Methods and models

June 19, 2019

It's a well-known fact that the ocean is one of the biggest absorbers of the carbon dioxide emitted by way of human activity. What's less well known is how the ocean's processes for absorbing that carbon change over time, and how they might affect its ability to buffer climate change.

For UC Santa Barbara oceanographer Timothy DeVries and graduate student Michael Nowicki, gaining a good understanding of the trends in the ocean's carbon cycle is key to improving current models of carbon uptake by the Earth's oceans. This information could, in turn, yield better climate predictions. Their paper on the topic is published in the Proceedings of the National Academy of Sciences.

"We started off looking at the rate at which CO2 was accumulating in the atmosphere, and then we compared that to the rate of emissions," DeVries said. "One would expect basically that if you're increasing emissions at 10 percent, the accumulation rate in the atmosphere should increase at 10 percent, for example.

"But what we found is that the rate at which CO2 was accumulating in the atmosphere doesn't necessarily track emissions," he continued. Indeed, after looking at two decades' worth of carbon emissions versus atmospheric carbon accumulation data, the researchers came away with some counterintuitive results.

"We saw in the 1990s that the accumulation rate in the atmosphere was increasing quite fast, whereas the emissions weren't increasing very quickly at all," DeVries said. "Whereas the opposite was true in the 2000s when the emissions increased quite substantially, but the accumulation rate in the atmosphere was steady."

That variability, the researchers said, is due in part to the ocean's carbon-absorbing activities, a range of physical and biological processes that move carbon from the surface to depth. Up to 40% of the decadal variability of CO2 accumulation in the atmosphere can be attributed to how quickly the ocean takes up carbon; the rest can be attributed to activities in the terrestrial biosphere.

"We used a few different methods that estimate how quickly the CO2 is accumulating in the ocean, and basically they all agreed that the ocean was absorbing CO2 slower in the 1990s, which is why it accumulated faster in the atmosphere," DeVries said. "And the ocean was absorbing CO2 faster in the 2000s, so it accumulated slower in the atmosphere."

The results in this paper underscore just how dynamic the ocean is, with any number of factors influencing its ability to act as a carbon sink, the researchers said. Among the primary physical factors that drive oceanic carbon absorption is ocean circulation -- CO2 is absorbed into surface water which then sinks as the currents take it to cooler parts of the world, sequestering the greenhouse gas away from the atmosphere.

Both natural and anthropogenic warming could affect the deep ocean currents, which are driven by differences in water density (cold, dense water sinks while warm water is less dense and rises), in effect, slowing these currents down, decreasing the rate at which carbon is absorbed and making the ocean less efficient as a carbon sink in the long term. Similarly, other natural phenomena such as El Niño and volcanic eruptions have the power to change temperatures and wind patterns which could, in turn, affect ocean currents.

The good news is that climate models generally have been pointing in the correct direction.

"It was interesting to see that the ocean models got the trend generally, but the magnitude was smaller (than what was noted in the observations)," Nowicki said. "But that brings up the next question: Why is that?"

"We need to do more research to look at what's driving this variability," said DeVries, who made the distinction between climate variability -- relatively short-term fluctuations from the general trend -- and climate change, a long-term trend in which the climate enters a new mean state, a "new normal." Capturing the ocean's changing carbon sink in models, the researchers said, will lead to more accurate climate predictions.
Research on this study was conducted also by Corinne Le Quéré and Oliver Andrews of University of East Anglia; Sarah Berthet and Roland Séférian of Centre National Métérologique, Toulouse, France; Tatiana Ilynina at Max Planck Institute for Meteorology, Hamburg, Germany; Andrew Lenton of Commonwealth Scientific and Industrial Research Organization, Australia; Ivan D. Lima of Woods Hole Oceanographic Institute; and Jörg Schwinger of Bjerknes Centre for Climate Research, Bergen, Norway.

University of California - Santa Barbara

Related Climate Change Articles from Brightsurf:

Are climate scientists being too cautious when linking extreme weather to climate change?
Climate science has focused on avoiding false alarms when linking extreme events to climate change.

Mysterious climate change
New research findings underline the crucial role that sea ice throughout the Southern Ocean played for atmospheric CO2 in times of rapid climate change in the past.

Mapping the path of climate change
Predicting a major transition, such as climate change, is extremely difficult, but the probabilistic framework developed by the authors is the first step in identifying the path between a shift in two environmental states.

Small change for climate change: Time to increase research funding to save the world
A new study shows that there is a huge disproportion in the level of funding for social science research into the greatest challenge in combating global warming -- how to get individuals and societies to overcome ingrained human habits to make the changes necessary to mitigate climate change.

Sub-national 'climate clubs' could offer key to combating climate change
'Climate clubs' offering membership for sub-national states, in addition to just countries, could speed up progress towards a globally harmonized climate change policy, which in turn offers a way to achieve stronger climate policies in all countries.

Review of Chinese atmospheric science research over the past 70 years: Climate and climate change
Over the past 70 years since the foundation of the People's Republic of China, Chinese scientists have made great contributions to various fields in the research of atmospheric sciences, which attracted worldwide attention.

A CERN for climate change
In a Perspective article appearing in this week's Proceedings of the National Academy of Sciences, Tim Palmer (Oxford University), and Bjorn Stevens (Max Planck Society), critically reflect on the present state of Earth system modelling.

Fairy-wrens change breeding habits to cope with climate change
Warmer temperatures linked to climate change are having a big impact on the breeding habits of one of Australia's most recognisable bird species, according to researchers at The Australian National University (ANU).

Believing in climate change doesn't mean you are preparing for climate change, study finds
Notre Dame researchers found that although coastal homeowners may perceive a worsening of climate change-related hazards, these attitudes are largely unrelated to a homeowner's expectations of actual home damage.

Older forests resist change -- climate change, that is
Older forests in eastern North America are less vulnerable to climate change than younger forests, particularly for carbon storage, timber production, and biodiversity, new research finds.

Read More: Climate Change News and Climate Change Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to