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Don Paul: Is there a connection to extreme-weather events and our warming climate?

My old standby, and that of thousands of meteorologists, used to be you really couldn’t connect most extreme individual weather events to our mean warming climate. In the aggregate, there was growing evidence of such a broad link, but tying the warming to a singular event was a much greater challenge.

That is changing. In my own case, the failure to see such a connection in the cases of Hurricanes Harvey and Florence was due to my own lapse in putting two and two together, as I pointed out in a September article.

Now, a new study published by the American Meteorological Society lends more evidence to the links between singular events and the climate.

Note the acceleration in the last three decades, which is exactly what 1980s climate models were predicting. The warming is greatest in the Arctic due to the loss of sea ice for longer periods, allowing dark blue waters in the ice’s place to absorb more solar energy. That is a very rapid change in the reflectivity versus absorption of solar input at those latitudes.

One of the analytical methods used by scientists is to examine what would have been the frequency of weather extremes in the absence of the accelerated warmth that has occurred. The frequency, minus the warming, is far lower for extreme droughts, flooding rains and heat waves. This is accomplished by model simulations that remove the warming and bring the climate back to that of the ’70s. As I’ve also written in past articles, model simulations run with CO2 levels found around the year 1900 show the world would have been slightly cooling in the mean this past 20th century and this century without the added greenhouse gases from human activity.

Warmer air means more water vapor enters into the system. For every 1 degree Celsius, which is the mean warming that has occurred since 1880, the air can hold an extra 7 percent water vapor. That is enough to produce more prolonged deluges than used to occur. In 2017, consecutive flooding downpours nearly led to what would have been a calamitous collapse of the Oroville Dam in California.

The year 2017 also brought the most extreme rainfall event in continental U.S. history, Hurricane Harvey.

Much of the well-forecast rainfall extreme came due to a stalling of Harvey caught in a weakened jet stream. The weaker jet is believed to be directly linked to Arctic warming. Less thermal contrast between the mid-latitudes and the high latitudes slows the jet stream flow. This is what models caught on to in their production of such an accurate forecast of disaster. Our meteorological models now incorporate some of the warming predicted by climate models (weather and climate models are quite dissimilar), making them more accurate.

In 2017, severe flooding also occurred in Bangladesh, China and Peru. The Bangladesh flooding was linked to a powerful monsoonal flow occurring six days early. The authors of the AMS report state human activity-caused warming was 100 percent responsible for the rare early Bangladesh disaster. Personally, I have never seen such high confidence expressed in a journal study by accomplished scientists.

Both the summers of 2017 and 2018 brought several extraordinary and deadly heat waves to different parts of the globe including, for example, the hottest San Francisco temperature ever recorded in 2017. "I’m virtually certain that nearly all heat waves have been made more severe by climate change," Michael Wehner, a senior scientist at the U.S. Department of Energy's Lawrence Berkeley National Laboratory, said this summer amid a flurry of global heat waves.

In 2017, a deadly heat wave in southern Europe brought record triple digit highs for days to the Balkans and Italy, and debilitating nighttime lows in the upper 80s. The authors calculate such extreme heat waves are now three times more likely to occur than in the 1950s. Similar extreme heat is now more likely even in northeast China than it had been in the 1950s.

Again, the stalled inland tropical cyclones, extreme rainfall events and more severe heat waves do have a tie-in to the decline in Arctic sea ice, as mentioned previously. The well-forecast arctic warming is now likely the cause for more common atmospheric blocking of systems like Harvey that normally move along, due to the weakened jet stream from the warming. And, the warming is giving such systems significantly more water vapor to work with, from more evaporation.

Finally, the increased loss of acreage to wildfires has multiple causes beyond warming. However, within the multiple causes, there is no doubt the warming is heating and drying fire fuels and making fires worse.

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