From the latin ‘albus', meaning ‘white', albedo is an indicator of reflectivity. Bright objects reflect much of the light that reaches them, and have high albedo. Dark objects have low albedo, because they absorb light. If you've ever climbed into a dark-colored car on a hot day, you already understand albedo. It's an unusual word, and an unusual factor in climate change, presenting both dangers and and opportunities.
The danger comes primarily from the Arctic, where vast sheets of white ice reflect sunlight away from the earth and have a cooling effect on the atmosphere. Ice is highly reflective, reflecting 80-90% of sunlight (pdf), but seawater is dark blue or green and reflects very little. As Arctic ice melts, seawater takes its place, which absorbs more heat and compounds the melting problem. Loss of albedo in this context becomes an accelerator of climate change.
This works in reverse in the case of deforestation, something I learned a few years ago when I had flying lessons in Kenya's Rift Valley. We flew across the transition from forested escarpment to desert valley floor, and the rising heat off the dusty plains created great waves of turbulence that tossed our little plane around. Forests, being dark, absorb 95% of the sunlight that falls across them, whereas cleared land or desert has a higher albedo and beams that light and heat back up. Interestingly, deforestation actually raises the earth's albedo.
Before we get the chainsaws out however, we should remember that forests use some of the sunlight in photosynthesis. They also help create clouds, and of course absorb CO2 from the atmosphere. Overall, their net effect is usually cooling.
I say usually because research by the Global Carbon Project found that there is in fact a case for deforestation (pdf) to cool the earth in some places. In tropical countries and mid-latitude areas forests have a cooling effect. But "high latitude forests have a strong warming influence, largely due to the presence of dark forest canopies in regions that would otherwise be snow covered."
As well as giving us some complicating factors in forestation and Arctic ice, albedo presents us with some interesting opportunities. If reflective surfaces have a cooling effect on the earth, can we create some more of them?
There are two immediate possibilities. The first of these is the urban environment, which covers around 1% of the earth's surface. In research earlier this year, two scientists at the Lawrence Berkeley National Laboratory in California discovered that whitening our cities (pdf) could have a considerable cooling effect. "Increasing urban albedo can result in less absorption of incoming solar radiation by the surface-troposphere system, countering to some extent the global scale effects of increasing greenhouse gas concentrations." It would also cool the air on the ground, reducing the need for air conditioning in the summer.
The report estimated that around 40% of the urban environment is pavement and 20-25% is roofs, both of which could be made more reflective. Most pavements and parking lots are laid in Portland cement concrete, which is expensive when totally white, but can be easily made lighter with different aggregates (pdf). White asphalt shingles, corrugated iron, or white acylic tiles can be used for roofing. You can read the maths for yourself, but the authors conclude that whitening the urban environment would offset the equivalent of 44 Gigatonnes of CO2, or 11 years worth of growth in CO2 emissions. That's an offset, so it would not affect climate change in the long term, but it would buy us some time while we bring emissions under control.
A second, more radical idea is the Global Albedo Enhancement Project, which suggests we cover large areas of the earth with white polythene film. According to Alvia Gaskill, the originator of the idea, covering a large enough area of the earth "could be expected to offset some or all of the projected additional radiative forcing and global warming from 2010 to 2070." Prime locations would be the Sahara, Arabian or Gobi deserts, where thousands of square miles of desert could be covered.
As geo-engineering goes, it's a fairly conservative idea, and the plastics technology already exists. On the downside, the cover would kill every living thing in the area, and would need to stay in place for at least a century. Local weather patterns may be affected. Windblown dust may lower the albedo of the plastic as it gets dirty, but Gaskill suggests robotic vacuum cleaners to keep them clean.
This may be the more outlandish end of albedo research, but expect to hear more about it - albedo is one of many new factors that will be added to climate change models in coming years. In the meantime, why not get some white paint, and get started on your own roof?