One of the favorite claims of anthropogenic global warming denialists is that Mars is also in the midst of a planetary warming trend. Since that warming is not caused by human activity, how can we claim that people are responsibile for similar warming on Earth? It must be changes in sunlight.
According to the April 7 issue of Science News, both worlds are warming in part due to a similar effect. But don’t blame it on “Old Sol.” Blame it on good old “Al”…
Albedo, that is: a measure of a planet’s reflectivity.
One of the biggest concerns about Earth’s current warming trend is that the melting of the sea ice around the North Pole is making Earth less reflective. Open water absorbs much more incoming sunlight than ice, thus the decrease in polar ice leads to even more warming, a positive feedback that we can certainly do without.
Mars also has experienced a decrease in one of its polar caps over the past few years. The southern ice cap has been shrinking. That causes a slight decrease in planetary albedo, but not enough to lead to significant changes in polar temperature. The culprit, it seems, is dust.
Martian windstorms redistribute the dust that covers the planet’s surface, sometimes revealing darker, less reflective rocks, and sometimes covering them with lighter, more reflective dust. Reporting on results described by planetary geologist Paul E. Geissler of the United States Geological Survey, Sid Perkins of Science News reports, “On balance, the planet grew darker between the late 1970s and the turn of the millennium.”
So the parallel global warming of Earth and Mars is caused by planetary phenomena, not solar ones, as the denialists of anthropogenic global warming like to claim.
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