Sunday, October 02, 2011

Nutrient limitation missing from an otherwise good NY Times story on forests and climate change

The NY Times published a lengthy article about the climate implications of the forest diebacks and fires. It is, on the whole, a great and all-too-rare example of longform science journalism.

The article does miss one important point about CO2 fertilization, the increase in plant growth thought to come from adding more CO2 to the air.

Climate-change contrarians tend to focus on this “fertilization effect,” hailing it as a boon for forests and the food supply. “The ongoing rise of the air’s CO2 content is causing a great greening of the Earth,” one advocate of this position, Craig D. Idso, said at a contrarian meeting in Washington in July.

Dr. Idso and others assert that this effect is likely to continue for the foreseeable future, ameliorating any negative impacts on plant growth from rising temperatures. More mainstream scientists, while stating that CO2 fertilization is real, are much less certain about the long-term effects, saying that the heat and water stress associated with climate change seem to be making forests vulnerable to insect attack, fires and many other problems.

The CO2 fertilization effect is limited, because plants require more than just CO2 to do their job:  photosynthesis. Water is certainly a limiting factor, but nutrients are just as important. In experiment after experiment, scientists find that the CO2 fertilization effect is short-lived without additional inputs of nutrients, particularly nitrogen.

One of the reasons CO2 fertilization may have accelerated plant growth in parts of Europe and North America over the past few decades may be the fact that we've inadvertently been fertilizing the plants with nitrogen, as well as CO2. We'll actually be talking about this in GEOB400 in a couple weeks. For the 6.7 billion or so of you who were unable to register this semester - yes, yes, the class is too small, I hear that all the time - I wrote about this on Maribo a few years ago, in a cross-post with Eli and Tamino:

One culprit is carbon’s chemical sibling nitrogen, that’s #7 on your periodic table if you’re scoring at home. Like many siblings, carbon and nitrogen are quite co-dependent, and, one might argue, a bit resentful about the whole thing. Carbon fixation - photosynthesis, plant growth – is limited by the availability of nitrogen. Though only up to a point. If there’s too much nitrogen, things get saturated, and the carbon-based plants pout and refuse to grow more.

You might find it strange that nitrogen is limited, given that N2 or di-nitrogen gas makes up the majority of the atmosphere. However, N2 is unreactive. It only becomes available to plants when converted to reactive form by microbes. In the process of making fertilizer and burning fossil fuels, we not only have increased the rate at which this conversion happens, leaving more nitrogen in our soils and waterways, we've emitted nitrogen in other reactive, gaseous forms, like nitrogen oxides or NOx

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