Two of Earth’s Ocean Carbon Sinks Shrinking, Studies Show

New research reveals that mangroves and kelp forests, crucial for carbon storage and cycling, are under threat from development and climate change

Kelp forest/Credit: Heal the Bay

It is well known that deforestation is shrinking the carbon storage capacity of tropical forests, one of the world's great land-based carbon sinks. Now a pair of studies confirms that two key marine CO2 stores — mangroves and kelp forests — are also in peril from human activity.

The findings, scientists say, highlight the need to protect carbon- and biodiversity-rich ocean forests from development and global warming by including them in existing forest-conservation programs.

Mangrove forests — trees and shrubs that thrive in brackish waters — have declined 30 to 50 percent in the past 50 years, according to one study out this week in the journal Nature Geoscience.

The seaside carbon sinks are generally being razed for coastal development such as houses, roads, railways, as well as to turn wetlands into farmland and expand fishing. Towns built on coasts nearby can also leach toxins into the forests and poison sediment, harming life-giving root systems.

Mangroves suck up carbon through the atmosphere during photosynthesis and store it in their leaves, branches and above-ground roots. Because their thick, mucky soils have fairly low oxygen levels, the natural decay of the biomass is slow, resulting in a steady but heavy buildup of carbon over time.

For the first time, scientists have crunched the numbers on the carbon sequestered in these trees, dead organic matter and soil in 25 sites across Micronesia, Indonesia and Bangladesh — which house a large portion of Earth's mangroves — and gleaned insight into salty forests worldwide.

The international team, led by Daniel Donato, an ecologist with the U.S. Forest Service Pacific Southwest Research Station in Hilo, Hawaii, said soils account for a massive 45 to 98 percent of carbon storage in mangrove ecosystems. Because of this, "coastal mangrove forests store more carbon than almost any other forest on Earth," the research station said via release.

Earlier research by Steven Bouillon, a mangrove ecologist with Catholic University of Leuven in Belgium, estimated that the world's mangroves take in more than 200 million metric tons of carbon dioxide annually, with some of it being stored and the rest getting cycled through to nourish other ecosystems.

Uproot the trees and degrade the soil, and the carbon gets spewed back into the atmosphere, the new study affirms. In total, mangrove destruction releases as many as 120 million metric tons of carbon dioxide annually, it finds — an amount nearly equivalent to the yearly emissions of Norway, Finland and Sweden combined.

The emissions footprint of clearing mangroves is disproportionate to their presence. While mangroves encompass only 0.7 percent of the world's tropical rainforests, their discharge of stored carbon accounts for roughly 10 percent of all deforestation-related emissions, Donato said in an interview. In total, forest loss is responsible for nearly a fifth of the greenhouse gases blamed for climate change.

Kelp Forests At Risk from Climate-Related Storms

Kelp plants, known as the "underwater forest," are another vital ecosystem at risk, though for different reasons, according to a report published online on Mar. 24 in the journal Global Change Biology.  

The large beds of seaweed, which can grow up to 50 feet high, are scattered across the globe in temperate and polar regions, including the entire west coast of North America. They are widely considered one of the most productive ecosystems on Earth because of a high rate of carbon cycling, the process by which carbon travels through the ecological community. 

In giant kelp plants, most cycling of carbon works this way: the seaweed takes in carbon through photosynthesis; marine life such as sea urchins graze on the kelp to provide them life, inhaling the carbon. It then moves through the food web relatively quickly as one organism consumes another.

While large swathes of carbon get recycled through the ecosystem, the kelp forests are believed to store as many as 60 million metric tons of carbon, according to a 2009 report by the International Union for Conservation of Nature (IUCN). But their long-term health is now very much in question.

Kelp is particularly susceptible to damage from storm waves, which rip the seaweed out of the earth as they tear through. The new research by a California-based team of ecologists, led by Jarrett Byrnes of the National Center for Ecological Analysis and Synthesis, finds increased storm strength and frequency from human-caused climate change will stop kelp from growing back after wave damage.

Wave height records collected over the last 60 years by government agencies and universities show that winter storms have on average been getting stronger and more frequent. Most climate science models argue this trend will continue as greenhouse gases accumulate and global ocean circulations get disrupted.

Using a decade of field records from the Santa Barbara region, satellite imagery, wave records and data on kelp forest food webs, Byrnes found that if strong storms start happening annually — as predicted by models — kelp won't be able to regenerate fast enough to replenish its stock. The result will be that the forests get replaced by other seaweeds.

The researchers say this will eventually shrink diversity and complexity of animal species in the ecosystem, slow carbon turnover and trigger a decline in storage carbon capacity.

Unlike mangroves, though, the loss of kelp won't necessarily lead to a sudden reduction in the sequestration of carbon from the atmosphere because the replacement algae also have absorptive capacity, Byrnes and co-author Dan Reed of the University of California, Santa Barbara told SolveClimate News.

However: "Other species of algae that grow in the absence of giant kelp do not appear to provide a comparable source of carbon," Reed said via email.

Research Supports Need for Conservation Programs

According to Byrnes and Reed, kelp plants have a large role in maintaining the biological health and carbon cycling in many other marine environments. "A key ecological function provided by giant kelp forests is the physical export of kelp carbon via wave and currents to other ecosystems," Reed said.

The IUCN says that projects like artificial reefs that provide the hard bottom structure necessary for kelp growth may help expand the kelp ecosystem, or at least restore those plants that have been lost.

As far as mangroves are concerned, climate change-fighting programs like the United Nations scheme that pays developing countries to limit CO2 emissions by keeping carbon locked in forests, known as Reducing Emissions from Deforestation and Forest Degradation, or REDD, do not cover mangrove protection.

Donato of the U.S. Forest Service says the reason for this was the lack of research on mangrove loss and global carbon dioxide emissions.

"There have been a few localized bits of evidence suggesting [mangroves are a major carbon sink], but there really hadn't been this sort of broad-scale, whole-ecosystem study," he told SolveClimate News. "I think that is a key reason they haven't been on the radar in terms of REDD programs. Hopefully this research will help change that."

Writing in a commentary accompanying the research in Nature Geoscience, Bouillon of the Catholic University of Leuven in Belgium said: The results "highlight the need for scientists and funding agencies to address uncertainties regarding the fate of carbon after land clearance" and  "provide a strong incentive to consider mangrove ecosystems as priority areas for conservation."

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