|Above: Local apartment residents cross high water on North Braeswood Blvd. to escape the flooding from Hurricane Harvey August 28, 2017 in Houston, Texas. Image credit: Erich Schlegel/Getty Images.
Hurricane Harvey of August 2017 brought the greatest rainfall event ever recorded in the U.S. by a tropical cyclone—an astonishing 60.58” in Nederland, Texas. The resulting flood disaster was the second costliest weather-related disaster in U.S. and world history--$125 billion, according to NOAA. Naturally, this gave rise to questions about whether the rains were made worse by global warming, and how we might expect tropical cyclone rainfall to change in a warming climate.
One of the more confident predictions hurricane scientists can make on the future of hurricanes in a warmer climate is that they will dump heavier rains, due to increased moisture in the atmosphere. There is a growing body of literature showing that heavy precipitation events of all kinds—including those from tropical cyclones (which include all hurricanes, tropical storms, and tropical depressions)—have already grown more common. Four papers in the past year have been published that found that human-caused global warming significantly increased the odds of heavy rains such as those Hurricane Harvey brought to Texas.
In this post, we take a comprehensive look at what the published peer-reviewed science says on the expected increase in heavy rains from tropical cyclones in a warmer world.
Little research has been done on observed tropical cyclone precipitation changes
A 2015 assessment by eleven hurricane scientists, Tropical cyclones and climate change, concluded that for the globe, “a detectable change in tropical cyclone-related rainfall has not been established by existing studies.” However, that paper explained that there have not been any global studies looking at the issue, and did not cite the three U.S.-based studies on the issue that had been done before 2015:
Groisman et al. (2004), Contemporary Changes of the Hydrological Cycle over the Contiguous United States: Trends Derived from In Situ Observation, found that the total number of daily rainfall events exceeding two inches associated with tropical cyclones in the Southeast U.S. from 1900 – 2000 did not change significantly.
Knight and Davis (2007), Climatology of Tropical Cyclone Rainfall in the Southeastern United States, found that between 1980 and 2004, tropical cyclones in the Southeast U.S. tended to grow wetter, with 11 of the 84 stations analyzed showing statistically significant increases in tropical cyclone rainfall. No stations had significant decreases. They also found that the number of tropical cyclones producing precipitation--and thus tropical cyclone rainfall amounts--had increased over much of the Southeast U.S. from 1980 to 2004. They found, though, that the increase in frequency of landfalling storms was a more important factor in the increase in tropical cyclone rainfall, rather than the fact that individual storms have tended to be wetter.
Kunkel et al. (2010), Recent increases in U.S. heavy precipitation associated with tropical cyclones, found that the number of Southeast U.S. tropical cyclone heavy precipitation events, defined as 1-in-5-year events, more than doubled between 1994 and 2008, compared to the long-term average from 1895 to 2008.
|Figure 1. Change in return period from 1950 to 2016 for a 7-day precipitation amount of 12.43” in Southeast Texas (the previous record before Hurricane Harvey), for the region extending from about 50 miles southwest of Galveston to 20 miles northeast of Port Arthur; 66% (dark band) and 90% (light band) conﬁdence intervals are included. The computation was done using an extreme value analysis with historical rainfall data. What was approximately a 1-in-100 year event in 1950 had shifted to be about a 1-in-25 year event by 2016. Image credit: Risser and Wehner (2017), Attributable Human‐Induced Changes in the Likelihood and Magnitude of the Observed Extreme Precipitation during Hurricane Harvey, Geophysical Research Letters 44, 12,457–12,464, https://doi.org/10.1002/2017GL075888
Decreasing return periods of extreme tropical cyclone precipitation events on the Gulf Coast
Since the publication of that 2015 assessment, there have been four studies published on Hurricane Harvey’s rains that found that global warming is likely making extreme hurricane rainfall events more likely along the U.S. Gulf Coast. Two of these studies used only observations, while two were modeling studies:
Wang et al. (2018), Quantitative attribution of climate effects on Hurricane Harvey's extreme rainfall in Texas, found--using the high-resolution (10 km) WRF model--that warming of the atmosphere and ocean since 1980 could have increased Hurricane Harvey’s extreme precipitation by 13% - 37%. They also found that August SST has warmed by about 0.7 °C since 1980, while the temperature of the lower atmosphere (below about 18,000 feet altitude) warmed by 1.4 °C between 1980 and 2017. The study noted that not all of the SST warming in the Gulf has necessarily been due to human-caused global warming, though—natural cycles such as the Atlantic Multidecadal Oscillation may also be involved.
Risser and Wehner (2017), Attributable Human‐Induced Changes in the Likelihood and Magnitude of the Observed Extreme Precipitation during Hurricane Harvey: Using only observational data, they found that human-induced climate change likely increased the chances of the observed 7-day precipitation accumulations during Hurricane Harvey in the most affected areas of Houston by a factor of at least 3.5, and that precipitation accumulations in these areas were likely increased by 38% (lower bound 18%). Their analysis shows that there has been a clear increase in the probability of extreme 7-day precipitation events along the Texas coast since 1950, with a 1-in-100-year event now being more like a 1-in-25-year event.
Emanuel (2017), Assessing the present and future probability of Hurricane Harvey’s rainfall, found that 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. Dr. Emanuel used a very high-resolution, specialized computational hurricane model embedded within six different state-of-the-art climate models that generated 3,700 predictions of hurricane tracks and rainfall amounts for the projected climate 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. See my November 2017 blog post for a detailed description of the paper.
van Oldenborgh et al., 2017, Attribution of extreme rainfall from Hurricane Harvey, August 2017, found that observations since 1880 over the region show a clear positive trend in the intensity of 3-day extreme precipitation events of between 12% and 22%--roughly two times the increase of the moisture-holding capacity of the atmosphere expected for 1 °C warming according to the Clausius–Clapeyron (CC) relation. Extrapolating these results to the 2017 event, they concluded that global warming made the precipitation about 15% (8%-19%) more intense, or equivalently made such an event three (1.5 - 5) times more likely.
While not focused specifically on Hurricane Harvey, van der Wiel et al., 2016, Rapid attribution of the August 2016 flood-inducing extreme precipitation in south Louisiana to climate change, examined the warm-cored low from August 2016 that caused catastrophic flooding in Louisiana. The authors concluded that anthropogenic climate change had made 3-day extreme precipitation events along the central Gulf coast 1.4 times more likely in 2016 compared to 1900.
NHC Director Ken Graham, @NWSWPC Director David Novak, and @NWSLakeCharles Meteorologist-In-Charge Andy Patrick inspect the rain gauge near Nederland, Texas, that recorded the U.S tropical cyclone rainfall record of 60.58 inches during Hurricane #Harvey. pic.twitter.com/Y8ZRhIgiV2— Natl Hurricane Ctr (@NWSNHC) May 8, 2018
Theoretical and modeling work predicts more rainfall in a warmer world
One of the more confident predictions we can make for hurricanes in the future is that they will dump more rain. 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 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.
The 2015 assessment by eleven hurricane scientists I mentioned above concluded that typical model results show tropical cyclone rainfall increasing by 20% within 100 km of the center by the end of the century, with a range of +3% to +37% between the various studies.
The most recent modeling study of hurricane rainfall was done by Ethan Gutmann of the U.S. National Center for Atmospheric Research (NCAR): Changes in Hurricanes from a 13-Yr Convection-Permitting Pseudo-Global Warming Simulation, published on May 1, 2018 in Journal of Climate. This study looked at 22 Atlantic hurricanes that occurred during the period 2001 – 2013, including Wilma, Rita, Sandy, Isaac, Ike, Ivan, Isabel and Gustav. The high-resolution WRF model (4 km grid spacing) was run for each storm using the current climate, and then for a climate approximately 4.5 degrees C (8 degrees F) hotter--the amount of warming expected by end of century if no attempt is made to control greenhouse gas emissions (scenario RCP 8.5). As a group, the 22 storms in the future simulation had 6 percent stronger average hourly maximum wind speeds than those in the past, moved at a 9 percent slower speed, and had a 24 percent higher average hourly maximum rainfall rate.
Kevin Trenberth, a senior scientist at NCAR, wrote a 2007 paper, “Water and energy budgets of hurricanes: Case studies of Ivan and Katrina”, which found that the precipitation from Hurricane Katrina (2005) increased by about 19% per 1°C increase in sea surface temperature (SST) in the computer model used, and that human-caused warming of the climate between 1970 – 2005 likely led to a 6 – 8% increase in hurricane rainfall.
|Figure 2. Departure of ocean heat content (OHC) from average, calculated by month (black) and year (red) for (a) the top 2,000 m for the global ocean and (b) the top 160 m in the Gulf of Mexico, in 108 J m−2. (c) The departure of sea surface temperature from average in the Gulf of Mexico. The last month is October 2017 and the last red dot is for January to October 2017; the baseline is 1961–1990. Record-high OHC levels were observed in the Gulf of Mexico in the summer of 2017, prior to the arrival of Hurricane Harvey. This heat energy contributed to Hurricane Harvey’s record rains. Image credit: Trenberth et al., 2018, Hurricane Harvey Links to Ocean Heat Content and Climate Change Adaptation. The paper concluded: “The bottom line is that the total observed OHC change is remarkably compatible with the total energy released by precipitation and, unsurprisingly, reflects strong energy exchanges during the hurricane. Accordingly, the record high OHC values not only increased the latent heat which fueled the storm itself, likely increasing its size and intensity, but also likely contributed substantially to the flooding caused by rainfall on land. The implication is that if the OHC had been less, then the rainfall amounts would also have been less.”
More rainfall due to more stalled tracks may occur
Hurricane Harvey's record rainfall amounts were primarily due to the fact that the storm stalled out and performed a loop over the coast for several days because of a collapse of the storm’s steering currents. There is not much research studying if a collapse of steering currents over land might be expected to change in the future, but the question is an important one. One study that did look at the issue was Wang et al. (2018), Quantitative attribution of climate effects on Hurricane Harvey's extreme rainfall in Texas. Using a global climate model that tracked Harvey-like stalled systems, the researchers found a predicted increase in the number and intensity of stalled tropical cyclones over southeast Texas through the mid-21st century. They cautioned, though, that the results may be different running other models.
A study from 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.”
Two recent papers have both found a shift in hurricane tracks away from the Caribbean to the northeast, though, which brings up the possibility that fewer such storms may make landfall in the U.S. in the future. 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).
|Figure 3. Example of a drought-busting tropical storm. Moderate drought (Palmer Drought Severity Index, PDSI, ≤ –2.0) was present in 52 percent of the Florida, Georgia, South Carolina, and North Carolina climate divisions in May 2006. The percentage decreased to 29 percent after Tropical Storm Alberto passed through on June 11 - 15, 2006, bringing widespread rainfall amounts of 3 – 5” to the region. Image credit: U.S. Drought Monitor.
The beneficial effects of increased tropical cyclone rainfall
Rains from tropical cyclones make up an important part of the annual precipitation budget of some regions of the globe, and heavy rains from landfalling tropical cyclones are often a key factor in busting high-impact droughts. For example, tropical cyclones account for approximately 20% of the observed monthly rainfall from June to November across the eastern U.S. Corn Belt (Wisconsin, Michigan, Illinois, Indiana, Ohio and Kentucky), and up to 15% of the precipitation in South Carolina and North Carolina. And Soule et al. (2012) found that tropical cyclones in the Southeast U.S. frequently “bust” droughts, with the majority of counties in Florida, Georgia, South Carolina and North Carolina seeing at least 20% of their droughts ended by a tropical cyclone between 1950 and 2008.
Super Clausius-Clapeyron scaling
The model-predicted increase of precipitation of about 20% expected in hurricane cores by the end of the century is higher than one would expect from Clausius-Clapeyron scaling (a 7% increase in atmospheric moisture per degree Centigrade of ocean warming). This result was also found in several of the Hurricane Harvey studies mentioned above—e.g., van Oldenborgh et al., 2017, Attribution of extreme rainfall from Hurricane Harvey, August 2017. There is also data from the Netherlands (Lenderink et al., 2017) showing that in the summer, we can expect to see an increase in hourly precipitation extremes (greater than the 99th percentile) of up to 14% per degree Centigrade of warming; at colder temperatures, the increase is about as one would expect from the Clausius–Clapeyron equation. The authors argued that the "super Clausius-Clapeyron scaling" of 14% more hourly rainfall per degree Centigrade of warming in extreme events in summer happens because of the dynamics of convective (thunderstorm) clouds--the extra moisture causes more rainfall formation and more latent heat release when the water vapor condenses into liquid, which forces stronger updrafts, invigorating the thunderstorm, and potentially leading to a stronger rate of condensation and even more rainfall formation.
Though there is still much work to be done quantifying the future risk from rainier hurricanes in a warmer world, the four papers published linking the catastrophic floods of Hurricane Harvey to global warming should serve as a strong wake-up call that we may already be experiencing a heightened risk of such disasters due to human-caused climate change. It doesn’t take much of a shift in the peak of a bell-shaped curve encompassing the range of precipitation amounts we expect from future storms to cause very significant changes in the odds of low-probability, high impact events at the tail end of the distribution, and the flood defense systems of the world are designed for the rains of the 20th-century climate, not the new more extreme rainfalls we are already observing in the warming 21st-century climate.
The conclusion of van Oldenborgh et al. 2017, Attribution of extreme rainfall from Hurricane Harvey, August 2017, is worth paying attention to: “And while fortifying Houston to fully withstand the impact of an event as extreme as Hurricane Harvey may not be economically feasible, it is critical that information regarding the increasing risk of extreme rainfall events in general should be part of the discussion about future improvements to Houston's flood protection system.”
Hurricane scientists Kerry Emanuel, Jim Kossin, Michael Mann and Stephan Rahmstorf wrote an excellent May 30, 2018 realclimate.org post, Does global warming make tropical cyclones stronger?
Will Global Warming Create Larger Hurricanes? (my April 2018 post)
Climatesignals.org has a Hurricane Harvey page summarizing the research linking heavier tropical cyclone rainfall to a warming climate.
Observed Slowdown in Tropical Cyclone Motion May Portend More Harvey-Like Rainstorms (my June 7, 2018 post)