High tides have been getting higher and low tides lower at cities around the Gulf of Mexico, according to a new study produced in part by scientists at the University of South Florida.Those extreme swings, caused at least in part by global climate change, have increased since the 1990s, the study found. The trend for sea level rise spells very bad news for anyone living along the coast if a hurricane hits during one of those higher high tides."The changes . . . have almost doubled the risk of hurricane-induced flooding associated with sea level rise since the 1990s for the eastern and northeastern Gulf of Mexico coastlines," noted the study, published by the scientific journal Geophysical Research Letters.If you live on the beach or in some other low-lying area, "you better pray somebody in Washington does something about this flood insurance situation," Mark Luther, associate professor of physical oceanography at the University of South Florida, said Wednesday. For their study, Luther and two colleagues spent about eight months comparing tidal measurements collected between 1900 and 2011 at 13 locations around the gulf, from Key West to Port Isabel, Texas.Not all the charts go back to the turn of the last century. St. Petersburg's for instance, begins in the mid 1920s, Luther said. Originally, the measurements were taken out on the city's Million Dollar Pier, he said, but then moved to the U.S. Coast Guard base on Eighth Avenue SE.The size of high and low tides naturally vary between summer and winter. But increases in greenhouse gases raise the global average temperature, disrupting the usual weather patterns and producing both record heat waves such as the one Australia just experienced and record cold snaps like the one occurring in the United States this week. By turning summers warmer and winters colder, climate change has altered the normal tidal variations, too, particularly in the past two decades.Lower low tides can be good news if that's when a hurricane makes landfall, Luther said. But if a hurricane hits during one of the higher high tides, it could push seawater much farther inland than ever before, he said.The study included a chart showing the percent change in the annual variation of the tides in each of those locations. For St. Petersburg, the percentage was nearly 30 percent, which the study called "a significant increase." In Key West, it was even higher. The tidal charts there, which date back to before the Civil War, have documented a sea level rise of 9 inches in the last century. South Florida officials say they expect to see a rise of 9 to 24 inches in the next 50 years. Tidal flooding that used to be a rare occurrence in Key West now happens so frequently that some businesses at the end of the city's famed Duval Street have stockpiled sandbags inside their front doors for quick deployment.In low-lying Florida, where 95 percent of the population lives within 35 miles of its 1,200 miles of coastline, rising seas can produce major changes. At Waccasassa State Park in Levy County, for instance, palm trees have been toppling over dead as saltwater creeps up the beach.Although climate change is accepted as fact by everyone from NASA to the Vatican, Gov. Rick Scott has expressed skepticism. "I've not been convinced that there's any man-made climate change," he said in 2011.Last week, Scott sent President Barack Obama a letter seeking a meeting on the upcoming hike in federal flood insurance rates and calling on him to prevent increases on Florida homeowners that were approved by a bipartisan vote in Congress in 2012.Information from the Associated Press was used in this report. Craig Pittman can be reached at [email protected] or follow him on Twitter at @craigtimes.