The warming of the planet is driving ocean levels upward through two processes: the melting of land-based ice and the thermal expansion of the water in the oceans. Due to the vast energies involved, both of these processes are slow, so the ocean levels have only been creeping up a few millimeters a year. That slow pace makes it difficult for anyone to perceive the changes.

But it's clear that those changes are taking place. In the latest indication, the National Oceanic and Atmospheric Administration (NOAA) has compiled data on what it calls "nuisance floods," cases where coastal communities have to deal with flooding as a result of high tides or minor storms. Over the last 50 years, instances of these floods along the East Coast have gone up by anywhere from 300 to 900 percent.

On the rare occasions where sea level rise reaches the public's consciousness, it's typically as a result of a catastrophic event like Hurricane Sandy. Sea level rise does exacerbate these events, as the flooding reaches higher levels and extends over a wider area than it would have a century earlier. But the rarity and magnitude of catastrophes like these make it difficult for people to associate them with a gradual process. At the same time, the immediate effect of the process itself—high tides being about an inch higher every decade—is difficult for humans to perceive. As NOAA's new report puts it, "neither changes in tidal datum elevations nor rare-event probabilities are readily apparent to the casual observer."

But there are minor flooding events that are much more common, such as high tides that cause roads and properties to be submerged by salt water. Although these nuisance floods don't cause widespread chaos, they do make areas inaccessible and cause damage to infrastructure that wasn't designed to deal with salt water. These events are often common at certain times of the year or become more common in cycles, as normal high tides interact with changes in the ocean circulation or events like El Niños.

You'd expect sea level rise to also enhance nuisance flooding, and NOAA maintains the records needed to identify any trend. In the new report, NOAA scientists have defined a level above normal high tide that constitutes nuisance flooding for each of the tide gauges it maintains along the East, West, and Gulf Coasts. (These numbers were strictly based on tide levels; things like the amount of infrastructure present played no role in them.) It then examined the frequencies of nuisance floods at these locations, going back as far as the 1920s in some cases.

At the earliest points, nuisance floods are extremely rare, happening every few years, if at all. But toward the end of the studied period, the floods had become regular events, happening several times a year on average. The rates of increase are staggering. In the last 50 years, Annapolis and Baltimore, Maryland, have seen rates of nuisance floods increase by 925 percent. New Jersey and Pennsylvania had rate increases in the 600s, while the nation's capital and San Francisco each saw the frequency rise by about 370 percent. Similar trends were found when the length of time the flooding persisted was tracked.

The different rates are caused by other local conditions, such as settling due to the compaction of sediment. And the rises were generally more pronounced on the East Coast than they were elsewhere. But the report shows that regardless of the location, these nuisance floods will become increasingly significant, and sea level rise has a non-linear effect on their frequency.

The reason for this is that the height of high tides is very variable, influenced by things like weather, orbital mechanics, and so on. But that variation is strongly grounded to a mean tide level. Push the mean tide level closer to where the nuisance flood level cutoff resides, and many more of the "typical" high tides will exceed it. This is easiest to see visually, and NOAA provides two ways of visualizing it.

A lot has been made about the expectation among scientists that the rate of sea level rise will increase over the course of this century, giving us less time to harden or shift infrastructure. The NOAA report, however, notes that the increase in nuisance floods will continue even if the rate of sea level rise remains stable. And because of the non-linear nature of the problem, it's possible for areas to go from rare flooding to regular inundations in just a few decades.

The full report is available online.