As concern grows over human-induced climate change, many scientists are looking back through Earth’s history to events that can shed light on changes occurring today. Analyzing how the planet’s climate system has changed in the past improves our understanding of how it may behave in the future.
It is now clear from these studies that abrupt warming events are built into Earth’s climate system. They have occurred when disturbances in carbon storage at Earth’s surface released greenhouse gases into the atmosphere. One of the grand challenges for climate scientists like me is to determine where these releases came from before humans were present, and what triggered them. Importantly, we want to know if such an event could happen again.
In a recently published study, my colleagues Katie Harazin, Nadine Krupinski and I discovered that at the end of the last glacial era, about 20,000 years ago, carbon dioxide was released into the ocean from geologic reservoirs located on the seafloor when the oceans began to warm.
This finding is a potential game-changer. Naturally occurring reservoirs of carbon in the modern ocean could be disturbed again, with potentially serious effects to Earth’s oceans and climate.
The past is prologue
One of the best-known examples of a rapid warming caused by release of geologic carbon is the Paleocene-Eocene Thermal Maximum, or PETM, a major global warming event that occured about 55 million years ago. During the PETM, the Earth warmed by 9 to 16 degrees Fahrenheit (5 to 9 degrees Celsius) within about 10,000 years.
Climate scientists now consider the PETM to be an analog for environmental changes taking place today. The PETM happened over a longer period and without human involvement, but it shows that there is inherent instability in the climate system if carbon from geologic reservoirs is released rapidly.
Scientists also know that atmospheric carbon dioxide levels rose rapidly at the end of each of the late Pleistocene ice ages, helping to warm the climate. During the most recent warming episode, 17,000 years ago, the Earth warmed by 9 to 13 degrees Fahrenheit (5 to 7 degrees Celsius).
However, hundreds of scientific studies have failed to establish what caused the rapid carbon dioxide increases that ended each ice age. Researchers agree that the ocean must be involved because it acts as a large carbon capacitor, regulating the amount of carbon that resides in the atmosphere. But they are still searching for clues to understand what influences the amount of carbon in the ocean during abrupt climate changes.
Lakes on the ocean floor
Over the past two decades, ocean scientists have discovered that there are reservoirs of liquid and solid carbon dioxide accumulating at the bottom of the ocean, within the rocks and sediments on the margins of active hydrothermal vents. At these sites, volcanic magma from within the Earth meets superheated water, producing plumes of carbon dioxide-rich fluids that filter through crevices in the Earth’s crust, migrating upward towards the surface.
When a plume of this fluid meets cold seawater, the carbon dioxide can solidify into a form called hydrate. The hydrate forms a cap that traps carbon dioxide within the rocks and sediments and keeps it from entering the ocean. But at temperatures above roughly 48 degrees Fahrenheit (9 degrees Celsius), hydrate will melt, releasing buoyant liquid or gaseous carbon dioxide directly into the overlying water.
Scientists have thus far documented reservoirs of liquid and hydrate carbon dioxide in the western Pacific near Taiwan and in the Aegean Sea. In shallower waters, where ocean temperatures are warmer and pressure is lower, researchers have observed pure carbon dioxide emanating directly from sediments as a gas and rising to the ocean’s surface.