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Arctic could become more biologically productive as ice melts

Yool, Andrew, E. E. Popova, and A. C. Coward. “Future change in ocean productivity: Is the Arctic the new Atlantic?.” Journal of Geophysical Research: Oceans (2015). DOI: 10.1002/2015JC011167

sea_ice_melting

Arctic sea-ice has been retreating for many years now. The rate at which it is going away could accelerate over the next century with far reaching consequences. Photo courtesy of NASA/Kathryn Hansen

Scientists have long known that receding Arctic sea-ice is a major consequence of climate change. Even parts of the Arctic icecap that were once thought permanent are melting away. The effects of this phenomenon are far-reaching; from physical changes like sea level rise to biological issues related to habitat reduction.

Recent work by Dr. Andrew Yool suggests yet another effect of sea-ice decline: big shifts in patterns of planktonic productivity. Plankton fix carbon in the upper ocean and photosynthesize, thus providing energy to the rest of the food web. But the plankton themselves need certain things to grow, namely nutrients and sunlight.

How much of these two necessities a plankter is exposed to is partially determined by how deep the turbulent surface layer is. The so-called mixed layer depth (MLD) is the parameter that defines how far down in the ocean the plankton might be. The deeper the organism goes, the more nutrients it might encounter. The closer to the surface, the more sunlight it receives.

Plankton tend to bloom when the MLD is at a happy medium. In the North Atlantic, this happens in the spring and summer months when the MLD shoals, or gets shallower, after its winter maximum. The plankton then get lots of sunlight and nutrients that have been mixed up from deeper waters.

The availability of both of nutrients and sunlight in the North Atlantic and Arctic oceans might change dramatically as a function of sea-ice reduction. As you might expect, plankton tend not to grow well under sea-ice since they wouldn’t see much sun. As the sea-ice declines, previously dark areas of the ocean will see more light. Likewise, a future dearth of sea-ice might affect ocean circulation and, by extension, the MLD. If less mixing is happening, fewer nutrients will make it to the sunny surface waters for plankton to use.

To predict where the plankton might grow in these regions over the next century, Yool and his colleagues at the University of Southampton ran a number of big, complex ocean models. In fact, they combined several models to look at all the variables they were interested in. The physical changes in the ocean were simulated using the Nucleus for European Modeling of the Ocean (NEMO) model. NEMO was then coupled to the Model of Ecosystem Dynamics, nutrient Utilization, Sequestration and Acidification (MEDUSA) to produce biogeochemical outputs.

The NEMO-MEDUSA system comprises a digital, idealized version of the world’s oceans. For the ocean model to run, it needs to be “forced” by atmospheric physical parameters. This particular instance of NEMO-MEDUSA was forced using the Intergovernmental Panel on Climate Change’s RCP 8.5 scenario. RCP 8.5 represents a comparatively high greenhouse gas emission situation. This, in other words, is what would happen if anthropogenic emissions continued to rise unabated.

fig1_yool

Figure 1: Decadally averaged mixed-layer depth (MLD) in the North Atlantic and Arctic Oceans drawn from the NEMO-MEDUSA model. The color scale is in meters. The black line represents the contour of the maximum sea-ice extent in the decade considered. The gray line shows the maximum sea-ice extent in the 2000s for comparison. (Adapted from Yool et al., 2015)

Yool and his team allowed the model to run from 2000 to 2099 and extracted parameters they were interested in from every year. They looked at a ton of environmental signals from NEMO-MEDUSA. For this post, we’ll focus on two of them: the MLD and how much primary production is happening.

Yool’s model found that the Arctic would be nearly ice free in the summers by the 2050s. As the sea-ice recedes, the annual maximum of the MLD moves poleward as shown in figure 1. Each plot in figure 1 is a one-decade average of the MLD in the North Atlantic and Arctic. The thick black line represents the contour of sea-ice concentration at the annual maximum in March. Notice how in the 2000s the MLD seems to track along the edges of the ice sheet and reaches depths of 1000 meters. By the 2090s, the MLD in seas off of Greenland dropped by an order of magnitude. Meanwhile, the peak of the MLD shifted from the Atlantic to areas of the Arctic where ice once was.

fig2_yool

Figure 2: Plots of decadally averaged primary production in the North Atlantic and Arctic Oceans from the NEMO-MEDUSA model. Color scale is in grams carbon fixed per square meter per day (gCm-2d-1) integrated over the whole water column. Again the black line shows the maximum sea-ice extent during the decade in question. The gray line shows what the maximum sea-ice coverage was during the 2000’s. (Adapted from Yool et al., 2015)

Figure 2 illustrates the attendant changes in decadally averaged primary productivity as the amount of carbon fixed in a given region per day. The black lines once again denote the contour of the maximum ice coverage. Notice how the amount of productivity rises in regions where ice has receded and the MLD has gotten deeper. But, unfortunately, the levels of productivity in the Arctic of 2090 do not approach those of the Atlantic in 2000.

Overall, the future Northern Atlantic and Arctic oceans will be less productive, less biologically diverse places under the RCP 8.5 climate scenario. These predictions, should they come to fruition, indicate a vastly different ocean landscape with implications for many human activities such as fishing and defense. Yool and his team note, however, that the model was only run under the “worst case” emission projections. And furthermore, these models cannot capture all the complexity of the ocean; the exact way these scenarios play out might not follow any model. But, hopefully, in light of advances during recent climate negotiations, none of these futures will come to pass.

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