Biofuel crops have the potential to offset the world's reliance on fossil fuels, increasing energy security and decreasing the risk of climate change. However, the choice of crops and target fuel can radically change their impact, as issues like fertilization and ease of processing come into play. But, as a paper released by PNAS points out, these aren't the only things that change when you shift the crop. The paper suggests that switching from corn to a perennial grass can alter the water cycle and sunlight absorption so dramatically that it will have a larger impact than the carbon emissions that biofuels avoid.

The researchers involved take advantage of some recent field work—literally, in a field—in which corn was planted side-by-side with a perennial grass that has been suggested as a potential biofuel stock, miscanthus. The results led them to model the switch to a perennial crop very simply: take the default values in an existing model (they used the Weather Research and Forecasting Model), and expand them by a month at each end. In other words, miscanthus (or another perennial grass, like switchgrass) would turn green earlier in the year, and last a bit later into the autumn.

At this point, their modeling gets a bit unrealistic. Instead of assuming a mix of crops, they simply convert all the cropland of the central US, a total of over 800,000 square kilometers, into miscanthus. Clearly, this is never going to happen, given that areas like Iowa and Illinois will need to keep providing the US with significant food. Still, it's an interesting thought experiment, and provides us with a sense of the trends we might see with a smaller-scale conversion of cropland.

One of the things that changes dramatically is the albedo, a measure of the amount of sunlight that's reflected back into space by the area in question. Since this is sunlight that doesn't heat the earth, a higher albedo can offset some degree of greenhouse-driven warming. And that's precisely what the authors see; each hectare converted to miscanthus reduces the Earth's energy budget to about the same extent that avoiding a significant amount of carbon emissions would. In fact, the effect is six times larger than the amount of carbon offset by harvesting the plant and converting it to biofuels.

Another way they phrase that is that it will take seven years before the reduced carbon emissions from harvesting the miscanthus are more significant than simply planting the grass in the first place.

But that's not all that happens. Perennial grasses tend to grow roots deeper into the ground and access water that's not at the immediate surface. If the authors extended the root depth in the model to two meters, there were dramatic effects on the water cycle. Since more water was mobilized, a lot more of the sun's energy went into evaporating it, either directly or through transpiration in the leaves. That led to increased humidity and more rain, which actually increased the moisture in the surface soil (the deeper roots dried things out below that).

All of the increased evapotranspiration significantly affected the local climate by cooling it, with a maximum effect of nearly a 1°C. For the next several decades, the authors calculate, the local climate wouldn't register the increased greenhouse effect caused by rising levels of atmospheric carbon dioxide.

These effects were very local, however. Over broader areas, the local cooling was swamped by the planetary trend.

This isn't the first paper to highlight the impact of evapotranspiration on local climate; a few months back, a study suggested that changes in plants' leaves could offset over a half-degree of warming, also by increasing the amount of sunlight that goes into driving water vapor out of the plants. That study covered just about all land with foliage, and not only cropland (the over-ocean areas were predicted to keep warming).

We're probably not at the point where we know enough to comfortably predict what the impact of plants will be as the climate warms; different areas will see changes in precipitation, temperature, etc., and human choices on what to plant will also have a profound effect. But the large magnitude of the predicted changes these papers are seeing suggests that this is an area where we should focus on reducing uncertainties.

PNAS, 2011. DOI: 10.1073/pnas.1008779108 (About DOIs).

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