Understanding the impact of climate change on the ocean

When deciding on a major, one thing was clear for Michelle Kornberg — she didn’t want to be stuck inside for four years. “I like the environment of working on something in the lab, but I grew up in a very outdoorsy family,” she says. “I definitely knew I didn’t want to be inside all the time.”

During MIT’s First-Year Pre-Orientation Program, Kornberg got a sense of which major would help her achieve this goal. One of her orientation counselors presented on their summer aboard the research vessel Nautilus Explorer. The counselor was studying Course 2-OE (Mechanical and Ocean Engineering). Later that year, Kornberg declared 2-OE.

“I think ocean engineering as a field is really interesting because it marries the holistic side of living on planet Earth with solving all the technical challenges mechanical engineers face,” explains Kornberg, now a senior.

This balance of using fundamental theories in areas like fluid dynamics, controls, and acoustics to solve problems in underwater environments has been a driving force throughout her academic career.

“I am interested in how we can apply specific ocean engineering solutions to larger global problems, particularly climate change,” Kornberg adds.

Throughout her time in Course 2-OE, Kornberg has tackled problems stemming from climate change through an ocean engineer’s lens in a number of ocean environments, including Boston Harbor and the Great Barrier Reef.

Measuring ocean acidification in Boston Harbor

By the middle of her first year, Kornberg started working with the MIT Sea Grant College Program as an undergraduate researcher working alongside Thomas Consi, research education specialist. In addition to building a model tugboat through her work with Consi, Kornberg worked on a project to track ocean acidification in Boston Harbor.

During coastal ocean acidification, higher levels of carbon in the water cause algae blooms that negatively impact organisms in the ecosystems. In Boston Harbor, shellfish suffer in particular. To understand the impact coastal ocean acidification has in Boston, Kornberg worked with Consi on developing a suite of sensors to take measurements below the water.

“Being able to track different elements of the coastal ocean environment is important to understanding the chain of events that leads to things like coastal ocean acidification,” she says.

In addition to mapping the topography of the ocean floor — known as bathymetry — the team measured variables such as water current speed. Their goal was to identify the best place to situate a measurement station that could track coastal ocean acidification in Boston Harbor.

Kornberg would have more opportunities to work on one of Boston’s waterways throughout her 2-OE classes. In the capstone classes 2.013/2.014 (Engineering Systems Design and Development) during her junior year, she worked with a team on designing and building a working prototype of an autonomous vehicle.

“The class was the first time I had the experience of working on a team with people with very different backgrounds,” Kornberg adds. “It was such a good experience — you had some people who focused on electronics and others who had a background in materials science.”

The student team built a prototype for an MIT Lincoln Laboratory project called the Ionobot. An autonomous surface vessel, the Ionobot measures changes in the ionosphere to help identify and prevent interference with GPS signals. 

On the day of the final project presentation, Kornberg and her team assembled on the docks of the MIT Sailing Pavilion, nervously awaiting the moment of truth. Would their prototype work as they planned?

An angry storm offered the team one additional challenge — windy conditions made for a choppy day on the Charles River. The team launched the Ionobot prototype and took a sigh of relief. It performed as they expected.

“It was objectively the worst weather day that semester, but it was still somehow the greatest day,” recalls Kornberg.

Several weeks later, Kornberg would be working in very different conditions along the northeast coast of Australia.

Tracking coral bleaching in the Great Barrier Reef

Last summer, Kornberg worked at the Australian Institute of Marine Science (AIMS) in Townsville, Australia through the MIT International Science and Technology Initiatives. Alongside Nicholas Fritzinger-Pittman, a senior studying mechanical engineering, Kornberg worked on developing a rail system that would enable a hyperspectral scanner to collect accurate data on thousands of coral samples in the AIMS Sea Simulator.

“In a regular camera, each pixel has a specific color. A hyperspectral camera, on the other hand, assigns a frequency distribution to each pixel, allowing you to track more than just physical light,” explains Kornberg.

This technology is particularly useful in predicting whether or not a coral is experiencing the early stages of bleaching — a condition often caused by climate change that has destroyed of large swaths of coral reefs globally.

“Coral depend on algae to stay alive. The algae can tell when conditions are suboptimal for photosynthesis before the coral can so they ‘get out of Dodge’ and leave the coral. This is the first sign of coral bleaching,” Kornberg adds.

Since researchers are unable to see algae leave the coral using the visual spectrum, they rely on a linear hyperspectral scanner to capture this process.

Before Kornberg and Fritzinger-Pittman arrived at AIMS, coral samples would have to be transported from one tank to the tank with the hyperspectral scanner on it. Since removing the coral from their environment could compromise the experiment, Kornberg and Fritzinger-Pittman worked with researchers at AIMS to develop a portable rail system that would bring the hyperspectral scanner to the coral — not the other way around.

As part of their time at AIMS, Kornberg and Fritzinger-Pittman joined a research cruise on the RV Cape Ferguson. Over the course of six days, the team visited Keeper Reef and John Brewer Reef. During breaks in research, they were able to snorkel explore the reef.

“Snorkeling along the reef was just an incredibly humbling and magical experience,” says Kornberg.

Later in the summer, the pair also took a scuba diving trip to Cairns, Australia. While they got to see colorful reefs and animals like clownfish, giant clams, and sea turtles, they also saw the effects of coral bleaching on some parts of the reef off the coast of Cairns.

“Seeing some coral bleaching in the reef itself was a really sad, sobering sight,” she adds.

Wave energy conversion in Spain

As Kornberg prepares for life after graduation this May, she is shifting her focus to renewable energy.

“I personally am most interested in looking at how we can build platforms that harness energy from the ocean environment,” she says.

Kornberg plans to work for several months this fall at Seaplace in Madrid, Spain. Her focus will be on designing and building marine turbines for wave energy conversion.

“Waves offer so much potential in terms of renewable energy. We’ve known this for a long time, but haven’t been able to figure out how to best harness this energy,” she explains. “I’m really interested in working on technologies that can help us utilize wave energy at scale.”

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Staring into the vortex

Imagine a massive mug of cold, dense cream with hot coffee poured on top. Now place it on a rotating table. Over time, the fluids will slowly mix into each other, and heat from the coffee will eventually reach the bottom of the mug. But as most of us impatient coffee drinkers know, stirring the layers together is a more efficient way to distribute the heat and enjoy a beverage that’s not scalding hot or ice cold. The key is the swirls, or vortices, that formed in the turbulent liquid.

“If you just waited to see whether molecular diffusion did it, it would take forever and you'll never get your coffee and milk together,” says Raffaele Ferrari, Cecil and Ida Green Professor of Oceanography in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS).

This analogy helps explain a new theory on the intricacies the climate system on Earth — and other rotating planets with atmospheres and/or oceans — outlined in a recent PNAS paper by Ferrari and Basile Gallet, an EAPS visiting researcher from Service de Physique de l'Etat Condensé, CEA Saclay, France.

It may seem intuitive that Earth’s sun-baked equator is hot while the relatively sun-deprived poles are cold, with a gradient of temperatures in between. However, the actual span of that temperature gradient is relatively small compared to what it might otherwise be because of the way the Earth system physically transports heat around the globe to cooler regions, moderating the extremes.

Otherwise, “you would have unbearably hot temperatures at the equator and [the temperate latitudes] would be frozen,” says Ferrari. “So, the fact that the planet is habitable, as we know it, has to do with heat transport from the equator to the poles.”

Yet, despite the importance of global heat flux for maintaining the contemporary climate of Earth, the mechanisms that drive the process are not completely understood. That’s where Ferrari and Gallet’s recent work comes in: their research lays out a mathematical description of the physics underpinning the role that marine and atmospheric vortices play in redistributing that heat in the global system.

Ferrari and Gallet’s work builds on that of another MIT professor, the late meteorologist Norman Phillips, who, in 1956, proposed a set of equations, the “Phillips model,” to describe global heat transport. Phillips’ model represents the atmopshere and ocean as two layers of different density on top of each other. While these equations capture the development of turbulence and predict the distribution of temperature on Earth with relative accuracy, they are still very complex and need to be solved with computers. The new theory from Ferrari and Gallet provides analytical solutions to the equations and quantitatively predicts local heat flux, energy powering the eddies, and large-scale flow characteristics. And their theoretical framework is scalable, meaning it works for eddies, which are smaller and denser in the ocean, as well as cyclones in the atmosphere that are larger.

Setting the process in motion

The physics behind vortices in your coffee cup differ from those in nature. Fluid media like the atmosphere and ocean are characterized by variations in temperature and density. On a rotating planet, these variations accelerate strong currents, while friction — on the bottom of the ocean and atmosphere — slows them down. This tug of war results in instabilities of the flow of large-scale currents and produces irregular turbulent flows that we experience as ever-changing weather in the atmosphere.

Vortices — closed circular flows of air or water — are born of this instability. In the atmosphere, they’re called cyclones and anticyclones (the weather patterns); in the ocean they’re called eddies. In both cases, they are transient, ordered formations, emerging somewhat erratically and dissipating over time. As they spin out of the underlying turbulence, they, too, are hindered by friction, causing their eventual dissipation, which completes the transfer of heat from the equator (the top of the hot coffee) to the poles (the bottom of the cream).

Zooming out to the bigger picture

While the Earth system is much more complex than two layers, analyzing heat transport in Phillips’ simplified model helps scientists resolve the fundamental physics at play. Ferrari and Gallet found that the heat transport due to vortices, though directionally chaotic, ends up moving heat to the poles faster than a more smooth-flowing system would. According to Ferrari, “vortices do the dog work of moving heat, not disorganized motion (turbulence).”

It would be impossible to mathematically account for every single eddy feature that forms and disappears, so the researchers developed simplified calculations to determine the overall effects of vortex behavior, based on latitude (temperature gradient) and friction parameters. Additionally, they considered each vortex as a single particle in a gas fluid. When they incorporated their calculations into the existing models, the resulting simulations predicted Earth’s actual temperature regimes fairly accurately, and revealed that both the formation and function of vortices in the climate system are much more sensitive to frictional drag than anticipated.

Ferrari emphasizes that all modeling endeavors require simplifications and aren’t perfect representations of natural systems — as in this instance, with the atmosphere and oceans represented as simple two-layer systems, and the sphericity of the Earth is not accounted for. Even with these drawbacks, Gallet and Ferrari’s theory has gotten the attention of other oceanographers.

“Since 1956, meteorologists and oceanographers have tried, and failed, to understand this Phillips model,” says Bill Young, professor of physical oceanography at Scripps Institution of Oceanography, “The paper by Gallet and Ferrari is the first successful deductive prediction of how the heat flux in the Phillips model varies with temperature gradient.”

Ferrari says that answering fundamental questions of how heat transport functions will allow scientists to more generally understand the Earth’s climate system. For instance, in Earth’s deep past, there were times when our planet was much warmer, when crocodiles swam in the arctic and palm trees stretched up into Canada, and also times when it was much colder and the mid-latitudes were covered in ice. “Clearly heat transfer can change across different climates, so you'd like to be able to predict it,” he says. “It's been a theoretical question on the minds of people for a long time.”

As the average global temperature has increased more than 1 degree Celsius in the past 100 years, and is on pace to far exceed that in the next century, the need to understand — and predict — Earth’s climate system has become crucial as communities, governments, and industry adapt to the current changing environment.

“I find it extremely rewarding to apply the fundamentals of turbulent flows to such a timely issue,” says Gallet, “In the long run, this physics-based approach will be key to reducing the uncertainty in climate modelling.”

Following in the footsteps of meteorology giants like Norman Phillips, Jule Charney, and Peter Stone, who developed seminal climate theories at MIT, this work too adheres to an admonition from Albert Einstein: "Out of clutter, find simplicity."

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Scientists quantify how wave power drives coastal erosion

Over millions of years, Hawaiian volcanoes have formed a chain of volcanic islands stretching across the Northern Pacific, where ocean waves from every direction, stirred up by distant storms or carried in on tradewinds, have battered and shaped the islands’ coastlines to varying degrees.

Now researchers at MIT and elsewhere have found that, in Hawaii, the amount of energy delivered by waves averaged over each year is a good predictor of how fast or slow a rocky coastline will erode. If waves are large and frequent, the coastline will erode faster, whereas smaller, less frequent waves will result in a slower-eroding coast.

Their study helps to explain the Hawaiian Islands’ meandering shorelines, where north-facing sea cliffs, experiencing larger waves produced by distant storms and persistent tradewinds, have eroded farther inland. In contrast, south-facing coasts typically enjoy calmer waters, smaller waves, and therefore less eroded coasts.

The results, published this month in the journal Geology, can also help scientists forecast how fast other rocky coasts around the world might erode, based on the power of the waves that a coast typically experiences.

“Over half of the world’s oceanic coastlines are rocky sea cliffs, so sea-cliff erosion affects a lot of coastal inhabitants and infrastructure,” says Kim Huppert ’11, PhD ’17, lead author of the study and a former graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “If storminess increases with climate change, and waves get bigger, we need to understand specifically how waves affect erosion.”

Huppert, who is now a senior research scientist at the German Research Center for Geosciences, has co-authored the paper with Taylor Perron, professor of earth, atmospheric, and planetary sciences and associate department head at MIT, and Andrew Ashton of the Woods Hole Oceanographic Institution.

Sink and carve

Scientists have had some idea that the rate of coastal erosion depends on the power of the waves that act on that coast. But until now, there’s been no systematic study to confirm this relationship, mainly because there can be so many other factors contributing to coastal erosion that can get in the way.

The team found the Hawaiian Islands provide an ideal environment in which to study this relationship: The islands are all made from the same type of bedrock, meaning they wouldn’t have to account for multiple types of rock and sediment and their differences in erosion; and the islands inhabit a large oceanic basin that produces a wide range of wave “climates,” or waves of varying sizes and frequencies.

“As you go around the shoreline of different islands, you see very different wave climates, simply by turning a corner of the island,” Huppert notes. “And the rock type is all the same. So Hawaii is a nice natural laboratory.”

The researchers focused their study on 11 coastal locations around the islands of Hawaii, Maui, and Kaho‘olawe, each facing different regions of the Pacific that produce varying sizes and frequencies of waves.

Before considering the wave power at these various locations, they first worked to estimate the average rate at which the sea cliffs at each coastal location eroded over the last million years. The team sought to identify the erosion rates that produced the coastal profiles of Hawaiian Islands today, given the islands’ original profiles, which can be estimated from each island’s topography. To do this, they first had to account for changes in each island’s vertical motion and sea level change over time.

After a volcanic island forms, it inevitably starts to subside, or sink under its own weight. As an island sinks, the level at which the sea interacts with the island changes, just as if you were to lower yourself into a pool: The water’s surface may start at your ankles, and progressively lap at your knees, your waist, and eventually your shoulders and chin.

For an island, the more slowly it sinks, the more time the sea has to carve out the coastline at a particular elevation. In contrast, if an island sinks quickly, the sea has only fleeting time to cut into the coast before the island subsides further, exposing a new coastline for the sea to wear away. As a result, the rate at which an island sinks strongly affects how far the coast has retreated inland at any given elevation, over millions of years.

To calculate the speed of island sinking, the team used a model to estimate how much the lithosphere, the outermost layer of the Earth on which volcanic islands sit, sagged under the weight of each Hawaiian volcano formed in the past million years. Because the Hawaiian Islands are close together, the sinking of one island can also affect the sinking or rising of neighboring islands, similar to the way one child may bounce up as another child sinks into a trampoline.

The team used the model to simulate various possible histories of island sinking over the last million years, and the subsequent erosion of sea cliffs and coastlines. They looked for the scenario that best linked the islands’ original coastlines with today’s modern coastlines, and matched the various resulting erosion rates to the 11 locations that they focused on in their study.

“We found erosion rates that vary from 17 millimeters per year to 118 millimeters per year at the different sites,” Huppert says. “The upper end of that range is nearly half a foot per year, so some of those rates are pretty fast for rock.”

Waves of a size

They chose the 11 coastal locations in the study for their variability: Some sea cliffs face north, where they are battered by stronger waves produced by distant storms. Other north-facing coasts experience tradewinds that come from the northeast and produce waves that are smaller but more frequent. The coastal locations that face southward experience smaller, less-frequent waves in contrast.

The team compared erosion rates at each site with the typical wave power experienced at each site, which they calculated from wave height and frequency measurements derived from buoy data. They then compared the 11 locations’ wave power to their long-term rates of erosion.

What they found was a rather simple, linear relationship between wave power and the rate of coastal erosion. The stronger the waves that a coast experiences, the faster that coast erodes. Specifically, they found that waves of a size that occur every few days might be a better indicator of how fast a coast is eroding than larger but less frequent storm waves. That is, if  waves on normal, nonstormy days are large, a coast is likely eroding quickly; if the typical waves are smaller, a coast is retreating more slowly.

The researchers say carrying out this study in Hawaii allowed them to confirm this simple relationship, without confounding natural factors. As a result, scientists can use this relationship to help predict how rocky coasts in other parts of the world may change, with variations in sea level and wave activity as a result of climate change.

“Sea level is rising along much of the world’s coasts, and changes in winds and storminess with ongoing climate change could alter wave regimes, too,” Perron points out. “To be able to isolate the influence of wave climate on the rate of coastal erosion gets you one step closer to going to a particular place and calculating the change in erosion rate there.”

This research was supported, in part, by the NEC Corporation, and by NASA.

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