How Humans are Breaking the Jet Stream and Changing the Weather

Pennsylvania's weather in 2023 has been strange at best and deadly at worst. Extreme drought in May gave way to historically dangerous air quality in June, with a dash of severe storms and tornadoes for good measure. Forecasters point to wind patterns to explain why the weather has been so extreme. But a deeper dive into what is causing these unusual events points to something more insidious: humans are disrupting the Earth's jet streams.

Let's start with the basics before this turns into a sequel to Day After Tomorrow.

What is a jet stream?

Jet streams are like twelve-lane highways in the atmosphere. They are fast-moving currents of air that circle the globe and are usually found at the altitude where commercial airlines fly. Jet streams exist because of temperature differences. The sun warms the Earth unevenly because our planet's axis is tilted, so regions around the equator receive more solar warmth than regions around the poles. The temperature difference between the colder north and south poles and the equator causes wind to blow in fast currents.

These jet streams play a major role in our daily lives. For example, Pennsylvania has faced numerous energy crises because of the polar vortex, when the northern polar jet dips far south, bringing arctic air with it.

The image above shows the jet streams on June 29, 2023. Notice the faster-moving lines closer to the top and bottom of the map. In the northern hemisphere, the subtropical jet cuts across the United States and through the northern Atlantic and Pacific Oceans.

For months, forecasters discussed how United States weather has been driven by what is called an Omega Block, a phenomenon in which the jet stream stops moving predominantly west to east across the country and instead forms large, block-like waves.

The second map zooms in on the United States during the same time period and highlights this wavy, omega-shaped jet stream pattern, which persisted throughout May and into July. This blockiness locked in dramatic weather patterns, including extreme heat in Texas, numerous tornado outbreaks across the Plains and Deep South, and a pathway for wildfire smoke to travel from Canada into Pennsylvania's airspace.

Changing Patterns

Omega Blocks are not uncommon, but what is unusual is how persistent and blocky the broader global jet stream pattern has become. Dr. Michael Mann, a climate scientist in Pennsylvania, noted that he is honestly at a loss to characterize the current large-scale planetary wave pattern and remarked that it looks like a Van Gogh painting.

Looking more closely at the first global jet stream map, you can see what Dr. Mann describes. The subtropical jet in the northern hemisphere appears fragmented, discontinuous, and slower compared to jet streams elsewhere on the planet. Over the United States, India, China, and Northern Africa, the lines are lighter in color, indicating slower speeds, and it is difficult to trace a continuous path around the globe. It is subtle but significant.

Why does this matter, and what is causing it?

Climate change research provides insight into what is happening. Human-caused global warming is rapidly heating the planet, and much of that heat is being concentrated at the poles. For example, the North Atlantic is experiencing unprecedented ocean temperature increases of 5 to 10 degrees Fahrenheit. Arctic sea ice has declined so dramatically that scientists expect it to be ice-free during summer months by the end of this decade. Meanwhile, global average temperatures in June have been nearly a degree warmer than any month in recorded history.

Rapid warming in the Arctic fundamentally alters the temperature difference between the poles and the equator. Without a strong temperature gradient, wind currents slow down. In other words, global warming driven by human activity is slowing and even disrupting the planet's jet streams. As the jet stream weakens, blocky wave patterns like the Omega Block grow larger and are more likely to stall.

Sound familiar to what Pennsylvania has experienced for more than two months?

In the short term, extreme weather is likely to worsen. Natural climate cycles are also shifting and may amplify human-induced changes, including the rapid warming of the eastern Pacific Ocean known as El Nino, which also alters jet stream behavior and United States weather patterns.

Human activity, particularly the burning of fossil fuels, is warming the planet and altering the fundamental dynamics of global jet streams. This adds to the growing list of extreme weather events harming the Commonwealth. The question used to be whether Pennsylvania could or should act on global warming. As climate impacts intensify, the question becomes how quickly we can do everything possible to stop further damage.