Bombshell: Land, Ocean Carbon Sinks Are Weakening, Making Climate Action More Urgent


One of several huge craters recently found in Sibera. They are "probably caused by methane released as permafrost thawed."

We are destroying nature’s ability to help us stave off catastrophic climate change. That’s the bombshell conclusion of an under-reported 2014 study, “The declining uptake rate of atmospheric CO2 by land and ocean sinks,” as coauthor Dr. Josep (Pep) Canadell recently explained to me.

Based on actual observations and measurements, the world’s top carbon-cycle experts have determined that the land and ocean are becoming steadily less effective at removing excess carbon dioxide from the atmosphere. This makes it more urgent for us to start cutting carbon pollution ASAP, since it will become progressively harder and harder for us to do so effectively in the coming decades.

As Canadell put it, “clearly Nature is helping us” deal with atmospheric CO2 right now much more than it will be decades to come. He said this was one more reason why delaying action to cut carbon pollution is a costly and dangerous mistake.

Canadell is executive director of the Global Carbon Project, a project by the international scientific community to “to develop a complete picture of the global carbon cycle, including both its biophysical and human dimensions together with the interactions and feedbacks between them.” Canadell notes that this paper includes co-authors who were previously skeptical that there was “a decreasing long-term trend in the carbon sink efficiency over the last few decades.”

Because this is one of the most consequential recent findings by climatologists, with significant policy implications, I’ll examine it in more detail.

The ocean and the land (including vegetation and soils) are carbon “sinks” that currently absorb more than half of all human-caused carbon dioxide emissions. Scientists have long been concerned that these sinks will become increasingly ineffective at absorbing CO2 — because of global warming itself. That would mean a greater and greater fraction of human-caused carbon pollution would stay in the air, which would speed up climate change, causing even more CO2 to stay in the air — an amplifying feedback. And that in turn means humanity will have to work harder and harder in the future to keep CO2 and methane from accumulating in the air.

For instance, the defrosting permafrost and the resultant release of carbon dioxide and methane (CH4) turns part of the land sink into a source of airborne greenhouse gases (with methane being much more potent at trapping heat than CO2). Similarly, as global warming increases forest and peatland fires — burning trees and vegetation — that also turns one part of the land carbon sink into a source of atmospheric CO2.

In September 2014, the World Meteorological Organization (WMO) reported:

The observations from WMO’s Global Atmosphere Watch (GAW) network showed that CO2 levels increased more between 2012 and 2013 than during any other year since 1984. Preliminary data indicated that this was possibly related to reduced CO2 uptake by the earth’s biosphere in addition to the steadily increasing CO2 emissions.

A similar conclusion was reached by the more comprehensive international study discussed above. That study, published in Biogeosciences, noted that, for the last five decades, roughly 44 percent of total human caused carbon dioxide emissions stay in the atmosphere. It defined the “The CO2 uptake rate by land and ocean sinks (kS, henceforth called the CO2 sink rate)” as “the combined land–ocean CO2 sink flux per unit mass of excess atmospheric CO2 above preindustrial concentrations.” This is a measure of the land and ocean “sink efficiency.” The study found that this uptake rate, kS, “declined over 1959–2012 by a factor of about 1/3, implying that CO2 sinks increased more slowly than excess CO2.”

What does declining sink efficiency mean in simple terms? As Dr. Canadell explained to me, “For every ton of carbon dioxide we emit into the atmosphere, we are leaving more and more in the atmosphere” each passing year.

Significantly, the study found that of the reasons for the decline in land and ocean sink efficiency, “intrinsic” carbon-cycle feedbacks were responsible for about about 40% of the drop:

Fifth, our model-based attribution suggests that the effects of intrinsic mechanisms (carbon-cycle responses to CO2 and carbon–climate coupling) are already evident in the carbon cycle, together accounting for ∼ 40 % of the observed decline in kS over 1959–2013. These intrinsic mechanisms encapsulate the vulnerability of the carbon cycle to reinforcing system feedbacks…. An important open question is how rapidly the intrinsic mechanisms and associated feedbacks will contribute to further decline in kS under various emission scenarios.

The study notes that “Many (though not all) of these [feedbacks] are fundamentally nonlinear.” It concludes that “Using a carbon–climate model, continuing future decreases in kS will occur under all plausible CO2 emission scenarios.” So the land and ocean sinks are projected to become increasingly less efficient. There’s uncertainty about exactly how fast that will happen, but there’s a very high probability it will happen faster than it has.

As noted above, some feedbacks — such as the permafrost melt and wildfires — are already well known to reduce the net uptake of carbon dioxide from the land sink. NOAA and the National Snow and Ice Data Center have estimated that the permafrost will turn from a carbon sink to a source by the 2020s. The permafrost feedback by itself has been projected to add up to 1.5°F to total global warming by 2100. Remember, no climate model used by the IPCC factors in the permafrost feedback!

A 2012 study led by the U.K. Met Office‘s Hadley Centre, “High sensitivity of future global warming to land carbon cycle processes,” used a major global climate model to systematically study potential land carbon-cycle feedbacks. The researchers found that those feedbacks were “significantly larger than previously estimated.” Those feedbacks are so large that they could add as much as a few hundred parts per million to carbon dioxide levels in 2100 compared to the no-land-feedback case — even in a scenario of moderate carbon dioxide emissions. That in turn could add 1°C or more to total warming in that case. And that is just for this century.

The oceans similarly have feedback processes that threaten to reduce their net uptake of carbon dioxide over time. For instance, global warming drives ocean stratification — the separation of the ocean into relatively distinct layers — which in turn reduces the ability of the oceans to take up carbon dioxide (as explained here).

The bottom line is that our best shot at stopping catastrophic warming is to start cutting carbon pollution immediately. The longer we wait, the less nature’s carbon sinks will be able to help us and the greater the risk that we cross tipping points that cause feedbacks like the permafrost melt to become “self-reinforcing.”

NOTE: The lead author on this 2014 study, Professor Mike Raupach, died last month. He co-founded the Global Carbon Project, and as the GCP tribute notes, he was “an extraordinary carbon cycle scientist and climate change communicator.” To learn more about Raupach, The Conversation has an excellent article on “the scientist who tallied the world’s carbon budget.” He will be missed.