Scaling regional sea level changes with climate forcings

Global sea level rise is one of the most distinct signs of a warming climate. The main causes for rising sea levels are the warming of the oceans and the resulting thermal expansion as well as the melting of glaciers and ice sheets. It is, however, the regional sea level changes that have serious consequences for the coastal population, infrastructure and economy, and that are therefore of great interest. Regional sea level changes may deviate greatly from the global mean simulations with complex climate models are necessary to understand them. Combined with estimates of future emissions, so- called scenarios, such models are used to project regional sea level changes. Scenarios describe the expected evolution of human-related greenhouse gas emissions and aerosols as well as changes in land use and ozone concentration. The exact future evolution of those forcings is unknown. Therefore, to estimate the range of possible sea level changes, it is necessary to run climate models for a large number of scenarios yet, this is an expensive and time consuming undertaking. Recent research revealed that regional sea level changes follow a spatial pattern, the strength of which can be derived from e.g. global mean surface temperature a variable that is relatively easy to model. For coastal communities this would provide the possibility to estimate the range of possible future sea level change relatively easily and to take action in time to prevent damage. However, before the spatial patterns can be used for such practical purposes, it is important to understand what causes them, what the underlying physical processes are and whether the patterns are robust and therefore reliable. These are the questions we want to answer in our research project. To do so, we will make use of the numerous existing model simulations of the climate of the past 150 years and investigate how the various forcing agents (e.g. greenhouse gases, aerosols or ozone) affect regional sea level change. How can we distinguish between the individual forcings? We will use dedicated climate simulations that are performed with selected forcings only. It is possible to force a climate model with volcanic eruptions only. Or with greenhouse gas emissions. By doing so, the effect of a single forcing on the climate system in general and sea level in particular can be estimated. Our goal is to identify a distinct spatial pattern that can be attributed to each forcing agent individually for each of the major contributors to sea level change (e.g. ocean warming and glaciers). We will also analyze how these patterns depend on the magnitude of the forcings. The forcing-specific response patterns can then be used to easily project regional sea level changes for a large number of future scenarios.

Global sea level rise is one of the most distinct signs of a warming climate. The main causes for rising sea levels are the warming of the oceans and the resulting thermal expansion as well as the melting of glaciers and ice sheets. It is, however, the regional sea level changes that have serious consequences for the coastal population, infrastructure and economy, and that are therefore of great interest. In this research project, we investigated the influence of past climate "forcing agents" on global and regional sea level rise. By "forcing agent", we mean the natural and anthropogenic (human made) processes that affect the climate, such as volcanic eruptions, solar activity, aerosols, land use change, and greenhouse gases. We developed new methods allowing us to disentangle between each forcing and each component of sea level rise, using observations and numerical models. We found that human influence on sea level rise, which has been shown to be dominant in the 20th century, is also detectable for all ocean basins at almost all latitudes even over the shorter timescale of the satellite era (1993-2015). This is significant because it means that the forced trend is stronger than the year to year variability. We then focused our interest on glaciers and ice caps, which have been the main land ice contributor to sea level rise in the past 100 years and, for the first time, we also suggest that they have been the dominant factor of change over the last 2000 years. Based on this new knowledge, we provided updated projections of future changes in sea level rise during the 21st century, for a large number of future climate scenarios. Our results have informed the recent Intergovernmental Panel on Climate Change (IPCC) Sixt Assessment Report published in 2021.