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Tracking Climate Change with Radar Eyes

Long-term measurements document sea level rise in the Arctic.

In recent years, there has been an enormous spotlight on the Arctic because of the fast changes in the region. Arctic sea level determination is challenging because of the occasional to permanent sea-ice spread, lack of regional coverage of satellites, satellite instruments ability to gauge ice, inadequate geophysical models, residual orbit errors, challenging tracking of satellite altimeter data.

Over the past 22 years, sea levels in the Arctic have risen an average of 2.2 millimeters per year. This is the conclusion of a Danish-German research team after evaluating 1.5 billion radar measurements of various satellites using specially developed algorithms.

In a collaborative effort, researchers from the Technical University of Denmark (DTU) and from the TUM have now documented sea-level changes in the Arctic over more than two decades.

Stine Kildegaard Rose, Ph.D., a researcher at DTU Space, said, “This study is based on radar measurements from space via so-called altimetry satellites and covers the period from 1991 to 2018. Thus, we have obtained the most complete and precise overview of the sea-level changes in the Arctic Ocean to date. This information is important in terms of being able to estimate future sea levels associated with climate change.”

Dr. Marcello Passaro said, “The challenge lies in finding the water signals in the measured data: Radar satellites measure only the distance to the surface: Albeit, vast areas of the Arctic are covered with ice, which obscures the seawater.”

Scientists developed algorithms to assess radar echoes reflected from the water where it achieves the surface through cracks in the ice.

Passaro used these algorithms and homogenized 1.5 billion radar measurements from the ERS-2 and Envisat satellites. Based on the signals tracked at the TUM, the DTU team worked on the post-processing of these data and added the measurements collected by the current CryoSat radar mission.

The researchers created a map with lattice points to represent the monthly sea level elevations for the period between 1996 and 2018. The sum of the monthly maps reveals the long-term trend: The Arctic sea level rose by an average of 2.2 millimeters per year.

There are, however, significant regional differences. Within the Beaufort Gyre, north of Greenland, Canada and Alaska, sea levels rose twice as fast as on average – more than 10 centimeters in 22 years.

The reason: The low-salinity meltwater collects here, while a steady east wind produces currents that prevent the meltwater from mixing with other ocean currents. Along the coast of Greenland, on the other hand, the sea level is falling – on the west coast by more than 5 mm per year, because the melting glaciers weaken the attractive force of gravity there.

Passaro said, “The homogenized and processed measurements will allow climate researchers and oceanographers to review and improve their models in the future.”

The study is published in Remote sensing.

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