Climate Change and Deadly Floods: What you need to know

With extreme snowstorms in the northeastern U.S. and extreme rain in Texas, and the southeast, the obvious question arises… are these events linked to climate change?

New research shows us the answer is a resounding ‘yes’.  It actually makes perfect sense.  Carbon dioxide traps heat in the atmosphere, and a warmer atmosphere holds more moisture.

When you combine warmer, moist air masses with disruption of the Jet Stream cased by a warming Arctic, you get huge, slow-moving weather fronts packed with moisture.  This is the exact recipe for epic, extreme storms.

As an example, on Memorial Day in 2015, a massive storm parked over southern Texas and dropped nearly 11 inches of rain on the Houston metro area in less than 24 hours, with devastating results.


Houston rain totals for Memorial Day 2015
24 Hour Rain Totals for the Houston Metro Area, Memorial Day, 24 May, 2015.  (From the


Downtown Houston was flooded, 23 counties were declared disaster areas, and at least 23 people lost their lives.  In one town to the north of the city, a whole family and their house were swept away in the middle of the night by a flash flood.  Tragic.  Real human consequences of extreme precipitation that, as you will see, are linked to climate change.


kids with grandparent in kayak on flooded street
Grandfather and two grandkids kayaking through a flooded Houston neighborhood. This image and cover image from the Houston Chronicle,


Of course, any single storm, no matter how epic, may not necessarily be due to climate change.  But, a great analogy by scientists at the National Center for Atmospheric Research in Boulder, Colorado, makes the point that steroids are to baseball, as global warming is to extreme weather; they don’t guarantee home runs — or epic storms — they just make them more likely.



Heavy Downpours Increasing Worldwide

Measurement of rainfall over the years shows us that the number of extreme rain storms is increasing across the U.S.

One measure of heavy precipitation events is a two-day precipitation total that is exceeded on average only once in a 5-year period, also known as the once-in-five-year event. As this extreme precipitation index for 1901-2012 shows, the occurrence of such events has become much more common in recent decades. Changes are compared to the period 1901-1960, and do not include Alaska or Hawai‘i. (Figure source: adapted from Kunkel et al. 2013)(7).


And, it’s not just the U.S. where epic rain and snow storms are happening, but all over the world.  A 2015 study, titled Increased record-breaking precipitation events under global warming“, by Jascha Lehmann, Dim Coumou, and Katja Frielerby, in the journal Climactic Change (July 2015), offers more direct evidence of climate change.  According to this work, intense precipitation events have increased 12% worldwide since the 1980s; and, have increased by whopping 56% in Southeast Asia.

From the official press release for this study: “An advanced statistical analysis of rainfall data from the years 1901 to 2010 derived from thousands of weather stations around the globe shows that over 1980-2010 there were 12 percent more of these events than expected in a stationary climate, a scenario without global warming.”

As a side note, lots of interesting things started happening to the global climate in the 1980s, including an unprecedented jump to more frequent, large forest fires.  That was also about the time that NASA’s Dr. James Hansen first testified before Congress about the dangers of climate change, and when tackling climate change was still a bi-partisan issue.

That is, before the oil companies started spending millions of dollars on what ended up being a successful campaign to misinform the public.  The same tactics used by tobacco corporations to deny a link between smoking and cancer.


What does the future hold?

In another study, titled Anthropogenic contribution to global occurrence of heavy-precipitation and high-temperature extremes, by E.M. Fischer and R. Knutti, in the journal Nature Climate Change, published 27 April 2015, the authors tried to figure out the impact of global warming on the frequency and intensity of extreme precipitation and heat waves, and also what part of that impact can be linked to human activity.

To do this, they compared the results of 25 different climate models to see what conditions would have been like if we had never burned any fossil fuels.  They then compared that to real events caused by the increase in global temperature of 0.85° C (1.53° F) that has occurred since the beginning of the Industrial Revolution.  The Industrial Revolution started about 175 years ago, and marks the time when us humans really started burning fossil fuels and pumping CO2 into the atmosphere in a big way.

The 25 models were also run into the future to see what happens if the temperature goes up by 2° C (3.6° F) and beyond.  Two degrees centigrade is the accepted value that we need to stay under to prevent catastrophic climate change, although many scientists believe that number is too high.

Here’s a summary of what they found…

  • For warming that’s already happened (0.85°C / 1.53°F)
    • 18% of the most extreme precipitation events are caused by global warming, and…
    • 75% of extreme heat events are caused by global warming
  • For warming that is likely to happen (2°C / 3.6°F)
    • 40% of extreme precipitation events will be due to global warming, and…
    • As the temperature rises, there will be an exponential increase in extreme heat events due to global warming


Figure 3 (below) from the paper, summarizes the change in precipitation (top) and heat events (bottom) due to climate change.  The left two panels show the present day chance for extreme precipitation (top) and extreme heat (bottom) caused by the warming that has already happened (0.85°C / 1.53°F).  The middle panels show a world that has warmed by 2°C (3.6°F), which is likely to happen by the year 2050, and the right side shows a world warmed by 3° C (5.4°F, which may happen by 2100.


Global map of extreme precipitation and heat


The color gradient on the maps represent the probability ratio (PR).  The PR shows how much the probability of an event (extreme rain or heat) changes with increasing temperature.  A PR of 1.0 means no change; the chance of extreme precipitation or heat is the same as it not happening.  However, a PR of 2.0 means that extreme weather in a warming world is twice as likely as the chance of no extremes happening.

As you can see, as the world warms to 2°C (3.6°F) we are in for a rough ride; while warming to 3° C (5.4°F) will be a nightmare scenario.  Under those conditions large parts of the Earth’s equatorial region may become uninhabitable during the summer.


Are we responsible?

One of the unique things about this study is that they borrowed a concept from epidemiology and public health called the Fraction of Attributable Risk (FAR).  (The next section gets into a bit of epidemiology, skip ahead if that’s not your cup of tea!)

The Fraction of Attributable Risk as a formula….

FAR = 1 – (X0 / X1)

  • 1 represents all events (100%) that actually happen
  • X0 is the probability of an extreme weather NOT happening, and
  • X1 is the probability of it happening.

So, If the chance for extreme weather is the same in a warming world as it was in a world with no fossil fuel burning, then the same number divided by itself is one, and one minus one is 0; meaning that no extreme events (0%) can be attributed to fossil fuel burning.  (  FAR = 1 – (1/1) = 0 )

However, if there is MORE extreme weather with the burning of fossil fuels, then the second part of the equation is less than one, and one minus less than one gives you THE number that tells you what fraction of all extreme events can be attributed to fossil fuel burning.

The closer the number is to 1 (100%), the more events can be attributed to human activity.  For example, if the probability for extreme events without warming is 50%, and the probability of extreme events with warming is 75%, then the part that can be attributed to human activity is 33%  ( FAR = 1 – (0.5/.75) = 0.33 or 33.3% ).

Keep in mind, we are talking about the most extreme events.  For example, think about the worst heat wave you have ever experienced, then think of that kind of heat happening more days than not, during the summer in a warming world.

I think the authors of the paper sum it up nicely…

“In a broader context, the approach here is reminiscent of medical studies, where it is not possible to attribute a single fatality from lung cancer to smoking.  Instead, a comparison of the lung-cancer-related mortality rate in smokers with the rate in non-smokers may allow attribution of the excess mortality to smoking.  Likewise, no single weather event exclusively results from anthropogenic (human) influence…but arises from complex interactions…. Warmer temperatures may influence some of those factors and favour extreme events to happen, and thus the probability of such events increases because of warming.”

It’s important to note that in this analogy we humans are the pathogen that is causing the disease;  by burning fossil fuels that release excess CO2, we are causing global warming and climate change — an interesting thought to ponder.


Here’s another key figure from this study…

The two graphs at the top of this figure show the probability ratio (PR) for extreme rain (left) and heat (right), for the world as a whole. The bottom part is the fraction of those extreme events that are caused by us humans (the FAR). Black line shows current global warming, red line shows future warming.


As discussed above, a FAR of 0 means that no extreme events can be attributed to humans, while a FAR of 1.0 (100%) means that ALL extreme events can be attributed to humans.  The collection of blue lines represent all the model runs that looked at extreme events that were greater than the historical 99th percentile, while the red lines are all the model runs looking at events exceeding the 99.9th percentile.

What this graph shows is that as the temperature rises, more and more of the overall extreme precipitation and heat will be due to us humans warming the atmosphere by the burning of fossil fuels.  What’s more, this graph also shows that we are already having a major impact on extremes of precipitation and heat.

This means that people are dying TODAY from the impacts of climate change, and as things get warmer, more and more people will suffer.  Also, heat waves are one of the things that affect people living in wealthy, developed nations just the same as those living in poor, developing nations.


What are the health consequences and what can we do about it?

Extreme precipitation causes flooding that can obviously cause drowning or injury, but there are also many health risks after the water goes away.  Flood waters contain a mix of sewage, petroleum, chemicals, and other nasty things.  After the water recedes, the muck that’s left is toxic.  Drinking water sources can also be contaminated with toxins and bacteria.  Mold grows in the walls of previously flooded buildings.  And of course there is massive property damage, a huge increase in insurance claims, and massive government intervention.

In a world where extreme weather events are happening more often, the costs of these sorts of events are growing ever larger.  Can individuals, businesses, and governments continue to function normally?  That remains to be seen.  Hopefully, we will have the collective foresight to stop the worst from happening, and be ready for it when it does.

Read in-depth about –>  the health impacts of flooding, how we can be prepared for it, and what to do if it happens to us.



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