Harvey, Rainfall, and Climate Change

What We Know

It may be months, if ever, before a formal modeling study is conducted to quantify exactly how much global warming fueled the unprecedented rainfall that drove the historic floods in Houston.

However, we already know quite a lot about how climate change is currently driving trends in extreme precipitation and affecting tropical storms generally. By looking at the trends behind Harvey and examining the simple physics involved, a pretty clear picture emerges — and it is extremely disturbing. One preliminary expert estimate puts the contribution of global warming to the record rainfall at up to 30%.

Our weather is now profoundly different. Record-breaking rainfall is a classic signature of climate change. And global warming is now helping to fuel deluges around the world. The fingerprint of climate change has been firmly identified in the observed global trend of increasing extreme rainfall. Warmer seas are giving up more moisture to the atmosphere above, and our warmer atmosphere is now holding more moisture. Much as a bigger bucket dumps more water when emptied, our atmosphere is dumping more rainfall when it rains.

The approach of looking at trends and physics to identify the role of climate change in extreme events is called statistical analysis of observation, and this approach has been validated by the National Academies of Sciences for attribution study when formal modeling results are not available.[1]

Following that approach, here is a run-down of what we know about the physics and trends behind Harvey:

Climate change is fueling extreme rainfall and dramatically increasing rainfall across many types of storms.[2][3]

One of the clearest changes in weather globally is the increasing frequency of heavy rain.[4] Over the past century the US has witnessed a 20% increase in the amount of precipitation falling in the heaviest downpours. Nationwide in the U.S., there has been a 40% increase in number of five-year rain events, extreme events that have only a 20% chance of occurring in any given year.[5]

Over the period from 1994–2008, extreme precipitation events linked to hurricanes accounted for more than 33% of the observed increase in heavy events across the US.[6] Estimates of the increased rainfall have been calculated for individual hurricanes such as Katrina,[7] and an increase in rainfall rates is one of the most confident predictions of the effects of climate change on tropical cyclones.[8]

A climate change attribution study on last-year’s tropical depression that delivered 1,000-year rains and drove historic floods in Louisiana found that climate change most likely doubled the odds for that event to occur.[9]

As the global average temperature increases, so too does the ability of the atmosphere to hold and dump more water when it rains.[10] Atmospheric water vapor has been increasing.[11][12] The observed increases in water vapor and extreme precipitation have been studied and the fingerprint of global warming in these trends had been formally identified.[13][14][15]

Over the last three decades, extreme and record-breaking rainfall events have significantly increased globally, and the fingerprint of global warming has been firmly documented in this pattern. Climate change has been to now be responsible for 17 percent of moderate extreme rainfall events, i.e. 1-in-3 year events, while a much larger share of very extreme events is attributed to warming, as climate change affects the frequency of the very extreme events the most.[16][17] An April 2017 study found that from 1961 to 2010, global warming increased the likelihood of occurrence of the wettest five-day periods on record in 41% of the observed areas globally.[18]

“…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”

— Fischer and Knutti, 2015

Storms reach out and gather water vapor over regions that are 10–25 times as large as the precipitation area, thus multiplying the effect of increased atmospheric moisture.[19] As water vapor condenses to form clouds and rain, the conversion releases heat that adds buoyancy to the air and further fuels the storm.[20] This increases the gathering of moisture into storm clouds and further intensifies precipitation.[21]

Inland cities near large rivers in the US now experience more flooding, and this shift is consistent with climate change.[22]

Houston

Since the 1950s, Houston has seen a 167% increase in the frequency of the most intense downpours.[23]

During the Harvey deluge, many areas of Southeast Texas witnessed 1,000-year rainfall (i.e. rainfall having a 0.1% chance of occurring in that year in that location), according to the Space Science and Engineering Center at the University of Wisconsin at Madison.[24]

Hurricane Harvey drove Houston’s third ‘500-year’ flood in 3 years.[25]

In the U.S southern climatic region (which extends from Mississippi through Texas) the number of daily heavy precipitation events has increased by 25% over the long-term average, and tropical cyclones contributed 48% of that increase. [While there has been a recent increase in the number of landfalling U.S. hurricanes, the increase in tropical cyclone associated heavy events is much higher than would be expected from the pre-1994 association between the two, indicating that the upward trend in heavy precipitation events is due to an increase in the number of heavy precipitation events per system, not just more landfalling cyclones.] Much of the Gulf and Atlantic coastal areas have experienced an increase in the frequency of such events associated with tropical cyclones.[26]

Harvey in particular was sustained by sea surface temperatures up to 2.7–7.2°F above average.[27] Last winter, the daily surface temperature of the Gulf of Mexico never dropped below 73°F for the first time on record.[28] As seas warm, more water evaporates to the atmosphere, and in turn a warmer atmosphere can hold more water, fueling extreme rainfall and increasing flood risk.

Kevin Trenberth, a senior scientist at the U.S. National Center for Atmospheric Research and one of the world’s leading experts on how climate change affects precipitation has offered:

“The human contribution can be up to 30 percent or so of the total rainfall coming out of the storm.” [29]

Another major contributor to the extreme rainfall totals was the stalling pattern that locked Harvey in place for several days over southeast Texas. Waves in the jet stream can stall in place (instead of moving eastward), leading to blocking and persistent weather patterns that fuel the intensity and duration of rainfall events. A pair of recent studies have documented the role of climate change driving these blocking patterns, and the study authors have pointed to Houston as an event fitting this pattern.[30] [31] One author, Michael Mann, noted that the stalled weather pattern during Hurricane Harvey “is precisely the sort of pattern we expect because of climate change.”[32]

All of the available evidence, and there is plenty of it, clearly points to a very strong role for climate change, driven by carbon pollution, in helping to fuel the unprecedented rainfall delivered by Hurricane Harvey. When, and if, a formal modeling study is done, we will learn more then.

But, this is what we know now.