Supermoons, King Tides, and Global Warming

December 6, 2017 | 10:04 am
The Moon is seen as is rises, Sunday, Dec. 3, 2017 in Washington. Photo by NASA. CC-BY-2.0 (Flickr)
Astrid Caldas
Senior Climate Scientist

Were you, like me, dazzled by the supermoon this weekend? Did you also stare in a state of wonder at the bright and shiny orb of color illuminating the night? Supermoons happen when a full or new moon is at its closest point to Earth. While we can’t see them during the new moon, supermoons that occur during a full moon are indeed something to behold. They bring thoughts of the universe, of space, stars and planets.

Flooding in Boston wharf.
Photo by: MyCoast, Christian Merfeld

But while we are turning our heads to the sky, we may not realize what’s happening at our feet. The moon might be out in space, but its movement has real impacts here on Earth, specifically on the oceans. I am talking about tides.

Tides are all about big masses of land and water pulling one another in a gravitational act. Tides are always higher at full and new moons — when the Moon, Earth, and Sun are aligned — and it follows that the gravitational pull is strongest when the masses are at their closest during a supermoon. That’s why we saw some unusually high tides, called king tides, across the country (and beyond) at the same time that we experienced the supermoon.

So, while we may not realize it when looking at the supersized moon, it is causing a great deal of disruption to people’s lives in the form of tidal flooding, also called “nuisance flooding.” As stated in one of my colleague’s earlier blogs, this localized tidal flooding has been steadily increasing due to sea level rise. And climate change is behind the sea level rise rates being observed.

The recently released Climate Science Special Report (CSSR) states with very high confidence that “global mean sea level (GMSL) has risen by about 7–8 inches (about 16–21 cm) since 1900, with about 3 of those inches (about 7 cm) occurring since 1993”, and rising will continue throughout the rest of the century at accelerated rates. Rates of sea level rise in many locations along the coast of the U.S. have been higher than the global average, and nuisance flooding is now 300% to more than 900% more frequent than it was 50 years ago in many of those locations.

Many cities have initiatives to track tides and some are specifically geared toward monitoring king tides. Volunteers with “Catch the King,” an initiative by the Virginia Institute of Marine Science, can use a smartphone app to map flooded areas in Hampton Roads in real time. The group then uses the collected data to improve predictions and forecasts, and to better understand the risks from tidal flooding.

Similarly, the “My Coast” project asks Massachusetts residents to submit pictures of areas inundated by king tides to catalogue the effects of these events on the state’s coastal areas. In California, volunteers with the global “King Tides Project” document king tides to bring attention to these events and show “what our daily tides may look like in the future as a result of sea level rise.” Ultimately, these types of initiatives are geared towards improving resilience and preparedness, informing residents of impassable areas and floodwater reach.

The amount of emissions currently released into the atmosphere has already committed us to a certain amount of sea level rise through midcentury, simply because these warming gases remain in the atmosphere for a long time. However, decisions made in the next few years will determine how much the sea will rise in the second half of the century – reducing emissions can reduce the rates of rise and potentially save hundreds of coastal communities from tidal flooding.

So next time you look up at a supermoon (in January 2018), while still marveling at the incredible phenomenon you are witnessing, remember to also look down. It may just make you think about the moon in a completely different way – and how as a nation, we need to do more to reduce emissions and prepare for coastal flooding.

[Update 12/7/2017 2:23pm: We added a sentence acknowledging the King Tides Project in California.]