Earth's Future Knowing the unknowns
The Earth's atmosphere is not the only source of radiative forcing and anthropogenic climate change. As surely as people and civilizations have carbon footprints, they have albedo footprints as well. By altering the reflectivity of roughly half the land surface of the Earth in the past, mankind has made inadvertent geoengineering a part of the landscape of history. This worldwide alteration of reflectivity raises questions about the future of climate change, for albedo is a first order determinant of the Earth's radiative equilibrium. As surfaces absorb roughly 100 times more solar energy than the CO2 in the atmosphere, future anthropogenic changes in both land and water albedo may figure significantly in climate policy outcomes. At the turn of the 20th century, scientific controversy raged over the varying color of the planet Mars. Although a pleasure to look at, Victorian telescopes were of little use in taking pictures, delivering thumb nail images so dim that astronomers were reduced to sketching maps that differed so greatly that the red planet's fixed geography amounted to little more than two bright polar caps that came and went with the passing of the seasons. Yet in retrospect, 19th century Mars gazers were better off than the 20th century geographers at least they could see what they were arguing about (Figure 1) Although Percival Lowell's "canals" remained pho tographically elusive, they could easily gage how much sunlight Mars reflected, its "albedo," by comparing its darker regions with its snow white poles.