Ph.d.-forsvar Wiebke Margitta Kolbe

Principal supervisor:

• Associate professor Rasmus Tage Tonboe, DTU Space

Co-supervisors:

• Professor Eigil Kaas, Copenhagen University, Denmark
• Ph.D. Gorm Dybkjær, DMI, Denmark

Examiners:

• Senior researcher Henriette Skourup, DTU Space
• Senior scientist Lars Kaleschke, Alfred Wegener Institute, Germany
• PD Dr. habil Matthias Drusch, European Space Agency, The Nederlands

Chairperson at defence:

• Senior researcher Sine Hvidegaard

Summary

Sea ice plays a critical role in the Earth’s energy balance and climate change. It influences how much energy of the sun is absorbed and limits the heat exchange between the relatively
warm ocean and cold air in the polar regions. The impact of changes in sea ice extent affects therefore temperatures, which especially in the Arctic have increased significantly during the
past 40 years by approximately four times as much as the global average temperature. Because of these large differences, sea ice and ice surface temperatures are important
indicators for climate change, that should be monitored and analyzed.

However, it is very difficult to observe sea ice in the Arctic and Antarctic due to the harsh environment near the poles. Therefore, measurements from satellites are the main source for our modern climate time series. Most of these time series are currently based on instruments launched in the 1980s and late 1970s. However, there are also older satellite data, from their predecessors in the early 1970s, which albeit not as advanced as modern instruments, still can be used to yield insights into sea ice development from the past.

This PhD project’s aim was the investigation, development and evaluation of historical satellite data to create and extend sea ice climate data records, which can be used to assess climate change. The 1970s especially are an important reference period for sea ice development and global climate change, as it seems from current climate time series, that global warming and sea ice decline started at that time.

Therefore, two sea ice concentration data sets were created by applying modern processing methods to satellite data from 1972-1977 and 1975-1976. The usage of modern methods
reduced noise in the data significantly and has made the data sets comparable to each other and to modern sea ice data records.

Furthermore, a third data record of ice surface temperatures for 1982-2023 is presented in this study, covering the past four decades, allowing for the coherent identification of regional
and temporal temperature trends.

Overall, this study contributes to a more consistent assessment of past and present sea ice and temperature developments, that can improve the physical understanding and can be
used to put future changes into perspective.