THEGREENGROK    Planetary Watch

As the World Warms: How Goes It In the Rice Paddies?

by Bill Chameides | August 25th, 2010
posted by Wendy Graber (Researcher)

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Rice yields’ sensitivity to rising maximum daytime and minimum nighttime temperatures varies through the growing season.

Can we expect to grow more or less rice in our warming world?

They say that the only two things you can count on in life are death and taxes. It seems to me that prior to the inevitability of death, food or the need for it should be on the list. You gotta eat and so you gotta have food. And until scientists crack the trick of mass producing food synthetically, that means that we need green plants; those organisms big and small that have the really cool ability to convert CO2 and water vapor into sugars and starches by tapping the energy in sunlight via photosynthesis.

Given our dependence on green plants, their continuing ability to carry out their photosynthetic thing is kind of important. So how are our green friends handling global warming? Are they more productive? Less productive? And how might that change as the globe continues to warm? A recent paper published in the Proceedings of the National Academy of Sciences (PNAS) sheds some light on these questions for rice.

Of Rice and Men

Along with wheat and maize, rice is one of the most important crops for meeting human needs for nutrition. On average over the globe, rice supplies about 20 percent and 15 percent of our respective caloric and protein input per capita. As a result there is great interest in knowing how rice is affected by climate change.

Thus far climate/rice studies have been largely limited to looking at field trials on experimental plots or in controlled greenhouses. These studies suggested that rice is sensitive to temperature changes, but questions remain. For example: (1) Little is known about the relative importance of changes in maximum daytime temperature (Tmax) and minimum nighttime temperarture (Tmin); and (2) It is unclear how applicable the results from experimental plots and greenhouses are to real world conditions.

Now a new study published in PNAS by Jarrod Welch of the University of California at San Diego and colleagues (including Nicholas School faculty member Jeff Vincent) has attempted to address those concerns. They used five years of non-aggregated empirical data from a large number of farmer-managed rice fields across six Asian countries (China, India, Indonesia, Philippines, Thailand, and Vietnam). The farms studied all used so-called Green Revolution technologies and practices, and collectively were representative of Asian cropping systems responsible for about 40 percent of global rice production.

Using these data, Welch et al. carried out a statistical analysis designed to tease out the impact of Tmax and Tmin, as well as other weather (radiation, rain) variables on rice yields throughout the growing season against the backdrop of other real-world variables that impact yields (e.g., economic factors such as the price of crop inputs).

The Answers?  Well…It’s Complicated

While increases in Tmax tended to increase rice productivity, increases in Tmin tended to decrease productivity. The reasons for these responses are not completely understood, but other work suggests that increases from Tmax are probably related to the higher temperatures accelerating photosynthetic processes, while the decreases from Tmin might be caused in part by higher nighttime temperatures accelerating respiration.

In comparison to Tmax and Tmin, the authors found that the effects of variations in solar radiation and rainfall were small. The result for rainfall might at first blush seem surprising. But the farms included in the study were all irrigated and not significantly affected by drought during the study period and so actually is to be expected.

What’s Been the Effect of Past Global Warming? Well…That’s Complicated Too

Ok, so rice yields increase with increasing Tmax but decrease with increasing Tmin. Both have been on the rise. What’s the net effect been on rice yields? The authors’ calculations for the period between 1979 to 2004 suggest that the effect has been mixed. Because of the competing effect of Tmax and Tmin and the fact that the relative changes in these two parameters varied from one rice-growing region to another, the net change in rice-yield has been quite variable. Rice yield is estimated to have decreased in some regions (e.g., Indonesia, the Philippines, North Vietnam) and increased in others (e.g., South Vietnam).

Two caveats to note:

  • The net impacts calculated by Welch et al. are all less than 1 percent of the total annual yield and occurred during a period when overall rice productivity was on the rise (most likely due to improved agricultural practices).
  • The authors don’t indicate the uncertainty in their net impact estimates; an especially important point since the net impact is largely calculated by subtracting two intrinsically uncertain numbers –the increase from Tmax and the decrease in Tmin–given the effect of other variables is small.

The Past May Not Be a Good Predictor of Future

But while the net impact over the preceding decades may have been mixed, the same might not be true for the future. Consider the following:

  • With climate change Tmin is expected to increase more rapidly than Tmax meaning that the negative impact of rising nighttime temperatures could become dominant in the future.  Indeed empirical datashow that globally that is exactly what is occurring—Tmin is increasing somewhat faster. (See figure.)

Historical variations in globally averaged Tmax, Tmin, and the difference between the two (DTR). A negative trend in DTR means that Tmin has increased more rapidly than Tmax. The results of Welch et al. suggest that a continuing decline in DTR could trigger a decline in rice yields(Source: http://www.ipcc.ch/publications_
and_data/ar4/wg1/en/ch3s3-2-2.html)

  • The increase in photosynthesis from increases in Tmax has a ceiling. Eventually temperatures get so hot that key enzymes used in photosynthesis begin to break down and further temperature increases slow instead of enhance photosynthesis. That break point between enhancing and impeding photosynthesis is generally found to be about 35 degrees Celsius (95 degrees Fahrenheit). But mean Tmax in Welch et al.’s analysis ranged from 20.9 to 34.1 degrees Celsius; i.e., they were below the threshold temperature of 35 degrees Celsius. If mean maximum daytime temperatures begin to exceed 35 degrees C in the coming decades, we could begin to see a much more precipitous and ubiquitous fall in rice yields.  How likely is that to happen? I’d say it’s pretty likely; 90 percentof the world’s rice is grown in this region where maximum temperatures are already close to the threshold.
    • Welch et al. found that rainfall had a minimal impact on rice yields. But that could change if droughts become more prevalent.
    • The human element (e.g., our physical response to warming) is a wild card. I was surprised to learn that the rate of heat-related deaths among crop-workers in the U.S. is nearly 20 times higher than the average for all workers.  Just speculating here, but the impact of global warming on the ability of farmers to cultivate rice in the developing Asian countries studied by Welch et al. could be as large as or perhaps even larger than the impact on the rice.

    I give this paper high marks – it used real world data taken from actual working farms to tease out how rice responds to climate change. The results are a little fuzzy, but they make a very cogent case that we may be confronting a serious rice-yield decline in the coming decades. I guess that’s good news if you’re heavily invested in rice futures for the 2020s, 30s, 40s, and beyond. For the rest of us…start stocking up because rice supplies could be capped and expensive. I’ll trade you two Rice-A-Roni’s for one Rice Krispies or perhaps more to the point, two bags of short-grained rice for one bag of basmati. Gosh, wouldn’t it be easier in the long run to do something like that for carbon emissions now?

filed under: agriculture, climate change, faculty, food, global warming, Planetary Watch
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