North Atlantic sea-level variability during the last half-millennium

Sea-level change is one of the most significant threats facing society over the next 100 years and beyond. Measurements of current sea-level change have shown that there has been a mean global sea-level rise of between 10 and 20 cm over the 20th century. A further rise in sea level of between 20 and 80 cm is predicted by AD 2100 due to future global climate change. However, such predictions of future change are subject to very large uncertainties because our understanding of the past behaviour of sea level is poor. It is essential that we quantify sea-level changes in the recent past if we are to provide more accurate and precise predictions for the future. It is clear from measurements and from sea-level reconstructions based on geological data that there has been a significant increase in the rate of sea-level rise from the 19th to the 20th century. We ask the question: Have similar accelerations of sea-level rise happened in the past? Some of our published geological reconstructions give us good reason to believe that there were pre-industrial sea-level accelerations and these require further investigation. We aim to establish the precise timing and magnitude of these rapid rises of sea level by constructing detailed 500-yr histories of sea-level changes in six sites around the North Atlantic Ocean. These records will be based on the remains of fossil plants and animals buried in coastal sediments which are excellent indicators of the past level of the sea. Timing is key, so we will use the most advanced dating methods, in particular ultra-high precision radiocarbon dating techniques, to find out when the rapid increases in sea-level rise occurred. If the changes we observe occurred in various sites at the same time, then it would imply that hitherto unknown episodes of land-based polar ice melt are responsible. There are important processes that obscure the sea-level signal derived from melting ice that may be observed in coastal sediments and tide gauges. These include changes in the density of sea water - leading to expansion/contraction - due to temperature and salinity variations and vertical movements of the coast. We will correct for these processes separately, using models and available tide-gauge and ocean temperature measurements. First, we will create a model that can calculate steric (density) changes along the coast. Measurements of ocean density are available for the past 50 years, but these were taken in the open ocean, not near the coast. Many processes operating on the continental shelves, such as tides, currents and winds, mix the water column in these areas and so using ocean records may be inaccurate. Our model will help us to predict how the water density changes at the coast following a measured change in the middle of the ocean. A second model can simulate ocean steric changes for the past 500 years, a period for which ocean density and temperature data are not available. Some additional corrections for wind, air pressure and tidal changes, are also necessary but these are relatively easy to do. Second, we need to remove the effects of long-term land movements from our records. We will do this by reconstructing sea-level trends over the last 2000-3000 years and subtracting these from the proxy reconstructions. There are also geophysical models and GPS data that can help with this correction. The 'corrected' records of sea level will be analysed to determine whether synchronous episodes of sea-level rise have occurred in the past 500 years. We believe the work is important because it will, for the first time, enable us to test whether accelerations in sea-level in the North Atlantic have occurred at the same time or not, and if they have, we can determine how big they were. These data will provide important 'baseline' constraints for future sea-level predictions.