Many regions suffer simultaneously from hot, dry conditions – ScienceDaily

According to the researchers, climate change has doubled the odds that a region is suffering from both a hot and dry year compared to the average for this region in the mid-twentieth century. It is also increasingly likely that dry, extremely hot conditions will hit key agricultural areas in the same year, which may make it more difficult to find surpluses in one area and reduce yields in another.

"When we look at historical data on the main crop and pasture areas, we find that before anthropogenic climate change, there is very little chance that two regions will simultaneously experience these extremely difficult conditions," he said. climatologist, Noah Diffenbaugh, from Kara. J. Foundation professor at the Stanford School of Earth, Energy and Environment Sciences (Stanford Earth) and lead author of the study published November 28 in Progress of science. The study is entitled "Multidimensional risk in a non-stationary climate: joint probability of increasingly hot conditions and drought".

"The global market offers protection against localized extremes, but we are already seeing an erosion of this climate buffer due to the increase in extremes in response to global warming," said Diffenbaugh, also a senior member of the Kimmelman family at the Stanford Woods Institute. For the environment.

The new study suggests a future in which multiple regions may experience low yields simultaneously. While some crops may flourish during a hot growing season, others – especially cereals – grow and ripen too quickly when the temperature rises, dry days are superimposed and heat continues all night long. As a result, dry and warm conditions tend to produce lower yields of major commodities, including wheat, rice, corn and soybeans.

The implications go beyond agriculture. These same hot, dry conditions can also exacerbate fire hazards, dry out vegetation in summer and fall, and fuel intense, fast-spreading fires such as those that burned more than 240,000 acres in California in November 2018.

Mobile targets

The fundamental trend of global warming – 1 degree Celsius or 1.8 degrees Fahrenheit since the end of the 19th century – lends an intuitive logic to the main conclusions of the study. "It's getting hotter all over, so it's more likely that it's hot in two places at once," explained Diffenbaugh, "and it's probably also more likely to get hot when it is also dry in two places at once. "

Yet, despite this simple intuition, taking into account continuous and interdependent changes in precipitation and temperature at different locations over time is a statistical problem. As a result, many previous analyzes have considered hot and dry events as independent phenomena, or different regions as independent of each other.

This approach may underestimate the additional risks associated with global warming, as well as the social, ecological and economic benefits of reducing emissions. "When these extremes occur simultaneously, they exacerbate the negative impacts beyond what any of them could have caused separately," said Ali Sarhadi, lead author of the study. and postdoctoral researcher from Diffenbaugh's Climate System and Land System Dynamics Group at Stanford Earth.

The extremes become normal

The new study used historical data from the last century to quantify the likelihood of different regions experiencing hot, dry conditions in the same year. The analysis shows that before 1980, there was less than a 5% chance that two pairs of regions would experience extreme temperatures during a dry year in both regions. However, over the past two decades, probabilities have increased to 20% for some pairs of regions.

For example, the probabilities that China and India – two of the world's largest agricultural producers and two of the most populous countries – experience low rainfall and extremely hot temperatures in the same year rose from less than 5% before 1980 to more than 15% today, said Diffenbaugh. "We can therefore expect what was once rare to happen with some regularity, and we have very good evidence that global warming is the cause."

In addition to their analyzes of historical data, the authors also analyzed climate model projections of possible global warming scenarios. They found that in a few decades, if the world continued on the path of emissions, the likelihood of average temperatures well in excess of the range normally observed in the middle of the 20th century could increase by 75% in many areas.

But achieving the goals set out in the UN climate agreement in Paris should significantly reduce these risks, said Sarhadi. While the White House has announced its intention to withdraw the United States from the agreement, the study shows that achieving the emission reduction targets in the 200-country compact would allow the world to significantly reduce the risk of drought, striking many cropland around the world. "There are still options to mitigate these changes," he said.

Real risk planning

The framework developed for this study represents a critical step in determining the risk associated with the combination of multiple climate extremes in a single region, where they can often combine. What were the chances, for example, that high temperatures, high winds and low humidity combined to create gigantic fire conditions in the past, and how did these probabilities change due to global warming? This is the kind of question that the team member can answer. This is an extremely urgent problem for public servants who now have fires of historical magnitude and intensity in California.

"A lot of the stressful events that infrastructure and our systems of disaster prevention and disaster response occur when several ingredients are gathered at the same place at the same time," said Diffenbaugh. Strong storm surges and wind speeds with heavy rains can mean the difference between a passing storm and a catastrophic tropical cyclone; Wind patterns and moisture levels in different parts of the atmosphere affect the severity of a rain storm and the associated flood risk.

A key challenge for policymakers is to understand what to expect in a changing climate. This means relying on common probabilities, which are at the heart of the calculations that engineers, policymakers, aid providers and insurers use to allocate resources, define building codes, design evacuation plans and other disaster response.

"People make practical decisions based on the probabilities of different combinations of conditions," said Diffenbaugh. "The flaw is to use historical probabilities, but our research shows that assuming that these historical probabilities will continue in the future does not accurately reflect current or future risk."