Meteorologist Topper Shutt: 'A 500 year flood does not mean that an area will see a flood of that magnitude once in 500 years. It means that in any given year there is a .2% chance of a 500 year flood and likewise a 1% chance every year for a 100 year flood. Think of it like this. If I flip a coin and it lands heads 5 times in a row the 6th time I flip the coin the odds are still 50/50. The odds are always 50/50.
There have been '100 year' floods in Houston in 1929, 1935, Allison in 2001 and Ike in 2008. Folks are probably scratching their heads wondering what in the world is going on. There are several explanations. Firstly, the maps drawn to depict these floods were educated guesses. Remember, we are talking about billions of years of climate and usually just a hundred years of actual, observational data. Secondly, urban development reduces the surface of the ground that allows the rain to permeate into the ground. Adding parking lots, more roads and driveways create more runoff. Thirdly, at least in the case of Houston 1000s of homes have been built close to streams, creeks and bayous that should have never been built in the first place. Houston is the fourth largest city in the U.S.'
'The number of significant trends was about the number expected due to chance alone.'
'Changes in the frequency of major floods are dominated by multidecadal variability.'
'The Intergovernmental Panel on Climate Change (IPCC) concluded (Hartmann et al., 2013) that globally there is no clear and widespread evidence of changes in flood magnitude or frequency in observed flood records.'
'The results of this study, for North America and Europe, provide a firmer foundation and support the conclusion of the IPCC (Hartmann et al., 2013) that compelling evidence for increased flooding at a global scale is lacking.'
Dr. Spencer: "Major floods are difficult to compare throughout history because the ways in which we alter the landscape. For example, as cities like Houston expand over the years, soil is covered up by roads, parking lots, and buildings, with water rapidly draining off rather than soaking into the soil. The population of Houston is now ten times what it was in the 1920s."
Are the rainfall totals unprecedented? "Even that question is difficult to answer. The exact same tropical system moving at, say, 15 mph might have produced the same total amount of rain, but it would have been spread over a wide area, maybe many states, with no flooding disaster. This is usually what happens with landfalling hurricanes. Instead, Harvey stalled after it came ashore and so all of the rain has been concentrated in a relatively small portion of Texas around the Houston area. In both cases, the atmosphere produced the same amount of rain, but where the rain lands is very different. Even with the system stalling, the greatest multi-day rainfall total as of 3 9 a.m. this Monday morning is just over 30 39.7 inches, with many locations recording over 20 inches. We should recall that Tropical Storm Claudette in 1979 (a much smaller and weaker system than Harvey) produced a 43 inch rainfall total in only 24 hours in Houston
So what makes this event unprecedented? The National Weather Service has termed the event unfolding in the Houston area as unprecedented. I’m not sure why...And don’t pay attention to claims of 500 year flood events, which most hydrologists dislike because we don’t have enough measurements over time to determine such things, especially when they also depend on our altering of the landscape over time.
Weather disasters happen, with or without the help of humans.
Professor Roger Pielke Jr. of the University of Colorado Boulder: "The world is presently in an era of unusually low weather disasters. This holds for the weather phenomena that have historically caused the most damage: tropical cyclones, floods, tornadoes and drought. Given how weather events have become politicized in debates over climate change, some find this hard to believe...The US has seen a decrease of about 20% in both hurricane frequency and intensity at landfall since 1900...Data on floods, drought and tornadoes are similar in that they show little to no indication of becoming more severe or frequent...Thus, it is fair to conclude that the costs of disasters worldwide is depressed because, as the global economy has grown, disaster costs have not grown at the same rate. Thus, disaster costs as a proportion of GDP have decreased. One important reason for this is a lack of increase in the weather events that cause disasters, most notably, tropical cyclones worldwide and especially hurricanes in the United States."
"It is not just droughts that are at or near record levels. On almost every measure of extreme weather, the data is not cooperating with the claims of the climate change campaigners. Tornadoes, floods, droughts, and hurricanes are failing to fit in with the global warming narrative."