If the graphic looks familiar it’s a mod of one I used in my June 27 post about the Low Moon. There I explained why the Moon’s orbital precession affects how high the Moon appears in the sky. In 2034, when the Full Moon reaches its most northern declination, it will trace its highest arc through the sky (for Northern Hemisphere observers). During research for that post I discovered something I had not known: In 2034, the Moon will also cause ocean tides higher and lower than normal in many locations. The graphic above shows the geometry. Note that the tidal bulge raised by the Moon is, on average, about twice that raised by the Sun.
If you’re not familiar with the science of tides, you might want to check out this post: The Moon’s Tidal Effect on Earth. In and around the year 2034 certain areas of the globe will experience amplified tides because of a complex interaction between the Moon’s declination, the superposition of its tidal bulge with that of the Sun, and the asymmetric distribution of continental land masses around the globe.
Oceanographers classify tides into one of three types:
- Semidiurnal: Two equal high and low tides each day.
- Diurnal: One high and low tide each day.
- Mixed Semidiurnal: Two unequal high and low tides each day.
Of course this ignores the smaller tidal bulge raised by the Sun, but more on that later. The following graphic shows the global distribution of each type of tide:
There were quite a few articles online that warned of “larger tides in the mid-2030s” but none really explained why. Several of them did, however, cite the same scientific paper by Ivan D. Haigh, Matt Eliot, and Charitha Pattiaratchi in their references. I cite it here for curious readers:
Here’s a summary of my takeaways from that paper:
- In general, a given point on Earth’s surface will experience four tides each day. On a rotating Earth, that point will pass through the two tidal bulges caused by the Moon, and two caused by the Sun. When the Moon is Full or New, its tide superimposes on that of the Sun yielding only two tides per day. These are commonly called spring tides because of their larger size.
- In reality, tides are much more complex. They are affected by long-term cycles in the Earth’s and Moon’s orbits. Also, continents and ocean currents introduce phase delays as a tidal bulge sweeps from east to west. And the topography of the coastline and seafloor at the point where the tide is experienced can focus or disperse the tide. This is why some locations have diurnal and mixed semi-diurnal tides.
- In 2034, when the Moon is at high declination and its orbital plane nearest the plane of Earth’s equator, it’s tidal bulge will better align with the tropics, amplifying both high and low diurnal tides in that region.
There is another lunar cycle that affects tides: perigeal precession.
Lunar tides are greatest when the Moon is at perigee — the closest point to Earth in its elliptical orbit. The graphic exaggerates the geometry for clarity. The major axis of the Moon’s orbit precesses 360° in 8.85 years. Because this is a longitudinal drift (instead of latitudinal), the effect is more evenly distributed and contributes more to semi-diurnal tides. If the Moon is at perigee when its New or Full, its tidal bulge will align with that of the Sun producing what is called a king tide. In 2034, several Full Moons will occur near perigee causing semi-diurnal king tides.
The theoretical model used to predict tides includes parameters for the Earth and Moon. After incorporating phase delays caused by continents and ocean currents, the model generates a mathematical sum of three sinusoidally varying components:
- Precession of the Moon’s orbital plane: T = 18.61 yr and T = 9.35 yr (subharmonic)
- Precession of the Moon’s orbital perigee: T = 8.85 yr and T = 4.425 yr (subharmonic)
- Orbit of Earth around the Sun: T = 1 yr and T = 0.5 yr (subharmonic)
The paper’s authors accumulated a database of archived tidal records for locations around the globe. Then they searched that database using Fourier analysis looking for signals matching any of those six cycles. What they found was that all six signals were present, but in varying degrees depending on the type of tide experienced at the location of interest.
The paper was published in June 2011. The authors were predicting large tides for the year 2015 for the same reasons we’ve been hearing warnings about 2034. Here’s what happened in 2015 in one community on the Gulf Coast of Texas:
Meteorologists refer to this phenomenon as a “nuisance tide” or “sunny day tide” because no hurricane surge was needed to flood normally dry areas. Unfortunately, in 2034 when that tidal cycle repeats, we’ll can expect even higher water levels because of the increasing contribution of sea level rise (SLR).
The current rate of SLR is around 3.6 mm/year (0.14 inches/year). So extrapolating from 2015, in 2034 there will be tides 67 mm (2½ inches) higher than what you see in that photo. That’s just an average prediction from a computer model, so some locations will see more or less. Further, that extrapolation assumes the rate of SLR remains constant, but many climate scientists expect that rate to increase.
Next Week in Sky Lights ⇒ JWST Teaser Image