# Long-Term and Decadal Sea-Level Trends of the Baltic Sea Using Along-Track Satellite Altimetry
## 1 Introduction  
Sea-level variation stands as a crucial benchmark for assessing global climate change, with approximately 410 million individuals residing in areas lower than 2 m above sea level. Thus, there exists a looming risk of sea-level rise [1] and, as a consequence, accurate and reliable methods are required for determining these changes. One of the main reasons behind these sea-level changes is that, starting from the mid-1960s, the oceans absorbed more heat and expanded, leading to a noticeable acceleration in the rate of the sea-level rise [2]. This consistent ongoing global warming has been a primary driver behind the persistent rise in global sea levels [3; 4; 5]. Since the 1990s, the satellite altimetry (SA) technique [6] has enabled observations of the absolute sea level (ASL), i.e., relative to the Earth's centre of mass (CM). However, it is important to note that relative sea level (RSL) observations primarily rely on data series from land-bound tide gauges (TGs), which may be susceptible to uncertainties due to vertical land motion (VLM) and instrumental limitations (e.g., [7]). Both measurement techniques have unveiled a significant acceleration in the Earth's average sea-level rise over recent decades, primarily attributed to the increasingly rapid loss of ice in Greenland and Antarctica [2; 8; 9]. Therefore, the accurate detection of contemporary changes and trends in sea levels is essential for preparing coastal communities and forecasting future sea-level projections [10]. While many researchers have investigated the trends of sea levels, the precise scale of thesetrenchs remains enshrouded in uncertainty. Most of the research conducted has focused on global mean sea level (GMSL) changes (e.g., [11; 12; 13; 14; 15]). The estimated GMSL trend varies between 2 and 4 mm/yr, thus depending on the data source and the time span of the datasets considered (e.g., [16; 17; 18; 19; 20; 21; 22; 23]). Although these studies attribute sea-level changes as a response to global-scale climate variability, they are limited in providing regional insights. Estimating regional sea-level changes seems more practical, but also, within a smaller domain, it may be challenging to decipher the noise compared to that of GMSL. This complexity arises from the fact that oceanic variability is typically larger on regional scales due to redistribution effects like wind. In addition, coastal regions are also more sensitive and more susceptible to errors that are frequently neglected when calculating the global average [24]. However, during the last decade, considering the advances in the more accurate estimation of sea levels, focus has shifted to regional sea-level trend assessments (e.g., [25; 26; 27; 28; 29; 30]). Also, the currently achievable accuracy and stability in estimating sea-level changes at the regional scale enable the analysis and attribution of sea-level changes as a response to climate variability more precisely. SA is one of the most widely used sources to study ASL and has been extensively used to understand sea-level variabilities. The SA working principle is to transmit short pulses of microwave radiation, which interact with the sea surface and return to the altimeter. From the two-way travel time of the pulses, the range between the satellite and the sea surface can be estimated. SA thus provides highly accurate spatiotemporal ASL measurements with respect to an earth-fixed geocentric coordinate system. Since October 1992, with the advent of the TOPEX/Poseidon mission, SA has provided repeated precise measurements of sea levels and continuously improved the observing system and associated data record over most of the world's oceans. Multiple SA missions have collected sea-level data with unprecedented coverage, resolution, accuracy, and stability. Hence, SA-derived sea-level estimates have become a needed reference for scientists, stakeholders, and decision-makers [5; 15; 28; 31; 32]. The question now arises on the ability of SA to be used independently, especially in complex coastal areas.