Humans’ greenhouse gas emissions throw next ice age off schedule

In The World Without Us, writer Alan Weisman speculates on what might happen to the Earth if people disappeared tomorrow. Much of the change is predictable: Cities crumble, plants proliferate and nobody has to worry about election-year politics anymore.

But what would happen to the heat-trapping gases people have pumped into the air with their machines? Would carbon dioxide get sucked down into newborn forests, quickly cleansing the air (if not the airwaves) back to a pristine state?

Well, no. Thanks to the long “memory” of the planet’s air, water and land, the extra carbon would hang around for centuries, slowly and inexorably raising the planet’s temperature long after humans vanished. In fact, our climate meddling has gone so far as to stave off the beginning of the next ice age. A growing body of research suggests that the cocoon of greenhouse gases swaddling the Earth has kept it from cooling as it would naturally, without us.

Geologic records of past climate, preserved in seafloor mud and gas bubbles in ice sheets, show that Earth has warmed and cooled in great glacial cycles starting 1.8 million years ago. These temperature swings can mainly be traced to three astronomical oddities, all changes in the way the planet orbits the sun.

First, the shape of Earth’s orbit becomes more stretched out and then more circular in a cycle that lasts around 100,000 years. Second, the angle at which the Earth’s axis tilts with respect to its orbital plane — currently 23.5 degrees — also changes over time, with a period of about 41,000 years. And the rotation axis itself wobbles like a top, repeating about every 23,000 years.

Each orbital cycle alters how much of the sun’s energy hits the planet. Cut back on sunshine, and the planet cools. Maximum chill hit most recently 20,000 years ago, when ice crawled south to the upper Midwest. Only about 11,500 years ago did the planet emerge from this big chill into its current “inter­glacial” state.

Humankind’s great advances, from agriculture to industry, took place in this comfortably warm period, the longest and most stable in the last 400,000 years. But time is running out. Because the planet is about halfway through its 23,000-year axial-wobble cycle, some scientists think we are due for the next ice age.

Except for greenhouse gases. In the past, levels of carbon dioxide, methane and other heat-trapping gases soared and plunged in concert with warm and cold spells. Now, they may overwhelm the natural orbital cycles and dictate what happens to our planet. At the start of the industrial revolution, Earth’s atmosphere contained around 280 parts per million of carbon dioxide. Today it has 390 ppm, and the number is climbing.

Computer simulations of future climate suggest that the world would need to be below about 280 ppm for the next ice age to kick in. In the January issue of Nature Geoscience, paleoecologist Chronis Tzedakis of University College London and his colleagues suggest a way this next ice age might begin.

The scientists traveled back in time by looking at a warm interglacial stage 780,000 years ago, which wound to an end as orbital cycles deprived the planet of sunlight. Once the polar ice sheets got big enough to spit icebergs into the ocean, there to melt and disrupt normal ocean circulation, the next ice age took hold. Going by this analogy, today’s next ice age would begin within the next 1,500 years, Tzedakis and colleagues say — had carbon dioxide not blanketed the Earth.

Not everyone agrees. Earth’s orbital factors were slightly different 780,000 years ago than today, and other calculations have come up with different results. A paper last year in The Holocene, for instance, suggests that the current interglacial period might have had another 10,000 years to go had people not interfered. And even Tzedakis’ team suggests that the next ice age could launch only if carbon dioxide levels were around 240 ppm, lower than preindustrial levels. (William Ruddiman of the University of Virginia, though, argues that levels were naturally at 240 ppm until people started cutting down trees and clearing land for crops thousands of years ago.)

To some extent, all this speculation about the beginning of the next ice age is precisely that. One 2009 paper in Geophysical Research Letters even calculated how humans could release pulses of carbon dioxide, by precisely controlling their fossil fuel burning, in order to parry the natural forcings of the next ice age and extend the current interglacial epoch by about 500,000 years.

It’s a thought experiment that will clearly never come to pass, in a world with or without us.

SN Prime January 30, 2012 | Vol. 2, No. 4

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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