In a microbrew induced haze I attempted this question a couple of nights ago but came across as coherent as a monkey, so…
My understanding of albedo is as a general unitless measurement for the reflectivity of a surface. With that in mind and with the following picture Dawei posted of my home town (ironic) at first glace it certainly appears that farms would have a higher albedo than forested land and thus the increase of farmland would lead to a negative forcing shown in the second link.
http://en.wikipedia.org/wiki/File:Radiative-forcings.svg
However the darker color of the leaves absorbing more light and radiating heat can’t be compared to, say, dark rock because leaves convert the solar energy into chemical energy instead of simply storing and radiating heat. In fact a scant percentage of light absorbed by leaves is radiated as heat.
A cursory review of the literature shows no indication of this being taken into account in albedo measurements so is the negative forcing of land use changes truly accurate? Does the absorption of light and radiation of heat energy by land in fact play less of a negative role and potentially more of a positive one when the heat island effect of concrete, asphalt and steel is taken into account? What are your thoughts?
@Dana, maybe I’m misunderstanding your response, but my impression is that the land use albedo is more concerned with dark green forests converted to lighter colored farms causing more reflection and thus a negative forcing instead of absorbing radiation in excess of normal levels
@Dawei – I’m not comparing it to a rock, my point is that plant leaves (often darker colored) absorb light and convert it to chemical energy whereas a rock absorbs the light and immediately radiates heat to the surrounding environment which increases warming. Although virtual guys description of conversion of biomass to energy contradicts the idea. And I’m technically from Gainesville, just NW of city limits by a few miles.
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Interesting question but my first reaction is that it doesn’t matter. Though the energy may be stored in a different form, it’s still not being reflected by the Earth’s surface. Regardless of the reason, you’re still increasing the Earth’s overall energy imbalance by absorbing more and reflecting less radiation. It’s that energy imbalance which results in the planet’s temperature increasing.
I’m not sure why you’re comparing it to rock. It’s not as if farms are planted on rocky land, they’re planted in areas that used to be forested.
Your point that you would expect some cooling since the energy is being converted into chemical energy instead of heat is good but I don’t understand why this effect would necessarily be greater for food crops than for wild forest.
And of course the land use forcing will be less than it would be without the contribution from urban development. I’m pretty sure that’s accounted for, but even if it isn’t the sheer area of urban land is such a tiny fraction compared to farm land that I can’t imagine it making a very big difference.
The amount that human beings have changed the color of the landscape is utterly negligible.
Here’s an experiment.. go to Google earth. Switch to sattelite view. Zoom out until the scale reads 20 miles. Look at the earth.
Come back here and say if you think people are significant on the face of the earth.
Albedo is indeed a dimensionless number – it’s energy out divided by energy in. It doesn’t matter to the calculation what happens to the energy that stays with the planet.
Energy absorbed can show up as immediately or it can be bound in a chemical reaction, as in photosynthesis. When chemically bound, the energy is not detectable as heat, it will only show up that way if the chemical reaction is reversed – as in burning or metabolism.
About 1-2% of the light hitting a cornfield gets used photosynthetically to turn CO2 into biomass. That 1-2% of the light energy won’t show up as heat until the biomass is used for fuel or food. Meanwhile, the biomass came from sequestering atmospheric CO2, which is no longer in the atmosphere being a greenhouse gas until the biomass gets used. The albedo will be temporarily decreased.
The heat simply absorbed by a mass, however, remains heat. It is making a direct contribution to global warming whether it is absorbed by a parking lot or a tree.
The Hadley Center, mentioned in the reference, has done some studies relating to the effects of photosynthetic use vs absorption, including the carbon sequestration effects.
Edit. Our eyes are not well-suited to determining how much we’ve changed the reflectivity/texture of a surface when viewed from satellite distances. To change the picture of the earth so that you can see how we’ve changed it, look at the nighttime pictures.