The rise in CO2 is now faster than ever

Ice core disk

Air bubbles in a thin slice from an Antarctic ice core. © Katherine Stelling/Oregon State University

Man-made greenhouse gas emissions are driving carbon dioxide levels in the atmosphere to new heights. A research team has now used an Antarctic ice core to investigate whether the rate of CO2 increase is still within the range of what is possible through natural processes. It turned out that today’s CO2 increase is ten times faster than even the strong, short CO2 jumps during the last ice age. The current growth rate is unprecedented in the last 50,000 years, as the team determined. At the same time, the data also provides new evidence of interactions between winds, ocean and CO2.

During the last ice age, global average temperatures fell by several degrees and large parts of the northern hemisphere were covered by thick glaciers. But even during this cold phase, which lasted several tens of thousands of years, there were repeated abrupt changes in the climate. These so-called Heinrich events were characterized by massive iceberg releases in the North Atlantic and rapid, although often short-lived, warming. At the same time, during these periods of upheaval there were brief, abrupt increases in methane and CO2 levels in the Earth’s atmosphere. But how severe these were and over what period of time they occurred was previously unclear.

Unprecedented in the last 50,000 years

To learn more about these CO2 jumps and relate them to current developments, Kathleen Wendt from Oregon State University and her colleagues used an ice core from Antarctica as a climate archive. “Our interest was piqued and we wanted to go back in time to understand more precisely what happened back then,” says Wendt. To do this, they analyzed the isotopes and gas contents in 249 different layers of the 3.2-kilometer-deep core from the center of the West Antarctic Ice Sheet. This enabled the research team to trace the development of atmospheric gases and the climate over the last 50,000 years, and in particular during the climate changes of the Ice Age.

The analyzes confirmed that there was a sharp increase in CO2 levels during each glacial Heinrich event. The largest of these CO2 jumps occurred 39,500 years ago during Heinrich Event 4 (HS4). It increased atmospheric CO2 levels by 14 parts per million (ppm) in just 55 years, the team found. The jump was almost as strong and rapid during the most recent Heinrich Event 1 around 16,800 years ago: the concentrations rose by twelve ppm in 75 years. “These rates of CO2 increase are the fastest in the entire ice core archive,” report Wendt and her colleagues. “But despite this, these natural events are still ten times slower than the current rate of anthropogenic CO2 increase.” Humanity currently only needs around five to six years to increase CO2 levels in the atmosphere by 14 ppm.

Shifting wind bands affect the Southern Ocean

The team was also able to use the data to determine what caused the abrupt CO2 jumps during the Ice Age and what consequences the Heinrich events had back then. “The ice cores reveal a decrease in the C13 carbon isotope during the abrupt CO2 jumps of Heinrich events 1 and 4,” the researchers report. This suggests a deeper change in the global carbon cycle. As the scientists found using additional analyzes and model simulations, there was likely a shift in Earth’s wind bands and climate zones during the Heinrich events. The westerly winds in the southern hemisphere became more intense and shifted poleward. “This increases the ventilation of the Southern Ocean and leads to a rapid release of CO2 from the depths of the ocean,” explains the team. “The rise of warmer, CO2-rich water masses from the depths in turn explains the release of heat and C13-poor CO2 from the Southern Ocean into the atmosphere.

The results also have relevance for today’s climate developments. According to measurement data and models, current climate change is again causing the westerly winds to shift and intensify. This could again lead to increased release of greenhouse gases from the Southern Ocean. “We rely on the Southern Ocean to absorb some of our CO2 emissions,” says Wendt. “But rapidly increasing winds in the southern hemisphere are weakening its buffering capabilities.”

Source: Kathleen Wendt (Oregon State University, Corvallis) et al.,
Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2319652121

Recent Articles

Related Stories