Upon Further Reflection

Just as a Full Moon reflects the Sun’s radiation to the Earth, the Earth reflects some of the Sun’s incoming radiation to space. If you were standing on the moon, the Earth would appear as a bright body in the heavens. Overall, the Earth reflects about 30% of the incoming solar radiation it receives. The Earth’s reflection, also known as albedo, produces Earthshine on the Moon.

The reflection of the Earth is not uniform. The polar regions, covered by ice, have high reflectivity—as much as 85% of incoming sunlight. The ocean’s surface has much lower reflectivity—about 10%. Similarly, barren lands have higher reflectivity than forests. Areas that are impacted by air pollution that consists of small particles in the atmosphere, have greater reflectivity than regions of cleaner air. When we create greater reflectivity over urban regions, we experience “global dimming.” Here, particles in the atmosphere absorb or reflect incoming light, so there is less light received at the Earth’s surface.

Averaged over the whole planet, there has been little overall change in albedo during the past few decades, but there have been certain periods and regions of variation. From the 1950s to the 1980s many areas experienced global dimming. In recent years, much of Europe and China has shown a reversal—a global brightening—perhaps as a result of air pollution controls. Fewer particles in the atmosphere are absorbing less of incoming solar radiation.

Changes in albedo in the Arctic have huge implications for our predictions of climate change. When a large portion of the sea ice in the Arctic melts, exposing ocean waters beneath, the albedo of the region decreases. The ocean waters warm, reinforcing the overall warming of Earth’s atmosphere. Each year there is 3 m2 less sea ice in the Arctic per ton of CO2 emitted to the atmosphere, and in the U.S. the average citizen is responsible for the emission of 16 tons of CO2/yr. Polar scientists estimate that the Arctic could be ice-free in the summer by mid-century. Already late summer sea ice cover has dropped 30% since 1979.

The current warming of the Earth due to rising concentrations of greenhouse gases, like CO2, in the atmosphere is about +2.3 W/m2 (watts per square meter). Human additions of particles to the atmosphere is thought to have reduced this warming by about 1.2 W/m2. Warming due to lower albedo associated with the loss of arctic ice is estimated to contribute +0.45 W/m2. Anticipated additional warming due to loss of sea ice in Arctic regions may contribute another +0.3 to +1.1 W/m2 for every degree of future temperature rise in that region.

Changes in albedo are central to accurate predictions of future climate, and there is good reason to believe that we are reducing Earth’s albedo with the current warming trends.

References

Flanner, M. G., K. M. Shell, M. Barlage, D. K. Perovich, and M. A. Tschudi. 2011. Radiative forcing and albedo feedback from the Northern Hemisphere cryosphere between 1979 and 2008. Nature Geoscience 4: 151–55.

Kellogg, W. W. 1992. Aerosols and global warming. Science 256 (5057): 598.

Notz, Dirk, and Julienne Stroeve. 2016. Observed Arctic sea-ice loss directly follows anthropogenic CO2 emission. Science 354 (6313): 747–50.
Perkins, S. 2019. Core concept: Albedo is a simple concept that plays complicated roles in climate and astronomy. Proceedings of the National Academy of Sciences 116: 25369-25371.

Schwarz, M., D. Folini, S. Yang, R.P. Allan and M. Wild. 2020. Changes in atmospheric shortwave absorption as important driver of diming and brightening. Nature Geoscience 13: 110-115.

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