Central Plains and Ohio Valley see dangerously hot and humid conditions

A high pressure system of heat that had been building in the central US for weeks, kicked-off on Wednesday, July 20, in Minnesota with scorching temperatures and “real feel” heat index values well over 100°F in the Twin Cities and much of the state.[1] By Thursday, July 21, heat indices were well above 100 degrees in large portions of the country.

In total, eight heat-related deaths have been reported,[2][3][4] and 122 million Americans in 26 states were under heat alerts, warning and advisories on Friday.[5] At one point Friday morning, the three largest metro areas in the United States—New York, Los Angeles and Chicago—were all under heat alerts for the first time in at least a decade.[5]

Observed changes in heat extremes

Due to global warming, the most extreme heat events now impact a global area 10 times greater than in the period 1951-1980.[6]

The impact on moderate heat waves is also dramatic, with a seventy-five percent share of moderate heat events now attributed to climate change.[7]

The connection between climate change and extreme heat events is well understood and well documented, reports the US National Academy of Sciences.[8] The Intergovernmental Panel on Climate Change (IPCC) reports that: “Overall, the most robust global changes in climate extremes are seen in measures of daily temperature, including to some extent, heat waves.”[9]

Global warming is the most obvious, well-documented effect of climate change. As a result, the signal is very strong so we can more easily detect it amongst noise of natural variability compared to other types of extreme events. - Stephanie Herring, NOAA[10]

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.[10][11]

Heat waves have generally become more frequent and intense 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.[12] There has also been a dramatic increase in nighttime temperatures in the US, reducing the number of critically important relief windows during heat waves.[12] These changes are consistent with the changes projected by models of a climate warmed by greenhouse gas emissions.[12]

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.[13] The northern hemisphere is tending toward increasingly warmer and more humid summers, and the global area covered by extreme water vapor is increasing significantly.[14]

The current heat wave is caused by a high pressure weather system that typically brings warm and dry conditions to the surface (as explained below), yet several factors are combining with climate change to help increase humidity levels over the Central Plains and Ohio Valley: recent rains, plant evapotranspiration, and warm and humid air from the Gulf of Mexico. These moisture sources combine with the additional moisture carrying capacity of a warmer atmosphere to raise humidity levels.

The rains are a mixed blessing, according to Bob Henson at Weather Underground, because they are helping to "keep surface air temperatures a notch lower than they’d otherwise be," while also "pump[ing] up the amount of water vapor in the air."[15]

In addition, the Midwest’s vast corn crops are at a stage where they add moisture to the air through evapotranspiration, which will likely increase heat index values to uncomfortable and dangerous levels over large parts of the central and eastern US.[15][16]

Humidity levels over the Central US are also boosted by moist winds coming off the Gulf coast[17] where sea surface temperatures are running hot, bumped up by global warming. On July 20, sea surface temperatures off the Texas coast were 1.6°F to 2.7°F (1°C to 1.5°C) above the 1961-1990 average.[18]

Intensification of US hot extremes linked to high-intensity heat domes

This heat wave is associated with a dome of hot air under high pressure that is straddling the central United States. High-intensity heat domes such as witnessed in this event are rare but have recently become more frequent, especially in the West. Looking through records back to 1958, researchers have found that almost all of the high-intensity heat domes in the West have occurred since 1983—with the overwhelming majority of them occurring since 1990.[19] Data extending back to 1979 illustrates that high-pressure areas in the form of larger ridges and domes have become more common across the northern hemisphere and globally as well.[14]

These heat domes are associated with anticyclones, winds that rotate around a center of high atmospheric pressure. Air at the center of an anticyclone is forced away from the high pressure and replaced in the center by a downward draft of air from higher altitudes. As this air moves downward, it is compressed and warmed. Warm and dry conditions are typically associated with anticyclonic weather patterns.

Looking forward, the projected intensification of hot extremes in the United States over the next three decades is associated with a shift toward anomalously dry summer conditions at the surface and "anomalously anticyclonic circulation in the mid-troposphere."[20][21] Recent extreme heat events associated with large-scale anticyclonic atmospheric anomalies, including the 2003 European heat wave and the 20th-century Sahel drought.[20]

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. Global warming has already driven 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 found 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.[6][14][22]

...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.[7]