A study released Friday by Metstat, a weather-analysis company specializing in “detailed precipitation analysis” and “weather frequency analysis,” found that Harvey delivered a stunning once-in-25,000-year deluge over much of southeast Texas.

Some places saw an unimaginable once-in-500,000-year deluge, which translates to a 0.0002 percent chance of this deluge occurring in any given year. Probabilities that extreme “are calculated by extrapolating the distribution curves for precipitation observed over the last century,” Metstat’s Shauna Bokn explained.

Since global warming has been making extreme precipitation events more likely, however, the U.S. won’t have to wait 25,000 years to witness the next event of Harvey’s proportion.

But, again, to be clear, that’s all based on historical precipitation data over the past century. Human-caused climate change is shifting the baselines. In regard to Harvey, MetStat found that “an interesting atmospheric setup was present that allowed it to stay alive for so long; bringing devastation over such a widespread area.” In particular, “the jet stream was located at very high latitudes, allowed for very light wind shear over Texas, and this aided in the lack of movement for Harvey.”

As climatologist Michael Mann explained during the storm, “the kind of stalled weather pattern that is drenching Houston is precisely the sort of pattern we expect because of climate change.” Climate science predicted a weaker jet stream, and Harvey stalled because of a weakened jet stream.

5. Climate change

Climate change gathers force. The evidence of the damage inflicted by man-made global warming grows stronger. "As recently as a decade ago, climate scientists had a motto that 'you can't attribute any single extreme event to global warming'," wrote three climate scientists in the Scientific American this week.

"By the time politicians and journalists started repeating that line, however, the science had moved on, so that we now can attribute individual events in a probabilistic sense," according to Michael Mann, Thomas Peterson and Susan Joy Hassal. "We know that the strongest storms are getting stronger, with roughly 8 meters per second increase in wind speed per degree Celsius of warming.

"And so it is not likely to be a coincidence that almost all of the strongest hurricanes on record (as measured by sustained wind speeds) for the globe, the Northern Hemisphere, the Southern Hemisphere, the Pacific, and now, with Irma, in the open Atlantic, have occurred over the past two years."

The UN secretary-general Antonio Guterres is due to meet Donald Trump next week and is expected to try to persuade him to keep the US committed to the Paris carbon treaty. If the evidence of Irma doesn't make an impression on Trump, it's hard to know what could. After all, it's received a lot of coverage.

https://www.theguardian.com/commentisfree/2017/sep/11/threat-climate-change-hurricane-harvey-irma-droughts

The problem is, our current business-as-usual trajectory takes us to a world that’s about 3.5C warmer. That is to say, even if we kept the promises we made at Paris (which Trump has already, of course, repudiated) we’re going to build a planet so hot that we can’t have civilisations. We have to seize the moment we’re in right now – the moment when we’re scared and vulnerable – and use it to dramatically reorient ourselves. The last three years have each broken the record for the hottest year ever measured – they’re a red flashing sign that says: “Snap out of it.” Not bend the trajectory somewhat, as the Paris accords envisioned, but simultaneously jam on the fossil fuel brakes and stand on the solar accelerator (and also find some metaphors that don’t rely on internal combustion).

Economic loss estimates

Based on current damage estimates made by multiple agencies, Hurricane Harvey is likely to be at least the second-most costly natural disaster in U.S. history, behind only Hurricane Katrina in 2005.^[5][125] Moody's Analytics has estimated the total economic cost of the storm at $81 billion to $108 billion or more; most of the economic losses are damage to homes and commercial property.^[5] USA Today reported an AccuWeather estimate of $190 billion, released August 31.^[126] On September 3, Texas state governor Greg Abbott estimated that damages will be between $150 billion and $180 billion, surpassing the $120 billion that it took to rebuild New Orleansafter Katrina.^[127][128] According to weather analytics firm Planalytics, lost revenue to Houston area retailers and restaurants alone will be approximately $1 billion. The Houston area controls 4% of the spending power in the United States.^[129]

A significant portion of the storm's damages may be uninsured losses. Regular homeowner insurance policies generally exclude coverage for flooding, as the National Flood Insurance Program underwrites most flood insurance policies in the US.^[130][131] Although the purchase of flood insurance is obligatory for federally guaranteed mortgages for homes within the 100-year flood plain, enforcement of the requirement is difficult and many homes, even within the 100-year flood plain, lack flood insurance.^[130] In Harris County, Texas—which includes the city of Houston—only 15% of homes have flood insurance policies issued by the NFIP. Participation in the NFIP is higher, but still low, in neighboring Galveston (41%), Brazoria (26%), and Chambers Counties (21%).^[130]