A new study from Stanford University suggests that the type of hot and dry conditions that can reduce crop yields, destabilize food prices, and lay the foundations for devastating fires, are rising simultaneously due to the warming climate.
According to researchers, climate change has doubled the probability that the region will suffer from a year that is warm and dry compared to the average of this site in the mid-20th century. It is also likely that dry and strictly hot conditions will hit key farming areas in the same year, making it more difficult for surpluses in one place to cope with low yields in another.
"Looking at historical data in key crops and pastures, we can see that before the anthropogenic climate change, there were very low assumptions that any two regions will experience the really tough conditions at the same time," said Noah Diffenbaugh, at the Stanford Earth School of Earth, Energy and Environmental Sciences, and lead author of the study published on November 28 Scientific advances. The study is called "Multidimensional Risk in a Non-Stationary Climate: A Common Probability of More and More Warm and Dry Conditions".
"The global market provides protection against localized extremes, but we are already seeing the erosion of this climate buffer because extremes have increased in response to global warming," said Diffenbaugh, who is also Family Commissioner Kimmelman at the Stanford Woods Institute for the Environment.
A new study points to the future in which more regions are faced with low yields. This is because while some crops can thrive in a warm growing season, others – especially grains – grow and learn too quickly as the temperatures rise, the consecutive dry days accumulate and the heat lasts overnight. As a result, dry conditions tend to produce smaller harvests of major commodities, including wheat, rice, maize, and soy beans.
Impacts go beyond agriculture. The same hot and dry conditions can also worsen the risk of fire, drying out vegetation in the summer and autumn, and promoting intense, rapid fires like those that burned over 240,000 acres in California in November 2018.
The basic trend of global warming – 1 degree Celsius or 1.8 degrees Fahrenheit since the end of the 19th century – brings intuitive logic to the basic findings of the study. "If it's warm everywhere, it's likely to be hot in two places at a time," Diffenbaugh explained, "and probably will be worse if it's also dry at two places at a time."
In spite of this simple intuition, representing the ongoing interdependent changes in both precipitation and temperature at different places over time, it presents a statistical challenge. As a result, in many past analyzes, hot and dry events have been dealt with as independent phenomena or in different regions independently of each other.
This approach may underestimate the added risk caused by global warming caused by people as well as the social, environmental and economic benefits of emission reductions. "When these extremes occur at the same time, they aggravate the adverse impacts beyond what would cause it separately," said Ali Sarhadi, lead investigator and postdoctoral scientist at the Diffenbaugh Climate and Earth Dynamics Group on Stanford.
Extremes become normal
A new study used historical data from the past century to quantify the probability that different regions will experience warm and dry conditions in the same year. The analysis shows that before 1980 there was less than 5 percent chance that two regions would result in extreme temperatures in a year that was dry in both areas. However, in the last two decades, the rates for some regions have increased to 20 percent.
For example, the chances that China and India – two of the world's largest agricultural producers and the two most populous countries – have both low rainfall and extremely hot temperatures in the same year fell from less than 5 percent before 1980 to more than 15 percent today, Diffenbaugh said. "So what has become a rare occurrence can be expected to occur with some regularity and we have very strong evidence that global warming is the cause."
In addition to analyzing historical data, the authors also analyzed the climate models of possible future global warming scenarios. They found that within a few decades, if the world continues its current emission trajectory, the probability of average temperatures rising from the range commonly occurring in the mid-20th century can increase by 75 percent in many areas.
Achieving the goals outlined in the UN climate agreement in Paris is likely to substantially reduce these risks, Sarhadi said. While the White House announced its intention to abolish the United States from the agreement, the study shows that achieving the emissions reduction targets in the 200 nation-wide agreement would allow the world dramatically to reduce the likelihood of a number of crops across the world in hot and dry conditions. "There are still possibilities to mitigate these changes," he said.
Planning for real risks
The framework built for this study is a crucial step to worsen the risk associated with extreme climatic extremes that link up in one region where they can often interact. What was the probability, for example, of high temperatures, strong winds and low humidity in the past, to create mega fire conditions in the past, and how did these courses change as a result of global warming? This is a question the team team will be able to answer. It is a very urgent task for officials who are now counting on historic scale and intensity fires in California.
"Many of the events that stress the infrastructure and our disaster prevention and response systems occur when more than one component is composed at one point," Diffenbaugh said. High impact storms and wind speeds with heavy rain can mean the difference between the storm and the calamity tropical cyclone; wind patterns and humidity in different parts of the atmosphere affect the severity of the storm and the associated flood risk.
A key challenge for decision makers is to understand what to expect in a changing climate. This means improving the common probabilities that are at the heart of the calculations that engineers, policy makers, humanitarian aid providers and insurers use to allocate resources, set construction regulations, and design evacuation plans and other disaster responses.
"People make practical decisions based on the likelihood of different combinations of conditions," Diffenbaugh said. "The starting point is the use of historical probabilities, but our research shows that assuming that these historical probabilities will continue into the future does not accurately reflect current or future risk."