If Earth's past climates tell us anything, it’s that ice will return. Over the last 2.6 million years, the planet has experienced a series of glacial periods separated by thaws, or interglacials. The next big chill could hit within two millennia—that is, if it weren’t for soaring levels of atmospheric carbon dioxide, driven by humans.“Climate modelers have been warning for many years now that the net impact of human activities would prolong the current interglacial,” says Chronis Tzedakis, a climate scientist at University College London.
A medley of forces influences the glacial-interglacial cycle, including the amount of solar radiation reaching the Earth, which is controlled mainly by Earth's orbital shape and axial tilt, the composition of gasses and aerosols and extent of cloud cover in the atmosphere, and the reflectivity of Earth’s surface (for example, the extent of ice and vegetation cover at high latitudes). A reduction in incoming summer solar radiation would be the primary trigger for glaciation, but atmospheric CO2 concentrations—the primary driver of climate change—must be relatively low, too.
How low is “relatively low”? Tzedakis and colleagues compared ice and marine records from previous interglacials and found that, given the current small decrease in summer solar radiation, CO2 concentrations would have to fall to around 240 ppm for the next glacial period to take place.
Since the Industrial Revolution, however, atmospheric CO2 levels have been trending higher and higher. They reached an estimated 395.09 ppm globally in January 2013. So, if business proceeds as usual, with carbon release being driven primarily by fossil-fuel burning, we likely have a long thaw ahead of us. Modeling work by geophysical scientist David Archer has shown, for instance, that burning all potential fossil carbon on Earth—5,000 gigatons—would be enough to delay the next glaciation by 500,000 years.
That kind of hold-up would be a major deviation from the glacial-interglacial cycle that has played out over the last couple million years, in which glacial periods lasted about 80,000–90,000 years and interglacials about 10,000–20,000 years. (The current interglacial began about 11,500 years ago.)
The real question, then, might have more to do with the next “age,” generally, that we face, rather than the next ice age. Some scientists consider the current era to be defined by human influence, what Dutch chemist Paul J. Crutzen dubbed the Anthropocene Epoch, which has its origins in the Industrial Age.
Regardless of what comes next, it’s probably safe to say that we can’t expect a resurgence of woolly mammoths any time soon. Unless we clone them.
