Nav: Home

Study finds increased ocean acidification due to human activities

September 07, 2016

Oceanographers from MIT and Woods Hole Oceanographic Institution report that the northeast Pacific Ocean has absorbed an increasing amount of anthropogenic carbon dioxide over the last decade, at a rate that mirrors the increase of carbon dioxide emissions pumped into the atmosphere.

The scientists, led by graduate student Sophie Chu, in MIT's Department of Earth, Atmospheric, and Planetary Sciences, found that most of the anthropogenic carbon (carbon arising from human activity) in the northeast Pacific has lingered in the upper layers, changing the chemistry of the ocean as a result. In the past 10 years, the region's average pH has dropped by 0.002 pH units per year, leading to more acidic waters. The increased uptake in carbon dioxide has also decreased the availability of aragonite -- an essential mineral for many marine species' shells.

Overall, the researchers found that the northeast Pacific has a similar capacity to store carbon, compared to the rest of the Pacific. However, this carbon capacity is significantly lower than at similar latitudes in the Atlantic.

"The ocean has been the only true sink for anthropogenic emissions since the industrial revolution," Chu says. "Right now, it stores about 1/4 to 1/3 of the anthropogenic emissions from the atmosphere. We're expecting at some point the storage will slow down. When it does, more carbon dioxide will stay in the atmosphere, which means more warming. So it's really important that we continue to monitor this."

Chu and her colleagues have published their results in the Journal of Geophysical Research: Oceans.

Tipping the scales

The northeast Pacific, consisting of waters that flow from Alaska's Aleutian Islands to the tip of southern California, is considered somewhat of a climate canary -- sensitive to changes in ocean chemistry, and carbon dioxide in particular. The region sits at the end of the world's ocean circulation system, where it has collected some of the oldest waters on Earth and accumulated with them a large amount of dissolved inorganic carbon, which is naturally occurring carbon that has been respired by marine organisms over thousands of years.

"This puts the Pacific at this already heightened state of high carbon and low pH," Chu says.

Add enough atmospheric carbon dioxide into the mix, and the scales could tip toward an increasingly acidic ocean, which could have an effect first in sea snails called pteropods, which depend on aragonite (a form of calcium carbonate) to make their protective shells. More acidic waters can make carbonate less available to pteropods.

"These species are really sensitive to ocean acidification," Chu says. "It's harder for them to get enough carbonate to build their shells, and they end up with weaker shells, and have reduced growth rates."

Protecting shells

Chu and her colleagues originally set out to study the effects of ocean acidification on pteropods, rather than the ocean's capacity to store carbon. In 2012, the team embarked on a scientific cruise to the northeast Pacific, where they followed the same route as a similar cruise in 2001. During the month-long journey, the scientists collected samples of pteropods, as well as seawater, which they measured for temperature, salinity, and pH.

Upon their return, Chu realized that the data they collected could also be used to gauge changes in the ocean's anthropogenic carbon storage. Ordinarily, it's extremely difficult to tease out anthropogenic carbon in the ocean from carbon that naturally arises from breathing marine organisms. Both types of carbon are classified as dissolved inorganic carbon, and anthropogenic carbon in the ocean is miniscule compared to the vast amount of carbon that has accumulated naturally over millions of years.

To isolate anthropogenic carbon in the ocean and observe how it has changed through time, Chu used a modeling technique known as extended multiple linear regression -- a statistical method that models the relationships between given variables, based on observed data. The data she collected came from both the 2012 cruise and the previous 2001 cruise in the same region.

She ran a model for each year, plugging in water temperature, salinity, apparent oxygen utilization, and silicate. The models then estimated the natural variability in dissolved inorganic carbon for each year. That is, the models calculated the amount of carbon that should vary from 2001 to 2012, only based on natural processes such as organic respiration. Chu then subtracted the 2001 estimate from the 2012 estimate -- a difference that accounts for sources of carbon that are not naturally occurring, and are instead anthropogenic.

Sinking carbon

The researchers found that since 2001, the northeast Pacific has stored 11 micromoles per kilogram of anthropogenic carbon, which is comparable to the rate at which carbon dioxide has been emitted into the atmosphere. Most of this carbon is stored in surface waters. In the northern part of the region in particular, anthropogenic carbon tends to linger in shallower waters, within the upper 300 meters of the ocean. The southern region of the northeast Pacific stores carbon a bit deeper, within the top 600 meters.

Chu says this shallow storage is likely due to a subpolar gyre, or rotating current, that pushes water up from the deep, preventing surface waters from sinking. In contrast, others have observed that similar latitudes in the Atlantic have stored carbon much deeper, due to evaporation and mixing, leading to increased salinity and density, which causes carbon to sink.

The team calculated that the increase in anthropogenic carbon in the upper ocean caused a decrease in the region's average pH, making the ocean more acidic as a result. This acidification also had an effect on the region's aragonite, decreasing its saturation state over the last decade.

While the total amount of anthropogenic carbon appears to be increasing with each year, Chu says the rate at which the northeast Pacific has been storing carbon has remained relatively the same since 2001. That means that the region could still have a good amount of "room" to store carbon, at least for the foreseeable future. But already, her team and others are seeing in the acidification trends the ocean's negative response to the current rate of carbon storage.

"It would take hundreds of thousands of years for the ocean to absorb the majority of CO2 that humans have released into the atmosphere," Chu says. "But at the rate we're going, it's just way faster than anything can keep up with."
-end-
This research was supported in part by the National Science Foundation Ocean Acidification Program, the National Institute of Standards and Technology, and the National Science Foundation Graduate Research Fellowship Program.

Massachusetts Institute of Technology

Related Ocean Acidification Articles:

New threat from ocean acidification emerges in the Southern Ocean
Scientists investigating the effect of ocean acidification on diatoms, a key group of microscopic marine organisms, phytoplankton, say they have identified a new threat from climate change -- ocean acidification is negatively impacting the extent to which diatoms in Southern Ocean waters incorporate silica into their cell walls.
Coral skeleton crystals record ocean acidification
The acidification of the oceans is recorded in the crystals of the coral skeleton.
Ocean acidification boosts algal growth but impairs ecological relationships
Shrimp fed on marine algae grown in acidic water do not undergo a sex change that is a characteristic part of their reproductive life-cycle, report Mirko Mutalipassi and colleagues at Stazione Zoologica Anton Dohrn in Italy in a study publishing June 26 in the open-access journal PLOS ONE.
Ocean acidification 'could have consequences for millions'
Ocean acidification could have serious consequences for the millions of people globally whose lives depend on coastal protection, fisheries and aquaculture, a new publication suggests.
Southern Ocean acidification puts marine organisms at risk
New research co-authored by University of Alaska indicates that acidification of the Southern Ocean will cause a layer of water to form below the surface that corrodes the shells of some sea snails.
Ocean acidification harms cod larvae more than previously thought
The Atlantic cod is one of the most important commercial fish species in the world.
Business as usual for Antarctic krill despite ocean acidification
A new IMAS-led study has found that Antarctic krill are resilient to the increasing acidification of the ocean as it absorbs more C02 from the atmosphere due to anthropogenic carbon emissions.
Ocean acidification may reduce sea scallop fisheries
Each year, fishermen harvest more than $500 million worth of Atlantic sea scallops from the waters off the east coast of the United States.
Can seagrass help fight ocean acidification?
Seagrass meadows could play a limited, localized role in alleviating ocean acidification in coastal ecosystems, according to new work led by Carnegie's David Koweek and including Carnegie's Ken Caldeira.
Study shows ocean acidification is having major impact on marine life
Carbon dioxide emissions are killing off coral reefs and kelp forests as heat waves and ocean acidification damage marine ecosystems, scientists have warned.
More Ocean Acidification News and Ocean Acidification Current Events

Top Science Podcasts

We have hand picked the top science podcasts of 2019.
Now Playing: TED Radio Hour

In & Out Of Love
We think of love as a mysterious, unknowable force. Something that happens to us. But what if we could control it? This hour, TED speakers on whether we can decide to fall in — and out of — love. Guests include writer Mandy Len Catron, biological anthropologist Helen Fisher, musician Dessa, One Love CEO Katie Hood, and psychologist Guy Winch.
Now Playing: Science for the People

#543 Give a Nerd a Gift
Yup, you guessed it... it's Science for the People's annual holiday episode that helps you figure out what sciency books and gifts to get that special nerd on your list. Or maybe you're looking to build up your reading list for the holiday break and a geeky Christmas sweater to wear to an upcoming party. Returning are pop-science power-readers John Dupuis and Joanne Manaster to dish on the best science books they read this past year. And Rachelle Saunders and Bethany Brookshire squee in delight over some truly delightful science-themed non-book objects for those whose bookshelves are already full. Since...
Now Playing: Radiolab

An Announcement from Radiolab