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

海平面变化是未来100年及以后社会面临的最重大威胁之一。对当前海平面变化的测量表明，20世纪以来，全球海平面平均上升了10至20厘米。由于未来的全球气候变化，预计到公元2100年，海平面将进一步上升20到80厘米。然而，这种对未来变化的预测受到非常大的不确定性的影响，因为我们对海平面过去行为的理解很差。如果我们要对未来提供更准确和准确的预测，我们就必须量化最近过去的海平面变化。从测量和基于地质数据的海平面重建可以清楚地看出，从19世纪到20世纪，海平面上升的速度显著增加。我们提出的问题是：过去有过类似的海平面上升加速吗？我们公布的一些地质重建让我们有充分的理由相信，在工业化之前，海平面曾加速上升，这需要进一步的研究。我们的目标是通过构建北大西洋周围六个地点500年来海平面变化的详细历史，来确定海平面迅速上升的准确时间和幅度。这些记录将以埋藏在沿海沉积物中的植物和动物化石遗骸为基础，这些化石和动物化石是过去海平面的极好指标。时间是关键，所以我们将使用最先进的测年方法，特别是超高精度的放射性碳测年技术，来找出海平面迅速上升的时间。如果我们观察到的变化同时发生在不同的地点，那么这将意味着迄今为止未知的陆地极地冰川融化事件是罪魁祸首。在沿海沉积物和验潮仪中可以观察到一些重要的过程，这些过程掩盖了冰融化产生的海平面信号。其中包括海水密度的变化--导致膨胀/收缩--由于温度和盐度的变化以及海岸的垂直运动。我们将使用模型和现有的验潮仪和海洋温度测量，分别对这些过程进行修正。首先，我们将创建一个模型，可以计算沿海地区的立体(密度)变化。在过去的50年里，人们可以测量海洋密度，但这些测量是在开阔的海洋上进行的，而不是在海岸附近。在大陆架上运行的许多过程，如潮汐、洋流和风，将这些地区的水柱混合在一起，因此使用海洋记录可能不准确。我们的模型将帮助我们预测在海洋中部测量到的变化后，海岸的水密度如何变化。第二个模型可以模拟过去500年的海洋空间变化，这段时期的海洋密度和温度数据是不可用的。对风、气压和潮汐变化的一些额外修正也是必要的，但这些都相对容易做到。第二，我们需要从我们的记录中消除长期土地流动的影响。我们将通过重建过去2000-3000年的海平面趋势并从代理重建中减去这些趋势来实现这一点。也有地球物理模型和GPS数据可以帮助进行这种校正。海平面的“修正”记录将被分析，以确定海平面在过去500年中是否发生过同步上升事件。我们认为这项工作很重要，因为它将第一次使我们能够测试北大西洋海平面的加速是否同时发生，如果发生，我们可以确定它们的大小。这些数据将为未来的海平面预测提供重要的“基线”约束。

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.