Could the American West become both drier and greener under climate change?
This is the perplexing subject of a recent study by scientists from NASA, Dartmouth College, and Columbia University. The study, led by Justin Mankin and colleagues and published in the Journal of Climate, explores the connection between the water cycle, plant growth, and climate change. Through their analysis, the researchers help explain, in their title’s words, “The Curious Case of Projected Twenty-First-Century Drying but Greening in the American West.”
To understand how the American West might become both drier and greener as the century progresses and the region continues to warm under human-caused climate change, Mankin and colleagues focus on summer soil moisture.
Summer soil moisture is exactly what it sounds like: moisture in the soil during the summer months. It’s also a good measure of how much water is available for plants to use. Drier summers make it more challenging for plants to photosynthesize and grow. To illustrate this drier/greener paradox, how it relates to climate change, and how Mankin and colleagues performed their analysis, let’s consider an analogy.
Think of soil moisture in the American West as a really big bucket of water. During the winter months, this enormous bucket gets filled with rain and snow. This is our supply.
Now let’s examine demands on that supply.
The authors considered three ways that water can leave the soil: runoff, evaporation, and transpiration by plants. (Water that seeps deep into and through soil ends up as ground water. But that’s another story.) Runoff is pretty simple to calculate. It is directly proportional to how much water is in the soil. Lower soil moisture means less runoff. Evaporation of soil moisture is fairly straightforward as well. The warmer air temperatures get, the more water tends to evaporate from the soil. (Imagine setting a cup of water outside on a hot summer day compared to a cool fall evening. Which cup loses more water to evaporation?) With transpiration by plants, things get a little more complicated.
To understand how plant transpiration affects soil moisture, we first need a quick refresher on photosynthesis, the process by which plants turn sunlight into usable energy.
To photosynthesize, plants need both water and carbon dioxide (CO2). Plants get their water from the soil via their roots, transporting the water up to their leaves where photosynthesis takes place. Plants get their CO2 from the air by opening little holes in the underside of their leaves called stomata. Inside their stomata, what was once soil moisture has been turned into little pools of water for CO2 from the air to dissolve into. Plants close their stomata once they have captured enough CO2 for photosynthesis to occur. It’s an effective method for collecting CO2, but there is one big downside.
While a plant’s stomata are open, tiny amounts of water from inside the plant can sneak out, evaporating into the atmosphere. This is called transpiration and it is a very important part of the cycling of water between our planet’s land, atmosphere, and oceans. Transpiration is another source of demand on soil moisture. In general, the warmer it gets outside, the more water plants transpire, the more moisture they extract from the soil. But here’s where things get tricky and where we can start to unravel the greener/drier paradox and how it relates to climate change.
Greenhouse gases drive climate change. Chief among these is CO2 from the burning of fossil fuels, which is now at levels high enough above the planet’s baseline of CO2 from natural sources that we can definitely say that this added CO2 is warming our world. This extra CO2 while making our climate warmer is also changing our planet’s vegetation. Plants, we have seen, need CO2 for photosynthesis. Having extra CO2 available in the atmosphere means plants are able to capture CO2 for photosynthesis faster.
This phenomenon is called CO2 fertilization and it has been well studied in actual garden greenhouses for decades. CO2 fertilization leads to less water demand. The reason: if plants can capture CO2 more quickly, then they don’t have to leave their stomata open for quite as long, and this means they lose less water through transpiration.
The relationship of the total amount of carbon photosynthesized to the amount of water transpired is called water use efficiency. Higher atmospheric levels of CO2 are expected to increase the water use efficiency of many plants. In plain English: climate change is expected to make plants more efficient water users.
By now you should see where we’re going and the question Mankin and colleagues faced. More CO2 means more efficient use of water by plants, but more CO2 also means warmer temperatures and less water available for those plants. The question then becomes how will CO2 fertilization and increased water use efficiency affect how much water plants “demand” from the soil? This is the question Mankin and colleagues set out to answer in their study.
To figure this out the researchers used simulations from an earth system model of a future that assumes humanity continues to emit CO2 at its current rate. This emissions scenario is called Representative Concentration Pathway 8.5 (RCP 8.5), sometimes called the “business as usual scenario.”
(For a refresher on emissions scenarios, see our page Human Choice, Warming, & Emissions: The Representative Concentration Pathways.)
In total, the researchers ran 35 simulations covering the American West for the years 1920 to 2100 using the Community Earth System Model developed at the National Center for Atmospheric Research in Boulder, Colorado. Using the earth system model, Mankin and colleagues were able to parse out the complex relationship between water supply and water demand and the role that plants play in these processes.
From the modeled results, the researchers found that CO2 fertilization increases photosynthesis—as one would expect—but rather than simply using less water, plants utilize the extra conserved water combined with the extra CO2 to make more leaves. Unfortunately, plants with more leaves tend to transpire more water. (There’s simply more leaves and surface area for water to escape from.) The result: under a high emissions scenario, plants will extract more water from the soil in the future.
The researchers conclude that CO2 fertilization enabled by that extra atmospheric CO2 allows plants to lose less water per leaf, but also to grow more leaves, which translates to additional transpiration. The net result is an increase in the demand for water during the growing season. This accounts for the curious case of the simultaneous greening and drying of soils projected for the American West under climate change.
Mankin and colleagues also found that the supply and demand components interact to further increase greening and drying.
With less snowpack and warmer spring temperatures, plants start to grow earlier in the spring, which contributes to greening. Early spring plant growth means more photosynthesis and more transpiration earlier in the year, which means soil moisture gets depleted earlier as well. The result is drier soils by August when the West’s fire season is in full swing. This is, in fact, what played out here in the Pacific Northwest last summer.
The winter of 2016/2017 saw record-breaking wet weather. This extra moisture led to extra plant growth in the spring. As summer set in, temperatures soared. The summer of 2017 was one of the warmest in the past four decades. The hot summer temperatures coupled with the extra plant growth from the spring meant more evaporation and transpiration, which dried out both soils and vegetation. The exceptionally warm and surprisingly dry conditions combined with the extra fuel, creating an explosive wildfire season.
Study: Journal of Climate
Citation: Mankin, Justin S., Jason E. Smerdon, Benjamin I. Cook, A. Park Williams, and Richard Seager. “The Curious Case of Projected Twenty-First-Century Drying but Greening in the American West.” Journal of Climate 30, no. 21 (2017): 8689-8710. https://doi.org/10.1175/JCLI-D-17-0213.1
Linnia Hawkins is a PhD candidate studying atmospheric science at Oregon State University. Working with the Oregon Climate Change Research Institute since 2014, Linnia’s research interests include, regional climate modeling and the impacts of climate change on forests in the western US. She is a regular contributor to The Climate CIRCulator. Other Posts by this Author.