Bio-geoengineering - Musings on a Satellite Photograph

Your humble blogger is in full-on real-estate exploration mode at the moment.  In doing so I have spent a fair amount of time staring at satellite pictures from Google Maps, much like the one above.  For the sake of my privacy, that particular picture is a randomly selected piece of the countryside not too far from Ithaca.  However, it's not too dissimilar from the kinds of property we have been looking at - plots that are former farms with 5 or 10 or 20 acres of land, some pasture, maybe a pond, maybe some woods.  The rest of my family's goal is to have a place of great natural beauty where our kids can grow up catching frogs in the stream, running in the field with the family dog (that we don't have yet, but are very eagerly anticipating), and so on.

However, I am thinking in terms of my goal to get us to the point of being climate neutral.  Obviously a lot of that involves thinking about the traditional issues of building energy efficiency, renewable power, etc.

However, staring at the aerial photos and thinking about the solar energy budget of a particular piece of property is starting to make me think about the problem rather differently.

The most striking thing about the picture to me is the albedo variation across the property.  Recall, if you've forgotten your high school science that the albedo of an object is defined thus:

The albedo of an object is a measure of how strongly it reflects light from light sources such as the Sun. It is therefore a more specific form of the term reflectivity. Albedo is defined as the ratio of total-reflected to incident electromagnetic radiation. It is a unitless measure indicative of a surface's or body's diffuse reflectivity. The word is derived from Latin albedo "whiteness", in turn from albus "white", and was introduced into optics by Johann Heinrich Lambert in his 1760 work Photometria. The range of possible values is from 0 (dark) to 1 (bright).

So in lots of these satellite pictures of old farms (or current farms for that matter), there are stock ponds, and they invariably show up as jet black.  So clearly the albedo of a pond is pretty close to zero.  That means that pretty much all of the incident radiation is absorbed.  Now, these kinds of farm ponds are generally only 0-15 feet deep, so this must be even more strongly true of the ocean, which is really deep.   Recalling that the oceans cover something like 2/3 of the planets surface, it's clear that the planet's energy absorption will be strongly dominated by the ocean.

Furthermore - recall the basic physics of the greenhouse effect.  The atmosphere is relatively transparent to incoming visible light, but carbon dioxide, water vapor, etc, make it less transparent to outgoing reradiated infrared.  By keeping some of this reradiated heat in, the greenhouse gases warm the planet.

Now, if we guess that the land in the picture above has average albedo of around 0.3 (the average for the land on earth generally), that means a third of the incoming solar radiation is immediately reflected back out into space still at visible wavelengths.  So that reflected portion is largely not subject to greenhouse gas capture.  By contrast, thermal radiation from the high albedo ponds is all subject to the greenhouse gas (it's important to note that the albedo also applies to emission of thermal radiation from objects, as well as their absorption - Kirchoff's law).

So the oceans not only dominate the earth's energy budget, but they are also the place where the bulk of the change in energy flow due to greenhouse gases is occurring.  The wiki actually has a picture that illustrates this:

The 'total sky' one include the effects of cloud, whereas the "clear sky" one is only the albedo without clouds.  Note how the world's deserts show up as bright spots, while the tropical oceans are dark.

Now, the oceans are very large and slow to change, so the effects of pumping extra energy into them will be slow to show up.  However, in the long term, it seems likely that's where we are making the most important changes to the energy balance.

Anway, returning to the individual property above:

The other thing that's striking is that the land-uses have clear and large effects on the albedo.  The forests are much darker than the pasture.  Generally, forests have an albedo of 0.08-0.18, while green grass has an albedo of about 0.25.  Note that these kinds of variations are large compared to human changes in the greenhouse gases.  For example, here's an illustration of the effects of various "climate forcings" since 1850 (from Hansen and Sato here):

These are W/m², and for comparison, the solar illumination averaged across day and night is about 342 W/m². So for example, the effect of CO₂ increases since 1850 is about 1.4/342 = 0.4%.  By contrast, changing from forest to pasture changes the amount of incoming solar radiation absorbed by around 10% - a much larger effect (locally).

This suggests that one can exert significant climate leverage by changing what is grown.  If somehow one could cover the pasture in plants that had pretty white flowers all summer, as well as pale waxy leaves, that could have a pretty substantial effect on the energy balance of a particular piece of property.

I googled around a little bit, and it seems climate scientists are starting to think along similar lines.  I found a paper Tackling Regional Climate Change By Leaf Albedo Bio-geoengineering from last year, which has this to say:

The likelihood that continuing greenhouse-gas emissions will lead to an unmanageable degree of climate change [1] has stimulated the search for planetary-scale technological solutions for reducing global warming [2] (‘‘geoengineer- ing’’), typically characterized by the necessity for costly new infrastructures and industries [3]. We suggest that the existing global infrastructure associated with arable agricul- ture can help, given that crop plants exert an important influence over the climatic energy budget [4, 5] because of differences in their albedo (solar reflectivity) compared to soils and to natural vegetation [6]. Specifically, we propose a ‘‘bio-geoengineering’’ approach to mitigate surface warm- ing, in which crop varieties having specific leaf glossiness and/or canopy morphological traits are specifically chosen to maximize solar reflectivity. We quantify this by modifying the canopy albedo of vegetation in prescribed cropland areas in a global-climate model, and thereby estimate the near-term potential for bio-geoengineering to be a summertime cooling of more than 1􏳢C throughout much of central North America and midlatitude Eurasia, equivalent to seasonally offsetting approximately one-fifth of regional warming due to doubling of atmospheric CO2 [7]. Ultimately, genetic modification of plant leaf waxes or canopy structure could achieve greater temperature reductions, although better characterization of existing intraspecies variability is needed first.

That sounds pretty promising.