Just 20,000 years ago—which is really nothing at all on geological timescales—the ice age that had gripped the Earth for the previous 100,000 years finally slipped. The end of the expansive ice sheets saw human populations blossom and our range expand as we pushed into the newly opened terrain.
Why did the ice leave after such a long reign? The answer is complicated and routed in an understanding of how the Earth’s oceans, atmosphere and land surfaces interact both with each other and with forces far beyond the edge of the planet. It’s an answer that is helping to shape what we know about where the Earth is headed as we continue to pour carbon dioxide into the atmosphere.
In New Scientist, Anil Ananthaswamy looks at the processes that pushed our planet from pleistocene ice to our modern period of plenty. It’s a story that starts with the Sun. Long-term periodic cycles in the orientation and orbit of the Earth, known as Milankovitch cycles, change how much sunlight reaches the surface. From this initial “tiny” blip in the amount of incoming energy, amplification systems and feedback loops in Earth’s climate took over.
Warming from the extra sunlight melted some of the glacier ice, dumping vast quantities of fresh water into the salty oceans. This sudden influx of fresh water changed ocean circulation patterns and disturbed the flow of energy around the planet.
As fresh water poured into the North Atlantic, the overturning circulation shut down, cooling the northern hemisphere but warming the southern hemisphere. These changes were mostly due to a redistribution of heat – by 17,500 years ago, the average global temperature had risen just 0.3 °C.
The shift in ocean and atmospheric circulation patterns drove long-buried carbon dioxide into the air, further enhancing warming.
The dump of fresh water into the North Atlantic that freed us from the ice age’s frigid grasp was of a scale that likely could not be reproduced today. But, many of the same systems that took a small shift in sunlight and pushed it into a planetary transformation are still in existence—a fact that could have a dramatic effect on our future climate.
It took just a small increase in sunshine and a gradual, 70-ppm rise in CO2 to melt the great ice sheets that once covered Eurasia and America. Since the dawn of the industrial age levels have risen by 130 ppm and counting. If we haven’t already pumped enough CO2 into the atmosphere to melt the ice sheets on Greenland and Antarctica, we might soon.
Indeed, current observations of how the Earth is responding to elevating carbon dioxide levels seem to suggest that we’ve been underestimating the effectiveness of many of these same amplification systems. In Scientific American, says John Carey, outlining some of the most recent research on Earth’s feedback loops,
“We are… shoving the climate harder than the known causes of various ice ages did.”
More from Smithsonian.com:
Melting Greenland Ice Has Consequences