Decades after Boston Harbor witnessed the events that launched the American Revolution records were collected there that could start a more modest revolution, one in flood prevention. Tidal records from the harbor have been used to verify what we suspected about centuries of sea level rise, but also reveal a two-decade cycle in ocean floods.

Since 1921 America's tide heights have been tracked in a comprehensive database that has played an essential part in our understanding of how global warming is affecting sea levels. Further back we depend on a mix of spotty records and imprecise proxies, interfering with our capacity to make predictions.

“It's really hard to come up with an accurate estimate of a 100-year storm based on only a few decades of data,” Dr Jonathan Woodruff of the University of Massachusetts, Amherst noted in a statement. 

Woodruff and Portland State University's Dr Stefan Talke learned of earlier records for Boston Harbor stored in the Harvard University and MIT archives and saw the potential.

The Harvard/MIT archives turned out to be very incomplete, but combined with some other sources, still very useful. "We recovered 50 years of instrumental data between 1825 and 1920,” Talke said, “but there are some gaps, for example, 1834-1846, 1877-1896 and 1912-1920."

The land around Boston is still sinking from after-effects of the last Ice Age and changes have been made to tide gauges, so the data had to be corrected before conclusions could be published in the Journal of Geophysical Research: Oceans.

Once these were allowed for, it became clear the average sea level in the harbor has risen by 0.28 meters (0.9 feet) over almost 200 years. Even this modest increase can greatly increase flooding during storms. Moreover, just as global warming models predict, most of the rise has happened in the last few decades, and it is speeding up. Already this year Boston has had the highest and third highest ocean floods since records began.

That much was predictable to anyone not in denial about human-induced climate change, but the work also found a pattern to the tides that will be very important for flood planning. “Our research shows that slow changes in the moon's orbit around the earth relative to the earth's orbit around the sun results in high tides increasing and decreasing over an 18.6-year cycle," Woodruff said. 

The cycle does not affect average sea level, but can make high and low tides 10 centimeters (4 inches) more extreme, something Woodruff said appears to make more difference than changes in storm intensity. For the next decade, east coast tides will be suppressed, before growing again in the 2030s. Since floods usually involve storms coinciding with high tides, the combination of more extreme tide heights and sea level rise could imperil Boston and similar cities in decades to come.