Another “super full moon” is coming May 26, 2021, and coastal cities like Miami know that means one thing: a heightened risk of tidal flooding.

Exceptionally high tides are common when the moon is closest to the Earth, known as perigee, and when it’s either full or new. In the case of what’s informally known as a super full moon, it’s both full and at perigee.

But something else is going on with the way the moon orbits the Earth that people should be aware of. It’s called the lunar nodal cycle, and it’s presently hiding a looming risk that can’t be ignored.

Right now, we’re in the phase of an 18.6-year lunar cycle that lessens the moon’s influence on the oceans. The result can make it seem like the coastal flooding risk has leveled off, and that can make sea level rise less obvious.

But communities shouldn’t get complacent. Global sea level is still rising with the warming planet, and that 18.6-year cycle will soon be working against us.

I am an atmospheric scientist at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science who keeps a close eye on sea level rise in Miami. Here’s what you need to know.

What the moon has to do with coastal flooding

The moon’s gravitational pull is the dominant reason we have tides on Earth. More specifically, Earth rotating beneath the moon once per day and the moon orbiting around Earth once per month are the big reasons that the ocean is constantly sloshing around.

In the simplest terms, the moon’s gravitational pull creates a bulge in the ocean water that is closest to it. There’s a similar bulge on the opposite side of the planet due to inertia of the water. As Earth rotates through these bulges, high tides appear in each coastal area every 12 hours and 25 minutes. Some tides are higher than others, depending on geography.

The sun plays a role too: Earth’s rotation, as well as its elliptic orbit around the sun, generates tides that vary throughout the day and the year. But that impact is less than half of what the moon contributes.

This gravitational tug-of-war on our water was discovered nearly 450 years ago, though it’s been happening for nearly four billion years. In short, the moon has very strong control over how we experience sea level. It doesn’t affect sea level rise, but it can hide or exaggerate it.

So, what is the lunar nodal cycle?

To begin, we need to think about orbits.

Earth orbits the sun in a certain plane – it’s called the ecliptic plane. Let’s imagine that plane being level for simplicity. Now picture the moon orbiting Earth. That orbit also lies on a plane, but it’s slightly tilted, about 5 degrees relative to the ecliptic plane.

That means that the moon’s orbital plane intersects Earth’s orbital plane at two points, called nodes.

The moon’s orbital plane precesses, or wobbles, to a maximum and minimum of +/- 5 degrees over a period of about 18.6 years. This natural cycle of orbits is called the lunar nodal cycle. When the lunar plane is more closely aligned with the plane of Earth’s equator, tides on Earth are exaggerated. Conversely, when the lunar plane tilts further away from the equatorial plane, tides on Earth are muted, relatively.

The lunar nodal cycle was first formally documented in 1728 but has been known to keen astronomical observers for thousands of years.

What effect does that have on sea level?

The effect of the nodal cycle is gradual – it’s not anything that people would notice unless they pay ridiculously close attention to the precise movement of the moon and the tides for decades.

But when it comes to predictions of tides, dozens of astronomical factors are accounted for, including the lunar nodal cycle.

It’s worth being aware of this influence, and even taking advantage of it. During the most rapid downward phase of the lunar nodal cycle – like we’re in right now – we have a bit of a reprieve in the observed rate of sea level rise, all other things being equal.

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These are the years to implement infrastructure plans to protect coastal areas against sea level rise.

Once we reach the bottom of the cycle around 2025 and start the upward phase, the lunar nodal cycle begins to contribute more and more to the perceived rate of sea level rise. During those years, the rate of sea level rise is effectively doubled in places like Miami. The impact varies from place to place since the rate of sea level rise and the details of the lunar nodal cycle’s contribution vary.

Like the “supermoon” in late April, the one on May 26 is a perigean full moon. Even with the lunar nodal cycle in its current phase, cities like Miami should expect some coastal flooding.

This story is part of Oceans 21
Our series on the global ocean opened with five in depth profiles. Look out for new articles on the state of our oceans in the lead up to the UN’s next climate conference, COP26. The series is brought to you by The Conversation’s international network.

Brian McNoldy, Senior Research Associate, University of Miami

This article is republished from The Conversation under a Creative Commons license. Read the original article.

For 149 years, Americans have marked Arbor Day on the last Friday in April by planting trees. Now business leaders, politicians, YouTubers and celebrities are calling for the planting of millions, billions or even trillions of trees to slow climate change.

As ecologists who study forest restoration, we know that trees store carbon, provide habitat for animals and plants, prevent erosion and create shade in cities. But as we have explained elsewhere in detail, planting trees is not a silver bullet for solving complex environmental and social problems. And for trees to produce benefits, they need to be planted correctly – which often is not the case.

Tree-planting is not a panacea

It is impossible for humanity to plant its way out of climate change, as some advocates have suggested, although trees are one part of the solution. Scientific assessments show that avoiding the worst consequences of climate change will require governments, businesses and individuals around the globe to make rapid and drastic efforts to reduce greenhouse gas emissions.

Moreover, planting trees in the wrong place can have unintended consequences. For example, planting trees into native grasslands, such as North American prairies or African savannas, can damage these valuable ecosystems.

Planting fast-growing, nonnative trees in arid areas may also reduce water supplies. And some top-down tree-planting programs implemented by international organizations or national governments displace farmers and lead them to clear forests elsewhere.

Large-scale tree-planting initiatives have failed in locations from Sri Lanka to Turkey to Canada. In some places, the tree species were not well suited to local soil and climate conditions. Elsewhere, the trees were not watered or fertilized. In some cases local people removed trees that were planted on their land without permission. And when trees die or are cut down, any carbon they have taken up returns to the atmosphere, negating benefits from planting them.

Focus on growing trees

We think it’s time to change the narrative from tree-planting to tree-growing. Most tree-planting efforts focus on digging a hole and putting a seedling in the ground, but the work doesn’t stop there. And tree-planting diverts attention from promoting natural forest regrowth.

To achieve benefits from tree-planting, the trees need to grow for a decade or more. Unfortunately, evidence suggests that reforested areas are often recleared within a decade or two. We recommend that tree-growing efforts set targets for the area of forest restored after 10, 20 or 50 years, rather than focusing on numbers of seedlings planted.

And it may not even be necessary to actively plant trees. For example, much of the eastern U.S. was logged in the 18th and 19th centuries. But for the past century, where nature has been left to take its course, large areas of forests have regrown without people planting trees.

Helping tree-growing campaigns succeed

Tree-growing is expected to receive unprecedented financial, political and societal support in the coming years as part of the U.N. Decade on Ecosystem Restoration and ambitious initiatives such as the Bonn Challenge and World Economic Forum 1t.org campaign to conserve, restore and grow 1 trillion trees. It would be an enormous waste to squander this unique opportunity.

Here are key guidelines that we and others have proposed to improve the outcomes of tree-planting campaigns.

• Keep existing forests standing. Global Forest Watch, an online platform that monitors forests around the world, estimates that the Earth lost an area of rainforest the size of New Mexico in 2020. It is much more effective to prevent clearing of existing forests than to try to put them back together again. And existing forests provide benefits now, rather than decades into the future after trees mature.

Protecting existing forests often requires providing alternative income for people who maintain trees on their land rather than logging them or growing crops. It also is important to strengthen enforcement of protected areas, and to promote supply chains for timber and agricultural products that do not involve forest-clearing.

• Include nearby communities in tree-growing projects. International organizations and national governments fund many tree-growing projects, but their goals may be quite different from those of local residents who are actually growing the trees on their land. Study after study has shown that involving local farmers and communities in the process, from planning through monitoring, is key to tree-growing success.

• Start with careful planning. Which species are most likely to grow well given local site conditions? Which species will best achieve the project’s goals? And who will take care of the trees after they are planted?

It is important to plant in areas where trees have grown historically, and to consider whether future climatic conditions are likely to support trees. Planting in areas that are less productive for agriculture reduces the risk that the land will be recleared or existing forests will be cut down to compensate for lost productive areas.

• Plan for the long term. Most tree seedlings need care to survive and grow. This may include multi-year commitments to water, fertilize, weed and protect them from grazing or fire and monitor whether the venture achieves its goals.

We encourage people who support tree-growing efforts to ask where the money is going – to the organization’s managers, or to landowners who are actually growing the trees? Who is monitoring the effort and how long will they track it?

Growing trees can help solve some of the most pressing challenges of our time. But it is important to understand that planting seedlings is just the first step.

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Karen D. Holl, Professor of Restoration Ecology, University of California, Santa Cruz and Pedro Brancalion, Professor of Forest Restoration, Universidade de São Paulo

This article is republished from The Conversation under a Creative Commons license. Read the original article.