The Earth's orbit isn't perfectly even. Its poles wobble a bit. The cyclical changes in these orbital features create small changes in the amount of sunlight reaching our planet, as well as its distribution across the surface. Despite their relatively small size, these changes have been driving our planet's climate for millions of years, causing events called hyperthermals 50 million years ago, and the entry and exit to glacial cycles more recently.

This raises a pretty obvious question: if the solar changes are so small, how do they drive such large changes in the global temperature? An answer was provided in part by ice cores taken from Greenland and Antarctica. These showed that the concentrations of greenhouse gasses like carbon dioxide and methane rose and fell in near-perfect synchrony with the rise and fall of the temperatures. These would provide an obvious amplification of the orbital cycles, and could account for the large, global change.

But the precise timing of the warming and rise in greenhouse gasses was a bit uncertain, as some ice cores indicated that the warming was well underway before the greenhouse gasses rose. This being climate change, the uncertainty has been used to cast doubt on the significance of the greenhouse effect. Skeptics argued that if the warming started without a rise in carbon dioxide, that carbon dioxide might not be needed at all. Now, researchers have performed a careful reconstruction of our planet's exit from the most recent ice age, and clarified the relationship between orbital and greenhouse climate forcings. Their data shows that CO 2 actually led the global rise in temperatures, and played a critical role in bringing the ice age to a close.

Tracking a global seesaw

To understand what was happening with the climate at the time, the authors gathered data from 80 different climate proxies, widely distributed around the globe. All of them covered 14,000 years that included the exit from the last ice age; they included things like the ice cores, microfossils in ocean sediments, and pollen from lake sediments. These were then used to reconstruct both the local conditions, such as temperature and ocean current strength, as well as the mean temperature for each hemisphere and the planet as a whole.

The proxies paint a complex picture. About 20,000 years ago, the sub-Arctic region of the Northern Hemisphere shows a gradual warming. This warming wasn't sufficiently large or widespread, so it didn't affect the global temperature significantly. What it did do, however, is slow down the Atlantic portion of the global conveyer currents. These currents redistribute warm water to the polar regions, where it cools and sinks to the deep water. Proxies in the Atlantic indicate that the Atlantic meridional overturning circulation (AMOC) began to weaken at this time.

This started off a pattern the authors call a "pronounced interhemispheric seesaw event." As the AMOC shut down, more warm water remained trapped near Antarctica, causing the Southern Hemisphere to heat up even as the northern one cooled. This rise in temperatures is recorded in the ice cores as part of a continuous rise in temperature that ended the glacial cycle. But, since the Northern Hemisphere was cooling at the same time, it had little effect on global temperatures, causing a rise of roughly 0.3°C. (In contrast, the global temperatures have risen by about three times that amount over the last century.)

It was during this initial warming of the Southern Hemisphere that greenhouse gasses started to rise, a rise that continued for about 5,000 years. The addition of greenhouse gasses to the mix seems to have sufficiently forced the rising temperatures to go global. With the rising warmth, the AMOC restarted, and temperatures in the Northern Hemisphere rapidly caught up with those in the South.

Then, the seesaw kicked in again, and the AMOC shut down. This corresponds with a cold period in the Northern Hemisphere called the Younger Dryas, which dropped the mean temperature there by nearly a full degree. Warming in the Southern Hemisphere, in contrast, only slowed down briefly. When the AMOC returned about a thousand years later, the seesaw ended. Both hemispheres warmed until they plateaued at preindustrial temperatures, which occurred about 8,000 years ago.

Remaining questions

To get a better sense of the role of various factors, the authors performed multiple runs of the warming using the Community Climate System Model version 3. They were able to trigger the AMOC shutdowns using pulses of fresh water injected into the North Atlantic. These pulses are known to occur from the release of massive freshwater lakes trapped behind ice sheets, and occur as the sheets melt. Alternately, they can occur when large volumes of freshwater ice is transported to the North Atlantic via icebergs

It's entirely conceivable that the early warming seen in the Northern Hemisphere triggered one of these injections. At the moment, however, a source for the meltwater hasn't been identified. The authors had to hand-tune the volume of water to get the behavior seen in their real-world data. Once they did that, however, the models accurately reproduced the sorts of behavior seen in the proxies.

To test the influence of various climate influences, the authors reran their model with only one of the factors in place: either only orbital forcings or only carbon dioxide forcings. Orbital forcings were able to drive a very slight increase in temperature over the course of about 8,000 years, and showed little of the variability seen in the real world data. In contrast, the CO 2 -only scenario captured most of the major features seen in the proxies.

There were some significant differences, however. The CO 2 -only results were less volatile than the real world data, and the temperature plateaued earlier and a bit lower than it did in the full model. This suggests that other factors, like changes in the reflection of sunlight caused by the vanishing ice sheets, also play a key role in shaping the trajectory of the warming. However, the researchers didn't tease these effects out.

The one thing the study didn't address at all is where all the carbon dioxide came from. There are a number of ideas out there, most of them focused on it being trapped deep in the Southern Ocean. So far at least, none of the ideas appear to have won the community over.

But the study goes a long way towards clarifying why the different ice cores showed warming that didn't match up precisely, and why none of it appeared to match up neatly with the rise of carbon dioxide. All of which will be a big help for the climate science community—even if it won't help stop the arguments raging on the Internet.

Nature, 2012. DOI: 10.1038/nature10915 (About DOIs).