Blue carbon refers to the carbon captured by the world's oceanic and coastal ecosystems. Protecting and restoring coastal ecosystems like mangroves and seagrass meadows has been heralded as a way to achieve net-zero emissions goals by storing vast amounts of "blue carbon."
However, a marine scientist is urging caution, citing major uncertainties that could undermine reliance on blue carbon offsets. There is a substantial gap in understanding and a potential overestimation of their impact on achieving net-zero greenhouse gas emissions by 2050.
Dr. Bradley Eyre, the foundation Director of the Centre for Coastal Biogeochemistry at Southern Cross University, Australia, outlined how variability in carbon burial rates, greenhouse gas emissions, and other factors significantly reduce the potential climate benefits compared to previous estimates.
"There's been too much hype around blue carbon and we need much more discussion around the uncertainties," Dr. Eyre stated. "Current uncertainties make it risky to rely on blue carbon ecosystems to help achieve net zero by 2050."
Contrary to popular belief, not all carbon processes in coastal ecosystems qualify as blue carbon. For instance, carbon stored in plant biomass and soil carbon are often incorrectly included in blue carbon estimates. The key criterion for blue carbon is long-term organic carbon burial, defined as carbon sequestered for over 100 years.
Eyre highlighted that previous studies may have overestimated the carbon burial rates of blue carbon ecosystems. A synthesis of seagrass studies showed that actual burial rates are much lower than earlier estimates.
In addition, the process of carbon burial itself can generate greenhouse gases such as methane and nitrous oxide, which have far higher global warming potentials than CO2. This can significantly offset the climate benefits of blue carbon. For instance, in seagrass ecosystems, methane and nitrous oxide emissions can reduce the net climate benefit by up to 33%.
"If we produce these greenhouse gases while we're burying that carbon, it reduces the net climate benefit of that carbon that we're burying," explained Dr. Eyre. Another significant factor is that calcifying marine organisms like mollusks release CO2 when producing their shells, counteracting some of the carbon storage benefits.
Perhaps most crucially, Dr. Eyre notes the temporal mismatch between fossil fuel emissions and blue carbon sequestration. While fossil fuel emissions release carbon stored for millions of years, blue carbon sequestration involves storing carbon for a comparatively short period (around 100 years). This discrepancy challenges the notion that blue carbon can directly offset fossil fuel emissions.
While still advocating for protecting these ecosystems due to their ecological value, Dr. Eyre cautions against corporations and industries overly relying on blue carbon to offset emissions instead of direct reductions. "It reduces the message and provides an alternative to businesses for immediate ambitious emission reductions and carbon taxes," he warned.
As global decarbonization efforts ramp up, scrutiny over the uncertainties and limitations of nature-based solutions like blue carbon will be crucial to guide policy and investment. More research is still needed before these coastal ecosystems can be considered a silver bullet for net-zero targets.
