Boston’s Long Wharf is flooding.
This flooding is part of a “king tide” – a tide higher than all others. When the sun, moon, and Earth are in alignment, and close to each other, the tides will have especially high amplitudes – that is, the highest high tides and the lowest low tides. If you want a general introduction to how tides work, check out this Crash Course Astronomy video.
So, with this week’s supermoon, the tides are especially strong – some of the strongest of the year. During high tide, salt water is sneaking through storm drains and spilling over seawalls all along our coasts.
This kind of flooding is often called “nuisance flooding” or “clear-day flooding” because it isn’t associated with storm surges or winds. But let’s be clear: this “nuisance flooding” is exactly how sea level rise works.
What’s more, these floods will become more common as sea level rises.
You might be thinking, “Wait, Emma said this was caused by the moon. Why is she talking about sea level rise?”
To answer this question, let’s use an analogy. As you age, your risk of heart attack increases. Let’s say that, when you’re 60 years old, you get a heart attack. It might result from diet or stress. However, the fact that you’re older means that a heart attack is already more likely to happen. In other words, while your age didn’t directly cause the heart attack, it certainly didn’t help.
Sea level rise works the same way: it’s a slow process that works behind the scenes to make other problems more common and more severe. When sea level rise is superimposed on high tides, it creates the floods like the ones we’ve seen this week.
Since 1920, Boston sea level has risen about two fingers’ width (3 cm) per decade. That might not sound like a lot, but it adds up to about one foot since 1920. That’s enough to swamp areas that were once comfortably above the high tide mark. Take a look at the tide gauge data for Boston, below (if you can’t see it, click here).
It’s one thing to see trends like this in data, and completely different to see in real life. As peak high tide approaches, the sea becomes an unstoppable force, spurting through cracks in the wooden wall, bubbling up the storm drain, and forming waves that spill onto the road behind. The granite walkway disappears beneath half a foot of water, and the pier becomes indistinguishable from the harbor behind it.
The flood subsides a few hours later, but it leaves behind a promise to return.