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Global Warming Post: Extreme Harvey-Like Rainfall 18 Times More Likely by 2090


Maybe it's not exactly like the movie "Waterworld", but flooding via extreme rainfall, as well as coastal flooding from sea level rise will continue to increase throughout this century.

Extreme Harvey-Like Rains in Texas 6 Times More Likely Today Than 25 Years Ago

Texas rainfall events as intense as that produced by Hurricane Harvey, which had about a 1 percent annual likelihood in the 1990s, had already increased in likelihood to about 6 percent annually in 2017, and by 2090 could be about 18 percent. That’s according to the paper "Assessing the present and future probability of Hurricane Harvey’s rainfall", published on Monday in Proceedings of the National Academy of Sciences by Dr. Kerry Emanuel of MIT, who is widely regarded as the world’s leading hurricane expert. Dr. Emanuel used a very high-resolution, specialized computational hurricane model embedded within six different state-of-the-art climate models to make his assessment. The model generated 3,700 predictions of hurricane tracks and rainfall amounts for the projected climate in at the end of the century, in the years 2081 - 2100. These predicted rainfall amounts were then compared to the rainfall amounts generated by the model for hurricanes simulated during the historical period 1979 – 2015.

Harvey's rains well beyond a 1-in-1000 year event

Using the past century of rainfall data, scientists can estimate the Average Recurrence Interval (ARI) of a given rainfall event. Using this metric, Harvey’s rains were truly biblical in intensity. According to MetStat, Inc., an area of Southeast Texas approximately 100 miles wide experienced 24-hour rainfall amounts that we would expect to occur on average every 1,000+ years (i.e., have a .1% chance of occurring in any given year). Some localized maximum recurrence intervals of 1 in 500,000 (a 0.0002% chance of occurring in any given year) were observed in the northern Houston Metro area based on a USGS statistical analysis of historical rainfall data from the past century. This likely overstates the true rarity of this event, since we are discussing recurrence intervals (> 1,000 years) that are greater than the length of time we have records for (about 100 years.)

Climate change is likely to increase hurricane rainfall

One of the more confident predictions we can make for hurricanes in the future is that they will dump more rain, with typical model results showing tropical cyclone rainfall increasing by 5 – 20% by the end of the century. It is uncertain whether or not we are already seeing this effect, though. Global warming increases the rate at which ocean water evaporates into the air, and increases the amount of water vapor the atmosphere contains when fully saturated. This result is about 6 – 7% more water vapor in saturated air for every 1°C of ocean warming. This increase in atmospheric water vapor can cause a much larger increase in hurricane rainfall than one might surmise, since water vapor retains the extra heat energy required to evaporate the water, and when the water vapor condenses into rain, this “latent heat” is released. The extra heat helps power the hurricane, making it larger and more intense, allowing it to pull in water vapor from an even larger area and thus dump more rain.

Climate change’s impact on making hurricanes stall out is uncertain

The main reason Harvey’s rains were so extreme is that the storm stalled out for several days just inland from the coast, allowing the portion of the storm’s circulation over water to pull in gargantuan amounts of moisture and dump it as rain. There is concern that such stalling behavior may become more common in the future climate. A study from March 2017, "Influence of Anthropogenic Climate Change on Planetary Wave Resonance and Extreme Weather Events", found that climate change is altering large-scale weather patterns, such as the jet stream, causing extreme weather conditions to stay locked in place for extended periods of time. A parallel 2015 study, "Increased record-breaking precipitation events under global warming", found a similar effect. According to one of the authors, Stephan Rahmstorf, “this is a consequence of the disproportionally strong warming in the Arctic; it can make weather systems move less and stay longer in a given location – which can significantly enhance the impacts of rainfall extremes.”

However, no published research has looked at whether or not an increase in stalling behavior in Atlantic hurricane tracks in recent decades can be seen in the historical database. Instead, two recent papers have both found a shift in hurricane tracks away from the Caribbean to the northeast. These studies were "The Impact of Anthropogenic Climate Change on North Atlantic Tropical Cyclone Tracks" (2013), and "Persistent northward North Atlantic tropical cyclone track migration over the past five centuries" (2016.)

Storm-total rainfall from Hurricane Harvey, August 24 – 31, 2017. Harvey dumped over 40” (yellow colors) in Houston, with isolated amounts in excess of 50” (pink colors) south of Houston and northwest of Port Arthur. Image credit: NOAA.

Preliminary maximum 5-day Average Recurrence Interval (ARI) of Hurricane Harvey’s rainfall in Texas from August 25, 2017 through August 30, 2017. An area over 100 miles in diameter (yellow-orange, orange, and magenta colors) experienced 5-day rains that we expect to occur on average every 1,000+ years (i.e., have a .1% chance of occurring in any given year), based on a USGS statistical analysis of historical rainfall data from the past century. Some localized maximum recurrence intervals of over 1 in 500,000 (a 0.0002% chance of occurring in any given year) were observed in the northern Houston Metro area. Note that a 1-in-1000 year rainfall event is not the same as a 1-in-1000 year flood, which depends on more than just rainfall. The black dots represent rain gauges used in the analysis. Source: MetStat, Inc.

All of the information in this post was obtained from a blog post co-written by Dr. Jeff Masters and Bob Henson from the Category 6 blog at wunderground.com


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