Projecting Global Sea-level Change

David Holland, professor of Mathematics at NYU New York's Center for Atmosphere Ocean Science (CAOS) and principal investigator of the Project on Global Sea-level Change at NYU Abu Dhabi is working to develop credible projections on future sea-level change.

Along the low-lying beachfront of the city of Abu Dhabi stands a skyline punctuated with high-rise towers, beach clubs, restaurants, and cafes. As with many of the world's leading cities, much of Abu Dhabi's development sits at just above sea level, making the question of rising global seas a matter of critical self-interest for the city.

According to the World Bank, nearly one half of the world's population lives in coastal environments, many of which would be impacted by changes in coastal sea levels as ocean temperatures warm and ice sheets near Greenland and Antarctica melt. This is a very real concern; over the last century, the global sea level has risen by approximately 30 centimeters and the atmosphere has warmed by approximately one degree Centigrade, according to a report by the Intergovernmental Panel on Climate Change. Yet for all of the potential consequence on urban developments across the world — most of which were planned within the context of present-day sea levels — credible projections on future sea-level change are not currently available.

David Holland, professor of Mathematics at NYU New York's Center for Atmosphere Ocean Science (CAOS) and principal investigator of the Project on Global Sea-level Change at NYU Abu Dhabi, aims to change this.

"The behavior of the climate system on our planet is far more complex than I had ever imagined," he said. "Only through observation of the natural system, coupled with theoretical developments implanted in computer models, can we come to an understanding and projective capability of sea- level change."

The complexity of such projective modeling stems from the range of causes for climate change — natural variation and forced (human-caused) variation. Separating these two factors as they impact the environment simultaneously will require a significant amount of data and resources, Holland said.

Breaking down this problem requires an understanding of the chain of events that impact sea-level change, he explained. "The ice is affected by the ocean temperature, and ocean temperature is affected by the atmosphere. The question really is, 'why does the atmosphere change?' It changes because of natural variation, greenhouse gases, ozone changes, and unanticipated natural causes such as volcanoes — and all of these signals are progressing in an interrelated pattern."

The Project on Global Sea-level Change team has already started to make strides in contributing to the record of reliable observational data through field research in the Ilulissat and Helheim ice fjords in Greenland, where a team of nine deployed three times over the course of 12 weeks in 2012. Holland was joined by Logistical Coordinator of Research and Project Leader at NYUAD Denise Holland (also his wife), one postdoctoral fellow from NYUAD, one postdoctoral fellow from NYUNY's CAOS, and four NYUNY graduate students.

During its expeditions, the team positioned a range of meteorological, glaciological, and oceanographic monitoring devices to record several types of metrics that relate to the two principal causes of rising sea levels: glacial melting and changing ocean temperatures due to atmospheric warming. These two factors are "the lynchpins upon which future global sea level rests," according to Holland.

Only through observation of the natural system, coupled with theoretical developments implanted in computer models, can we come to an understanding and projective capability of sea-level change.

David Holland, professor of Mathematics at NYUNY's Center for Atmosphere Ocean Science and principal investigator of the Project on Global Sea-level Change at NYUAD

Through the placement of automatic weather stations — masts with weatherproof enclosures and meteorological sensors — the team is now able to monitor atmospheric data such as air temperature, air pressure, wind speed, and radiation, in addition to keeping a photographic record of calving (or melting) glaciers. Meanwhile, oceanographic data is collected through CTD (conductivity, temperature, depth) instruments that can be deployed in a number of ways, resulting in versatility of the information provided. They can be slung over the side of a ship to provide a vertical profile of the ocean's appearance and to help reveal the location of areas of warm water that are melting glaciers. Autonomous underwater vehicles, known as AUVs, can be used to carry CTD instruments as they travel throughout the ocean collecting data. Perhaps the most inventive solution to effectively deploy CTDs has been through the assistance of marine mammals, such as ringed seals, which can serve as CTD carriers to gather information in previously uncharted territory. When the seal dives, the instrument records data, and when it surfaces, a portable satellite telephone attached to the seal places a call to a satellite and transmits the recorded data to a research center in New York. The device simply drops off the animal when it molts. Whether carried by AUV or seal, the CTDs remain active for the duration of approximately one year.

The team will conduct a similar exercise in Antarctica in 2013 and will continue the expeditions on an annual basis in both locations to gather repeated data over a five-year timespan.

Holland describes the endeavor as a significant contribution to a global resource of data that is being collected. Computer modeling of the earth started in the 1950s and observations about the earth are tracked back to approximately a century ago. However, given the long-scale nature of tracking changes in climate and water temperatures, continuing the process of recording data is important both for developing projective models and for enabling future generations to continue this work.

"It's a global problem that is going to require enormous global resources," he said.

Holland has been investigating issues of climate and sea-level change for more than a decade; he made his first trip to Greenland and Antarctica in 2007 and has been returning ever since. During these expeditions, Holland said he was at first struck by the breathtaking beauty of the landscapes, characterized by expansive open land, endless horizons, and massive icebergs. However, the harsh environment also creates significant challenges; despite months of planning, on occasion, decisions have to be made on a moment's notice due to unpredictable weather conditions.

"If, for example, the weather changes and a helicopter cannot fly, you are stranded on the ice; or a storm might come in and your ship has to seek harbor," he explained. "Our team always has this in mind, and so we learn to adapt and remain flexible in our plans."

This field research is just one element of the ambitions of the Project on Global Sea-level Change team; back on dry ground in New York and Abu Dhabi, members of the team will work on theoretical and computational components of the project. This will include developing new theory or building upon existing theory that is underdeveloped. Ice sheet behavior, for example, is not well understood scientifically and will be an important area of theoretical focus. The research team will also work on combining field observations with climate-change theory to develop sophisticated, predictive computational models. This will be approached by taking an existing global climate model, such as one used for projecting future air temperatures, and building in a sea-level projective capability.

Currently, the absence of concrete, scientifically sound models makes appropriate management and planning for this issue a difficult endeavor. The impact of having reliable projections will be considerable; science-based recommendations will help policymakers target the causes for sea-level change and make more informed decisions about safeguarding existing developments.

Holland recognizes that this is a long-term project. Given both the massive scale and long periods over which environmental changes incrementally appear, it will take a considerable amount of time to understand the impact of human causes in isolation of natural changes. Reliable computational projections may be a long-term ambition, but the Project on Global Sea-level Change is taking important steps to tackling this challenging issue, making Abu Dhabi a new global hub for research in climate-change modeling.