If you’ve been in the Northern Hemisphere lately, you have probably experienced some extreme weather. From extreme cold and snowy snaps to deadly heat waves, droughts, and flooding, the weather in the north feels more intense and variable than ever. This winter, for example, northern countries have been posting their warmest winter temperatures on record  —  from the United States to Finland, this winter has been abnormally mild. Even Antarctica has hit its warmest recorded temperature this season!

Warm winters aren't the only weather weirdness we've experienced more often. Over the past decade, extreme weather events have made headlines all over the world: the midwestern Polar Vortex and Snowmageddon in Washington, D.C., Pacific Northwestern and European heatwaves, flooding in the Midwest and south Asia, droughts in California, high-intensity hurricanes in the North Atlantic… the list seemingly goes on and on. 

To better understand what’s happening with the weather in the US and Europe, climate scientists have been looking a little farther north. They have been tracking Arctic warming and sea ice loss for decades, noting correlations between high-latitude warming and extreme weather events at lower latitudes. A recent article in Science Advances reviewed, in depressing detail, the most alarming changes that sensitive Arctic regions are undergoing as a result of climate change. The Arctic is warming at a faster rate than the rest of the world, resulting in less sea ice, early snowmelts, and melting permafrost that releases greenhouse gases. The shifts in Arctic climate are dramatic, and their effects are not limited to the Arctic. 

Extreme weather events in the midlatitudes (roughly from the top of Florida to the southern tip of Greenland) – including heatwaves, flooding, and cold snaps –have been linked to the Arctic’s rapid warming and sea ice loss through changes in atmospheric circulation patterns. These patterns (the way in which air masses move through the atmosphere) depend on a number of factors, but are mostly controlled by two fundamentals of physics: temperature and pressure. Temperatures in the troposphere – where weather happens, the lowest level of the atmosphere, from the ground up to about 15 kilometers – affect the stratosphere, the middle layer, about 15-50 km up, and vice versa, because air pressure and density depend on temperature. Warm air rises and cold, dense air sinks; the bigger the temperature difference between two air masses, the bigger the energy imbalance is. Those temperature and pressure imbalances drive air circulation and wind. 

Arctic warming can also affect the jet stream, a fast west-to-east flow of air that hovers high in the troposphere, by weakening it and causing it to meander. That meandering can be compounded by a phenomenon known as Rossby waves, wavy atmospheric patterns that encircle the middle of the northern hemisphere. Picture a cosine wave stretching from Seattle to St. Louis to Buffalo, then curving into the Atlantic. They form because the sun heats the planet unevenly; landmasses of different sizes and oceans absorb heat differently, creating an energy imbalance. Air masses flow around the planet, transferring heat. As the air masses flow, they form wavelike patterns with peaks and troughs that progress, slowly, from west to east.

The shape and movement of Rossby waves depend on temperature and pressure gradients throughout the atmosphere, which can be affected by – you guessed it – long-term Arctic temperatures. When these waves stretch farther south, they also tend to move eastward more slowly, letting weather conditions linger over midlatitudes longer than they might otherwise. The longer weather conditions linger, the more likely extreme weather events are. That’s when we experience dangerous heatwaves and apocalyptic snowstorms. Or, as much of the eastern and midwestern US has experienced so far this season, the Rossby waves can linger higher up (say, above Ontario instead of Ohio), creating an unusually mild winter for us. This effect can be amplified during an El Niño year, when ocean surface temperatures in the eastern Pacific are warmer than usual and weather patterns across the U.S. change. 

Beyond having to pull out our warmest parka or pay an exorbitant heating bill, the extreme weather events linked to Arctic warming have a variety of negative effects on people. If a heat wave hits, especially in a region ill-prepared to cope with it – like where people tend not to have air conditioning, for example, or with a large elderly population – the risk of loss of life is high. In a “business as usual” emissions scenario as outlined by the International Panel on Climate Change, where we don't alter our rate of greenhouse gas emissions, at least twice as many heat-related deaths are likely to occur in U.S. cities than under a lower-emissions scenario. The "urban heat island" effect, where cities are hotter than surrounding rural areas, can exacerbate heat waves and increase mortality rates.

Along with all of these ill-effects come a suite of ecological changes. As temperatures warm, climate patterns are essentially shifting northward – which is why vintners in France are panicking and trying to adapt, and those in England are eyeing a prosperous future their current climate doesn’t allow. Bees are suffering in heat waves., with potential ramifications for agriculture. In the southeastern US, an invasive, rapidly-spreading leafy vine called kudzu has been spreading for decades, choking out forests. In the Arctic, as warming changes environments, the ranges of both plants and animals may grow or shrink, and the spread of diseases may threaten local populations. Antarctica, which hosts unique ecological communities, may be subject to invasive species as it becomes more habitable. Worldwide, as climate change drives ecosystem shifts, plants, animals, and people will be forced to respond. Some will survive. Others won't.

The question of what the consequences of extreme weather events will be is complicated to answer. There are no one-size-fits-all answers in climate change. What we do know is that if we maintain our business-as-usual approach to greenhouse gas and aerosol emissions, extreme weather events will likely become more intense, more frequent, and harder to predict – as will the economic consequences.