Abstract/Summary

This thesis is concerned with the effects of climate change on ocean heat content in recent decades and the 21st century. We examine whether significant changes in ocean temperature can be detected in recent decades and attributed to anthropogenic or other factors, and we consider the recent and future pattern of sea level change due to ocean density change, which is dominated by temperature change. We compare ocean temperature change for 1960-2005 in four observational datasets and in historical simulations by atmosphere-ocean general circulation models (AOGCMs) from the Coupled Model Intercomparison Project phase 5 (CM1P5). Observations and CMIP5 models show that the upper 2000m have warmed and the signal gradually propagates to deeper layers over time with a defined geographical pattern determined by the regional processes of ocean heat uptake. Greenhouse gas forcing has contributed most to increase the temperature of the ocean which has been offset by anthropogenic aerosols, and volcanic eruptions cause episodic cooling. Using optimal fingerprinting, we show that observed changes may be attributed to greenhouse-gas, other anthropogenic and natural (mainly volcanic) forcings, with all three of these being detectable. Multi-model mean fingerprints are constructed using the time-depth structure of the warming by considering multiple depth levels, which decreases the uncertainty of the results. Limiting the observations and model fields to locations where there are observations benefits the detection of signals. By comparison of historical and pre-industrial control simulations using the AOGCMs of the CMIP5 project, we conclude that the observed geographical pattern for 1993-2012 of sea level change due to ocean dynamics and density changes is generally dominated by unforced (internal generated) variability, although some regions, especially in the Southern Ocean, may already show an externally forced response. Applying the method of pattern scaling to projections of sea level change we show that it gives accurate estimates of future local sea level change in response to anthropogenic forcing as simulated by the AOGCMs under RCP scenarios, and determine that the forced signal will be detectable above the noise of unforced internal variability within the next decade globally and may already be detectable in the tropical Atlantic. This work points to the need to continue developing AOGCMs in order to improve their simulations of past ocean temperature and sea level change and reduce their uncertainty in projections of the future.