Current climate models say we're on track for some pretty drastic changes if we don't reduce our carbon footprint. But a new study has found a huge potential source of carbon emissions that are so far not accounted for in climate models: reservoirs of gases trapped at the seafloor, which could be released as the oceans warm up. And it's happened before, with the team suggesting that these gases belched up from the deep thousands of years ago and put an end to the last ice age.
Towards the end of the Pleistocene period, atmospheric greenhouse gas levels shot through the roof, putting an end to the last ice age. But where did all those gases come from? It's long been thought that the ocean's regular carbon cycle might have been responsible, but according to more recent calculations, that process is too slow to account for the sudden uptick.
The next idea was that seafloor reservoirs of carbon dioxide could have been the culprit. These reservoirs tend to build up around hydrothermal vents, where carbon dioxide and methane is released by volcanic activity underground. The gases mix with water and other materials to form a slurry, which hardens into a cap. There they can sit idle for thousands of years, until something comes along and disturbs them.
One such trigger seems to be warming waters – essentially, as the ocean captures more heat from the atmosphere, those caps melt and release the large stores of carbon dioxide. The gases in turn bubble up to the surface and escape into the atmosphere, exacerbating the greenhouse effect.
The new study found evidence that this is may have been what happened at the end of the last ice age. Researchers from the University of Southern California, Australian National University and Lund University examined a region known as the Eastern Equatorial Pacific (EEP). This area is a hotspot for carbon being released from the ocean into the atmosphere.
Studying ancient marine sediments, the researchers found that hydrothermal materials seemed to be deposited in higher amounts around the end of the last ice age. In particular, there was about four times more zinc in the shells of microscopic sea creatures from the time, indicating hydrothermal activity.
The ages of these marine microorganisms were correlated with data on variations in the atmospheric carbon dioxide levels, and the link was clear: the EEP released large amounts of carbon just before the end of the ice age, contributing to increases in the surface temperature.
But the most worrying thing about the study is that the oceans are currently warming again, and our climate models don't account for extra CO2 held in these reservoirs. If it were to be released, the outcomes could be much worse than we had planned for.
"We're using the past as a way to anticipate the future," says Lowell Stott, lead author of the study. "We know there are vast reservoirs of carbon gas at the bottom of the oceans. We know when they were disrupted during the Pleistocene it warmed the planet. We have to know if these carbon reservoirs could be destabilized again. It's a wild card for which we need to account."
The researchers say that much more study is needed to determine the level of the threat. Since so much of the ocean floor remains unexplored, we don't yet know how much carbon dioxide is held in these reservoirs. The oceans also don't warm in a uniform way, so we don't know which reservoirs are most vulnerable.
"This study shows that we've been missing a critical component of the marine carbon budget," says Stott. "It shows these geologic reservoirs can release large amounts of carbon from the oceans. Our paper makes the case that this process has happened before and it could happen again."
The research was published in the journal Environmental Research Letters.
