Climate variability and change: Trends and impacts on California agriculture
Dr. Tapan Pathak looks at how climate variability, increasing temperatures, and changing precipitation could affect California agriculture
In this presentation, Dr. Tapan Pathak, a Cooperative Extension Specialist with University of California Division of Ag and Natural Resources, discusses climate variability and climate impacts on California agriculture.
Dr. Tapan Pathak began with some basic facts about California agriculture. California has 76,400 farms producing more than 400 commodities with farm-gate value of $54 billion. California is a leading producer of almonds and pistachios in the world, a leading fresh market vegetable producer, as well as leading the nation in milk production. California’s top ten valued commodities are milk, almonds, grapes, cattle/calves, strawberries, lettuce, walnuts, tomatoes, pistachios, and hay, and their values range from $9.4 billion to $1.3 billion.
“It’s a significant component of world’s economy and changing climate poses many challenges to agriculture in California,” he said.
Climate variability is by definition a measure of short-term climate fluctuations above or below the long-term averages, said Dr. Pathak. The top figure is a multi-variate ENSO index, one of many indices have been developed to identify El Nino Southern Oscillation patterns; these oscillations in the time series range anywhere from six to eighteen months, so it’s a rather short-term fluctuation, he said.
“When the sea surface temperature in the equatorial Pacific region is above normal for a certain period of time, that phase is called El Nino; what we’ve seen this year is one of the strongest El Nino on record,” he said. “As opposed to that, when the sea surface temperature in the equatorial Pacific region is below normal for longer period of time, that phenomenon is called La Nina. Together it’s called El Nino Southern Oscillations. When we are not even in a El Nino or La Nina phase, the third phase is called neutral, where sea surface temperatures are neither warmer nor cooler. When you look at El Nino or La Nina impacts on climate across the globe, it varies significantly, but their time frame is rather short term.”
The figure on the bottom is the Pacific Decadal Oscillation, and its oscillation range is anywhere from ten to twenty years, so it’s a rather longer-term oscillation. With respect to warmer or cooler conditions, they can impact climate significantly across the globe, including here in the United States, he said. He explained that when the Pacific Decadal Oscillation is in a warmer phase, the coastal area from Alaska to the equator experiences warmer than normal sea surface temperature, but at the same time, it’s cooler than normal in the tropical Pacific region; in a cooler phase of Pacific Decadal Oscillation, the whole process is the other way around. When the El Nino Southern Oscillation and the Pacific Decadal Oscillation are in a similar phases, like positive or negative or warmer or cooler, that can signify impacts of climate or it can vary climate across the globe quite significantly.
“These are just two examples of large scale teleconnection patterns influencing the climate across the globe; it’s a naturally occurring phenomenon that does vary climate quite a bit, and that is defined under the category of climate variability,” Dr. Pathak said. “But climate change by definition is a measure of longer-term, statistically significant continuous change. It can be an increasing trend or decreasing trend in the measure of climate. For example, temperature increases, frequency of extreme precipitation, or reducing snowpack are some of the examples of climate change. So the major difference between climate variability and climate change is the time frame that we look at in terms of identifying their significant changes.”
He then presented a diagram showing how El Nino can impact climate across the globe. The top figure shows climate impacts of El Nino from December to February. “The impacts vary quite substantially based on which location we are looking at,” he said. “For example, during the wintertime, we can expect cooler and wetter conditions in the southern part of the United States, but at the same time, if you look at Asia and most parts of India and in some other parts of Asia, this climate could be warmer and drier, and similar in some parts of Africa as well as Australia. So depending on which location we are looking at, El Nino impacts can vary substantially.”
The bottom figure shows El Nino’s impact on climate during the months of June through August. “There are no signals here in the United States, it’s because ENSO signals for summertime in the United States are very weak and we sometimes don’t get signals in this area, but if you look at South America, during the summertime, you can expect warmer conditions and dry conditions in this area, similar in parts of Asia and India as well.”
As opposed to El Nino conditions, during the La Nina phases in the months of December to February, you can expect wetter conditions in the southwest Pacific area, versus warmer and drier conditions for southeast or the southern part of the United States, but at the same time, you can expect cooler conditions in the northwest part of the United States, he said. In June through August with La Nina conditions, the Gulf of Mexico can expect cooler conditions but parts of Australia could expect warmer than normal conditions, and South America is cooler and other parts are drier, so depending on the location, La Nina impacts could be significantly different, he said.
“If it is not a strong El Nino or a strong La Nina, there are a lot of uncertainties and variability in terms of what we expect in terms of precipitation for California,” said Dr. Pathak. “So in terms of predicting how the water year is going to be, if it is a moderate or weak El Nino or La Ninas, we might not get reliable skills in terms of predictions.”
Dr. Pathak then turned to climate change, starting with the basics. He presented a basic diagram showing how greenhouse gas emissions create warmer conditions due to the process called greenhouse effect. He explained that when the energy from the sun is coming towards the earth’s surface, part of this energy is reflected back into space, but half of this energy is absorbed by the earth’s surface and then earth reemits that energy in terms of infrared radiation or longer wavelength radiation; these greenhouse gases have a tendency to absorb those longer wavelength radiation, and then reemit it in all those different directions through a process called scattering, so when you have a higher concentration of greenhouse gases in the atmosphere, it traps this energy and warms the globe, and the whole process is called the greenhouse effect. “Due to the increasing greenhouse gases and due to the interaction of temperature with other parameters, we can expect significant changes in the climate,” he said.
He then presented a map showing the rate of temperature change from 1901 to 2014 as compared to climate normals from 1980 to 2010. “Average temperature across the U.S. has increase at the rate of .13 degrees Fahrenheit per decade. The southwestern United States has experienced substantial warming in last hundred years, anywhere from 2 to 3.5 degrees Fahrenheit for the hundred years period; at the same time, some parts of the southeast United States did experience somewhat cooling effect. Overall in general, we did have increased for the entire United States, and that is similar for global conditions as well. Seven out of the top ten warmest years on record have occurred since 1998, so in the recent years, this warming has been substantially higher as it used to be. In 2014 year was the warmest year on record as well globally.”
He then presented a map of California, showing the rate of both maximum and minimum temperature increases. “For the San Joaquin Valley, which is heavily agricultural area, there is not a significant increase in terms of maximum temperatures, but looking at the rate at which minimum temperatures have increased, it’s substantially higher,” he said. “It’s a similar trend for almost entirety of California; the rate at which minimum temperatures are increasing are substantially higher than the rate at which maximum temperatures are increasing.”
“This specifically can have a significant impact on agriculture,” he said. “One is that with this rate of increases, you might have more respiration and that can adversely impact yield. Another consequence would be lower accumulation of chill hours, which is very significant for many of the nut crops that are grown in this region.”
Dr. Tapak then presented a figure from the National Climate Assessment showing changes in number of hot nights, which also corresponds to the increase in minimum temperatures. Hot nights are defined as nights where the minimum temperature is higher than 98% of the minimum temperatures between 1971 to 2000. “The southern part of the United States and some parts of California have significantly higher number of hot nights compared to other parts of the United States, and with the higher nighttime temperatures, it can reduce grain yields and increase stress on animals, resulting in a reduced rate of meat, milk, and ag production along with reduced crop production.”
He then presented a slide showing the changes in the numbers of consecutive dry days, noting that the Southwest in general has more number of consecutive dry days compared to the other parts of the United States. “Annual maximum number of consecutive dry days is projected to increase, especially in the western and southern part of the United States,” he said. “Consecutive dry days and higher temperatures will increase evaporation and stress to limited water resources, which is one of the significant impacts of climate change for California as well, affecting irrigation and other water users.”
Next, he presented a graphic showing changes in frost free season length, which is the duration between last spring freeze and first fall freeze. “That duration has been increasing across the United States and especially on the western half of the United States,” he said. “The number of frost free dates are substantially higher than the eastern part of the United States, and that can also have a positive impact in terms of adaptation. For example, if you are looking at an annual cropping system, you can adapt or change the planting dates in the cropping pattern with respect to this frost free season so you can have a longer growing season. On the other side, for perennial systems, it can negatively impact because some of the specialty crop require specific chill hours and with this trend, it might be negatively impacting in terms of them not getting enough chill hours.”
He then presented a chart showing the declining trend in chill hours for the Central Valley. “Around 1950, growers in the Central Valley could rely on between 700 to 1200 chill hours,” he said. “By 2000, that number had reduced by 30% already, so that’s a significant reduction in number of chill hours accumulated for California. Under future projects, or high emission scenarios, chill hours are projected to decrease significantly and that could be ranging anywhere from 200 to 400 by 2080 to 2090, and 400-700 for 2041 to 2060, so we could expect significantly lower number of chill hours in the future climate projections.”
He then presented a chart showing precipitation trends with respect to last 100 years. “Over the last 100 years, overall for the United States, the rate of precipitation has increased at the rate of .15” per decade, so overall, we have seen wetter than normal conditions,” he said. “On the other side, the southwest has seen decrease in the rate of precipitation for last hundred years, so even with those areas where we did experience wetter than normal conditions, at the same time we are also seeing increase in number of single day precipitation events.”
“Eight out of ten years of extreme single day precipitation events has occurred since 1990, so in the recent years this extreme single day precipitation events has been occurring quite often as opposed to how it used to be in the past,” he said. “The number of extreme single day precipitation in events, in general from 1980 onwards has increased substantially, and that could also have an adverse impact on agriculture.”
Snowpack is California’s natural water storage reservoir. “So far, we have seen approximately 10% reduction in snowpack for California, and under low emission scenario, we could expect 40% of remaining snowpack,” Dr. Pathak said. “In a medium warming range or medium climate scenarios, that could result in 20% remaining snowpack by 2070 to 2099. That’s a substantial lower snowpack that we could expect in future climate change projections. According to the scientists, we could also expect 25% of reduction of snowpack by 2050, so all these models are showing decreasing trends in the snowpack which could adversely impact total water supply for the state.”
Dr. Pathak next presented a graph of the Palmer Drought Severity Index, noting that based on this index, the southwestern United States had been in persistent drought for the last decade. “We could expect to see more of these frequent drought conditions in future climate change projections as well.”
Even with one of the strongest El Ninos on record, while it did provide some relief to the five year drought conditions that we had, it was not enough to completely get the drought out of California. Still, many parts of California, especially the southern portion remain under exceptional drought conditions, and most of the other parts of California are still in moderate to severe drought conditions as well heading into the summer of 2016, he said.
In terms of wildfire, compared with the historic average from 1960 to 1999 under low warming range, the risk of wildlife can increase by 11%; the same risk under medium warming range could increase up to 55%, and that could be a significant economic impact for the state, he said.
He then presented results from modeling study looking crop yield responses to warming in California’s Central Valley. “With this particular modeling study, they kept the water at an adequate level so there was no water stress under these conditions,” he said. “If you look at just the warming ranges and their impacts on crop yields, alfalfa, safflower are not expected to decrease in their yield specifically, but if you look at crops such as tomato, rye, wheat, cotton, and sunflower, their yield under both high and low emission scenarios could reduce anywhere from 10 to 30%, which is significant reduction in yield.”
Dr. Pathak concluded by presenting a summary slide of projected global warming impacts between 2070 to 2099, noting there are three different scenarios: low warming, medium, and high. “Even under the low warming scenarios, we might expect 30 to 60% loss in Sierra snowpack, 6 to 14” of sea level rise, 2 to 2.5 times as many heat wave days in major urban centers, four to six times as many heat-related deaths in urban centers, up to 1.5 times more critically dry years, 7 to 14% decrease in forest yield, and 10 to 35% increase in risk of large wildfires. All these impacts intensify with medium and high emission scenario ranges, so the message is that we have to be really proactive, both in climate change mitigation and adaptation in order to make California agriculture more resilient to these climate risks.”
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