In an interesting new article in Climatic Change, Christopher Doughty and colleagues at Stanford consider whether raising crop albedo (reflectivity) could decrease solar  absorption at the Earth’s surface and cool regional climates.  One might consider this a kind of climate “bio”engineering.

How could you do this, and would it work?

Many desert plants have hair-like projections that reflect excess energy and keep the leaves cool.  As the authors suggest, perhaps crops could be developed/switched such that agricultural landscapes become more reflective:

Agricultural scientists have modified crop morphology with concomitant
increases in albedo. Leaf pubescence in soybeans was increased fourfold over normal varieties to increase crop water use efficiency, thereby increasing surface albedo by ∼0.01. Switching from a potential biofuel crop such as corn (albedo: 0.20–0.23) or soybean (albedo: 0.21) to a higher albedo potential biofuel crop, such as sunflower (albedo: 0.24–0.30) can increase surface albedo by ∼0.06.

Using a computer to simulate how climate is affected by changing plant albedo, here’s what they found:

Simulations indicate that planting brighter crops can decrease summertime maximum daily 2 m air temperature by 0.25◦C per 0.01 increase in surface albedo at high latitudes (>30◦).

Based on the numbers shown above, a 6% rise in albedo would be enough to cool regional climate by 1.5◦C.

However, there are a few challenges:

(1) It turns out that when you plant high albedo crops a low latitudes close to the equator, the increased reflectivity means that plants don’t absorb as much sunlight, they don’t heat up as much, and they don’t lose as much water to the atmosphere via their leaves. In the computer model, this leads to less cloud cover and more sunlight reaching the soil surface, which heats air temperatures and counteracts any cooling effect of the more reflective leaves.

(2) When they looked at different varieties of soybeans, they could only muster a 1.6% increase in albedo based on differences in existing types, meaning that there’s only about a 0.4◦C cooling possible by changing from a less-hairy to a more-hairy soybean crop.

Bottom line:  Although substantial regional cooling could be achieved at high latitudes from switching crop types, the authors argue that it’s unlikely that we will be able to significantly offset future warming with the kinds of crop albedo changes currently available.

Doughty, C., Field, C., & McMillan, A. (2010). Can crop albedo be increased through the modification of leaf trichomes, and could this cool regional climate? Climatic Change DOI: 10.1007/s10584-010-9936-0

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Photo credit: autan

When reviewing the most popular words of 2009, I was surprised to see that “albedo” didn’t crack the top 5—Tweet, Obama, H1N1, Stimulus, and Vampire.  I bet you were equally shocked.

Albedo is a simple concept—the reflectivity of a landscape—but it’s hugely important in understanding how the surface of the Earth impacts climate.  As we saw in a recent post, things like thawing sea ice, northward advancing treeline, and asphalt paving all darken landscapes, causing more solar radiation to be absorbed and temperatures to climb—one of the reasons for the so-called urban heat island effect.

So what would happen if we were to install white roofs?  In a forthcoming article¹ in Geophysical Research Letters (subscription required), Keith Oleson and colleagues use biophysical models to address this.

Their answer:  White roofs reflect more sunlight and cool buildings.  Averaged over all urban areas in the world, the urban heat island effect declines by 33%, causing maximum and minimum daily temperatures to decrease by 0.6 and 0.3 degrees C, respectively.

At face value, this sounds great.  But, there’s a potential hidden cost of cool buildings—heating.  Interestingly, they found that white roofs caused space heating to increase more than air conditioner use declined, suggesting that energy use might actually increase with white roofs!

¹Oleson, K. et al. (in press) The effects of white roofs on urban temperature in a global climate model. Geophysical Research Letters.

Related post:   New ideas about how changing vegetation at high latitudes can cause climate warming to accelerate

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Photo credit:  http://www.flickr.com/photos/calliope/ / CC BY 2.0