Talk to a climate denier — someone who believes global warming has purely natural causes — and chances are you’ll soon hear the phrase “Milankovitch cycles.” These, not humans, are responsible for climate change, they’ll say. Actually, this is what your regular climate denier will claim. In the extreme version, some of these benighted souls deny that Earth is actually warming, despite 19 of the last 20 years being the hottest on record. Those deniers are beyond the pale, or at least beyond the reasoning skills of this writer.
Around 97 percent of climate scientists believe the data show that the main driver of global warming is the increase in the concentration of greenhouse gases, in particular carbon dioxide (which rose by 24 percent from 1979 to 2024). They further maintain that we humans are the responsible party, hence the common term Anthropogenic Global Warming, or AGW. Not so fast, say the deniers, who claim Earth is naturally warming because we’re getting more heat energy from the sun, mainly because of the Milankovitch cycles.
Milutin Milankovitch (1879-1958) was a brilliant Serbian mathematician, astronomer and geophysicist who founded the discipline of planetary climatology. In the 1920s, he published an analysis of how changes in Earth’s movements in space over thousands of years affect the amount of sun energy reaching the surface of the planet. He singled out three major variables — eccentricity, axial tilt and precession — which have affected our climate over the long term, resulting in, for instance, the ice ages.
Taking these one at a time: Eccentricity is the change in the shape of Earth’s orbit around the sun — from nearly circular to mildly elliptical — on a 100,000-year cycle. It’s caused by gravitational forces from other planets, in particular Jupiter. Axial tilt, or obliquity, is the change in the angle of Earth’s axis from 22 degrees to nearly 25 degrees and back, over a 41,000-year cycle. Finally, precession is the wobble of Earth’s axis (think of a spinning top), which changes on a 26,000-year cycle.
The combined effect of these three variables is sometimes enough to kick-start extreme changes in Earth’s climate. For instance, once the three variables were sufficiently in sync to initiate a warm period, ocean ice began to melt and the oceans warmed, leading to more CO2 in the atmosphere in a self-reinforcing feedback cycle. Thousands of years later, a low in the combined effects of Milankovitch cycles caused the planet to revert to a cooling phase.
But these cycles can’t explain the recent warming trend, for two reasons: First, they cause long-term climate changes, not the rapid, exponential warming Earth has experienced since the beginning of the industrial age; Second, the amount of solar radiation reaching Earth has been slightly decreasing over the past 10,000 years as a result of the combined effects of Milankovitch cycles.
Climate deniers sometimes cite two other natural factors: the sun’s variable energy output and volcanoes. But the sun’s output (as its magnetic field reverses polarity) only changes by about 0.1 percent over its regular 11-year cycle. And volcanoes do indeed affect Earth’s temperature (in the accompanying graph, the “all natural factors” dips result from major eruptions like Krakatoa in 1883), but they’re short-lived impacts.
The graph compares human factors (mainly, CO2 from fossil fuels and deforestation) and natural factors (Milankovitch cycles + sunspot activity + volcanoes) with the average (land and sea) measured temperatures. Clearly, the natural factors cited by deniers are minimal, while human activities match the observations rather well. As Walt Kelly’s Pogo put it: “We have met the enemy and he is us.”
Barry Evans (he/him, barryevans9@yahoo.com, planethumboldt.substack.com) notes that Humboldt Bay is experiencing the fastest rate of relative sea level rise on the West Coast. (Warm water being less dense than cool water, sea levels rise as Earth warms.)
This article appears in Portrait of a Prairie.