Natural Variability and Extreme Weather

Natural Variability and Extreme Weather

Dried ground leading to a shrinking water source

Extremes in weather are nothing new. Weather varies naturally because of many factors, and those factors can combine to produce an extreme drought or a devastating flood, stronger hurricanes or record snowfalls.

Dec 2010-Feb 2011 Statewide Temperature Ranks

Sometimes, the same general climate pattern can produce drastically different weather conditions in different locations or different years. For example, the unusually cold winter of 2010-2011 in the U.S. was influenced by a La Niña pattern—a quasi-cyclic cooling of the eastern tropical Pacific ocean.

Photo of the Mississippi River overflowing and submerging agricultural fields  in Spring 2011 along the border between Tennessee and southeastern Missouri

It is very likely that this pattern produced an unusually cold spring with heavy precipitation in the Upper Missouri River Basin. Heavy rains in May fell on deep, late-spring snow pack, which caused flooding that devastated parts of several states.

Dec 2011-Feb 2012 Statewide Temperature Ranks

No two El Niño/La Niña-Southern Oscillation events are alike, however. The La Niña pattern persisted through the winter of 2011-2012, although it was somewhat weaker. Surprisingly, that winter was one of the warmest on record in the U.S. A number of factors conspired to produce the two dramatically different results. Part of the difference related to the high latitude pressure patterns (called the Arctic Oscillation) that dominated in the two winters. In the warmer 2011-2012 winter, the pattern tended to trap the cold air near the poles, but why that was the case is still being studied.

Land surface temperature anomalies for a given month (March) compared to the average conditions during that period between 2000-2008. Places that were warmer than average are red, places that were near normal are white, and places that were cooler than average are blue.

Departures of temperature (anomalies) from average 2000-2008 conditions during the month of March.

We have a good understanding that natural modes of climate variability affect the probability and intensity of weather extremes. So how does global warming come into the mix? In some cases it can mitigate extremes. For example, a record cold winter might be a little less cold than it otherwise would have been. However, climate change could also amplify the effects of a natural cycle, producing more severe extremes. So, for example, a natural pattern that produces warmer-than-normal annual conditions could be enhanced by global warming and result in increased night-time temperatures and more frequent heat waves.

Conceptual graphic showing many of the influences on climate

Climate is determined by a complicated interplay of many factors that affect the oceans, land surfaces, and the atmosphere. A change in one of them—such as increasing atmospheric concentrations of greenhouse gases—can ripple throughout the system.

Water drips from an icicle
City snowplows parked in a garage, awaiting a snowstorm

Of course, extreme weather events can have some positive effects. The warm winter of 2011-2012 in the U.S. resulted in lower heating bills, and many communities also saved by not needing snow removal services.

A Red Cross volunteer comforts the sister of fire victim, while friends and volunteers sift through the resident's home searching for valuables.

It is an individual's or community's vulnerability to the adverse effects of extremes that largely determines whether a particular event is a disaster or something beneficial.