Human-induced warming is sending Earth into frightening and uncharted climate territory — but humans are not the first force to cause colossal changes to our climate.
Other celestial bodies, including planets, tug at Earth causing it to move in ways that affect our ice caps in history-shaping ways, as discussed by narrator and astrophysicist Neil deGrasse Tyson on Sunday’s Cosmos: A Spacetime Odyssey.
For example, the pull of the planets influences Earth’s tilt. They cause Earth’s axis to wobble in a circular motion similar to the spin of a top. You can can see these movements in the Cosmos GIF to the left.
If you could stick a pen out of Earth’s north pole, it would draw a circle about every 26,000 years. In 14,000 AD for example, our north star was not Polaris, as it is now, but Vega. In 12,000 years , Vega will be our north star again.
The pull of the planets also cause the Earth’s tilt to change between 22.1 and 24.5 degrees over 41,000 years. When our tilt is more extreme, seasons can be more severe, with warmer summers and cooler winters. When the tilt is less, we get cooler summers and milder winters. Currently our tilt is about 23.4 degrees — near the middle.
In the GIF below, you can see another effect of planetary pull. Earth’s orbit is not round, but slightly egg-shaped or eccentric.
“The shape of the Earth’s orbit changes from being elliptical (high eccentricity) to being nearly circular (low eccentricity) in a cycle that takes between 90,000 and 100,000 years,” according to NASA’s Earth Observatory.
Currently, Earth is at its closest to the sun at the beginning of January when it is about 91.5 million miles away and at its furthest in July when it is about 94 million miles away. This corresponds to about a 6% difference in how much solar radiation the Earth receives.
Earth, planetary pull, and ice ages
During a highly elliptical orbit, the Earth would receive 20 to 30% higher sunlight intensity between its closest and furthest passes by Earth, “resulting in a substantially different climate from what we experience today,” according to NASA.
The pull of every celestial body combines to create Earth’s eccentricity, wobble, and tilt. These movements determine how our planet is positioned to receive sunlight and how much sunlight it can receive. Which of these is the dominant determiner of ice ages, however, is still not completely understood.
“There is still some discussion about how exactly this starts and ends ice ages, but many studies suggest that the amount of summer sunshine on northern continents is crucial: if it drops below a critical value, snow from the past winter does not melt away in summer and an ice sheet starts to grow as more and more snow accumulates,” said the Intergovernmental Panel On Climate Change in their 2007 report.
One glacial period, or lack thereof, was particularly significant to human history: Fifteen to twenty-five thousand years ago, the northern ice cap shrunk, revealing a land bridge that our ancestors would use to cross from Eurasia into North America.
How ice forms civilizations
When Earth’s northern region is ill-positioned to bathe in the sun’s rays, the top third of the world becomes entombed in ice. You can see the northern polar ice cap expand all the way down over California and consume the better part of North America:
When the ice grows, it siphons water from the sea, lowering sea levels 400 feet, said deGrasse Tyson. This exposes more land in coastal regions.
When Earth is at a prime angle for sunbathing, this ice thaws and sea levels rise. When Earth thawed about 15,000 to 25,000 years ago, receding ice exposed a land bridge connecting Eurasia and North America. This bridge would later be humanities route into North America.
Roughly 10,000 years ago, “the manic swings of the climate and sea levels,” subsided, said Tyson, “a new and gentler climate age began.”
We entered an interglacial period that we still live in, “one of those balmy intermissions in an ice age,” he said. Rivers transported fertile sediments into deltas, where some humans made their homes.
Once solely wanderers, humans had time and resources in this interglacial period to settle and build the civilizations at the dawn of our history.
“The way the planets tug at each other, the way the skin of the earth moves, the way those motions affect climate, and the evolution of life and intelligence, they all combined to give us the means to turn the mud of those river deltas into the first civilizations,” said Tyson.
Now go enjoy the beautiful interglacial period. You’ve only got about another 50,000 years. That is, if we don’t throw it all out of wack with our greenhouse gas emissions.
GIFs from Cosmos: A Spacetime Odyssey, The Lost Worlds Of Planet Earth. Watch the full episode here.
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