For years, climate contrarians have pointed to snowfall and cold weather to question the scientific reality of human-induced climate change.
Such misinformation obscures the work scientists are doing to figure out just how climate change is affecting weather patterns year-round.
Is global warming really happening?
Absolutely. Global warming refers to an upwards trend in temperatures observed around the planet. Federal agencies, universities, and research centers around the world have all concluded that Earth is warming. That is based in hard data, such as temperature measurements on land and water around the world over the course of decades.
Two major reports released since 2014, the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) and the Climate Science Special Report (2017) have stated unequivocally that temperatures are rising.
What’s causing this observed warming? The primary driver is an increase of heat-trapping emissions in the atmosphere when we burn coal, oil, and gas to generate electricity, drive our cars, and fuel our businesses.
Atmospheric temperature measurements show an average warming of 0.85 degrees Celsius (1.53 degrees Fahrenheit) from 1880 to 2012, according to the IPCC Fifth Assessment Report (AR5). Globally-averaged temperatures in 2016 were 1.78 degrees Fahrenheit (0.99 degrees Celsius) warmer than the mid-20th century mean. According to researchers at the National Oceanic and Atmospheric Administration (NOAA), the rate of warming during the first 15 years of the twenty-first century has been at least as great as the last half of the twentieth century. 2016 was the warmest year on record globally, with 16 of the 17 warmest years on record occurring since 2001.
What about all this snow? Shouldn’t warmer temperatures prevent snow from forming?
Even though the planet is getting warmer, cold weather still happens in winter or at very high elevations or high latitudes year-round. Northern hemisphere winter weather patterns are a complex interplay between the upper atmosphere conditions over polar regions and mid-latitude conditions over the oceans and on land.
Understanding the difference between climate and weather
Weather is what’s happening outside the door right now; today a snowstorm or a thunderstorm is approaching.
Climate, on the other hand, is the pattern of weather measured over decades.
Factors that come into play for regional weather (and indeed global weather) are Earth’s seasons, ocean patterns, upper winds, Arctic sea ice, and the shifting shape of the jet stream (see below). These factors can lead to extreme weather in various portions of northern mid-latitudes—such that some places get tons of snow repeatedly while others are unseasonably warm.
The seasons we experience are a result of Earth’s tilted axis as it revolves around the sun. During the North American winter, our hemisphere is tilted away from the sun, leading sunlight to hit us at an angle that makes temperatures lower. While climate change does not affect Earth’s tilt, it does have potential implications for many of the other factors that influence winter weather in the US, including Arctic sea ice, the polar jet stream, the polar vortex, and El Niño.
The connection between Arctic sea ice cover and more extreme weather events
Scientists are looking into the connection between changes in Arctic sea ice cover and more extreme weather events across the mid-latitudes. Many conditions are unprecedented, including the record low extent of Arctic sea ice.
The AR5 states that Arctic sea ice surface extent has decreased by 0.17 to 0.2 million square miles per decade, changes considered unprecedented in at least the last 1,450 years. 2017 set a new record for the lowest winter maximum sea ice extent observed. The 2017 Arctic Report Card stated that ice is shrinking faster compared with the prior 500 years, and that “observations in 2017 continue to indicate that the Arctic environmental system has reached a 'new normal', characterized by long-term losses in the extent and thickness of the sea ice cover, the extent and duration of the winter snow cover and the mass of ice in the Greenland Ice Sheet and Arctic glaciers, and warming sea surface and permafrost temperatures.”
A recent study on the North Pacific circulation patterns over the past 1.2 million years determined that sea ice on coastal areas can be an important factor in ocean circulation, therefore influencing climate at global and regional levels. In addition to affecting the ocean circulation patterns, Arctic sea ice is melting more rapidly and for longer periods each year, and is unable to replenish itself at the historical thickness levels in the briefer, warmer winter season. This can destabilize the polar vortex (see below) and raise the barometric pressure within it.
The jet stream, polar vortex, and El Niño
The jet stream is short for “tropospheric polar jet stream,” and is a current of fast-moving air found in the area of the atmosphere where weather occurs and jets fly. The jet stream is formed along the boundaries between hot and cold air and becomes more pronounced during winter, when the Arctic and mid-latitude air masses more strongly contrast with each other.
The polar vortex, or stratospheric jet stream, is a large area of low pressure and cold air surrounding both of Earth’s poles. It can expand or shift, such as during the very cold winters of 2013/2014 and 2014/2015, when it sent cold, Arctic air down into the central and eastern US, all the way to Texas, Mississippi, and Alabama. The expansion and shifting of the polar vortex to various degrees is regular occurrence during the winter.
The tropospheric jet stream is commonly seen on TV weather forecasts during winter, in the shape of a migrating wave. The bottom of the wave can be shallow, covering only the northern US states, or it can be deep, reaching states much farther south, as is illustrated in the animation above.
An elongated polar vortex can lead to a more wavy jet stream with higher latitude peaks and lower latitude troughs. When a deep trough occurs over eastern North America, this creates colder than usual weather for those areas as a general rule. The opposite can also happen, when a shallow trough brings milder weather across the northern and eastern US
The polar vortex was notably unstable during several recent winter seasons, which in turn influenced the jet stream. For example, during the 2011/2012 winter, there was a shift in the position of the jet stream. Typically New England, the Great Lakes, and parts of the Great Plains sit north of the jet stream and remain cold in the winter season. However, the 2011/2012 position of the jet stream meant these regions were south of it for most of the winter, which helped produce the fourth warmest US winter on record. The lack of snowfall and snowpack for the winter of 2011/2012 and the following spring was a precursor to the large drought episode that impacted two-thirds of the nation during the following summer and autumn.
During the winter of 2013/2014, an opposite shift of the jet stream left parts of the US with freezing weather and parts of Russia experiencing much warmer weather than usual. Similar conditions in the winter of 2014/2015 saw Boston break its all time seasonal snow record, as well as a record stretch of 43 days with temperatures below 40⁰F.
Finally, the El Niño Southern Oscillation (ENSO) is a climate phenomenon characterized by the warming of the eastern waters of the equatorial Pacific Ocean, mainly west of northern Latin America. (Usually the warmest waters are found in the western Pacific Ocean, near Australia and Indonesia.) This shift can have important consequences for global weather, and is intimately related to precipitation and snowpack in California during the winter, as well as precipitation patterns in other regions.
El Niño tends to create a subtropical, low-latitude jet stream that brings a lot of moisture to the southern United States and up the Mid-Atlantic region. The interactions between the polar jet stream and the subtropical jet stream determine a large portion of the weather in the continental US For instance, California tends to get a very wet winter in an El Niño year. 2015/16 was one of the strongest El Niño events since 1997/98, and was in part responsible for the freakish warmth of December 2015.
Global warming is already having measurable effects on our weather
Global warming is causing more intense rain and snowstorms in the United States, and making extreme events such as the January 2016 snow storm that crippled most of the Mid-Atlantic and Northeast more likely. Studies also find that it made the extreme amount of rain falling over the Houston area during hurricane Harvey more likely.
Global warming means hotter air, and hotter air can hold more moisture. This translates into heavier precipitation in the form of more intense rain or snow, simply because more moisture is available to storms. Therefore, less of a region’s precipitation is likely to fall in light storms and more of it in heavy storms.
In the US, the amount of rain or snow falling in the heaviest one percent of storms has increased nationally over the last half century—with the largest increases in the Northeast, Great Plains, Midwest, and Southeast. The Third National Climate Assessment shows that some regions of the country have seen as much as a 71 percent increase in the amount of rain or snow falling in the heaviest storms between 1958 and 2012.
The 2017 Climate Science Special Report adds that “Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901 (high confidence). There are important regional differences in trends, with the largest increases occurring in the northeastern United States (high confidence).”
Global warming is also causing warmer spring weather to arrive earlier than it used to. Overall, spring weather is already arriving 10 days earlier than it used to. A recent study estimated that the median onset of plant growth in spring will happen three weeks earlier over the next century, as a result of rising global temperatures. In California's Sierra Nevada, the onset of spring has already been happening three weeks earlier than historical records, with an immense amount of variability—meaning it is hard to plan for the average earlier spring onset.
We can slow global warming
The good news is that we have the technology and practical solutions at hand to effectively address global warming.
A crucial part of that effort is to significantly reduce the amount of heat-trapping emissions we are putting into the atmosphere.
As individuals, we can help by taking action to reduce our personal carbon emissions. But to fully address the threat of global warming, we must demand action from our elected leaders to support and implement a comprehensive set of climate solutions.