The latest satellite data based on 25 years of observation shows that sea level rise by 2100 will double the original projections based on climate models. Originally the increase was expected to be a steady rate. But the data says something entirely different.
The cause is accelerated melting in Greenland and Antarctica with current projections indicating mean global sea levels will rise by 65 centimetres (26 inches). The implications for cities lying on coastlines are significant.
Steve Nerem, a professor of Aerospace Engineering Sciences at The University of Colorado – Boulder, who is working with NASA’s Sea Level Change project, believes the projected rise is a conservative estimate based on changes seen in the Arctic and Antarctic.
How is sea level measured? Since 1992, a series of satellites have been remotely monitoring the ocean surface using onboard altimeters. They initially observed an annual rise of 1.8 millimetres (0.07 inches) in 1993. Today the rise is 4.2 millimetres (0.17 inches) per year. The total rise since 1993 is 9.4 centimetres (4 inches).
The satellite data has been supplemented by tide gauge data collected for more than three decades. What is a tide gauge? It is an Earth-bound piece of monitoring equipment with sensors to record surrounding water level heights. Before we started measuring global sea level rise, tide gauges were used to support safe navigation along coastlines.
Gary Mitchum, at the College of Marine Science, University of South Florida, notes that the tide gauge data is “essential for determining the uncertainty in the global mean sea level acceleration estimate.” These Earth-based metrics provide backup to the satellite observations.
In the past, sea levels have dropped after volcanic eruptions and La Niña Southern Ocean oscillations, and have risen from thermal expansion during El Niños like the current strong one which no doubt has raised ocean levels temporarily.
The melting of polar ice from Greenland and Antarctica isn’t just contributing to rising seas. A 2021 Harvard study notes that when the glacial ice from these two areas melts it impacts the land lying beneath. We are still witnessing a phenomenon called isostatic rebound in North America from the last Ice Age which started 2.6 million years ago and ended in geological time very recently a mere 11,000 years ago.
Sophie Coulson is the lead author of the 2021 Harvard study published in Geophysical Research Letters. In an article in The Harvard Gazette, Coulson talked about the various forces imposed on Earth’s crust by tectonic movement. She stated, “We need to be able to separate…motion generated by modern-day ice-mass loss.” In addition, she noted that “Earth is actually still rebounding from that ice melting.”
Coulson is making reference to an observed phenomenon, that parts of North America continue to rise from the disappearance of the ice causing warping of the continental landmass hundreds of kilometres away. A good example is the land around the Chesapeake Bay warping downward as Pennsylvania does the opposite and rises to the northwest.
Another consequence of melting polar ice is changes to the composition of ocean surface water. Freshwater ice melt is diluting the saltwater. Freshwater’s lower specific gravity causes it to overlay heavier saltwater layers.
In addition, the land ice melt contributes to changes in the amount of sea ice exposing more surface water to the atmosphere with its increasing burden of carbon dioxide (CO2) from anthropogenic sources of pollution. The greater exposure means a warmer ocean surface which can then absorb more CO2 which creates carbonic acid which makes the Arctic Ocean more acidic, threatening regional biodiversity.
For humans on land, these dynamic forces represent a variety of challenges. If sea levels are accelerating, coastal communities will be further subjected to flooding. It means coastal freshwater aquifers will experience more saltwater intrusion. And if biodiversity within the ocean is negatively impacted, it means that communities dependent on the largesse of the ocean will face greater food insecurity.
]]>We know the Moon as it circles our planet is a major influencer of ocean tides. But sometimes when the Moon and the Sun align, we get higher than normal tides. When they are on opposite sides of the planet then tides are lower. And as much as we think of the Moon orbiting Earth as a constant, it’s not. Its orbital plane does not match up with the Earth’s axial rotation. Hence it appears some nights to be higher in the sky, and on other nights much lower. That is the wobble NASA scientists are talking about, an 18.6-year cycle. The implications for our planet, say the scientists, are simple. Low-lying areas need to look out for a dramatic increase in coastal flooding by the mid-2030s.
In June 2021, a published study appeared in Nature Climate Change entitled “Rapid increases and extreme months in projections of United States high-tide flooding.” Scientists from Hawai’i, NOAA, Jet Propulsion Laboratory Pasadena, and the Scripps Institution of Oceanography, UC San Diego, were the authors. In it, they write that by the mid-2030s there will be a rapid increase in high tide flooding on coastlines around the world. The article’s focus, however, is largely the continental U.S.
In a press release issued by NASA, Bill Nelson, Administrator stated, “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.”
Principal author of the study, Phil Thompson from the University of Hawai’i notes in the release that “the accumulated effect over time…will have an impact.” What impact exactly?
The Moon wobble cycle of 18.6 years either suppresses daily ocean tides or amplifies them. First observed and recorded in 1728, what’s making the 2030s more significant is the accumulated and accelerating rise in sea levels caused by a warming atmosphere and ocean. Currently, we are in the tide-amplifying latter half of a wobble cycle where sea levels right now haven’t yet reached a point to noticeably change flooding thresholds. Even now, however, we see increased sunny day flooding in places like Florida, the Chesapeake Bay area, and the Carolina coast. But in the 2030s with another decade of accelerating sea-level rise under our planet’s belt, that frequency will increase to what may become a chronic climate condition.
What exactly is the historic and current rate of sea-level rise? Observations by tidal gauge from the 19th through the 20th century, and in the latter few decades from satellites, indicate a global mean rise of 16 centimetres (approximately 6 inches). But what is of greater concern is the accelerating rate of the rise in the last decade, calculated at 3.6 centimetres (1.45 inches). That means 24% of the total rise over a century and a half has happened in the last ten years. And the acceleration is continuing.
How will this play out? For mid to low-latitude coastal communities, it will mean a leap in the number of flood days each month. Thompson talks about “floods 10 or 15 times a month.” At that rate, how sustainable will existing coastal communities be? Unless they are developing mitigation and adaptation strategies, we will see more of what you see in the image that appears with this posting.
NASA is as vulnerable to sea-level rise as the rest of the U.S. coast. Cape Canaveral where the agency launches most of its rockets is barely above sea level. The same is true about Wallops Island where other launches occur.
So NASA has created a series of high-tide flood tools and made them accessible through a sea-level portal on the web. The portal provides flood day projections, data analysis, sea-level evaluation and assessment, and a virtual Earth system laboratory.
The NASA release noted that the coastlines of the entire lower-48 states and Hawai’i will be most impacted by sea-level rise in the 2030s. Only Alaska will be spared. Why is that? Because Alaska is still rising from being weighed down by the thousands of metres of glacial ice that covered it until 11,700 years ago. Alaska along with northern Canada, Scandinavia, and northern Russia have been experiencing an isostatic rebound, a slow release and rise of land formerly buried by ice. The land rise will more than offset the sea rise.
For the U.S., however, from the Texas Gulf coast to the Chesapeake Bay, expect chronic coastal flooding. New York City and Boston will not be spared. Nor will the Atlantic provinces of Canada, lowlands in Northern Europe, Asia, Africa, South America, and Australia. Even inland bodies of water like the Great Lakes of both Africa and North America will see shoreline inundations as the combination of increasing volumes of larger warmer bodies of water get pulled by amplified lunar gravitational forces.
Ben Hamlington at the Jet Propulsion Laboratory in Pasadena, California, one of the authors of the paper talks about the importance of the NASA sea level portal stating, “From a planning perspective…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.”
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What they report in their paper is, that polar ice sheets exert a gravitational pull on the water to which they are adjacent. That these slight pulls make sea levels higher at the intersection point where ice meets ocean. So as a glacier loses mass the pull weakens causing adjacent sea levels to fall. And as ice mass decreases, the land underneath it experiences isostatic rebound.
What is isostatic rebound?
It is the rise in elevation of continental land masses following melting of glaciers and ice sheets lying on top of them. Once the weight of the ice is gone, the land literally bobs back up. That rebound further lowers sea levels adjacent to the land where the ice mass melts. The combination of melt and rebound alters Earth’s gravitational field causing the extra volume of water to migrate far from its points of origin. For example, the researchers note that Antarctic ice melt correlates to a 52% faster rate off sea level rise off California and Florida than elsewhere on the planet.
Velicogna, the lead scientist of the paper, notes that lower latitudes exhibit the highest sea level rise because of gravitational field changes. The data that supports this comes from NASA’s GRACE satellites which measure gravitational changes on the Earth’s surface. Covering 12 years, from 2002 to 2014, scientists analyzed GRACE findings and then correlated them to other measurement sources including pressure stations on the ocean floor. The latter measure the mass of water above their locations and therefore give an accurate vertical height of the ocean surface.
What are the implications of this research?
Since sea level rise is variable across the planet it is important that scientists measure as many points as possible to understand the implications of further ice melts from Antarctic, Greenland and the Canadian Northern Archipelago. There should be a call for the widespread deployment of ocean floor pressure stations feeding continuous measurements so that we have a better picture of which coastlines will be most impacted by these gravitational shifts. For example, we know that the northern parts of North America and Siberia are still experiencing isostatic rebound from the last Ice Age, more than ten thousand years after the ice retreated. Geology and geological time move slowly.
Another phenomenon complicates issues further. Those northern areas of the hemisphere that rebound cause areas of the continent adjacent to them to be impacted in reverse. For example, the subsidence of The Chesapeake Bay area and Florida is in part because of continental rebound experiencing to the north.
Global Thermal Expansion and Weather Phenomena
Back in August, I wrote about another paper appearing in Geophysical Research Letters that described sea level rise three times faster on the Eastern Seaboard of the United States than the global average. The paper’s authors argued that two phenomena, the North Atlantic Oscillation combined with an El Nino were the contributing factors to this accelerated rise.
Add to this thermal expansion, a phenomenon caused by density decreases in water when it is heated causing it to expand. Ocean water columns are showing increased warming throughout. This phenomenon is contributing to sea level rise, but to what extent, remains an unknown. represents an additional threat to coastlines, particularly during high tides and storm surges.
For cities like Miami and Miami Beach, the combination of all of these phenomena, glacial ice melt, gravitational shifts, isostatic rebound, confluences of weather phenomena, and thermal expansion, represent the reasons why there are increasingly frequent sunny-day flooding events in these communities.
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