How do vertical movements affect the measurement of global sea level?

Vertical movements, such as land subsidence or uplift, significantly complicate estimates from tide gauge data when measuring global sea level. Tide gauges are designed to measure the height of the sea surface relative to the land on which they are installed. If the land is sinking (subsidence) or rising (uplift), the readings from these gauges may not accurately reflect changes in actual sea level. This misalignment creates challenges in understanding whether observed changes in sea level are due to global phenomena such as climate change or simply the result of local geological processes.

For instance, if a tide gauge in a coastal city experiences subsidence while global sea levels are rising, it may show a decrease in relative sea level when, in fact, the sea level may be rising faster than what is indicated by the gauge. This complicates not only the interpretation of the data but also the formulation of policies aimed at mitigating the impacts of sea level rise on coastal communities.

Overall, recognizing and correcting for vertical movements is essential for accurate assessments of sea level changes over time and for understanding the broader implications of sea level rise on global climate and ecosystems.