Global warming dramatically increases frequency of the most extreme heat events

Extreme heat events, such as this heat wave, are the kind of weather events that increase the most as the climate warms. The more extreme the heat wave, the more likely it is due to the change in the climate. And global warming has led to a dramatic surge in the frequency of the most extreme heat events.

Because of the way global warming shifts the climate, the very most extreme events are the weather events most affected by climate change. As the average global temperature rises and the climate shifts, temperatures that were extreme under the old climate are closer to the middle of the new temperature range. Under the earth's climate system events closer to the midpoint occur much more frequently than events closer to the extremes, as shown in the graphic on the right. Thus a small shift in climate leads to a dramatic increase in the frequency of temperatures at the high end. The shifting bell curve also leads to the occurrence of never-before-seen extremes in high temperatures.[1][2][3]

"...it is the rarest and the most extreme events - and thereby the ones with typically the highest socio-economic impacts - for which the largest fraction is due to human-induced greenhouse gas emissions."[4]

Due to global warming, the most extreme heat events now impact a global area 10 times greater than in the period 1951-1980.[2]

The impact on moderate heat waves is also dramatic, with a seventy-five percent share of moderate heat events now attributed to climate change.[4]

Many urban areas across the globe have witnessed a significant increase in the number of heat waves, with the largest number of heat waves occurring in the most recent decade studied, 2003-2012.[5] 

Heat waves have generally become more frequent across the US in recent decades, with western regions setting records for numbers of these events in the 2000s. Recent multi-month extreme heat events in the US are unprecedented since the start of reliable instrumental records in 1895.  There has also been a dramatic increase in nighttime temperatures in the US, reducing the number of critically important relief windows during heat waves.[6]

"Dangerous" high temperatures reported across the Southwest 

Southwestern states are experiencing record-breaking heat over the extended weekend, with the worst impacts in Arizona and southeastern California.[7] The highs in Phoenix, which the National Weather Service called "very dangerous," approached 120°F, a temperature the city has only seen three times since record-keeping began.[8] 

In Phoenix, nine of the first 22 days of June were 110°F or hotter, including five consecutive days. Periods of days over 110˚ in Phoenix have been on the rise; of the five instances with 10 or more consecutive days at or above 110°F in Phoenix, four have come since 1989. The other was in 1974.[9]

Records tend to be broken when natural variability runs in the same direction as climate change, in this instance towards higher temperatures. New record high temperatures now outnumber record low temperatures by a ratio of 2:1 in the US, a classic signature of climate change.[10]

The NWS estimated that from June 16 through June 22, 32 million people would see temperatures in excess of 100°F, 7 million would see temperatures at or above 110°F, and 159,000, temperatures would see temperatures of 120°F.[11]

Though the southwestern US frequently witnesses high temperatures, climate change is increasing the risk of extreme heat in the region through increased global temperature, increased humidity and elevated nighttime temperatures.[12]

The high pressure system over the Southwest in mid-June 2016 was record-breaking

This particular event was driven by a massive heat dome settling over the southwest that grew to become unprecedented in strength.

Usually the 500mb level is around 18,000 feet (5500m). On June 19, above Arizona, 500mb was reached at above 19,500 feet (almost 6000m), a daily record.[13]

Heat domes of this intensity are extremely rare but have recently become more frequent in the West. Looking through records back to 1958, researchers have found that almost all of the high-intensity heat domes have occurred since 1983 — with the overwhelming majority of them occurring since 1990.[14][3]

Climate change driving humid heat waves, elevating the risk of heat stress

Climate change is amplifying the intensity of extreme heat through increased humidity. A warmer atmosphere can hold more water vapor, and the global atmosphere has become moister due to warming.

The fingerprint of climate change has been found in the increase of wet bulb temperature since 1973, driving heat stress globally and in most land regions analyzed.[15] The northern hemisphere is tending toward increasingly warmer and more humid summers, and the global area covered by extreme water vapor is increasing significantly.[3]

In California, climate change has driven an increase increasingly severe, humid nighttime-accentuated heat waves.  The events are characterized by high atmospheric pressure in the Great Plains and low pressure off California’s coast, drawing warm moist air from the south, particularly the coastal waters west of Baja California. These waters have become unusually warm in recent decades as part of a global warming pattern. This ocean warming fuels the rare great California heat waves that bring warmer, more humid air.[12]

During the initial stages of this event, forecasts projected an above-average amount of moisture in the atmospheric column for the later half of the event, due to airflow from the south, typical of heat waves impacting the Southwest later in the summer, fueled in part by moist air from the south.[16]

Coastal waters in the Eastern Pacific are an important source for this humid air, and these waters have become unusually warm in recent decades as part of a global warming pattern.[12]

This ocean warming has been partially responsible for bringing warmer, more humid air into the weather patterns associated with great California heat waves.[12][17]