Module 2: Acidification in Alaska

Ocean acidification is a global process that is affecting Alaska more intensely than other places, primarily because of its cold water. Within the waters around Alaska, there is also variation in water chemistry because of seasonal cycles and local influences. While a rise in CO2 concentrations due to human emissions is affecting all ocean basins around the globe, natural factors in the foreground have a big impact on local variability. 

Imagine a marching band: these local factors are the melodies that come and go throughout the song, while the global signal is the overall volume of the band that is increasing as it approaches.

The main factors that influence local ocean acidification conditions in Alaska are: 

• Water temperature, circulation, sea ice cover, and freshwater outflow

• Phytoplankton blooms, the biological pump, and upwelling

• Seasonal cycles

Water Temperature and Circulation

Gasses like carbon dioxide (CO2) are more easily dissolved and absorbed in cold water compared to warmer water. That means Alaska’s chilly waters start off with a higher content of CO2 leading to more acidic baseline conditions. These cold waters play an important role in the global circulation of the ocean.

Ocean currents circulate like a global conveyor belt due to differences in temperature and salinity in different regions. The conveyor belt begins on the surface of the ocean near the pole in the North Atlantic. Here, the water is chilled by Arctic temperatures. It also gets saltier here because when sea ice forms, the salt does not freeze and is left behind in the surrounding water. The cold water is now more dense due to the added salts, and sinks toward the ocean bottom. Surface water moves in to replace the sinking water, thus creating a moving current. It is estimated that any given cubic meter of water takes about 1,000 years to complete the journey along the global conveyor belt. This process is called thermohaline circulation ("thermo" for temperature and "haline" for salt content)

In the ocean currents in the Atlantic, the southward flow of the ocean’s current continues all the way to the Antarctic, and then north again through the Pacific to the Arctic. Those surface waters are exposed to the effects of climate change (absorbing more CO2) along the route. Thanks to the topography of the Arctic Ocean floor, this far-traveled CO2-rich seawater tends to clump up around the North Pole. This is another reason why the Arctic ocean and sea ice are bearing the brunt of climate change. 

Watch this demonstration of cold water’s ability to absorb more CO2 than warm water!

Upwelling

The main driver of ocean carbon chemistry is a process called upwelling. Winds that blow from the north across the ocean surface tend to push surface waters away from the shore. Water then rises up from deep in the ocean to replace the water that was pushed away. The water that rises to the surface as a result of upwelling is typically much colder and is rich in nutrients. Remember, colder waters are naturally high in CO2. This means that the upwelling process is pulling more acidic waters up through the water column.

This video shows an easy at-home demonstration of how upwelling works.

As climate change intensifies, summer wind speed is strengthening throughout Alaska. Stronger winds and storms cause more frequent mixing of the water column, which brings colder, older, and more acidified waters to the surface more frequently. 

In the past, upwelling diminished each autumn when sea ice returned to Alaska’s waters and formed a type of “cap”.  However, now that sea ice is consistently forming or arriving later in the year, the ocean’s surface is increasingly open during late-autumn storms. This allows the upwelling process to continue and intensify, which exacerbates ocean acidification. 

Seasonal Cycles

Plankton photosynthesis, meltwater, other freshwater inputs, sea ice cover, storms and other winds that mix the water column and cause upwelling all have different timing based on location. Generally speaking, the phytoplankton activity and weaker winds of the summer typically make the water less acidic than in the fall and winter. In Alaska, unique seasonal factors like extended periods of summer sunlight, proximity to glacial input, and extreme tidal forces can lead to significant fluctuations in the chemistry of seawater. These factors impact ocean acidification conditions in all of Alaska’s coastal regions.

Does the marching band metaphor make more sense now? Ocean acidification is increasing all over the globe, like a band that is playing at a louder and louder volume. While the whole song is getting louder, local factors that change water chemistry are coming and going like melodies throughout the song. Those local factors that influence OA conditions in Alaska are: 

• Water temperature, circulation, sea ice cover, and freshwater outflow

• Phytoplankton blooms, the biological pump and upwelling

• Seasonal cycles 

While a rise in CO2 concentrations due to human emissions is affecting all ocean basins around the globe, these natural factors in the foreground have a big impact on local variability.