11 February 2019

Analyzing the Effect of Ocean Internal Variability on Depth-Integrated Steric Sea-Level Rise Trends Using a Low-Resolution CESM Ensemble


Ocean heat uptake is a key indicator of climate change, in part because it contributes to sea-level rise via thermal expansion. Quantifying the uncertainties surrounding ocean heat uptake and sea-level rise are important in assessing climate-related risks. Researchers under a multi-institutional Cooperative Agreement led by Stanford University have analyzed multiple sources of uncertainties influencing steric sea-level rise estimates, due primarily to changes in ocean temperature. The study focuses on model uncertainties using the Coupled Model Intercomparison Phase 5 (CMIP5) and “natural” variability using low-resolution CESM ensemble accounting for joint internal variability of the coupled system (including the deep ocean). 


Analysis shows that both internal “natural” variability and model uncertainties contribute substantially to spread in multi-decadal steric sea-level trends.  Results provide useful constraints on global and regional sea-level rise estimates, in particular for upper bound estimates relevant to coastal flood risk assessments.


Comprehensive global climate model ensembles are used to evaluate uncertainties surrounding decadal trends in depth-integrated global steric sea-level rise due to thermal expansion of the ocean. Results are presented against observational estimates, which are used as a guide to the state of recent literature. The first ensemble uses the Community Earth System Model (CESM), which samples the effects of internal variability within the coupled Earth system including contributions from the sub-surface ocean. The researchers compare and contrast these results with an ensemble based on the Coupled Model Intercomparison Project Phase 5 (CMIP5), which samples the combined effects of structural model differences and internal variability. The effects of both internal variability and structural model differences contribute substantially to uncertainties in modeled steric sea-level trends for recent decades, and the magnitude of these effects varies with depth. The 95% range in total sea-level rise trends across the CESM ensemble is 0.151 mm·year−1 for 1957–2013, while this range is 0.895 mm·year−1 for CMIP5. These ranges increase during the more recent decade of 2005–2015 to 0.509 mm·year−1 and 1.096 mm·year−1 for CESM and CMIP5, respectively. The uncertainties are amplified for regional assessments, highlighting the importance of both internal variability and structural model differences when considering uncertainties surrounding modeled sea-level trends. Results can potentially provide useful constraints on estimations of global and regional sea-level variability, in particular for areas with few observations such as the deep ocean and Southern Hemisphere. 

John Weyant
Stanford University