Are 2015’s extreme weather events driven by climate change or El Niño?

Record warmth, increased precipitation, and more intense tropical cyclones.

These are just a few of the consistent predictions from models investigating our future in a world with climate change. Or, it’s a list of some of the impacts of the periodic weather pattern called El Niño.

So which one has been driving some of this year’s extreme weather events?

A record year for Pacific tropical cyclones

The National Hurricane Center reports that eight major hurricanes (Category 3 or higher) have developed in the eastern North Pacific Ocean so far this season. This is consistent with the characteristics of El Niño that have been shaping up over the course of the year.

During an El Niño year, the surface ocean in the Eastern Pacific basin warms (it’s usually very cold) and the trade winds in the area weaken. These two meteorological developments favor the formation of tropical cyclones, the general term that includes hurricanes and related systems. Hurricane Patricia achieved record strength in a record short period of time this month, becoming the strongest Pacific hurricane to make landfall. Meanwhile, climate change will probably not change the number of hurricanes overall, but warmer ocean surface temperatures and higher sea levels are expected to intensify their impacts.

Driver: El Niño

Remember when Boston was buried in snow?

The Northeast saw record amounts of snowfall during the 2014-2015 winter. The extreme snow amounts were caused by two factors. First, a weakened polar vortex (the global wind pattern that usually keeps Arctic-cold air trapped over the poles) brought extended periods of bitter cold weather to the region. Then major storm systems arrived, bringing large amounts of moisture that fell as snow.

It’s unclear if or how climate change affects the polar vortex. But it’s very clear that a warmer world is a wetter world. Warmer air causes more water to evaporate over the oceans, forming storm systems heavy with moisture. When these systems hit cold air, as they did repeatedly this winter, that means lots of snow.

Driver: Climate change

More record high temperatures

Final global average temperature data is in for September 2015, and it was the hottest September ever measured. Moreover, this year is on track to be the hottest year ever recorded. While El Niño makes many (but not all) parts of the world warmer than normal, the effect is strongest in winter months, so it probably had little effect in September.

A record-setting month for temperature probably sounds like familiar news because records for high temperatures have been broken over and over again in recent years. This is one of the most certain outcomes of climate change. As the climate warms, it means not only hotter summers, but warmer autumns and springs, too.

Driver: Climate change

While climate change isn’t the cause of all extreme weather events, it contributes to many of them. And it is playing a role in making other long-term weather phenomena like El Niño more impactful. That’s why it’s critical to both build resilience to climate impacts and also reduce climate-altering emissions so that those impacts aren’t more than we can bear.