When you plant a garden, you need good dirt, seeds, water, and some kind of fertilizer, whether it be manure, compost, mulch, or granules you buy from your local nursery. Anyone who’s gardened for more than a few years knows that it’s good to fertilize your garden every so often because, eventually, the garden plants stop growing as well as they used to. This happens because the plants slowly consume nutrients in the soil that need to be replaced by some form of fertilizer. The same basic thing happens with cultivated crops regardless of whether they’re grown in fields or greenhouses – eventually, the soil nutrients are depleted and need to be replenished.

Unlike gardens and crops, wild plants lack human caretakers providing fertilizer. Wild plants have to scrounge for their soil nutrients wherever and however they can get them, and it is often the case that soil nutrients, especially nitrogen, phosphorus, and potassium, limit how fast forests, grasslands, etc. can grow. A paper in the journal Geophysical Research Letters shows that the availability of soil nutrients will probably limit how much human-emitted carbon dioxide (CO₂) plants can absorb. This limit will prevent plants from absorbing as much CO₂ as climate scientists have modeled, and so global warming will likely be worse than current projections.

Even though the bulk of the atmosphere is nitrogen, plants are not able to make use of that nitrogen directly. Instead, most plants rely on bacteria known as “nitrogen fixing” bacteria to convert nitrogen in the air into the nitrite and nitrate that plants then use to grow. Nitrogen-fixing plants such as peanuts, soybeans, and alfalfa form symbiotic relationships with nitrogen fixing bacteria, and it’s this relationship that is the reason farmers rotate crops like corn with soybeans – the corn uses the nitrogen in the soil while the soybeans put at least some if the nitrogen back.

According to the paper, most nitrogen fixing occurs in the tropics (defined by the GRL paper as between 30°N and 30°S latitude, roughly between New Orleans in and the country of Uruguay in South America), where temperatures are nearly optimal for nitrogen fixing bacteria. Areas outside of the tropics produce about 1/10^th of the fixed nitrogen that is produced in the tropics, largely because of cooler temperatures. As anthropogenic CO₂ emissions drive up global mean temperature, the paper projects that nitrogen fixing outside the tropics will rise, but the increase will be more than offset by a reduction of nitrogen fixing in the tropics. As a result, the total amount of available soil nitrogen will drop as a result of anthropogenic CO₂ emissions.

Without increasing the amount of fixed nitrogen in the soil, plants will not be able to convert the anthropogenic CO₂ into growth. As a result, recent estimates of CO₂ absorption are too high. The paper estimates that insufficient soil nitrogen will lead to between 16 and 149 Gigatons of unabsorbed carbon (GtC) by 2050 under best case model parameters. The worst-case parameters produce between 158 and 253 GtC unabsorbed carbon by 2050.

A couple of different climate research groups have tried to determine how much CO₂ human activity can emit into the atmosphere without exceeding 2° C temperature rise by 2100. Their “carbon budget” is no more than 750 or 1000 GtC by 2050. However, these budgets do not appear to include the likely reduced absorption by plants globally.

If CO₂ absorption by plants is too high by as much as the GRL paper suggests, then it will be even more difficult to stay below the projected danger threshold of 2° C warming. In fact, if emissions grow at an annual rate of approximately 2.4% (2.4% is the average global growth from 2004 to 2008 as calculated from the Energy Information Administration), then human activity will emit 750 GtC by 2029 and 1000 GtC by 2035. Removing about 250 GtC from the carbon budget means that human activity exceeds its budget approximately five years earler, 2024 and 2030 respectively.

The paper’s authors also estimate the warming potential of the unabsorbed anthropogenic CO₂. The authors estimate that global warming will increase between 0.38 and 0.72 C by 2050 over the approximately 1.0-1.5° C warming estimated by the IPCC AR4 WG1, Chapter 10. Put another way, the warming could be between 40% and 50% greater than currently estimated by climate models, depending on what model scenario is used for comparison. Right now, CO₂ are greater than the high emissions case (A2) model from the IPCC AR4.

This paper shows that plants are not just going to grow because of an abundance of CO₂ like some people claim. In fact, the claim that CO₂ is “plant food” is based on greenhouse farmers who increase CO₂ concentrations in their greenhouses in order to grow larger crops. But greenhouse farmers don’t just add CO₂ – they also fertilize their crops with abundant soil nutrients like nitrogen. And as this paper shows, nitrogen-limited natural ecosystems will not respond to elevated CO₂ in the same way that nitrogen-rich greenhouses do.

Thanks to Dr. Ying-Ping Wang of CSIRO for the providing me a copy of his paper for this post.

Image credits:
vivergreen.com