Are our carbon cycle feedback projections under net-negative emissions too low?

SFU researchers propose a novel feedback quantification approach that more accurately determines the effectiveness of carbon dioxide removal technologies at drawing down CO2.

September 07, 2023

To reach the Paris climate target of remaining below 1.5 or 2 degrees Celsius of warming, we need to reach net-zero emissions by 2050, and net-negative emissions by 2100. This means we need to dramatically reduce current CO2 emissions and use carbon dioxide removal technologies to offset residual CO2 emissions.

Currently, land and ocean carbon sinks play a huge role in taking up our CO2 emissions governed by the concentration-carbon and climate-carbon cycle feedbacks. However, not much is known about the strength of these feedbacks as we enter a net-negative emission phase.

In a new study published in Biogeosciences, SFU researchers take a critical look at the standard method for calculating carbon cycle feedbacks under negative emissions and propose a more accurate approach for quantifying them.

Rachel Chimuka, a PhD student in SFU’s Climate Research Lab and lead-author of the study, says that the question is whether we can apply our understanding of these feedbacks under positive emissions (increasing CO2 levels) to negative emissions (decreasing CO2 levels).

Illustration credit: Rachel Chimuka

“We have a good understanding of carbon cycle feedbacks under increasing CO2 concentrations — which is all we’ve ever really known. But now we’re talking about scenarios with decreasing CO2, and we don’t quite understand these feedbacks in that context,” says Chimuka.

According to their research, the standard approach used to quantify feedbacks under negative emissions underestimates the amount of CO2 that will be released by land and ocean carbon sinks and therefore overestimates the ability of carbon removal technologies to draw down CO2.

Chimuka explains that while the climate-carbon feedback promotes carbon sequestration under negative emissions, the concentration–carbon feedback — the dominant carbon cycle feedback responsible for moderating atmospheric CO2 levels — will drive greater land and ocean carbon release under negative emissions. Chimuka adds that this is due to reduced atmospheric CO2 decreasing the rate of photosynthesis and causing land carbon release, and the ocean releasing surface carbon into the atmosphere in an attempt to balance CO2 concentrations at the atmosphere-ocean interface.

“Essentially this means the dominant feedback that was helping us to keep carbon in the land and ocean now returns carbon to the atmosphere,” she says. “For us to know how effective these carbon dioxide removal technologies are going to be, we need to know how strong that feedback will be.”

Chimuka emphasizes that if we don’t adopt a more accurate approach to quantifying carbon cycle feedbacks under negative emissions, we’ll underestimate the amount of carbon that will be released back into the atmosphere and therefore, overestimate the effectiveness of carbon dioxide removal technologies at drawing down CO2.