surprising was that at the time astronomers fantasized that sunspots were like clouds that blocked radiation leaving the Sun. Folks assumed that more clouds meant more blocking of radiation, and cooler temperatures on Earth.

Careful measurements quickly put that idea into its grave with a stake through its heart! The reason is another feature of sunspots, which the theory conveniently forgot: they’re surrounded by relatively bright areas (called faculae) that pump out radiation at an enhanced rate. It turns out that the faculae associated with a sunspot easily make up for the dimming effect of the spot itself.

That’s why we carefully measure details before jumping to conclusions!

Anyway, the best solar-output (irradiance) research I was able to find was by Charles Greeley Abbott, who, as Director of the Smithsonian Astrophysical Observatory from 1907 to 1944, assembled an impressive decades-long series of meticulous measurements of the total radiation arriving at Earth from the Sun. He also attempted to correlate these measurements with weather records from various cities.

Blinded by a belief that solar activity (as measured by sunspot numbers) would anticorrelate with solar irradiation and therefore Earthly temperatures, he was dismayed to be unable to make sense of the combined data sets.

By simply throwing out the assumptions, I was quickly able to see that the only correlation in the data was that temperatures more-or-less positively correlated with sunspot numbers and solar irradiation measurements. The resulting hypothesis was that sunspots are a marker for increased output from the Sun’s core. Below a certain level there are no spots. As output increases above the trigger level, sunspots appear and then increase with increasing core output.

The conclusion is that the Little Ice Age corresponded with a long period of reduced solar-core output, and the Maunder and Sporer minima are shorter periods when the core output dropped below the sunspot-trigger level.

So, we can conclude (something astronomers have known for decades if not centuries) that the Sun is a variable star. (The term “solar constant” is an oxymoron.) Second, we can conclude that variations in solar output have a profound affect on Earth’s climate. Those are neither surprising nor in doubt.

We’re also on fairly safe ground to say that (within reason) global warming is a good thing. At least its pretty clearly better than global cooling!