Tracking climate change means (among other things) tracking annual changes in global greenhouse gas emissions and the corresponding increases in the atmospheric CO 2 concentration. This can get confusing, though, because there isn’t a perfect year-to-year correlation between the two.

Our CO 2 emissions are released into the atmosphere, but the atmosphere interacts with other parts of Earth’s carbon cycle, which pull some CO 2 out. In the short term, the two sinks that matter most are the oceans and the ecosystems on land. CO 2 dissolves into seawater to maintain an equilibrium with the air, and photosynthetic organisms on land and in the oceans take in CO 2 . Shifting ocean currents or weather on land that affects plant growth will alter the amount of CO 2 being taken out of the atmosphere.

It’s typically been thought that land ecosystems were the dominant source of this variability. But a team of researchers led by Tim DeVries at the University of California, Santa Barbara and Corinne Le Quéré at the University of East Anglia decided to investigate just how much of a role the oceans are playing.

A confusing picture

Changes in land and ocean sinks lead to a significant amount of variability in how much CO 2 increases each year. Even though human-caused emissions have gone up pretty smoothly, one year can see the concentration go up 2.5 parts per million, while the next it increases 1.7 parts per million.

This is especially challenging because the two key numbers can be used to check each other. Estimating emissions from each country each year can be tricky, so the atmospheric concentration is, in a way, the ultimate “proof in the pudding” that we have those estimates correct. But just watching the concentration change can’t tell you exactly what emissions were that year.

Every year, a group of researchers publishes updated estimates of the activity of all the different parts of the carbon cycle, doing the math that helps show how human-caused CO 2 emissions fit into the big picture. That task relies on measurements and national emissions accounting estimates, but it also relies on model simulations of the oceans and ecosystems on land. These help show how the year’s weather and ocean circulation patterns ought to be affecting their uptake of CO 2 .

Of course, no model is a perfect representation of reality, so it's worth checking those numbers.

In this study, the researchers compared the models with two other methods of tracking the land and ocean carbon cycle. One takes measurements of carbon in the ocean, as well as things like the CFCs that cause ozone depletion, and uses an independent model to work out how much of these gases has to move into the ocean over time in a way that matches all the data. Another method uses available measurements of CO 2 in the air just above the sea surface and in the water just below it to figure out how much the ocean is taking in.

A model comparison

Some interesting things pop out when comparing results from each of these methods over the last three decades. The overall trends line up pretty well. The decade-to-decade variation, however, is smaller in the models typically used for the annual carbon cycle updates. The other two approaches show larger swings in the amount of CO 2 going into the ocean during the 1990s and 2000s.

Land ecosystems had swings in the same directions, but the oceans contributed a larger influence than we had generally realized, which means a little less came from the land.

In the 1990s, a couple interesting things happened. The major eruption of Mount Pinatubo in 1991 affected climate around the world for a couple years, and the land and ocean consequently soaked up a little extra CO 2 during that time. But the 1990s also saw mostly El Niño conditions in the Pacific (including an incredibly strong El Niño in 1998), which reduced the amount of carbon flowing into the oceans and land ecosystems.

The different methods showed that the oceans were responsible for roughly 10 percent of that variation, with most of it coming from changes on land.

In the 2000s, on the other hand, the uptake of carbon from the atmosphere climbed in years dominated by La Niña conditions in the Pacific. And here, the methods put the ocean’s contribution at about 40 percent.

In both cases, the models used for the annual updates had the ocean’s role at half that (or less). The annual studies come close to balancing the books, but there’s always a slight mismatch between the totals of each part of the carbon cycle and the measured change in the atmosphere. If the models for ocean and land ecosystem uptake aren’t varying enough, they’re probably contributing to that mismatch.

For now, this study can help the climate curious make sense of atmospheric CO 2 numbers. But beyond that, a better scientific understanding of how the carbon cycle responds to climate variability can help improve all kinds of model projections into the future. More of certain measurements being made around the world would certainly make that easier.

PNAS, 2019. DOI: 10.1073/pnas.1900371116 (About DOIs).