First it was wildfires raging through Southern California, forcing evacuations and turning LA skies orange. Record-breaking hurricanes slamming into Florida. Heat waves baking large parts of North America. Tornadoes ripping through the Midwest. Catastrophic floods across Texas and the Gulf Coast, killing hundreds and displacing thousands. And most recently, storms in the Washington, DC, region, which caused our very first National Weather Service Flash Flooding Emergency.  

This year’s weather seems anything but normal. But given the realities of global climate change, are these events actually our new normal? 

We asked AU Professor Stephen MacAvoy in our Department of Environmental Science to answer some of our questions about shifting—and disturbing—weather patterns.  

PH: Should we expect extreme weather to be our new normal? Can you explain the science behind it all?  

SM: Yes, extreme weather is our new normal, with some caveats. Overall, we’re seeing more frequent instances of regionally extreme weather. The subtropical jet stream—a fast-moving westerly wind in the upper troposphere of the Northern Hemisphere—is becoming increasingly “wavy,” with larger northward crests and deeper southward troughs (dips). This pattern is linked to the fact that the polar regions are warming more rapidly than the tropics, which reduces the temperature gradient between them. That weaker contrast, in turn, allows the jet stream to meander more widely instead of staying tightly confined. 

The larger crests associated with the “wavy” jet stream are allowing tropical air to shift northward in Western North America, which is why British Columbia had a string of record- setting warm temperatures in 2021. Likewise, the troughs can drag cooler air from the arctic down the eastern side of North America, which is why Texas had a large freezing event the same year.  

PH: What’s driving the rise in storms, heavy rain, and flooding in places like Texas—and now in our area too? 

SM: In addition to changes in the subtropical jet stream, the warmer temperatures are increasing the amount of water vapor in the atmosphere (itself a greenhouse gas), leading to increased rainfall totals and greater rainfall intensity on average in the United States.  

There are exceptions, but overall, precipitation amounts and storm intensity have been increasing almost everywhere in the United States. The tragic situation with the Guadalupe River in Texas this month was partly a result of the incredible intensity of the rainfall, which was unprecedented in the region (in addition to the fact that the river floods regularly). 

PH: Can you also explain the science behind the heat and humidity in DC this summer? 

SM: The Washington, DC, region has been experiencing very high temperatures this month, as well as high amounts of precipitation and very high humidity (DC summers are always humid, but this summer is extra intense). In DC's case, what is causing our recent weather is a very stable high-pressure system in the Atlantic east of Florida (a Bermuda High), and that is diverting very humid Gulf of Mexico air to the southern and mid-Atlantic states (including Texas, tragically).  

PH: So, while these extremes are becoming more common, will different regions experience climate change in different ways? 

SM: We must keep in mind that not all regions of the planet will experience the same types of odd weather. So, even if Texas is experiencing high intensity rainfall, you can be sure that somewhere else there will be a drought.  

We can all expect more dramatic weather in the coming years. Anthropogenic climate change (change caused or produced by humans) isn't necessarily causing particular storm events, but it is exacerbating the extremes. 

About Professor Stephen MacAvoy

Working in biogeochemistry and ecology, Professor Stephen MacAvoy has been particularly interested in how both nutrients and contaminants flow through, and are incorporated by, aquatic ecosystem components. Research and consulting activities have taken him to the Gulf of Mexico hydrocarbon seeps, Barbados, 11 USA States, and other locations. Professor MacAvoy obtained his PhD from the University of Virginia in 2000 and conducted post-doctoral work at the University of Georgia. He joined the faculty of American University in 2003 was chair of the Department of Environmental Science from 2016 to 2022. He has been the Graduate Program Director for 22 years. Recent publications have appeared in Applied Geochemistry, Environmental Science and Pollution Research, Ecological Engineering and Marine Mammal Science. He is currently conducting research on the inorganic and organic geochemistry of the Anacostia River in Washington, DC.