How does Earth’s relationship with the sun affect our planet’s climate? (Image Credit: Space.com)
In 1645, the sun shut off.
Not literally, of course; it was still shining. But that year marked the beginning of the Maunder minimum, a time of remarkably low sunspot activity. That event coincided with the so-called Little Ice Age, a period of exceptionally cooler temperatures throughout the North Atlantic that led to harsher winters and briefer summers throughout much of Europe. Although it may be a mere coincidence that the two events occurred together, astronomers and geologists have since concluded that the relationship between the sun and Earth plays a major role in our planet’s climate.
During a typical 22-year period, consisting of a couple of solid sunspot cycles, we’re likely to count 30,000 to 40,000 sunspots. During a part of the Maunder minimum, right at the end of the 17th century, however, there were zero sunspots. Scientists still aren’t sure what caused the Maunder minimum, which eventually eased by 1715. But they do know that it overlapped with the Little Ice Age. The overlap may just be a coincidence; after all, the Little Ice Age started well before and lasted long after the Maunder minimum, and the rest of the planet seemed to be unaffected.
However, the number of sunspots does relate to the overall brightness of the sun. So during the Maunder minimum, the sun was a little less intense, and Earth’s northern latitudes are more susceptible to even tiny changes in the sun’s output. There’s far more land area there than at the equivalent southern latitudes, and land changes temperature far more quickly than water does. And because higher latitudes experience stronger seasons, small changes to the sun can have large ripple effects that wouldn’t be experienced throughout the rest of the globe.
There’s still no consensus on the link between the Maunder minimum and the Little Ice Age. But when we zoom out on a geological timescale, we do find a very strong — and very unexpected — connection.
The first person to point out the effect of the Earth-sun system on our planet’s climate was Serbian physicist and astronomer Milutin Milankovitch, who, in the 1920s, discovered several natural cycles in Earth’s orbit that might be responsible for major climatic shifts.
The first natural cycle is that Earth’s orbit slowly changes from elliptical to circular and back again roughly every 100,000 years. These changes are due to slight gravitational nudges from Jupiter and Saturn. Currently, Earth’s eccentricity (a measure of the ellipticity of an orbit) is 0.0167 and decreasing. These changes to our planet’s orbit influence the length and magnitude of the seasons, because when Earth is farther away from the sun, it moves more slowly than when it’s closer to the sun. So if Earth is at maximum eccentricity — and if Earth’s farthest point lines up with summer in the Northern Hemisphere — that year’s summer will last longer than usual.
Another cycle changes the axial tilt of our planet by 22.1 to 24.5 degrees roughly every 41,000 years. Earth’s current tilt is 23.44 degrees and decreasing. This cycle also affects the magnitude of the seasons: More tilt means more time in direct sunlight or more time hidden away from the sun, which makes the season more extreme.
A third cycle is known as axial precession. Our planet is spinning like a top every 25,700 years, with Earth’s axis of rotation drawing a lazy circle in the sky. This changes which hemisphere gets more sun. Right now, Earth’s closest approach to the sun happens to line up with Southern Hemisphere summers, making those seasons extra toasty.
These cycles weave in and out of each other — sometimes reinforcing each other, and sometimes canceling each other out. Sometimes, multiple cycles add up to create a big effect; other times, they don’t. But no matter what, Earth’s position relative to the sun has a big influence on our planet’s climate.
Comparisons of the Milankovitch cycles with temperature records taken from ice core samples reveal a very tight connection. Periods of glaciation, colloquially known as “ice ages,” line up with periods of the Milankovitch cycles when Earth, especially the northern latitudes, receive less sunlight than usual, and warm periods line up with more sunlight received in the North.
The last time the glaciers retreated was roughly 12,000 years ago, which coincided with a slight increase in overall sunlight due to the Milankovitch cycles. Everything that shift entailed — from the extinction of many species, such as woolly mammoths, to the spread of humanity throughout the Americas — followed directly from the small change in our planet’s orbital configuration.
According to the Milankovitch cycles, Earth should actually be experiencing a cooling period right now — but the consequences of humanity’s carbon emissions have completely swamped that eons-old relationship between the sun and Earth.