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Climate Change

Sea ice and Albedo: Should We Be Worried?

Pistone, K., I. Eisenman, and V. Ramanathan (2014). Observational determination of albedo decrease cause by vanishing Arctic sea ice. PNAS, v. 111(9), pp. 3322-3326. Doi: 10.1073/pnas.1318201111

Introduction:

The definition of albedo is simple: how well the Earth’s surfaces reflect the sun’s energy. Bright surfaces, such as glaciers, have a high albedo and thus reflect a lot of the sun’s energy (Figure 1).   Dark surfaces, like the ocean, are less reflective and have a lower albedo.   The larger the bright surface the greater the albedo effect. Consequences of altering Earth’s albedo are the changes brought upon by the intensification or dampening of solar energy. This can lead to global changes related to weather patterns, ocean circulation, and biodiversity.   Variation in the Earth’s albedo is not an unheard of event; it can be affected daily by cloud cover, seasonally with the formation and melting of sea ice, and in the long term (10kyrs) with glacial maxima and minima. Changes in Earth’s albedo are enhanced by a positive feedback mechanism. This means that where a reduction in the albedo occurs an increased rate of loss will be observed, and vice versa. Understanding changes in the globe requires careful monitoring of the Earth’s albedo state.  With such knowledge scientists can outline potential dangers and changes that may be expected in the future of the natural world.

 Cartoon illustrating that dark surfaces (oceans and forests) will absorb more solar energy than they reflect, and that bright surfaces (ice) reflect more solar energy than they absorb.  (http://www.sciencebuzz.org/sites/default/files/images/ill_maps-Albedo-Effect.jpg)

Cartoon illustrating that dark surfaces (oceans and forests) will absorb more solar energy than they reflect, and that bright surfaces (ice) reflect more solar energy than they absorb. (http://www.sciencebuzz.org/sites/default/files/images/ill_maps-Albedo-Effect.jpg)

Arctic sea ice has been declining for 30 years.   The temperature change in the Arctic since 1970 has been three times more drastic (2 C) than the rest of the world. The summer sea ice is almost half what is used to be, exposing more ocean surface. The consequence is a change in the albedo due to a decrease in the Earth’s ability to reflect energy and an increase in its ability to absorb it. Modeling and estimates of albedo change have previously been used to access the impact of decreased sea ice on albedo. This study sets itself apart from previous studies because it uses satellite radiation budget data and sea ice fraction data, as opposed to indirect estimates, to quantify the impact a change in albedo due to sea ice has on the changing climate.

Methods:

Researchers created a simulation with the goal of determining how dark Earth’s surface has become from the reduction in sea ice so that they could address the questions: what effect the darkening has had on the Earth’s albedo? and what its role is in the changing climate?.   Data from the Clouds and Earth’s Radiant Energy System (CERES) satellite program from 2000-2011 and microwave satellite sea ice observations from1979 to 1999 were analyzed. An advantage to using CERES is the direct quantification of Arctic darkening, and both all sky information, meaning that there are clouds that can mask a decrease in albedo, and clear sky (without clouds) information sets could be considered.   For years prior to CERES (1979-1999) the satellite microwave sea ice data was used as a proxy for darkening.  Data was analyzed initially as six regions on a monthly scale and then averaged to reflect an Arctic annual albedo.  The final step in the investigation compared the results of the simulation with the results of climate models to assess how well they work.

Results:

As suspected, scientists observed a relationship between albedo and sea ice (Figure 2). The non-linearity of the relationship is attributed to seasonal variation. For instance, in months that there is snow the albedo is increased, compared to months with melt ponds, which have a decreased albedo.   Researchers also determined that since 1979 there has been a %15 decrease in clear sky albedo whereas all sky albedo has decreased by only 8%.   The all sky decrease is associated with an additional 6.4 ±.9 W/m2 of solar energy infiltrating the Arctic Ocean; of this, 4.2 W/m2 is from 2000-2011. The impact of darkening on climate change is estimated to be almost a quarter of the impact from increased atmospheric CO2 during the same period.

sea ice cover compared with clear sky albedo.  Results of the simulation reveal that when sea ice cover is highest the greatest percent of albedo is observed.   The non-linearity of the relationship is due to seasonally variability in sea ice.   NCAR CCSM4 model output is the grey line.

sea ice cover compared with clear sky albedo. Results of the simulation reveal that when sea ice cover is highest the greatest percent of albedo is observed. The non-linearity of the relationship is due to seasonally variability in sea ice. NCAR CCSM4 model output is the grey line.

Investigators compared their observations with the results of current climate models. When compared with National Center for Atmospheric Research Community Climate System Model version 4 (NCAR CCSM4) the results were indistinguishable from each other (Figure 2). A comparison of the increase in solar energy input from sea ice changes from this study with a second model revealed that value determined in this study is nearly double a previous estimate, .47W/m2 compared to .22W/m2, although the earlier estimate has large uncertainty and was estimated as a lower bound. A third comparison of energy absorbed into the ice free Arctic revealed that although this study estimated 6.4W/m2, a previous study estimated only 5.6 W/m2 between 1979 and 2005. However, the two values are not directly comparable because of different boundary conditions.

 

Discussion

Investigations like this one are important for understanding global albedo and climate changes in the past, modern day, and into the future.   For one, they can provide validation of our understanding of feedback mechanisms and models. They also touch upon changes that current generations will be and have been witnessing and affected by. In just 50 years, the Arctic has seen drastic climate change compared to the rest of the world, and these changes and will continue to impact on Earth’s climate on global scale.

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