A concerning text I received from my gardener friend.

[The_jackalope]

24. It depends on one's definition of "global crop failure"

The entire world's cereal crop output doesn't need to decline to 0 in order to qualify, IMO. The majority of such crops are grown in the northern hemisphere:



Would a caloric reduction of 25% or 30% across the northern hemisphere qualify? I can see such a problem occurring as a result of extreme weather events (droughts,floods and heat waves) around the world during a single growing season.

Mechanisms have been identified that could lead to simultaneous (same-year) reductions in output.

Summer weather extremes linked to stalled Rossby waves in the jet stream

Early summer heatwaves in Western Europe and North America set new temperature records in 2018, while other regions of the northern hemisphere were hit with torrential rain and severe flooding. Now researchers in the UK, Germany and the Netherlands say that these events were linked by a pattern of stalled waves in the jet stream. They add that this wave pattern appears to have increased in frequency and persistence in recent years and may occur more frequently in the future due to climate change.

The northern jet stream is a river of fast-moving air that circles the northern hemisphere in the mid-latitudes. Travelling from east to west at an altitude of around 10 km, these winds drive large-scale weather systems around the globe.

Jet-stream winds generally travel at the same latitude, but they can shift into a wave-like pattern, known as Rossby waves, where they meander from north to south and back again. When this happens, warm air fills the peaks of the wave, while cold polar air drops into the troughs. Rossby waves normally continue to move from east to west – shifting high- and low-pressure weather systems with them. However, they can also stall – which can lead to heatwaves, droughts and floods as the regions of hot and cold air hover over the same regions for days, or even weeks.

In June and July 2018 extreme heatwaves hit North America, Western Europe and the Caucasus, while south-east Europe and Japan experience heavy rain and flash flooding. Norway set a new maximum temperature record and received just half of its average July rainfall, the United Kingdom experienced the second hottest July since records began, and various locations in the western United States broke temperature records. Meanwhile in Japan severe floods and landslides caused by heavy rain destroyed more than 10,000 houses.

The northern jet stream is already disrupted (at less than a 1.5C average temperature rise) exhibiting increased waviness and slowing that support this hypothesis.