During the summer of 2018, the future of climate change became the present. Highly amplified jet stream patterns remained stuck in place for unusually long periods of time, bringing the planet an onslaught of remarkable weather catastrophes—for example, unprecedented heat waves and drought in East Asia and Northern Europe, the start of the deadliest and most expensive fire season on record in California, and Japan’s deadliest floods since 1982.

The extreme summer weather helped bring the 2018 tally of billion-dollar weather-related disasters to 39--the fourth highest such total for any year since 1990, according to insurance broker Aon Benfield. Among these were seven billion-dollar droughts--the highest number of billion-dollar droughts on record (previous record: six in 1999 and 2015). Total damages from drought in 2018 were near $33 billion—tied for the fifth-highest level of global drought damage since 1975.

Unfortunately, extreme jet stream patterns like those of 2018 may be getting more common and more extreme, representing a significant danger to global food security. An April 26 paper, Extreme weather events in early summer 2018 connected by a recurrent hemispheric wave-7 pattern, by climate scientist Kai Kornhuber of Columbia University and co-authors, found that the 2018 extremes were associated with a particular mode of “stuck in place” jet stream behavior—one that has increased in frequency and persistence in recent decades.

A just-published December 9 follow-up study, Amplified Rossby waves enhance risk of concurrent heatwaves in major breadbasket regions--also led by Dr. Kornhuber--found that stuck jet stream patterns like seen in 2018 are prone to bringing simultaneous heat waves and associated drought conditions to multiple important grain-producing regions of the world. The authors wrote that these stuck jet stream patterns can cause “reductions of 4% in crop production in the affected regions, with regional decreases up to 11%. Given the importance of these regions for global food production, the identified teleconnections have the potential to fuel multiple harvest failures posing risks to global food security.” (A teleconnection is a causal connection or correlation between meteorological phenomena which occur a long distance apart).

In a press release that accompanied the most recent paper, Dr. Kornhuber said, “We found a 20-fold increase in the risk of simultaneous heat waves in major crop-producing regions when these global-scale wind patterns are in place. Until now, this was an underexplored vulnerability in the food system. During these events there actually is a global structure in the otherwise quite chaotic circulation. The bell can ring in multiple regions at once.”

Figure 1. Average crop production from 2003-2007 (in colors) for the major commodity crops maize (corn), wheat, soybean, and rice in units of kcal (based on data from Ray et al., 2012). Areas affected by heat waves and their associated droughts during stuck jet stream patterns of wave-5 (shown in brown-bordered regions) and during wave-7 (red-bordered regions) are also shown. Affected areas include the major breadbaskets in central North America, plus Western and Eastern Europe. The regions affected by wave-5 and 7 account for about a quarter of global food production. The U.S., France and Russia produce 42% of the world’s wheat; for maize, the U.S. and France alone produce 57% of the total. Credit: Kornhuber et al., 2019, Amplified Rossby waves enhance risk of concurrent heatwaves in major breadbasket regions, Nature Climate Change

Earth’s changing jet stream behavior

Earth's atmosphere features a series of wave-like troughs and ridges in the jet stream called planetary (or Rossby) waves with a wavelength of several thousand miles, which march from west-to-east around the globe at about 15 - 25 mph. Their long wavelength means that typically 5 – 10 of these Rossby waves circle the globe at any particular time.

Kornhuber and colleagues found that between 1979 and 2018, Rossby waves with specific wavelengths, termed wave-5 and wave-7—meaning that north-south wobbles in the jet stream that produce either 5 or 7 peaks and corresponding troughs around the planet’s circumference--can lock into a grid of symmetric large meanders centered over predictable regions. The wave-5 patterns tend to get stuck in place with ridges of high pressure (and their associated heat waves and drought) located over central North America, eastern Europe and eastern Asia; the wave-7 patterns bring heat waves and drought to western-central North America, western Europe and western Asia.

“Normally, low harvests in one region are expected to be balanced out by good harvests elsewhere,” said study coauthor Dim Coumou of the Institute for Environmental Studies at VU University Amsterdam, who has co-authored several papers on extreme jet stream behavior in recent years. “These waves can cause reduced harvests in several important breadbaskets simultaneously, creating risks for global food production.”

Though the current study did not find that wave-5 and wave-7 stuck jet stream patterns changed in frequency during the 1979 – 2018 period, a number of other studies have found evidence of an increase in stuck jet stream patterns in recent years, including Mann et al. (2018), Projected changes in persistent extreme summer weather events: The role of quasi-resonant amplification, Mann et al. (2017), Influence of Anthropogenic Climate Change on Planetary Wave Resonance and Extreme Weather Events, and Lehmann et al. (2015), Increased record-breaking precipitation events under global warming.

An October 2018 realclimate.org post by Michael Mann gave a good summary of these studies, which have generally found that human-caused climate change may be to blame for this highly concerning change in jet stream behavior. Dr. Mann predicted that our future climate is likely to bring a significant increase in stuck summertime jet stream patterns capable of bringing a rise in extreme destructive weather events like we experienced in 2018.

Even if stuck jet stream patterns do not increase in frequency, heat waves and their associated droughts are virtually certain to grow more intense in a warming climate. This behavior will bring a significant threat to global food security when a stuck jet stream brings intense long-lasting drought conditions to multiple grain-producing regions. As I wrote in a 2016 post, Food System Shock: Climate Change’s Greatest Threat to Civilization, simultaneous droughts hitting multiple breadbasket regions are capable of delivering a dangerous global “food system shock”. A "Food System Shock" report issued in 2015 by insurance giant Lloyds of London outlined a plausible extreme shock to global food production involving multiple droughts that could cause rioting, terrorist attacks, civil war, mass starvation and severe losses to the global economy. Lloyds gave uncomfortably high odds of this occurring--significantly higher than 0.5% per year, which works out to at least an 18% chance of occurrence in the next 40 years. Another excellent discussion of the dire food supply risks we have due to climate change is given in a very sobering book published in October called Food or War, by science writer Julian Cribb. I plan to review the book in a future post.

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