Here comes the flood

With rapid industrialisation and increased technological complexity over the last two centuries, we seem to have lost touch with the magnitude of our effect on our surroundings. Today, our fingerprints on the environment can be found everywhere—the atmosphere, the oceans, and the Earth's surface. Not to be outdone, there is now another player, up in the sky—the moon, whose periodic "wobble" together with rising sea levels on Earth will make high tides even higher in the near future, eventually leading to devastating floods around the world.

High tides are caused mostly by the gravitational pull exerted by the moon on Earth. But because our planet's lone natural satellite wobbles ever so slowly over a predictable 18.6-year cycle, the power of the pull varies from year to year. Nevertheless, the result is two simultaneous high tides: one on the side of Earth facing the moon, and the other on the opposite side.

Earth rotates on its axis much faster than the moon revolves around it. Our planet's rapid rotation coupled with higher orbital speed about the Sun drags the tidal bulge ahead of the moon's orbit, while the moon's gravitational attraction tries to pull it backward. The Earth, therefore, feels a drag force known as tidal friction, which has slowed down its rotational period from five hours almost four billion years ago to 23 hours 56 minutes today.

The moon takes the same time—27.3 days—to rotate once on its axis as it takes to orbit the Earth. This is called synchronous rotation. In other words, the moon is tidally locked with Earth, which means we always see the same side of the moon, irrespective of where the moon is in its orbit.

Additionally, there is an apparent wobbling and nodding of the moon, collectively called "libration." The wobbling is similar to the back-and-forth motion of a simple pendulum. It is an effect of the moon's non-uniform speed along its elliptic orbit around Earth. The orbital speed increases as the moon approaches the perigee, which is the point closest to the Earth. It decreases as it recedes from the perigee and approaches the apogee, the point farthest from the Earth. This gradual increase and decrease in the orbital speed causes the moon to wobble back and forth—at least to our eyes—by roughly seven degrees in the east-west direction.

The wobbling motion also arises from the fact that the moon's rotational axis is not perpendicular to its orbital plane; it is tilted by 1.5 degrees. Furthermore, moon's orbital plane is inclined to Earth's orbital plane by about five degrees. The net effect of these inclinations is an up-and-down wobbling, or nodding, in a north-south direction.

Alongside global warming, lunar wobbling can either exacerbate the severity of tides or counteract them. During one half of the 18.6-year wobbling cycle, high tides are lower than normal and low tides are higher than normal, so the difference between the two is rather small. As a result, it negates the effect of rising seas to some extent. During the other half of the cycle, high tides get bigger and low tides get lower, thereupon increasing the range between the highest and lowest tides. This cycle boosts the effects on the sea level rise.

The moon is now in the tide-amplification cycle, but the sea level has not risen enough yet due to climate change for the effect to be pronounced. However, a new study by scientists from Nasa and the University of Hawaii published in the peer-reviewed scientific journal Nature Climate Change, only days before the recent destructive flood in Western Europe, warns that Earth may experience record flooding in the mid-2030s, when the moon's next tide-amplification cycle is set to happen. By then, global sea levels will have risen enough to make the already elevated high tides especially troublesome.

The study is the first to take into account all known oceanic and astronomical causes for floods. According to the study, these floods will exceed flooding thresholds in the coastal countries more often—as frequently as every day or every other day. They may even occur in clusters and can last as long as a month or more, depending on the positions of the moon, the Earth and the Sun. "It's the accumulated effect over time that will have an impact," the study warns. However, it also notes that the prediction does not apply uniformly to every coastline everywhere.

Climate change has already increased the frequency and severity of hurricanes, floods and other extreme weather events around the world. This impending threat on the horizon could wreak additional havoc, thereby worsening the already grim predictions of climate change for people living along the coastlines. Some of the biggest cities on the US Atlantic and Gulf coasts, including New York City, Miami and New Orleans, will be hard hit. Osaka in Japan is also a major-risk city; climate models show it could "disappear" with rising tides. Other high-risk cities are Alexandria in Egypt, Rio de Janeiro in Brazil and Shanghai in China.

It took Noah years to build an ark to save him and his family from a godless society that God was going to destroy by sending a catastrophic flood. Unlike Noah, we have 15 years, more or less, to build the ark of mitigation and adaptation necessary to save us from the calamitous effects of climate change. It is thus a race against time, because the flood is coming faster than many once thought. Keeping this in mind, 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." Unfortunately, if poor low-lying countries like Bangladesh become the victims of high tides caused by lunar wobbling, they will suffer immensely because they may not have the technology and/or sufficient fund to implement Nasa's recommendations.

Finally, our moon, until now the quiescent mystical object majestically shining in the sky, has been "faithful to its nature and its power is never diminished." One of the powers it always flexed (albeit unnoticed by us) and will flex again approximately 15 years from now—but this time working in tandem with climate change—is its ability to create record floods. The floods will perhaps give our leaders, who are not doing enough to combat climate change, an alibi to throw in the towel by blaming the wobbling moon.

Dr Quamrul Haider is a professor of physics at Fordham University in New York, US.