This image, from a simulation in which oxygen levels were limited, reveals methane concentration in the Arctic Ocean after 30 years of clathrate dissociation due to ocean warming. The colors indicate depth-integrated methane concentration in millimoles per square meter. The marine environment is no longer able to break down some of the methane, as indicated by spikes in methane concentration at all eight plumes, most notably at the plumes in the Okhotsk Sea and Bering Sea at the bottom of the image. Further research will explore how much of this methane will reach the surface. North America is on the right, Russia is on the left. Image: Lawrence Berkeley National Laboratory |
A two-part study by scientists from the U.S Department of
Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and Los Alamos
National Laboratory paints one of the most detailed pictures yet of how climate
change could impact millions of tons of methane frozen in sediment beneath the
Arctic Ocean. Methane is one of the most potent greenhouse gases.
The initial phase of the project found that buried deposits
of clathrates, which are icy crystalline compounds that encase methane
molecules, will break apart as the global temperature increases and the oceans
warm.
In the second phase, the scientists found that methane would
then seep into the Arctic Ocean and gradually
overwhelm the marine environment’s ability to break down the gas. Supplies of
oxygen, nutrients, and trace metals required by methane-eating microbes would
dwindle year-by-year as more methane enters the water. After three decades of
methane release, much of the methane may bubble to the surface—where it has the
potential to accelerate climate change.
“Our simulation found that large methane releases erode the
ocean’s ability to consume methane. At this scale, resource limitations come
into play,” says Matthew Reagan of Berkeley Lab’s Earth Sciences Division.
Reagan is a co-author of an article on this research that was published in a
recent issue of the Journal of Geophysical Research. The initial
results of the project were published last year in Geophysical Research
Letters.
Their research counters the view held by some scientists
that the oceans will always consume big plumes of methane. Indeed, small-scale
methane releases have been seeping from seafloor vents for eons. In these
cases, hungry ocean-dwelling microbes quickly oxidize most of the methane
before it escapes into the atmosphere.
But this cycle will be disrupted if the Arctic region’s vast
stores of clathrates break apart and unleash a rash of new methane seeps, the
scientists found.
“Large-scale methane releases have a greater impact than we
anticipated,” adds Reagan. “When this happens, microbes cannot consume all of the
methane because there isn’t enough oxygen to fuel them.”
This image reveals simulated dissolved oxygen concentration, in micromoles, at a depth of 300 meters after 30 years of clathrate dissociation. Regions of severe oxygen depletion are indicated by white and purple shades. Image: Lawrence Berkeley National Laboratory |
Their finding is based on a first-of-its-kind combination of
two computer models. One model, developed by Reagan and Berkeley Lab’s George
Moridis in 2008, simulates methane release rates from warming clathrates. Next,
these methane release estimates were applied to a marine biochemistry and ocean
circulation model developed by Los Alamos National Lab’s Scott Elliott and
Matthew Maltrud.
The scientists plugged initial conditions into the
simulation, such as the ocean’s background concentration of methane and seabed
fluid flow. They then sprinkled a few hypothetical methane plumes around the
Arctic continental shelf and in the Okhotsk
Sea and Bering
Sea. These areas hold extensive shallow clathrate deposits that
are considered by scientists to be very susceptible to instability during the
first few decades of global warming. In fact, some may already be dissociating.
They turned on the methane plumes and ran the simulation for
three decades to predict what would happen during the early stages of climate
change-driven ocean warming.
The result is a scenario that could be all-too real in the
future: In some places, such as near plumes in the Okhotsk
Sea and Bering
Sea, the oxygen level plummets. Localized acidification also sets
in. The environment becomes inhospitable for many organisms, including microbes
that like to consume methane.
“The amount of methane entering the ocean is huge and it changes
the water chemistry dramatically,” says Reagan. “It consumes oxygen, the
microbes stop eating, and methane can reach the surface.”
The scientists hope to conduct further simulations to better
estimate the amount of methane, now locked in clathrates under the Arctic Ocean, which could reach the atmosphere due to
ocean warming.
