Saturday, February 02, 2008

Land use

One of the soap boxes I like to get on is about land use. Hmm, that's not a very appealing introductory sentence! Today's NASA Earth Observatory Newsroom image caught my eye. It's a map of tree canopy heights from 1650 to now. The tree canopy is just what it sounds like--the canopy of leaves formed by a forest. Trees have a large impact on local climate. A dense forest prevents much of the falling snow from reaching the ground, and also causes the snow that is there to melt faster (the tree trunks are far warmer than open land would be). A thick canopy of leaves is one of the darkest (most light absorbing) natural ground covers, absorbing up to 90% of the sunlight that falls on it. Contrast that to a snow-covered area which has been cleared of trees, and reflects 90% of the sunlight that falls on it.

You can start to imagine that clearing a forest might have a pretty big impact on the local climate. There is an interesting hypothesis that was put forward a few years ago by William Ruddiman that we humans started tinkering with climate long before our coal-burning frenzy began in the 1800s. His idea is that by clearing vast forests into crop and pasture land during the invention of agriculture 8000 years ago, we started altering climate unknowingly. Changing land use on a large scale changes many things--the sunlight absorbed, the depth and duration of snowcover, but also the chemistry of the atmosphere (those all-important "greenhouse gases"). Trees can store a lot of carbon, so cutting them all down changes how much carbon is absorbed from the atmosphere. Some cultivated plants, particularly rice, emit a lot of methane, which is another greenhouse gas. Domesticated farm animals also "emit" a lot of methane (actually, so do we bean-eating humans, although I'm not sure if anyone's tried to calculate our personal, direct contribution to global warming...). He even finds evidence for small climatic changes caused by the bubonic plague--enough people die that large amounts of land are abandoned and the forest returns.

This idea caused, well, it caused a shitstorm in the science world. A lot of people freaked out, somehow imagining Ruddiman's hypothesis contradicted current human-induced climate change. To me his idea, if true, only strengthens the case for how much we are changing things unknowingly. Modellers are furiously running simulations to see if they can quantify some of these effects, and it remains a big debate in the community.

I think it's a pretty cool idea--certainly food for thought. I'm also interested in the continuing land-use choices that we make. Last fall I applied for a faculty position in environmental science (which was, apparently, far out of my league...), and my proposed research project was to install a ton of little weather sensors around the urban area and try to understand how local choices affect local climate. For example, we know that adding a green roof acts to cool the building itself, but also prevents a bit of urban heat island because the moist, living, breathing greenery can be 25 degrees cooler than a searing hot asphalt roof. I wanted to explore this in more detail, as there are many of these ultra-local choices that can change local climate. Simple things like construction materials and the presence of parks or other green spaces or bodies of water can make a large difference on how miserably hot a city gets in summer. I'm really inspired by bringing the climate issue into the local realm. Yes, global carbon emission limitations are important, but I tend to feel pretty impotent in that process. I'm much more excited about things individuals and communities can work on. Not only would these help climate change, but they would make cities a lot more pleasant to live in.



Note: This material will not be on the final exam. Ha ha*. Hope someone found the science lesson interesting!!


*That is really me, although not my most spontaneous laugh (hard to make yourself laugh on demand for the mic). My (frequent) laugh is the one thing, bar none, I get complimented on the most and some friends were even threatening to make this into their ringtone.....

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