Tag Archives: tide

NASA says U.S. coastal cities should expect frequent high-tide floods by mid-2030, and the Moon is partly to blame

The US will see a dramatic increase in high-tide coastal flooding in the coming decade, according to NASA. This increase will be powered by the interaction between the moon’s gravitational influence (which causes tides) and higher sea levels caused by climate change.

Image credits Tim Hill.

High-tide floods are an already familiar problem in several areas of the US, mainly in cities on the Atlantic coast and a few of those dotting the Gulf coast. According to the National Oceanic and Atmospheric Administration (NOAA), more than 600 such floods occurred in the States in 2019.

But such events are likely to pick up in frequency around the mid-2030s, a new paper reports, mainly due to changes in climate — although astronomy does play a part, as well. The research was led by researchers at the NASA Sea Level Change Science Team from the University of Hawaii. Apart from warning of an increased frequency of such events in the future, the team also explains that the floods will likely occur in clusters of about a month or so at a time, depending on the relative positions of the Earth, Moon, and Sun. When our planet, the Sun, and the Moon line up in specific ways, they note, the resulting gravitational pull is likely to cause floods, especially considering the higher sea levels of the future.

Floods-to-be

“Low-lying areas near sea level are increasingly at risk and suffering due to the increased flooding, and it will only get worse,” said NASA Administrator Bill Nelson. “The combination of the Moon’s gravitational pull, rising sea levels, and climate change will continue to exacerbate coastal flooding on our coastlines and across the world. NASA’s Sea Level Change Team is providing crucial information so that we can plan, protect, and prevent damage to the environment and people’s livelihoods affected by flooding.”

The paper’s lead author Phil Thompson, an assistant professor at the University of Hawaii, explains that this process won’t be devastating due to the severity of the floods themselves, but rather, through their sheer number. Taken individually, high-tide floods involve relatively small amounts of water. “But if it floods 10 or 15 times a month, a business can’t keep operating with its parking lot under water. People lose their jobs because they can’t get to work. Seeping cesspools become a public health issue.”

The root of the issue is the Moon’s wobble. Our planet’s satellite has a regular wobble in its orbit caused by the specifics of how it rotates, how the Earth rotates, and how the two move relative to one another. This wobbling cycles every 18.6 years, and it’s a phenomenon known since the early 1700s. What changed, however, is sea level — rising seas interact with this wobble to make flooding way more likely, and way more common.

Throughout half of the cycle, the daily tide on our planet is suppressed. High tides aren’t as high as they should be, and low tides are not as low. In the other half, however, they’re amplified — higher at their highest, lower at their lowest. But since climate change is pushing sea levels up, it’s also lifting the tide higher at all times. Overall, this means that one half of the 18.6-month cycle will be counteracted, while the other will see higher high tides.

Right now, the Moon is in the tide-amplifying part of the cycle, but sea level rise has not been significant enough for it to translate into flooding across U.S. coastlines. However, by the next time this phase comes around, in the mid 2030s, the mean sea level will be higher than today. Under the Moon’s gravitational effect, we’re very likely to see frequent flooding on almost all U.S. mainland coastlines, Hawaii, and Guam, the team explains. Northern coastlines such as Alaska’s will likely not see frequent flooding in the mid-2030s, as they are rising generally due to geological processes, but they will likely suffer the same by the mid-2040s.

For the estimations, the researchers studied recordings from 89 tide gauge locations in every coastal U.S. state and territory except Alaska. Factoring in NOAA’s sea-level rise scenarios and flooding thresholds, they created a statistical model that estimates the number of times these thresholds have been exceeded annually, and how this related to the lunar wobble cycle. Other processes known to affect tides, such as El Niño events, were also factored in. Then, this model was used to project changes in flooding (defined as sea levels exceeding flooding thresholds) up to 2080.

“From a planning perspective, it’s important to know when we’ll see an increase,” said co-author Ben Hamlington from NASA’s Jet Propulsion Laboratory in Southern California, leader of NASA’s Sea Level Change Team. “Understanding that all your events are clustered in a particular month, or you might have more severe flooding in the second half of a year than the first – that’s useful information.” A high-tide flood tool developed by Thompson already exists on the NASA team’s sea level portal, a resource for decision-makers and the general public. The flood tool will be updated in the near future with the findings from this study.

Tides are turning in earthquake discoveries

It’s been suggested for a while that tides can have an impact on earthquakes that occur along mid-ocean ridges. However, no one knew why the frequency increased during periods of low tides. However new research might have found an answer.

A study published in Nature Communications by Christopher Scholz and Columbia University has found that it comes down to the magma below the mid-ocean ridges. The research was made possible by a network of seafloor instruments along the Pacific’s Juan de Fuca ridge — a mid-ocean spreading center and divergent plate boundary located off the coast of the Pacific Northwest region of North America.

“Everyone was sort of stumped, because according to conventional theory, those earthquakes should occur at high tides,” explained Scholz, a seismologist at Columbia University’s Lamont-Doherty Earth Observatory. “It’s the magma chamber breathing, expanding and contracting due to the tides, that’s making the faults move.”

Since most mid-ocean ridges feature vertical faults — those featuring steeply inclined planes — scientists assumed earthquake-generated slips would most likely occur at high-tides since the upper block slides down with respect to the lower one during movement. But seismic data was actually showing that the opposite occurred; the fault slips down during low tide, when forces are actually pulling upwards “which is the opposite of what you’d expect,” said Scholz.

In the end, it all came down to the volcano’s magma chamber, a component no one had yet considered as part of this mechanism. The team realized that when the tide is low, there is less water sitting on top of the chamber, so it expands. As it puffs up, it strains the rocks around it, forcing the lower block to slide up the fault, and causing earthquakes in the process.

When the team charted the earthquake rate versus the stress on the fault, they realized that even the smallest amount of stress could produce an earthquake. The tidal data helped to calibrate this effect, but the triggering stress could be caused by anything — such as the seismic waves from another earthquake, or fracking wastewater pumped into the ground.

“People in the hydrofracking business want to know, is there some safe pressure you can pump and make sure you don’t produce any earthquakes?” said Scholz. “And the answer that we find is that there isn’t any — it can happen at any level of stress.”

Scholz also adds that the tidal earthquakes in this region are “so sensitive that we can see details in the response that nobody could ever see before.” Of course, the scale of things should also be considered: small stresses over a small area isn’t going to trigger a massive earthquake.

The moon’s phases affect rainfall, says first-of-its-kind study

The moon does more than cause tides and delight lovers – according to a new study, it can also affect how much rainfall falls down on the ground.

Image via Wikipedia.

Since ancient times, the moon has been an object of fascination for people, both romantics and scientists. Now, researchers from the University of Washington found that when the moon is high in the sky, it creates “bulges” in the planet’s atmosphere, slightly affecting falling rain. This is the first study to document the effect of the moon on rainfall.

“As far as I know, this is the first study to convincingly connect the tidal force of the moon with rainfall,” said corresponding author Tsubasa Kohyama, a UW doctoral student in atmospheric sciences.

 

Satellite data over the tropics, between 10 degrees S and 10 degrees N, shows a slight dip in rainfall when the moon is directly overhead or underfoot. University of Washington

He started noticing something was up while studying atmospheric waves, periodic disturbances of pressure, temperature or wind velocity. He noticed slight, but consistent oscillations in the air pressure. He and co-author John (Michael) Wallace, a UW professor of atmospheric sciences, spent the next two years tracking these oscillations and attempting to explain them.

It’s not the first time atmospheric variations have been tied with the moon. Air pressure changes were connected to the phases of the moon back in 1847 and temperature in 1932. Furthermore, a 2014 paper from the same University showed how air pressure varies with the phases of the moon.

“When the moon is overhead or underfoot, the air pressure is higher,” Kohyama said.

After that, it seemed highly likely that rainfall is also affected by the moon, and this did turn out to be the case. When the satellite is overhead, its gravitational attraction tugs and pulls a slight damper on the rain. Higher pressure also creates warmer temperatures in the air parcels below. Warmer air holds more moisture, and having a higher moisture capacity means they don’t shed as much water.

“It’s like the container becomes larger at higher pressure,” Kohyama said. The relative humidity affects rain, he said, because “lower humidity is less favorable for precipitation.”

They used data collected from 1998 to 2012 to show that the rain is indeed slightly lighter when the moon is high. It’s a very slight and subtle change, of only about 1% total rainfall. It is consistent, but you shouldn’t really worry about it.

“No one should carry an umbrella just because the moon is rising,” Kohyama said.

However, this could have an effect on weather and climate models. It’s another valuable piece of information to piece into extremely complicated prediction models. Wallace plans to continue exploring the topic to see whether specific types of rain are more affected by lunar phases.

Journal Reference [open access] – Rainfall variations induced by the lunar gravitational atmospheric tide and their implications for the relationship between tropical rainfall and humidity.

The science behind the supertide that stranded the French Abbey

A shocking image went viral a few days ago, with an 11th-century French abbey completely surrounded by water following a super tide. Naturally, many similar photos followed, and many “explanations” emerged, vaguely referencing the solar eclipse, something about gravitational cycles, and some even stated it was caused by the “tide of the century”, although the “tide of the century” comes by every 18 years. So let’s take a look at what actually happened.

A supertide envelopes Mont Saint-Michel. Associated Press

Mont Saint Michel (Saint Michael’s Mountain) is a tidal island in Normandy, France. In prehistoric times, it was a part of the continent, but as the ice age faded away and sea levels rose, much of it was eroded or covered by water. Today, the Mont has a circumference of about 960 metres (3,150 ft), and is 92 metres (302 ft) above sea level at its highest point.

The island now hosts an abbey, which is a major touristic attraction, both for pilgrims and for people interested in the spectacular setting. In low tide, you can actually walk to the abbey, but in high tide, you can’t. In super high tides, water completely surrounds it – like was the case this March. So what caused the supertide? A strong clue here is the “March” that keeps popping up.

Mont Saint Michel during the low tide in 2005. Image via Wikipedia.

Tides are caused by the Moon’s attraction (and to a lesser extent, the Sun’s). When the sun and the Moon align properly, their gravitational attraction adds up, creating a stronger pull – the so-called spring tides, that occur not only in March, but also in September (around the spring and the autumn equinoxes). This year, the Sun and the Moon aligned particularly well – this is why we got the solar eclipse.

“If we see a solar eclipse there will be a spring tide,” Hal Needham, a climatologist at Louisiana State University, told me. “But if we see a spring tide it doesn’t mean we will see a solar eclipse.”

So this explains why we get strong tides in March, but there’s still something more – the 18 year cycle I mentioned above. This is a bit more complicated, and we have to understand so-called lunar nodes. The Moon exerts a stronger pull when it is in the same plane as the equator. But as the Moon revolves around the Earth, it doesn’t do so smoothly, but it also wobbles, in a precession type of movement. It takes the Moon 18.6 to complete a revolution and end in the same plane – in our case, the equatorial plane, where the pull is greatest. This is called a draconic or nodal period. It’s also important to note that weather can have a much stronger impact than these cycles – note Hurricane Sandy, for example.

The lunar nodes are the points where the moon’s path in the sky crosses the ecliptic, the sun’s path in the sky. Image via Wikipedia.

So to sum it up, every March or September we get very strong tides, and the effect is even stronger once every 18.6 years. This was visible not only in France, but in many other parts of the world, including the Bay of Fundy in Nova Scotia. But clearly, the most spectacular sight was this thousand year old abbey in France.

 

Still hot inside the Moon? Earth gravity creating a hot layer

A new study has shown that there is still an extremely hot layer deep inside the moon, with heat generated by the gravity from the Earth. If this is indeed the case, then the inside of the Moon has not yet entirely solidified, providing an insight on to how the Earth-Moon system evolved.

Credit: Image courtesy of National Astronomical Observatory of Japan

There is still a lot of debate regarding the Moon’s nature – is it a true satellite, or is it in fact a planet of its own, trapped by the Earth’s gravity system? When discussing the nature of such a celestial planet, you must understand how it was born and how it evolved. But studying the early stages and evolution of the Moon is no easy feat, and that’s why researchers were thrilled to find this hot layer. But how do you “find” a hot layer in the depths of the Moon? The key here is gravity.

We can get a good indication of what’s happening inside a celestial body by studying slight modifications in its shape. The shape of a celestial body being changes by the gravitational force of another body is called tide; we see this on Earth, in the oceans. High and low tides occur mostly due to the Moon’s gravity (the Sun also plays a smaller role), because water is so deformable that its desplacement can be easily observed. But even the solid parts can be displaced, though to a much smaller extent. Observing the degree of deformation enables us to infer several things about the interior.

But there are more ways to study the internal structure. When the Apollo program landed people on the Moon, they also left seismological sensors on the surface – because the Moon also has earthquakes (perhaps moonquakes would be more accurate though). Through these sensors, they showed that the satellite has two main parts: the “core,” the inner portion made up of metal, and the “mantle,” the outer portion made up of rock. Based on this data, and previous shape deformation observations, Dr. Yuji Harada and his team managed to show that there is also an intermediate, hot layer, wrapping and warming the core. But this study, while it may very well revolutionize what we know about the moon, it actually poses more questions than it answers, researchers say:

“I believe that our research results have brought about new questions. For example, how can the bottom of the lunar mantle maintain its softer state for a long time? To answer this question, we would like to further investigate the internal structure and heat-generating mechanism inside the Moon in detail. In addition, another question has come up: how has the conversion from the tidal energy to the heat energy in the soft layer affected the motion of the Moon relative to the Earth, and also the cooling of the Moon? We would like to resolve those problems as well so that we can thoroughly understand how the Moon was born and has evolved.”

Another investigator, Prof. Junichi Haruyama of Institute of Space and Aeronautical Science, Japan Aerospace Exploration Agency also emphasizes the significance of this study:

“A smaller celestial body like the Moon cools faster than a larger one like the Earth does. In fact, we had thought that volcanic activities on the Moon had already come to a halt. Therefore, the Moon had been believed to be cool and hard, even in its deeper parts. However, this research tells us that the Moon has not yet cooled and hardened, but is still warm. It even implies that we have to reconsider the question as follows: How have the Earth and the Moon influenced each other since their births? That means this research not only shows us the actual state of the deep interior of the Moon, but also gives us a clue for learning about the history of the system including both the Earth and the Moon.”

Scientific Reference: Yuji Harada, Sander Goossens, Koji Matsumoto, Jianguo Yan, Jinsong Ping, Hirotomo Noda, Junichi Haruyama. Strong tidal heating in an ultralow-viscosity zone at the core–mantle boundary of the Moon. Nature Geoscience, 2014; 7 (8): 569 DOI: 10.1038/ngeo2211