What are the Primary Direct Causes of Sea Level Rise?
Between 1880 and 2022, average global sea levels have risen by between 8 and 9 inches.
This is concerning news as it puts nearly 1/3 of the US population that resides in the growing coastal city regions at significantly greater risk for flooding and weather-related disasters such as hurricanes.
And the problem is not getting better.
The rate of sea level rise has accelerated significantly in recent years.
During the period 2006 – 2015, seas rose by 0.14 inches annually – which is more than double the average of 0.06 inches a year measured throughout the 20th century.
What are the primary direct causes of sea level rise?
Before we begin, we can assume that the amount of water globally remains constant – what changes is where it’s located, e.g. in ice, in the oceans, on the land, or in the atmosphere (if only temporarily).
To be more specific, some water sources are locked in glaciers and ice sheets (known as the cryosphere), water is stored on land in lakes and reservoirs, and some water resources are retained in underground aquifers. And of course, water does evaporate and collect in the clouds, eventually returning to earth as rainwater.
Now let’s talk about the two primary contributors to sea level rise.
Volumetric Thermal Expansion of Water
The first requires a quick physics recap. We’re familiar with hot air balloons – they can fly upward as the air inside the balloon heats up and becomes less dense than the air around it.
Water has some similar properties; its volume and density change depending on its temperature, a property known as volumetric thermal expansion.
At 4C (40F), water is at its most dense – 1000kg/m3.
But as the temperature of water rises, it becomes less dense.
At 26C (79F), the temperature that hurricanes can form, the density of water is only 996.8 kg/m3.
On its face, it may seem like a very subtle difference, but this decrease in density (and the corresponding increase in volume) causes oceans to expand as they get warmer, leading to increased sea levels.
(This change in water density/volume is also responsible for the Atlantic Meridional Overturning Circulation, or AMOC, more commonly known as “the Gulf Stream,” which shuttles warm water from the Gulf of Mexico region to the northern Atlantic, keeping temperatures in Europe warmer despite their high latitude.)
Melting Ice
The second primary cause of sea level rise is melting ice, typically glacier ice on land or ice sheets extending over ocean waters.
According to NOAA researchers, from the 1970s to the early 2000s, the primary causes of sea level rise were evenly split between ice melting and thermal expansion of the oceans due to increased water temperatures.
However, between 2005 and 2013, melting ice became the dominant cause of sea level rise, contributing more than double the effect compared to thermal expansion, a trend that is accelerating.
We can also mention a third cause of sea level rise here, although it has a relatively minor contribution. In recent decades, there has been a large increase in pumping water out of underground aquifers, particularly in areas hard hit by droughts, such as the Western US. A large portion of this groundwater does not return to replenish the aquifers from where it came; instead, much of it eventually ends up in the ocean through riverine drainage systems or as rain.
What are the Indirect Factors that Contribute to Sea Level Rise?
As it turns out, the primary causes of sea level rise (e.g. melting ice and thermal expansion of water) are fairly easy to describe – the underlying factors that cause these effects are not, however.
To create models that account for these factors, climate researchers need to take into consideration many different phenomena, including:
- Increased atmospheric heat due to greenhouse gases.
- Natural water temperature cycles, such as El Nino and La Nina.
- Cloud coverage density, including condensation trails (contrails) created by jet aircraft, can change the amount of energy absorbed from the sun or reflected into space.
- Greenhouse gas emissions (especially methane and carbon dioxide) can increase global temperatures.
- Aerosol and chemical pollution that expands the “Ozone hole.”
- Contact between ice sheets and glaciers and running meltwater, which can accelerate further melting or even complete collapse into the ocean.
- Reduction in foliage, such as forests, that can capture carbon pollution.
- Increased or decreased rainfall due to droughts, hurricanes, agricultural irrigation, etc.
- Permafrost melting can potentially release large amounts of sequestered carbon into the atmosphere.
- Localized changes to land elevation, such as long-term tectonic plate movements, land rise due to rebound from melted ice sheet coverage, and land subsidence due to pumping groundwater, oil and gas extraction, etc.
Are There Mitigating Factors that Could Reduce the Rate of Sea Level Rise?
A few indirect factors could also help slow down ocean level rise.
Some of them, such as volcanic activity, come with their own set of problems!
The indirect factors include:
- Ejection of fine particles into the atmosphere by explosive volcanic eruptions – these can reduce the amount of sunlight reaching the Earth.
- Some researchers have argued that particulate pollution, such as from diesel engines, can also reflect solar energy into space, limiting temperatures in the atmosphere. (See our article published on September 18, 2024, for more analysis on this controversial stance.)
- Heat sequestration in the deep oceans. Some climate scientists point out that the oceans have absorbed much of the atmosphere’s heat to date, but there is a limit beyond which temperatures could rise even more quickly.
- Planting trees to absorb carbon and reduce urban heat island effects.
- Transitioning toward energy sources that do not emit greenhouse gases.
- Capturing and sequestering greenhouse gases, such as carbon dioxide. Possible proposed measures include pumping C02 in underground wells or deep in the ocean and/or finding ways to use C02 in industrial processes that keep it out of the atmosphere.
- Artificially seeding clouds to reduce temperatures and increase rainfall is a form of geoengineering. (See our upcoming November 27, 2024, article on this topic.)
Data Collection Methods to Measure Rising Sea Levels
As you can see from the last two sections, many competing indirect factors can either exacerbate or mitigate sea level rise.
This creates a big challenge for climate researchers who A) need to quantify, measure, or anticipate these effects as data inputs, and B) try to develop meaningful models that create useful simulations that account for the interactions between these different factors.
Before we address these complex interactions, let’s first take a look at the basic collection methods for measuring sea level.
Tides have been measured at ports around the world for centuries. NOAA has access to tidal measurements dating back to the time of Thomas Jefferson using tide gauges that measure each day’s high and low tide levels.
Beginning in the early 1990s, satellites flying overhead also began measuring sea levels using radar altimeters. The first was the TOPEX/Poseidon satellite, launched in August 1992. This was followed by Jason-1, Jason-2, and currently Jason-3, a joint venture between NASA and the French space agency CNES, which launched in 2016.
What about sea temperatures?
To measure surface sea temperatures (SSTs), researchers rely on a worldwide network of fixed and driving buoys, as well as samples taken from passing ships and measurements made by overhead satellites.
Aquatic robots measure the underwater oceanic temperatures in the upper levels of the ocean, while ships are used to lower down and retrieve temperature sensing instruments in the deepest ocean areas.
Ice Measurements
The measurement of ice lost to melting is accomplished in a variety of ways.
The first is old-fashioned field work, with researchers traveling to measure changes in glacier formations and ice sheet coverage. Photography taken by aerial and satellite surveillance helps assist in documenting changes.
The second method is to rely on satellite instruments that can be used to infer the loss of ice.
One way to do this is to measure the height (altitude) of land masses from space in areas where glacier ice has melted. With less weight on top of the earth’s crust, it will spring back up.
However, in practice, there is a better method. Launched in 2002, NASA’s pair of GRACE satellites, working in tandem, can measure gravitational anomalies across the earth’s surface. As ice melts and makes its way to the ocean, the amount of gravity over the land area decreases while it increases over the ocean.
Putting it all Together in a Complex Equation to Predict Global and Local Sea Level Rise
The challenge for climate researchers seeking to predict sea level rise in the future is how to create a working model that accounts for the different relationships between factors such as rising global air and sea temperatures, changes in greenhouse gas concentrations, and potential tipping points (such as Antarctic ice sheets collapsing into the ocean), beyond which sea level rise increases suddenly and dramatically.
Creating these complex thermodynamic mathematical models that can predict climate change and sea level rise is very challenging.
This may be a good use case for artificial intelligence and machine learning.
In a talk presented at the Nvidia 2021 GTC conference, Anima Anandkumar, a director of machine learning research at NVIDIA and professor of computing at the California Institute of Technology, presented a new AI-driven approach to climate modeling that speeds up calculations dramatically (by a factor of 100,000) while improving accuracy. This approach uses what they are calling a Fourier Neural Operator or FNO, which allows AI to solve a family of partial differential equations without any training data (known as a zero-shot in AI speak).
NOAA’s 2022 Sea Level Rise Predictions
Hopefully, NOAA will be able to incorporate more AI-based modeling predictions in their next intra-agency sea level predictions, which are updated every 4 or 5 years.
The most currently available predictions from NOAA for global sea level rise were issued in 2022.
In it, NOAA predicts that, on average, the sea level along the US coastlines will rise between 10 and 12 inches between 2020 and 2050. To underscore the increased rate of sea level rise, this 30-year estimate calls for as much additional sea level increase as occurred over the previous 100 years!
Some areas, such as the US Gulf Coast region and the area around Norfolk/Newport News, could see sea levels rise even more – between 14 and 18 inches due to ongoing subsidence along these regions of coastline.
Beyond 2050 there is a lot of variability in the forecast, depending on how well we can contain rising temperatures in the atmosphere.
If we can reduce current greenhouse gas emissions, sea level rise by 2100 would be only 2 feet higher than year 2000 levels.
But if we continue to increase greenhouse emissions, the 2022 report concludes global average sea levels in 2100 could be 7.2 feet higher than the year 2000 levels – a very sobering thought indeed.
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