A variety of ocean processes are important for controlling the timing and characteristics of climate change. For a given climate forcing scenario, the timing of atmospheric warming is strongly dependent on the north-south transport of heat by ocean currents and mixing of heat into the ocean interior. Changes in the large-scale meridional overturning circulation could also have a significant impact on regional and global climate and could potentially lead to abrupt changes (Alley et al., 2003; NRC, 2002a). The relative scarcity of ocean observations, especially in the ocean interior, makes these factors among the more uncertain aspects of future climate projections. Changes in ocean circulations and heat transport are also connected to the rapid disappearance of summer sea ice in the Arctic Ocean. A better understanding of the dependence of ocean heat uptake on vertical mixing and the abrupt changes in polar reflectivity that follow the loss of summer sea ice in the Arctic are some of the most critical improvements needed in ocean and Earth system models.
Ice dynamics and thermal expansion are the main drivers of rising sea levels on a global basis, but ocean dynamics and coastal processes lead to substantial spatial variability in local and regional rates of sea level rise (see Chapters 2 and 7). Direct, long-term monitoring of sea level and related oceanographic properties via tide gauges, ocean altimetry measurements from satellites, and an expanded network of in situ measurements of temperature and salinity through the full depth of the ocean water column are needed to quantify the rate and spatial variability of sea level change and to understand the ocean dynamics that control global and local rates of sea level rise. In addition, oceanographic, geodetic, and coastal models are needed to predict the rate and spatial dynamics of ocean thermal expansion, sea level rise, and coastal inundation. The need for regionally specific information creates additional challenges. For example, coastal inundation models require better bathymetric data, better data on precipitation rates and stream flows, ways of dealing with storm-driven sediment transport, and the ability to include the effects of built structures on coastal wind stress patterns (see Chapter 7). Such improvements in projections of sea level changes are critical for many different decision needs.
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