Bob Henson ·

Above: Yana Kibyakova looks at her car parked along a flooded street in Miami Beach, FL, on September 29, 2015. The flooding was caused by seasonal “king tides” atop long-term sea level rise. City officials hope that a five-year, $400 million storm water pump program and other projects will help keep routine flooding at bay. Image credit: Joe Raedle/Getty Images.

Disruptive tidal flooding that now affects the U.S. Gulf and Atlantic coastlines on 3 to 6 days per year will strike as often as 80 to 180 days a year by the 2040s, according to a major report from NOAA’s National Ocean Service released in February. These increases will be driven mainly by global sea level rise, the report notes—although human-produced climate change itself is not mentioned in the report (see commentary below).

The new NOAA Technical Report, “Patterns and Projections of High Tide Flooding along the U.S. Coastline Using A Common Impact Threshold” (see PDF), builds on projections of global and regional sea-level rise that were released in a separate NOAA report in January (see PDF). The first sentence of that earlier report makes no bones about the situation: “Long-term sea level rise driven by global climate change presents clear and highly consequential risks to the United States over the coming decades and centuries.”

Though it’s often called “nuisance” flooding, since it poses little threat to life or limb, high-tide flooding is a fast-growing threat to the economies and the built environment of coastal areas, especially along the Gulf and Atlantic shores. In South Florida alone, hundreds of millions of dollars are being spent to combat not only long-term sea level rise but also routine “king tides” that are getting worse.

“The possibility that diffuse, low-cost incidents will aggregate over time into extremely high-cost outcomes…is a daunting challenge for policymakers and politicians in many domains,” noted a 2017 study in Earth’s Future led by Hamed R. Moftakhari (University of California, Irvine).

Figure 1. ​Annual summation of days with high tide flooding at 27 U.S. tide gauge locations from 1950 - 2016. Five of the gauges used to make this plot were on the coast of the Gulf of Mexico, three on the West Coast, five on the Southeast U.S. coast, and fourteen on the Mid-Atlantic and Northeast U.S. Coast. Image credit: Figure 2(b), NOAA Technical Report NOS CO-OPS 086, “Patterns and Projections of High Tide Flooding along the U.S. Coastline Using A Common Impact Threshold” (February 2018).

How climate change turns minor flooding into a big deal

High-tide flooding is distinct from extreme storm surges related to tropical cyclones and nor’easters, although they can overlap. By definition, high-tide floods happen at predictable points in the tidal cycle, such as the period from late summer into autumn when astronomical tides are at their highest. They can be enhanced by seemingly innocuous weather features, such as strong high pressure offshore that pushes high water toward the coast under sunny skies.

Current NOAA tidal tables show that the highest yearly swings from low to high astronomical tide are only 1-2 meters along the Atlantic coast and as little as 0.5 meters along the western Gulf coast. It’s plain to see that even a mildly elevated tide will pose increasing trouble when it occurs atop rising sea levels.

The new report detailed the three types of coastal flooding the National Weather Service (NWS) defines. They issue a flood advisory for minor flooding (which is more disruptive than damaging), and a flood warning for floods of moderate (damaging) or major (destructive) potential.

The thresholds for these three types of floods vary from location to location, due to the extent of infrastructure vulnerabilities, which vary by topography and relief, land-cover types or existing flood defenses. However, using data from the 99 non-Alaskan U.S. tide gauges with more than 20 years of data, the flood risk at almost all U.S. locations can be very accurately approximated thusly:

—minor flooding begins with water levels about 1.6 feet above the high tide mark

—damaging moderate flooding begins at 2.6 feet above the high tide mark

—destructive major flooding begins when water levels exceed 3.9 feet above the high tide mark.

Based on these values, just a modest one-foot increase in sea level will be enough to transform minor nuisance floods to damaging moderate floods. A further increase in sea level by 1.3 feet will make today’s minor flood a destructive major flood.

According to the official NWS flood thresholds for stations along the coast with tide gauges, the most vulnerable locations to sea level rise are Charleston, SC and Corpus Christi, TX, where a 1.8’ increase in sea level will cause damaging moderate flooding every day, and a 2.3’ increase in sea level will lead to destructive major flooding every day.

Figure 2. Change in decadal annual high tide flood frequencies at 99 U.S. tide gauge locations outside Alaska with at least 20 years of data, through 2016. Accelerating trends were found at 30 stations (shown in red-orange colors); linearly increasing trends were found at 31 stations (yellow-orange colors), and 38 stations had no trend (black dots). High tide flooding has been accelerating in frequency along most of the East Coast, and has been increasing at a slower linear rate along most of the Gulf Coast. Along the entire West Coast, the frequency of high tide flooding has remained nearly constant (no trend) with only a few locations, namely San Diego, La Jolla, Los Angeles, Humboldt Bay and Seattle, experiencing a 25% to 50% increase. Image credit: Figure 12(a), NOAA Technical Report NOS CO-OPS 086, “Patterns and Projections of High Tide Flooding along the U.S. Coastline Using A Common Impact Threshold” (February 2018).

Hundreds of days of high-tide flooding each year

Climate change raises the stakes greatly when it comes to high-tide flooding. As oceans expand and ice sheets melt, global sea level has risen by an average of about 3 mm (0.12”) per year since the 1990s (see our February 15 post), but the rate has been accelerating. If this acceleration were to continue through the end of the century, global sea level rise between 2005 and 2100 would be about 65 cm (26”), more than double the rise of 11” that would occur if sea level rise stayed constant at 3 mm/yr. There’s a real risk that global sea level could rise by a meter (39”) or more this century, especially if Antarctic ice sheets poking into the ocean are destabilized by warming waters.

The new NOAA report draws on the “intermediate low” and “intermediate” scenarios from the earlier report, which assume that global sea level will rise 0.5 m (19.7”) or 1.0 m (39.4”) by 2100. Along parts of the Atlantic coast, especially from the Carolinas to New England, regional factors—including subsidence and the knock-on effects of a weakening Gulf Stream—will increase sea level at a faster pace than the global average.

Here are some of the projected number of days per year when high-tide floods can be expected in 2041-2050 and 2091-2100, based on the new report:

Even under the intermediate low scenario, which the planet may well vault past, we can expect high-tide flooding virtually every day by the 2090s along the western U.S. Gulf Coast, and on two out of three days along the northeast U.S. Atlantic coast. Under the intermediate scenario, high-tide flooding is possible every other day by the 2040s along the western Gulf Coast and one out of every three days along the Northeast coast by the 2040s, which arrive just 22 years from now.

The U.S. West Coast and the Pacific and Caribbean Islands are also in line for a dramatic boost in high-tide flooding, although the more rugged coastal topography means it will take till later this century for the more significant effects to kick in. High-tide flooding is projected to reach near-daily levels by the 2090s along the West Coast and in the U.S. Caribbean Islands.

The report notes: “By definition, ‘every other day’ high tide flooding would bring to fruition the saying championed by NOAA’s (late) Margaret Davidson: “Today’s flood will become tomorrow’s high tide.”

Figure 3. Projected annual frequencies of high-tide flooding of at least minor strength in response to scenarios of global sea level rise (shown in color bar at center), estimated at NOAA tide gauges in a) New York City (The Battery), b) Miami (Virginia Key), Florida and c) San Francisco, California. The values shown include observed patterns of combined tidal and nontidal water level components. The water levels above the high-tide mark that are assumed to produce at least minor high-tide flooding (using the technique described in the article above) are 0.56 meter (1.8 feet), 0.53 m (1.74 ft), and 0.57 m (1.87 ft). Image credit: From Fig. 14, NOAA Technical Report NOS CO-OPS 086, “Patterns and Projections of High Tide Flooding along the U.S. Coastline Using A Common Impact Threshold” (February 2018).

Commentary: When climate change speaks for itself

In this powerful and sobering NOAA report, the lack of direct mention of the main cause of current global sea level rise—climate change produced by fossil fuel emissions—is itself striking, especially in the context of other developments. For example, Bloomberg reported on March 15 that FEMA’s strategic plan for 2018-2022 omits all mention of climate change, which had been prominent in the agency’s prior strategic plan.

NOAA oceanographer William Sweet is lead author on the February report from NOAA as well as its predecessor report from January, which (as noted above) does mention climate change prominently. In an email, Sweet said: “This [February] report was focused primarily on the flood threshold aspect, and I tend to let sea level rise speak for itself as a direct consequence of climate change." NOAA spokesperson Chris Vaccaro said in an email that there have been no recent changes to NOAA policy involving the freedom of scientists to refer to climate change directly in scientific and technical reports.

The new NOAA and FEMA documents are hitting the streets after more than a year of climate-change dismissal and denial fostered at the top levels of the current presidential administration, most notably by President Donald Trump and EPA Administrator Scott Pruitt. Similar forces have been at play at the state level over the past few years. For example, legislators in North Carolina voted in 2012 to allow use of only historical trends rather than the expected climate-change-driven acceleration when planning for the risk of coastal sea-level rise. And in Florida, state officials reported being ordered not to use the terms “climate change” and “global warming” in official communications, including reports.

Following an interview in August with FEMA Administrator Brock Long, the news site Emergency Management reported that Long “broke from environmentalists and Obama-era officials on one point: He declined to say whether he believed that human activity is warming the planet, instead citing deep-water ocean currents, El Nino and other ‘intrinsic cycles’ as reasons for changes in weather patterns. He said his job was to help state and local governments prepare for the risks they face, no matter their cause.”

Figure 4. Results from the last 18 years of public polling by the Gallup Organization on several questions pertaining to climate change. Image credit: Gallup, "Global Warming Concern Steady Despite Some Partisan Shifts" (March 28, 2018).

Based on years of polling from Gallup and the Yale Program on Climate Change Communication, a stubborn 30% to 35% of Americans do not believe or aren't sure that human activity is the main cause of global warming—a stance completely at odds with well-established science.

It’s human nature to ignore warnings unless they are reinforced in a variety of ways. Scientists ought to be free to invoke human-produced climate change as little or as much as they see fit in a given context. However, as long as a significant fraction of Americans continues to push climate change out of mind, there’s a case to be made for explicitly noting that it’s a real thing.

Dr. Jeff Masters contributed to this post.

Related Cat 6 posts

A Weaker Gulf Stream Means Trouble for Coastal New England (March 5)

Retreat From a Rising Sea: A book Review (February 16)

Sea Level Rise is Accelerating, Says Unnerving New Research (February 15)

Our December review of Jeff Goodell’s 2017 must-read book on sea level rise, The Water Will Come: Rising Seas, Sinking Cities, and the Remaking of the Civilized World.

Why Is It So Hard to Fix the National Flood Insurance Program?, our November 2017 post.

Blockbuster Assessment: Humans Likely Responsible For Virtually All Global Warming Since 1950s, our November 2017 post detailing the U.S. government’s official science findings on sea level rise.