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The warming of Earth is primarily due to accumulation of heat-trapping greenhouse gases, and more than 90 percent of this trapped heat is being absorbed by the oceans. Water volume rises with temperature because of thermal expansion—another major driver of sea level rise. The estimated rate of thermal expansion, or thermosteric sea level rise, from 1971 to 2010 is 0.4 to 0.8 millimeters per year; the estimate carries a confidence level of 90 to 100 percent [Rhein et al., 2013]. This corresponds to a warming rate of 0.015 degrees Celsius per decade in the upper 700 meters of the global ocean between 1971 and 2010. By comparison, an estimate using Argo floats found the thermosteric component of sea level rise above a depth of 2000 meters to be 0.5 millimeters per year, plus or minus 0.5 millimeters, between January 2005 and September 2010 [Leuliette and Willis, 2013]. Temperature measurements of the sea surface, taken by ships, satellites and drifting sensors, along with subsurface measurements and observations of global sea-level rise, lead researchers to conclude that this warming of the upper ocean over four decades is virtually certain. A contribution to sea level rise of about 0.1 millimeter per year by warming of ocean waters at depth, 700 meters to 2,000 meters, is considered likely, with about another 0.1 millimeter caused by warming deeper than 2,000 meters also considered likely [Rhein et al., 2013].
The expansion record is a short one; before 1971, ocean measurements were too few to allow meaningful estimates. Still, the record is robust enough to reveal that temperature-driven changes in seawater volume vary seasonally, as well as across decades. Combined with seasonal movement of rainfall, which brings shifts in water mass, such changes can cause sea levels in a given hemisphere to fluctuate by amounts approaching a centimeter [Chen et al., 2005].
The magnitude of expansion’s effect leads some researchers to urge adoption of sea level rise as the true “global warming number.” While we might instinctively look to globally averaged, surface air temperature as the benchmark of climate change, the sea level signal, these scientists argue, closely tracks the vast majority of the planet’s heat absorption.
Argo Floats & In-situ Observations
The deployment of floating sensors across the world’s oceans, known as the Argo project, reached a critical mass in 2007, with some 3,000 of the devices set adrift to measure temperature and salinity in the ocean’s upper 2,000 meters [Leuliette and Willis, 2011]. These sensors profile ocean expansion, the thermosteric sea-level rise that, as we have seen, is a consequence of ocean heat absorption. The Argo floats play critical roles in recent studies of trends in ocean heat content, which generally show increased warming over decades. One recent estimate, covering 1955 through 2010 [Levitus et al., 2012], relied upon historical data and more modern readings from the World Ocean Database 2009, additional data from NOAA through 2010, and Argo data that became available in early 2011. Some of the Argo data had been corrected by Argo quality-control teams, although uncorrected data also were used. (The authors say that temperature measurements from the floats, unlike salinity measurements, show few instances of data drift.)
Another recent study [Nieves et al., 2015] used Argo float data to determine that a decade-long slowdown in the rate of surface air warming—a “hiatus” reported by several researchers—can be explained by more heat accumulation in the Indian and Pacific oceans, at depth, than had been previously recognized.
Argo data also has been combined with ocean temperature measurements from the World Ocean Circulation Experiment in the 1990s to suggest a marked increase in thermosteric sea level through 2008 [Freeland and Gilbert, 2009]. The result comes with significant uncertainty, however, due to sparse sampling in the earlier project [Rhein et al., 2013].
Prior to deployment of Argo, ocean temperature analyses relied on bathythermographic instruments including expendable ones (XBTs) not designed for long-term monitoring of such tiny signals, conductivity-temperature-depth (CTD) probes and other methods [Levitus et al., 2009, Rhein et al., 2013]. Repeated hydrographic sections by ship (using ‘bottles’ that capture a small amount of water, with reversing thermometers) also have been used to create a reliable picture of temperatures below 2,000 meters, finding substantial indications of warming at depth [Purkey and Johnson, 2010].
Radar altimetry from orbit revolutionized global sea-level observations, capturing variations across most of the planet’s oceans every 10 days, as well as at other time intervals. The first measurements precise enough to track changes in global mean sea level began with the launch of the NASA-CNES TOPEX/Poseidon satellite in 1992 (CNES is the French space agency). The spacecraft was equipped with two altimeters and a microwave radiometer, which corrected for the effects of water vapor on radar signal transmission; other instruments ensured precision tracking of the satellite’s orbital position [Mitchum et al. 2010].
TOPEX Poseidon’s mission ended in 2006, but the launch of Jason-1 in 2001 and Jason-2 in 2008 allowed long-term sea-level measurement to continue. Transmitter failure brought the Jason-1 mission, a collaboration of NASA and France’s Centre National d’Etudes Spatiales, to an end in 2013; a second collaboration, Jason-2, remains in orbit, and a Jason-3 mission has been launched. The data from the TOPEX-Jason series are nicely complemented by data from the European Space Agency’s (ESA) ERS-1, ERS-2, ENVISAT and Cryosat altimetric satellites, as well as the more recent SARAL/AltiKa, a collaboration between ISRO, the Indian space agency, and CNES.
The orbital view brought seasonal and yearly variations into sharper focus—for example, a significant rise in global mean sea level during the powerful 1997-98 El Niño event [Nerem et al., 1999], and a sea-level drop in 2011 associated with La Niña [Boening et al., 2012].
Keeping watch on the global sea-level signal requires validation of satellite measurements against tide gauge data [Mitchum, 2000], as well as a modest upward adjustment in sea-level height because of the shifting shape of ocean basins due to glacial isostatic adjustment, occurring within the satellite coverage area [Douglas and Peltier, 2002]. Careful calibrations were needed to avoid potential data bias, for example, after a change was made to the TOPEX altimeter electronics in 1999 and after a time-series shift from TOPEX to Jason-1 in 2002[Mitchum et al., 2010].
The continuous 22-years-plus record of altimetry measurements helped identify a likely rise in the rate of globally averaged sea level between 1993 and 2010 that is comparable to a similar rise between 1920 and 1950 [Rhein et al., 2013]. A recent study [Watson et al., 2015] used tide gauges to correct for a drift in the earlier TOPEX/POSEIDON record; the combined dataset is more powerful than either one separately. These authors also found an acceleration in the rate of sea level rise over the past 22-plus years.
The Earth keeps its own records of sea level change across thousands and even millions of years, though one that is often fragmentary and that tells a tale of relative sea level (that is, combining sea level rise or fall due to changes in ocean volume with the rise or fall of coastal land). Such sea level reconstructions must then be fitted into a shifting context of vertical land movement, including ongoing glacial isostatic adjustment, tectonic uplift, and the deformation of ocean basins by movement of water mass among ice sheets, terrestrial water sources and oceans [Lambeck et al., 2010].
These proxy records are preserved in a variety of marine and terrestrial settings: sediments and organisms in deep ocean cores, for example, or once-submerged shorelines and terraces, uplifted and exposed. Analysis of oxygen isotope ratios in the shells, or tests, of tiny ocean organisms called foraminifera reveal oscillations in relative volumes of ocean and ice [Shackleton, 1987]. Dating of uplifted fossil reefs using radiometric techniques, such as uranium-series measurements, have increased the precision of sea-level estimates for the Quaternary period, from about 2.6 million years ago to the present [Lambeck and Chappell, 2001].
Biological traces of sea level change can be seen in fossil creatures that lived in intertidal zones, distributed over a reliably vertical spread according to their species’ requirements. Barnacles, mussels, corals, tube worms, oysters and boring mollusks are among the organisms that have yielded enough data to track sea level changes across centuries and millennia [Lambeck et al., 2010]. One intriguing life form, the coral microatoll, can preserve complex records of changing relative sea level, written in the undulating shape of its growth bands. These disk-shaped colonial formations, as much as 20 feet across, consist of a dead interior ringed by living coral, which can grow no higher than low tide. Falling sea levels can kill the upper part of the organism, while the lower portion continues to grow outward, leaving a distinct pattern. Rising sea levels lead to new growth over the previous dead interior. And sea level fluctuations leave wavy patterns in the growth rings that again are legible to scientists [Woodroffe and McLean, 1990; see also Lambeck et al., 2014].
Geological markers for sea level change include tidal notches, surf benches and other erosional features [Lambeck et al., 2010]. Under certain conditions, archaeological sites, especially in the Mediterranean, can track relative sea levels, including the Phlagrean columns. The oldest known such site is Cosquer cave with its flooded paintings in southern France, inundated some 10,000 years ago [Lambeck and Bard, 2000]. Roman fish tanks carved from coastal rock as long ago as the first century B.C., 8,000-year-old Israeli wells, and sunken cities such as Baia provide valuable records of changing sea levels [Lambeck et al., 2010].
Projections of global sea level rise by 2100, the year upon which climate modelers typically focus, vary widely depending on modeling methods and on assumptions—the rate of increase in greenhouse gas emissions, for example, and especially how ice sheets will respond to warming air and ocean water. Recent projections range from 0.2 meters to 2.0 meters (0.66 to 6.6 feet) [Melillo et al., 2014; see sections 13.5.1 and 13.5.2 of the 2013 IPCC report for detailed discussion].
The projections for the century ahead focus on the two largest contributors: thermal expansion of seawater and melting land ice. The consensus projections in the most recent IPCC report, called the Fifth Assessment or AR5, include dynamic changes in the great ice sheets—an improvement over the previous assessment, AR4, although much remains uncertain in the young field of ice sheet modeling [Church et al., 2013].
The latest assessment provides a range of projections for a variety of greenhouse gas emissions scenarios and associated radiative forcing (the energy injected into the climate system by the action of these gases). The four Representative Concentrated Pathway scenarios, or RCPs, rise from low to high emissions, each applied to CMIP 5 models to produce possible future sea-level changes.
AR5 expresses “medium confidence” in these projections, derived from process-based models—that is, attempts to simulate the mechanics and interactions of the factors driving sea level rise and land ice changes. But coupled general circulation numerical models—considered “process-based”—explain 90 percent of the observed sea level rise between 1971 and 2010, as well as that observed during a shorter period, 1993 to 2010 (see “By the Numbers”). This increases confidence that these models are reliable under present-day conditions, despite the fact that the models’ current rate of rise, 3.7 millimeters per year, is significantly higher than shown by observations. Since these coupled models do not include ice sheet instabilities, their projections very likely represent a “lower bound” for future sea level rise.
Process-based models project a rise of 0.26 to 0.55 meters, with a median value of 0.4, for the RCP 2.6 scenario, in which gas emissions decline after a peak, while carbon dioxide levels remain below 500 parts per million. For the RCP 8.5 scenario, with its higher concentrations of greenhouse gases and with carbon dioxide above 700 parts per million, the projected rise is 0.52 to 0.98 meters, with a median value of 0.6. [Church et al., 2013].
Ocean warming and ice-sheet losses are “very likely” to drive the rate of sea level rise higher in the 21st century than the rate measured from 1971 to 2010, according to AR5 [Church et al., 2013]. For the 2081-2100 period, compared to 1986-2005, the report considers it likely, with medium confidence, that global mean sea-level rise will fall between five and 95 percent of the range projected by process-based models. Only the collapse of marine-based portions of the Antarctic ice sheet could drive sea level above these “likely” ranges, the authors concluded, and no more than a few tenths of a meter [Church et al., 2013].
And while the IPCC report acknowledges a newer, alternative approach known as semi-empirical modeling, its projections earn only “low confidence” from the IPCC [Church et al., 2013]. The report’s authors could not evaluate the probability that semi-empirical models, or SEMs, would come true, and believed the scientific community lacked consensus on their reliability.
SEMs [Rahmstorf et al., 2012 and references therein] take a simple approach—a kind of shortcut—to simulating future sea level rise. Instead of trying to model the processes underlying sea level change, these models rely on sea-level changes observed in previous decades and their relationship to global temperature. Then they apply that same relationship to the century to come. The resulting projections tend to be significantly higher than those derived from process-based modeling.
An illustrative example can be found in a recent study contrasting the projections of process-based and semi-empirical models [Perrette et al., 2013]. Global mean sea level rise from major sources—thermal expansion, glaciers, and the Greenland and Antarctic ice sheets—total 0.42 meters by 2100 in the process based RCP 6.0 model, considered a mid-range, standard-type emission scenario. But updated with the semi-empirical approach, the same model yields a total of 0.86 meters, more than twice the process-based value.
For scenario RCP 2.6, the median projection of the SEMs is about 0.75 meters by century’s end, and about one meter for scenario RCP 8.5. At the high end of the confidence intervals (95%), sea level reaches above 1.5 meters for the latter scenario, mostly based on the works of Rahmstorf and of Jevrejeva. Another study of modeling reliability, in which Rahmstorf et al. performed an extensive analysis of their SEMs [ [Rahmstorf et al., 2012], concluded that a rise of about one meter, produced by a warming of 1.8 degrees Celsius, represented a robust result, derived from published data and their model.
Since the publication of AR5, newer ice-sheet observations also are suggestive of the higher values for sea level rise. Measurements of grounding line retreat in West Antarctic glaciers [Rignot et al., 2014] yielded evidence of rapid retreat between 1992 and 2011. More importantly, the researchers did not find a “major bed obstacle that would prevent the glaciers from further retreat and draw down the entire basin [Rignot et al., 2014].” Bedrock along the discharge channels grows deeper in the inland direction, helping the grounding line move farther inland. A complementary study [Morlighem et al., 2014] found that the glacial valleys through which Greenland discharges ice to the ocean are deeper than previously believed, making them more vulnerable to melting by adjacent, warmer ocean waters.