SOURCE: Phys.org

DATE: December 9, 2019

SNIP: Scientists have identified systematic meanders in the globe-circling northern jet stream that have caused simultaneous crop-damaging heat waves in widely separated breadbasket regions-a previously unquantified threat to global food production that, they say, could worsen with global warming. The research shows that certain kinds of waves in the atmospheric circulation can become amplified and then lock in place for extended periods, triggering the concurrent heat waves. Affected parts of North America, Europe and Asia together produce a quarter of the world food supply. The study appears this week in the journal Nature Climate Change.

“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,” said lead author Kai Kornhuber, a postdoctoral researcher at Columbia University’s Earth Institute. “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.”

Kornhuber warned that the heat waves will almost certainly become worse in coming decades, as the world continues to warm. The meanders that cause them could also potentially become more pronounced, though this is less certain. Because food commodities are increasingly traded on a global scale, either effect could lead to food shortages even in regions far from those directly affected by heat waves.

The jet stream is a fast-moving river of air that continuously circles the northern hemisphere from west to east. It generally confines itself to a relatively narrow band, but can meander north or south, due to a feature scientists call Rossby waves. Among other effects, these atmospheric wobbles may pull frigid air masses from the polar regions, or hot ones from the subtropics, into the populous midlatitudes. The wobbles strongly influence daily weather. When they become particularly large, they can bring prolonged heat waves, droughts or floods in summer; or in colder seasons, abnormal cold spells.

Because the earth’s atmospheric circulation is so vast and complicated, only in recent years have scientists been able to pick out global patterns in the Rossby waves. The new study builds on previous discoveries of such patterns, and links them to measurable losses in crop production.

Many scientists believe that Rossby waves will grow and stall more often as the planet warms. Kornhuber said that this scenario is quite plausible-almost all the global events have occurred since 2000- but that says is not yet enough data to form a consensus on this. Regardless, he said, “even if the frequency or the size of the [Rossby] waves doesn’t change, the heat extremes linked to the patterns will become more severe, because the atmosphere as a whole is heating.”