Using Pacific benthic foraminiferal δ18O and Mg/Ca records, we derive a Cenozoic (66 Ma) global mean sea level (GMSL) estimate that records evolution from an ice-free Early Eocene to Quaternary bipolar ice sheets. These GMSL estimates are statistically similar to "backstripped" estimates from continental margins accounting for compaction, loading, and thermal subsidence. Peak warmth, elevated GMSL, high CO2, and ice-free "Hothouse" conditions (56 to 48 Ma) were followed by "Cool Greenhouse" (48 to 34 Ma) ice sheets (10 to 30 m changes). Continental-scale ice sheets ("Icehouse") began ~34 Ma (>50 m changes), permanent East Antarctic ice sheets at 12.8 Ma, and bipolar glaciation at 2.5 Ma. The largest GMSL fall (27 to 20 ka; ~130 m) was followed by a >40 mm/yr rise (19 to 10 ka), a slowing (10 to 2 ka), and a stillstand until ~1900 CE, when rates began to rise. High long-term CO2 caused warm climates and high sea levels, with sea-level variability dominated by periodic Milankovitch cycles.

Cenozoic sea-level and cryospheric evolution from deep-sea geochemical and continental margin records.

The rate of global-mean sea-level rise since 1900 has varied over time, but the contributing factors are still poorly understood1 Previous assessments found that the summed contributions of ice-mass loss, terrestrial water storage and thermal expansion of the ocean could not be reconciled with observed changes in global-mean sea level, implying that changes in sea level or some contributions to those changes were poorly constrained2,3 Recent improvements to observational data, our understanding of the main contributing processes to sea-level change and methods for estimating the individual contributions, mean another attempt at reconciliation is warranted Here we present a probabilistic framework to reconstruct sea level since 1900 using independent observations and their inherent uncertainties The sum of the contributions to sea-level change from thermal expansion of the ocean, ice-mass loss and changes in terrestrial water storage is consistent with the trends and multidecadal variability in observed sea level on both global and basin scales, which we reconstruct from tide-gauge records Ice-mass loss-predominantly from glaciers-has caused twice as much sea-level rise since 1900 as has thermal expansion Mass loss from glaciers and the Greenland Ice Sheet explains the high rates of global sea-level rise during the 1940s, while a sharp increase in water impoundment by artificial reservoirs is the main cause of the lower-than-average rates during the 1970s The acceleration in sea-level rise since the 1970s is caused by the combination of thermal expansion of the ocean and increased ice-mass loss from Greenland Our results reconcile the magnitude of observed global-mean sea-level rise since 1900 with estimates based on the underlying processes, implying that no additional processes are required to explain the observed changes in sea level since 1900

The causes of sea-level rise since 1900

We review the characteristics of sea level variability at the coast focussing on how it differs from the variability in the nearby deep ocean. Sea level variability occurs on all timescales, with processes at higher frequencies tending to have a larger magnitude at the coast due to resonance and other dynamics. In the case of some processes, such as the tides, the presence of the coast and the shallow waters of the shelves results in the processes being considerably more complex than offshore. However, ‘coastal variability’ should not always be considered as ‘short spatial scale variability’ but can be the result of signals transmitted along the coast from 1000s km away. Fortunately, thanks to tide gauges being necessarily located at the coast, many aspects of coastal sea level variability can be claimed to be better understood than those in the deep ocean. Nevertheless, certain aspects of coastal variability remain under-researched, including how changes in some processes (e.g., wave setup, river runoff) may have contributed to the historical mean sea level records obtained from tide gauges which are now used routinely in large-scale climate research.

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Forcing Factors Affecting Sea Level Changes at the Coast

The energy radiated by the Earth towards space does not compensate the incoming radiation from the Sun leading to a small positive energy imbalance at the top of the atmosphere (0.4-1.Wm-2). This imbalance is coined Earth’s Energy Imbalance (EEI). It is mostly caused by anthropogenic greenhouse gases emissions and is driving the current warming of the planet. Precise monitoring of EEI is critical to assess the current status of climate change and the future evolution of climate. But the monitoring of EEI is challenging as EEI is two order of magnitude smaller than the radiation fluxes in and out of the Earth. Over 93% of the excess energy that is gained by the Earth in response to the positive EEI accumulates into the ocean in the form of heat. This accumulation of heat can be tracked with the ocean observing system such that today, the monitoring of Ocean Heat Content (OHC) and its long-term change provide the most efficient approach to estimate EEI. In this community paper we review the current four state-of-the-art methods to estimate global OHC changes and evaluate their relevance to derive EEI estimate on different time scales. These four methods make use of : 1) direct observations of in situ temperature; 2) satellite-based measurements of the ocean surface net heat fluxes; 3) satellite-based estimates of the thermal expansion of the ocean and 4) ocean reanalyses that assimilate observations from both satellite and in situ instruments. For each method we review the potential and the uncertainty of the method to estimate global OHC changes. We also analyze gaps in the current capability of each method and identify ways of progress for the future to fulfill the requirements of EEI monitoring. Achieving the observation of EEI with sufficient accuracy will depend on merging the remote sensing techniques with in situ measurements of key variables as an integral part of the Ocean Observing System.

/pdf/measuring-global-ocean-heat-content-to-estimate-the-earth-5gf7wfadjn.pdf

Measuring Global Ocean Heat Content to Estimate the Earth Energy Imbalance

. The Greenland ice sheet is one of the largest contributors to global mean
sea-level rise today and is expected to continue to lose mass as the Arctic
continues to warm. The two predominant mass loss mechanisms are increased
surface meltwater run-off and mass loss associated with the retreat of
marine-terminating outlet glaciers. In this paper we use a large ensemble of
Greenland ice sheet models forced by output from a representative subset of
the Coupled Model Intercomparison Project (CMIP5) global climate models to project ice sheet changes and sea-level rise
contributions over the 21st century. The simulations are part of the
Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We estimate the
sea-level contribution together with uncertainties due to future climate
forcing, ice sheet model formulations and ocean forcing for the two
greenhouse gas concentration scenarios RCP8.5 and RCP2.6. The results
indicate that the Greenland ice sheet will continue to lose mass in both
scenarios until 2100, with contributions of 90±50 and 32±17  mm to sea-level rise for RCP8.5 and RCP2.6, respectively. The largest
mass loss is expected from the south-west of Greenland, which is governed by
surface mass balance changes, continuing what is already observed today.
Because the contributions are calculated against an unforced control
experiment, these numbers do not include any committed mass loss, i.e. mass
loss that would occur over the coming century if the climate forcing
remained constant. Under RCP8.5 forcing, ice sheet model uncertainty
explains an ensemble spread of 40 mm, while climate model uncertainty and
ocean forcing uncertainty account for a spread of 36 and 19 mm,
respectively. Apart from those formally derived uncertainty ranges, the
largest gap in our knowledge is about the physical understanding and
implementation of the calving process, i.e. the interaction of the ice sheet
with the ocean.

/pdf/the-future-sea-level-contribution-of-the-greenland-ice-sheet-1so6wwhr11.pdf

The future sea-level contribution of the Greenland ice sheet: a multi-model ensemble study of ISMIP6

Changes in sea level lead to some of the most severe impacts of anthropogenic climate change. Consequently, they are a subject of great interest in both scientific research and public policy. This paper defines concepts and terminology associated with sea level and sea-level changes in order to facilitate progress in sea-level science, in which communication is sometimes hindered by inconsistent and unclear language. We identify key terms and clarify their physical and mathematical meanings, make links between concepts and across disciplines, draw distinctions where there is ambiguity, and propose new terminology where it is lacking or where existing terminology is confusing. We include formulae and diagrams to support the definitions.

/pdf/concepts-and-terminology-for-sea-level-mean-variability-and-133rrym0h3.pdf

Ichiro Fukimori

Papers

Concepts and terminology for sea level: mean, variability and change, both local and global