Sea-level Rise

Variability and trends

Observational evidence points to a warming global climate accompanied by rising sea levels which impose significant impacts on island and coastal communities. Studies suggest that internal climate processes can modulate projected future sea level rise (SLR) regionally. It is not clear whether this modulation depends on the future climate pathways. Here, by analyzing two sets of ensemble simulations from a climate model, we investigate the potential reduction of SLR, as a result of steric and dynamic oceanographic affects alone, achieved by following a lower emission scenario instead of business-as-usual one over the 21st Century and how it may be modulated regionally by internal climate variability. Results show almost no statistically significant difference in steric and dynamic SLR on both global and regional scales in the near-term between the two scenarios, but statistically significant SLR reduction for the global mean and many regions later in the century (2061-2080). However, there are regions where the reduction is insignificant, such as the Philippines and west of Australia, that are associated with ocean dynamics and intensified internal variability due to external forcing. 

Hu A. and S. Bates, 2018, Internal climate variability and the projected future regional steric and dynamic sea level rise, Nature Communications, Doi: 10.1038/s41467-018-03474-8

Previous studies suggest that anthropogenic warming has affected the multi-decadal trend patterns of sea level over the Indian Ocean (IO). This effect, however, has not been quantified. Using observational datasets combined with large ensemble experiments from two climate models, this paper assesses the effects of natural internal variability versus external forcing on the observed, multi-decadal trend pattern and the decadal sea level anomaly (SLA) of the IO since the 1960s. Because the global mean sea level rise (SLR), which results largely from external forcing, has been removed before the examination, the paper focuses on the regionally uneven distribution of trend and SLA. The impacts of climate modes are quantified using a Bayesian Dynamic Linear Model. For the regional trend pattern of 1958–2005, the effects of internal variability dominate external forcing. Over the Seychelles area where sea-level variations obtain the maximum, internal variability (external forcing) contributes 81% (19±2.4%) of the observed trend. For decadal SLA, internal variability is the predominant cause, with a standard deviation (STD) ratio of externally forced/observed SLA being 18±17% over Seychelles and 17±11% near the Indonesian Throughflow (ITF) area. Climate modes account for most observed SLA during boreal winter, with the total effects of decadal ENSO, Indian Ocean Dipole (IOD), and monsoon accounting for 78–86% of the observed STD near the Seychelles region, ITF area, and coasts of Sumatra and the Bay of Bengal. During summer, climate modes explain 95% of observed STD near the ITF but only 58–67% in other regions. Decadal ENSO dominates the SLA in the south tropical IO for both seasons and near the coasts of Sumatra and the Bay during winter. Decadal IOD and monsoon, however, control the coastal SLA during summer. Remote and local winds over the IO are the main drivers for decadal SLA, while the Pacific influence via the ITF is strong mainly in the southeast basin.

Han, W., D. Stammer, G. A. Meehl, A. Hu, F. Sienz and L. Zhang, 2018, Multi-Decadal Trend and Decadal Variability of the Regional Sea Level over the Indian Ocean since the 1960s: Roles of Climate Modes and External Forcing, Climate, 6, 0, Doi: 10.3390/cli6020051


Sea level rise acceleration in 25-year satellite sea level record

Global sea level rise is not cruising along at a steady 3 mm per year, it's accelerating a little every year, like a driver merging onto a highway, according to a powerful new assessment led by CIRES Fellow Steve Nerem. He and his colleagues harnessed 25 years of satellite data to calculate that the rate is increasing by about 0.08 mm/year every year -- which could mean an annual rate of sea level rise of 10 mm/year, or even more, by 2100.

If the oceans continue to change at this pace, sea level will rise 65cm (26 inches) by 2100 -- enough to cause significant problems for coastal cities, according to the new assessment by Nerem and several colleagues from CU Boulder, the University of South Florida, NASA Goddard Space Flight Center, Old Dominion University, and the National Center for Atmospheric Research. The team, driven to understand and better predict Earth's response to a warming world, published their work in the journal Proceedings of the National Academy of Sciences.

Rising concentrations of greenhouse gases in Earth's atmosphere increase the temperature of air and water, which causes sea level to rise in two ways. First, warmer water expands, and this "thermal expansion" of the oceans has contributed about half of the 7 cm of global mean sea level rise we've seen over the last 25 years, Nerem said. Second, melting land ice flows into the ocean, also increasing sea level across the globe. "This study highlights the important role that can be played by satellite records in validating climate model projections," said co-author John Fasullo, a climate scientist at the National Center for Atmospheric Research. "It also demonstrates the importance of climate models in interpreting satellite records, such as in our work where they allow us to estimate the background effects of the 1991 eruption of Mount Pinatubo on global sea level."

R. S. Nerem, B. D. Beckley, J. T. Fasullo, B. D. Hamlington, D. Masters, and G. T. Mitchum PNAS February 27, 2018 115 (9) 2022-2025


Geoengineering approach may fail to halt sea level rise

Even if efforts to artificially cool the planet successfully capped average surface temperatures around the globe, altered ocean currents could threaten vulnerable ice sheets, new research finds. Geoengineering the climate by blocking some incoming sunlight would speed up a major current that pumps warm ocean water from the tropics to the poles, the study shows. As a result, massive ice sheets in Greenland — and potentially Antarctica — would continue to shed water, contributing to damaging levels of global sea level rise.

The new research underscores the uncertainties of relying on technology to protect society from some of the most damaging aspects of climate change. Before any geoengineering scheme is attempted, more research is needed to increase understanding of the possible ramifications of trying to alter Earth's climate, the authors said. The study drew on cutting-edge computer models that simulated the climate system's response to hypothetical sulfur dioxide injections into the stratosphere, intended to stabilize temperatures as greenhouse gases increase. It compared the effects of such a cooling scheme to the warming that would occur this century if society followed a business-as-usual approach of continuing to emit greenhouse gases in the atmosphere. The authors included scientist from NCAR, PNNL, and Cornell. They conclude that even more advanced computer simulations are needed to understand the full impacts of geoengineering Earth's climate and to quantify such details as the response of ice sheets to warming waters.

Fasullo, J.T., S. Tilmes, J.H. Richter, B. Kravitz, D.G. MacMartin, M.J. Mills, and I.R. Simpson, Persistent polar ocean warming in a strategically geoengineered climate, Nature Geo., 2018