We died and the Earth got cooler

The Cold We Caused

Steven Stoll, Harper's: Excavate the Middle Ages, and one unearths a geological event with enormous implications for how we think about and respond to climate... The Northern Hemisphere fell into a frigid rut around 1350 that lasted until the nineteenth century... [It] came and went in a geological instant, too brief to have been caused by the wobble of Earth...

Around 1400, gases and temperatures both plunged, then recovered, then plunged again after 1500. Atmospheric carbon declined by a statistically significant 10 parts per million (ppm) in a period only slightly longer than a single human lifetime. For decades, climate scientists have been unable to offer an adequate explanation for this drop...

William Ruddiman, the environmental scientist at [the University of] Virginia, has hit upon a solution to the riddle that is biological instead of geological. Even the best climate models, he realized, had ignored humans. Economic activity until the eighteenth century consisted of farming, hunting, handicrafts, and trading. To whatever degree these practices affected the global carbon budget, they did nothing to lower it. But Ruddiman saw that humans did something else during these centuries. They died in tremendous numbers.

The Little Ice Age coincided with a series of astonishing pandemics. The best documented began in October of 1347... the first victims of the Black Death. Over the next four years, as many as 50 million people perished... By 1400, woody growth had occupied at least 25 percent -- and as much as 45 percent -- of arable Europe. Birches and hazels squatted in the ecological real estate left vacant by human loss.

And the Black Death was not the only pandemic of the late Middle Ages. The same bacterium had arrived in China a decade before, killing perhaps as many as 50 million. When Hernan Cortes invaded the Valley of Mexico in 1519, his armies brought smallpox, influenza, and mumps, setting off among never-before-exposed people a series of devastating infections that, as the disease moved north and south, killed between 50 and 60 million over the following two hundred years...

Globally, an estimated 125 million people died of pandemic disease between 1200 and 1750, representing 25 percent of the total population in 1500. According to Ruddiman's hypothesis, the deaths of so many in such a short time, over terrain extending from the Po Valley to the Incan Empire, left hundreds of millions of hectares abandoned to reforestation...

How the Little Ice Age ended is perhaps even more revealing than how it began. As population lurched toward recovery, settler cultures felt the tension between lands and hands, sending ax-wielding farmers into the forests of Massachusetts, the Volga River Valley, and Manchuria. Between 1700 and 1920 the world's forests lost 537 million hectares, as agrarian societies increased their land use more than threefold. The carbon in all of those trees -- together with soil itself, the greatest source on the surface of the Earth -- wafted up to thicken the eight-mile-high envelope that distinguishes this planet from Mercury. The world counted few coal-burning factories in 1850, but their numbers followed an accelerating curve as petroleum joined coal to provide the hydrocarbons that would generate two more centuries of economic growth. Under the new energy regime, atmospheric carbon levels rose by 100 ppm between 1750 and the present.

If our crop-planting, animal-herding, forest-and-savannah-burning ancestors could trigger the rapid cooling of the atmosphere through their sudden absence, then we can achieve the same effect by abandoning other practices...

Once we accept the human capacity to reconfigure the climate, the rich nations will become directly responsible for the suffering of the poor... What is a just climate and what an unjust one? Which climate represents the insatiable demands of corporate growth rather than the healthy and stability of everyone else? By confirming the human role in climate change, and by declaring a warming world injurious to the public good, the EPA has swung a club against perhaps the grandest capitalist conceit of the twentieth century: that society forms part of the economy, not the other way around.

Image source here.