What Is the Connection between Growing Crops and Climate Change?
Ask a Scientist - June 2010
C. Manning from Oakland, CA, asks "I've read about various proposals to give farmers credit for practices that reduce global warming emissions. Are these approaches scientifically sound? What is the connection, if any, between growing crops and climate change?" and is answered by Food and Environment Senior Scientists Noel Gurwick, Ph.D.
Before getting into specifics, let me outline a few broad points. First, if we're really going to avoid the worst effects of climate change, we need to reduce emissions of carbon dioxide that result from burning fossil fuels. Second, there's widespread agreement among the research community that choices about agriculture can help reduce human contributions to climate change, even though smart choices about agriculture won't solve the problem by themselves. And finally, it's critical to remember that farming practices impact many aspects of the environment in addition to climate. Wise choices from a climate point of view frequently also yield major benefits for protecting human health and ecosystems.
To understand how agriculture can be part of the solution, we need to think about how plants and soils work, how they're connected to each other, and how they take in and release heat-trapping gases.
Plants take in carbon dioxide—one of the main heat-trapping emissions that humans are releasing to the atmosphere at unprecedented rates. Carbon dioxide enters plants through small pores in their leaves, where it combines with hydrogen to form sugars. It’s a little bit like this: You get hungry and you go eat a sandwich. A plant gets hungry and it’s stuck in the soil, unable to move. But out comes the sun and the plant opens the pores in its leaves and, by combining the carbon dioxide with hydrogen, voila, it makes its own food.
With other nutrients acquired from the soil, plants convert the carbon dioxide and hydrogen to other organic compounds that perform many different roles. For example, they form new plant tissues, which means that some of the carbon gets tied up in the plant itself. Some of these compounds are shuttled below ground. There, some are used to form new roots, while some leak out of the roots into the soil where microorganisms feed upon them. Microorganisms also come into the picture when the plant dies. Anytime microorganisms consume organic matter, like dead plants, some of the carbon is released back into the atmosphere, while some is incorporated into the microorganisms—and eventually into the soil.
You can think of the plant as a pump moving carbon out of the atmosphere—but a very leaky pump because a lot of this carbon doesn’t make it into the soil (which you can think of as a storage tank).
Is all the carbon in soil stored there forever? No. It can be stored a few hours or a few years, and some for hundreds or thousands of years, before it’s released back into the atmosphere. The longer the carbon remains in the soil, the greater the reduction in carbon dioxide concentrations.
Another key link between agriculture and climate change is nitrogen. Nitrogen is an essential nutrient; we would die without it and so would plants. We tend to add nitrogen—whether as synthetic fertilizer, manure, or compost—to agricultural fields to help plants grow. But, in general, we’re feeding them more nitrogen then they have the capacity to absorb and often at the wrong time when they’re not growing very fast. Some of the excess nitrogen that plants don’t use is released back into the atmosphere as nitrous oxide—a heat-trapping gas that’s more than 300 times more potent than carbon dioxide.
So if we implement smart farming practices we can reduce the heat-trapping gases emitted from farming and increase the amount of carbon stored in the soil on those farms.
What types of farming practices are we talking about, and how strong is the science supporting them? The answer is that some are more certain to reduce global warming emissions than others.
We have strong evidence that, in general, using less nitrogen fertilizer would reduce nitrous oxide emissions. Since nitrous oxide is such a potent heat-trapping gas, this step would significantly reduce our impact on the climate. Researchers are also optimistic that we can reduce nitrous oxide emissions by applying nitrogen fertilizer when plants are germinating or growing fast.
Farmers can also take a number of steps to increase the amount of carbon stored in soil. For example, cover crops are plants deliberately grown in fields—frequently during the off season—to maintain positive conditions for cash crops in the long term. Planting cover crops increases the amount of time during which plants move carbon into the soil. Farmers and gardeners can also plant perennial grasses instead of annuals. Perennials tend to have larger, deeper root systems than annuals, and therefore tend to store more carbon.
The climate benefits of reduced nitrous oxide emissions are more certain than those of enhanced carbon storage in soil. If we reduce nitrous oxide emissions for a year, we can effectively count that as a permanent reduction in emission of heat-trapping gases. For soil carbon, the carbon now in the soil could be released next year or in five years, for any number of reasons, such as changes in land management. If that happens, we’ve bought ourselves a little time but not permanent storage.
We’re still learning a lot about how the many decisions farmers make are likely to impact our climate, but it’s clear that there are agricultural methods available that could play a significant role in reducing heat-trapping emissions.
It’s also important to remember that while farming practices can impact our climate, they also impact many other aspects of our environment.
I'd like to end by coming back to one of the points I made at the beginning: agriculture affects many aspects of the environment, not just climate. For instance, over-fertilizing not only leads to release of nitrous oxide into the atmosphere, but also to the release of other nitrogen-containing compounds like nitrate into groundwater and surface water. Excessive amounts of nitrate can harm both human health and coastal ecosystems. Nitrogen from agricultural fields washes into rivers and eventually runs into coastal waters, where it contributes to dead zones—areas where many kinds of marine life die from lack of oxygen. So agricultural systems that don't leak excess nitrogen have multiple advantages. This is just one example, but it illustrates the urgent need to take advantage of agricultural systems that protect human health and ecosystems, and the opportunity to do that while also helping to avoid the worst effects of climate change.
Noel Gurwick is a climate–agriculture scientist. His Ph.D. in Biogeochemistry and Environmental Change from Cornell centered on carbon and nitrogen cycling in shallow groundwater near streams. As a post-doctoral fellow at the Carnegie Institution's Department of Global Ecology, Dr. Gurwick studied responses of plant–soil ecosystems to multiple, co-occurring aspects of global environmental change, including carbon dioxide enrichment, warming, changes in precipitation, and increased nitrogen deposition.
