So, the national conversation is pivoting more intently to climate change. Granted some have been having the conversation more than others. And, in the broader conversation what I always find ironic is how little people outside agriculture realize farmers and ranchers have always had this conversation.

Yes, perhaps with a different slant, but you can’t run into a farmer or rancher that doesn’t track the weather and gauge how anticipated (or lack of) precipitation will impact his or her production for the year. To this day, my retired farmer dad daily records information from his digital weather station. Regarding rain, he can go back several years to report annual rainfall for a specific year. 

As we continue the climate change conversation, I discovered Bruce A. Kimball, Ph.D., (Soil Physics major and Crop Physiology minor, 1970, Cornell University, Ithaca, NY.), retired Collaborator with U.S. Arid-Land Agricultural Research Center USDA-ARS in Maricopa, Arizona. He knows dad; they’ve had those broader conversations. 

Though retired, he’s currently vice president for the Greenleaf Group, a firm providing technical and scientific consulting services for clients. For most of Kimball’s professional career, he’s studied the effects of elevated CO2 concentrations on plant growth and water relations including assembling and analyzing the published literature on this topic, as well as the likely effects of global change on water resources. Initial warming improves crop yields, his research found and perhaps the reason Arizona grows such amazing crops. 

Kimball led or co-led large cooperative multi-variate experiments using open-top chambers (OTC) and free-air CO2 enrichment (FACE) to expose field-grown crops to elevated CO2, including sour orange, cotton, wheat, and sorghum, all of this in Arizona. Besides his management role, he was responsible for the measurement of canopy microclimate, energy balance, and evapotranspiration. Kimball relates that dad, Pat Murphree, helped him with the FACE experiments. 

And if you thought that was a lot, Kimball also established the feasibility of using arrays of infrared heaters to study the effects of global warming on open-field plots (T-FACE); led a 2-year Hot Serial Cereal experiment wherein wheat was serially planted every six weeks and exposed to infrared warming on some of the planting dates, thereby producing a dataset for wheat response to a huge range of natural and artificial temperatures under open-field conditions. He facilitated the use of the FACE and T-FACE data for validating crop growth models to extrapolate the knowledge gained to other locations and future times.

Most recently he helped other groups to start new T-FACE experiments to simulate global warming in various managed and natural ecosystems. He worked to improve aspects of plant growth models that involve canopy temperature, energy balance, and water use. This ongoing research helps develop strategies to adapt agricultural productivity in the U.S. and around the world to the increasing atmospheric CO2 concentration and associated global warming, and therefore it benefits all consumers of food.

In our conversation below, we start with the basics moving to more in-depth questions as it relates to weather changes and future outlooks. Of note, there are solid scientists on each side of the climate change debate, Arizona Farm Bureau hopes to hear from all compelling and competing voices throughout the year. 

Arizona Agriculture: Is there a difference between climate change and global warming and if so, what is it?

Kimball: To most people, I suspect there’s not much difference. To me, an agricultural scientist studying the likely effects of the increasing atmospheric CO2 concentrations and warming on crop production, there are subtle differences. “Global warming” captures the increasing temperature and its worldwide extent. In contrast, “climate change”, besides warming, captures changes in precipitation patterns, including changes in storm frequency and intensity.

I prefer the term “global change” because it implies all of the above, as well as the increasing atmospheric CO2 concentration, which has direct effects on plant growth in addition to the climatic effects. In my research, I have generally had a global perspective rather than focusing on Arizona, where the continuing urbanization indicates more interest in growing people than agricultural crops. That said, Arizona is a great place to do agricultural research because we experience a huge range in temperatures over which plants grow from a few freezes in winter to summertime temperatures that exceed what is projected for most of the Earth with global warming. Further, with our irrigation systems and little rain, we have control of the water supply most of the time. 

Arizona Agriculture: How are we able to know the greenhouse gas and temperature levels of the distant past?

Kimball: One way is that scientists have obtained long ice cores from ice sheets in Greenland and Antarctica that have an annual growth pattern related to seasonal changes in precipitation. There are bubbles encased within the ice cores that retain air from the time a particular layer was formed. By analyzing the air from these bubbles, a record of past CO2 atmospheric concentrations was obtained. Further, the ratio between isotopes of oxygen in the bubbles gives a proxy for past temperatures.

Arizona Agriculture: What are the core disagreements between scientists on the climate issue?

Kimball: About 50 years ago, Earth was in a slight cooling trend, but then that turned around and began to rise. Thus, there were concerns about whether the warming trend was just a fluctuation or a real trend that would continue. However, it has continued, and the rate of rise is faster than historical fluctuations. Thus, there is no doubt now that global warming is real, but there are skeptics who yet question whether the warming is caused by man’s activities, especially the burning the fossil fuels. Besides the alarming rate of rise, the warming is consistent with projections from global climate models, so I think it is man-caused, and in any event, it is certainly prudent to presume so. 

Arizona Agriculture: In Arizona, what’s the critical issue or issues we must face in agriculture?

Kimball: As always – water. The early climate models did a poor job of predicting how precipitation patterns will change, and there was little agreement among them. However, over the years, they have gotten better, and now they tend to agree that areas that are now wet are going to get wetter, while areas that are now dry like Arizona and southwestern U.S. likely will get drier. Further, warming will decrease high elevation snowpacks, so the free reservoir they represent will no longer retain as much water for slow release into summertime growing seasons. 

With the Colorado River already over-allocated, a drying trend will exacerbate the need to cut back on water usage. Further, Arizona farmers are low persons on the totem pole when it comes to competing with cities for the available water. Also, at the same time, warmer temperatures will increase the irrigation water requirements of crops.

Arizona Agriculture: For most of your professional career, you have studied the effects of elevated CO2 concentrations on plant growth and water relations. In a nutshell, what conclusions are you reaching? What would be interesting for our Arizona crop farmers to learn from you? 

Kimball: Yes, I conducted experiments with elevated CO2 on crops for about 25 years, as well as about another decade doing warming experiments. During the 1980s and early 1990s, my colleagues and I studied the effects of elevated CO2 on cotton. We found that seed cotton yields were increased about 40% (lint yields even more) with an increase of CO2 to about 550 ppm, a concentration expected by mid-century (410 ppm now). Increasing CO2 also partially closed the stomata reducing water use per unit of leaf area, but with the large growth increase of the cotton, water use per land area did not change.

During the mid-1990s, we found that wheat yields increased about 15% with a similar increase in CO2 concentration, while water use was reduced by about 7%.

During the late 1990s, we found that sorghum yield did not increase at all with elevated CO2 when it had ample water, although water use was reduced by about 12%. The lack of a yield response of sorghum is because it is a so-called C4 crop (tropical grasses including corn, sugar cane, Bermuda grass) with a different photosynthetic pathway that concentrates CO2, which contrasts with most crops called C3. Interestingly, when water was in short supply, the sorghum yields were increased by about 25% due to water conservation, which could be important for rainfed agriculture.

We also did a 17-year-long experiment on sour orange trees (sour orange is a commonly used rootstock in Arizona) with CO2 enrichment to about 650 ppm. The orange trees were highly responsive, with annual yield increases averaging about 70% once the trees were mature.

All the above research was studying the effects of increasing atmospheric CO2 without considering the possible effects of global warming which are occurring simultaneously. In the 2000s I devised a way to warm open-field plots using arrays of infrared heaters. My colleagues and I conducted a major field warming study on wheat (about 3°F in daytime and 6°F at night) where we also varied planting dates about every six weeks through two years. When the wheat was planted at the normal early December time for Arizona, warming had little effect on yields, whereas for late (March) planted wheat, yields were markedly reduced. Surprisingly, for early planted (September), the warming reduced the amount of frost damage compared to unheated plots. A large group of crop modelers took our data to improve the high-temperature aspects of their models, and using the models, they concluded that for every 2°F of warming, world-wide wheat production would be reduced by about 6% (not considering the beneficial effects of elevated CO2).

Arizona Agriculture: Can you highlight how technology has greatly improved agricultural emissions relative to the quantity of food produced?

Kimball: Well, I don’t think technology has “greatly” improved, i.e. reduced, emissions. When tractors replaced horses, emissions went up greatly, However, in recent years, there have been some improvements. One example is the adoption of conservation tillage, which has reduced the number of times equipment crosses the fields. Similarly, more effective methods of pest control have similarly reduced the numbers of sprays and associated number of times equipment is on the fields.

Arizona Agriculture: As a desert state we’ve already made remarkable technology adjustments to handle our climate challenges. But, what’s next for us to do in agriculture? 

Kimball: Wow, this is a hard one, and I don’t have good answers. Of course, irrigation is a remarkable technology, and we need to continue to improve the efficiency of water use. Pressurized systems are more efficient, but they require more power and increase emissions. More adoption of conservation tillage, as well as fine-tuning fertilizer applications, will help. Continued plant breeding (aided by modern genetic techniques) can continue to improve yields and pest resistance. Genetic engineering, such as Bt cotton, will help.

Arizona Agriculture: In a recent video we produced on Arizona agriculture’s contributions to a better climate, NASA pointed out in 2014 that American agriculture during the peak growing season produces more oxygen than the Amazon rain forest. Can agriculture play a key role in improving our atmosphere?

Kimball: Of course, as described in the example, agricultural crops take CO2 out of the air and replace it with the O2 we animals need to breathe. Further, to the extent the crops can sequester the C from the CO2 in the soil or in the trunks of tree crops, agriculture can mitigate climate change.

Arizona Agriculture: If climate change is this big of an issue, from your scientific perspective, are you hopeful or concerned and if so why? 

Kimball: I am somewhere between cautiously optimistic and concerned. Looking from the perspective of agriculture, the increasing CO2 will be generally beneficial for agricultural productivity, but there are diminishing returns as concentrations rise above about 800 ppm. Warming will be generally beneficial in present-day cool climates, but in warm climates where plants are now often growing at the upper ends of their optimum range, further warming will depress yields. Thus, it appears that our northern tier of states (and Canada and Russia) will benefit, whereas the southern tier of states and low latitude countries will be hurt. The decreasing yields in low latitude countries that are least able to cope will increase political stress.

Arizona Agriculture: Finally, does carbon-sequestering have potential in Arizona agriculture and to the degree it does in the Midwest?  Please explain.

Kimball: No. In much of the Midwest, the original vegetation was prairie, and the soils had a high organic matter content. With the advent of the plow and other similar tillage, much organic matter was oxidized, and the carbon was lost from the soil. Therefore, with conservation tillage, there is potential to re-build the higher organic matter concentrations and associated carbon sequestration. 

In contrast, the desert soils in Arizona originally had little organic matter. Then the growing of irrigated crops added considerable root and plant residue material to the soil, which increased soil organic matter and carbon sequestration. If Arizona farmers switched to conservation tillage, there is some potential for increasing carbon sequestration here. However, the increase due to the start of irrigated cropping wasn’t nearly as large as the higher concentrations of the Midwest, so the potential for conservation tillage to sequester carbon in Arizona is smaller. Furthermore, the acreage of irrigated agriculture in Arizona is small compared to the acreage of regular agriculture in any of the Midwest states.

Lastly, if water supplies are reduced so that land can no longer be irrigated, as it reverts back to the desert, soil organic matter concentrations will decrease, and CO2 will be released into the atmosphere.

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