Environment Counts | Sea level change – An EC perspective on the evidence presented in IPCC’s 5th Assessment Report
Author: Rick Higgins – Published At: 2014-11-16 10:51 – (2797 Reads)
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More than seventy percent of the World’s surface is covered by water. Between 1901 and 2010 AR5 states that it is “very likely†the global mean rate of sea level rise was 1.7 (1.5 to 1.9) millimetres per year (mm/yr) for a total sea level rise of 0.19 (0.17 to 0.21) metres (m) or 19 centimetres (cm). In recent decades global mean sea level (GMSL) has continued to rise, and at an increased rate. Between 1993 and 2010, the rate was 3.2 (2.8 to 3.6) mm/yr.
For the first time observations are now available for each of the contributions to the observed GMSL change (from such contributing factors as thermal expansion, melting glaciers and ice sheets), as a result of improved and new observations of the ocean since the introduction of satellite radar altimetry in 1993. Observations indicate that 90% of anthropogenic heat added to the climate system has been in the oceans. During the last interglacial period (~129,000 to 116,000 years ago) global mean sea level (GMSL) was, for several thousand years, between a minimum of 5m (“very high confidence”) and a maximum of 10m (“high confidence”) higher than present. AR5 concludes with “high confidence†that sea level data indicate a transition in the late 19th century to the early 20th century from relatively low mean rates of rise over the previous two millennia to higher rates of rise. Over the past 2,000 years there is “medium confidence” that fluctuations in GMSL did not exceed ~ +/- 0.25m (or 25cm). Sea Level Change This article is one of a series of eight providing an EnvironmentCounts.org (EC) perspective on various aspects of IPCC’s AR5. Each article focuses on the primary data and related evidence presented and specifically excludes coverage of projections, as per EC’s editorial policy and guidelines. |
.Recent sea level change in perspective
More than 70% of the Earth’s surface is covered by water and around 96% of the Earth’s water is in the oceans. The total volume of water the ocean is approximately 1.34 billion cubic kilometers. Sea level (referred to as global mean sea level or GMSL) has changed significantly in the past. The two biggest contributing factors to changing sea level is thermal expansion (or contraction) of the oceans as a result of ocean warming (or cooling) and melting (or accretion) of ice from (or to) glaciers and the World’s two major ice sheets of Greenland and the Antarctic.
Over the last 140,000 years global mean sea level varied over a range of more than 120 metres. The period of the last ice age was from around 60,000 to 20,000 years ago, and the most recent major change in sea level was an increase of more than 120 metres starting at the end of that period as indicated in the following chart.
When water is added to the ocean (eg via melting glaciers) the resulting increase in the sea level is transmitted around the globe rapidly so that all regions experience a sea level change within days of the water being added. While the increase in sea level is transmitted rapidly, freshwater introduced to the ocean can change both salinity and temperature, but the impact of these changes can take up to decades to be transmitted around oceans.
Since AR4, important progress has been made in understanding the amplitude and variability of GMSL during past intervals when the climate was warmer than pre-industrial, since around 1700. Understanding past sea level change provides insight into the sensitivity of sea level to past climate change as well as a context for understanding current changes and for evaluating projected changes.
The following table has been assembled from data presented in Chapter 13 of AR5 to provide some context for recent changes in GMSL.
| Sea level change – paleo climate to current | |||
| Time (Period) | GMSL rise | ||
| Geological and sedimentary record | |||
| 424,000 and 374,000 years ago (Marine isotope stage II) | 6 to 15m higher than present (medium confidence). | ||
| ~129,000 to 116,000 years ago (Last Interglacial Period – LIG) | 5m (very high confidence) to max 10m (high confidence) higher than present. (Greenland ice sheet â€very likely†contributed between 1.4 and 4.3m sea level equivalent.) (“High confidence†there were intervals when rates of GMSL rise during the LIG exceeded the 20th century rate of 1.7 (1.5 to 1.9) mm/yr.) | ||
| last 7,000 years (Late Holocene) | From 7,000 to 3,000, GMSL “likely†rose 2 to 3m to near present levels. Last 2000 years, there is “medium confidence†that fluctuations in GMSL during this interval have not exceeded ~ ±0.25m. | ||
| ~1700–2012 Instrumental record, including tide gauges and satellite record | |||
| 1901-1990 | 1.5 (1.3 to 1.7) mm/yr | ||
| 1971-2010 | 2.0 (1.7 to 2.3) mm/yr | ||
| 1993-2010 | 3.2 (2.8 to 3.6) mm/yr | ||
The instrumental record (since 1700)
The instrumental record of sea level over the past three centuries is based on tide gauge measurements and recently from satellite altimetry. Tide gauge records go back to around the year 1700, and the geographic coverage and accuracy of these records has steadily improved, particularly over the past century. A major change in technology for measuring GMSL change was introduced in the early 1990s with the use of satellite-based radar altimeter measurements. The instrumental record is considered under these two different technology regimes; the tide gauge record and the satellite altimetry records. Prior to around 1700, sea level estimates rely primarily on geological and sedimentary records.
Tide Gauge Record (~1700–2012)
The number of tide gauges has increased since the first gauges at some northern European ports were installed in the 18th century. Southern Hemisphere (SH) measurements started only in the late 19th century. Estimates of 20th century GMSL rise based on these records indicate it is “very likely†that the long-term trend estimate in GMSL is 1.7 (1.5 to 1.9) mm/yr between 1901 and 2010, for a total sea level rise of 0.19 (0.17 to 0.21)m by 2010.
AR5 states it is “likely†that the rate of sea level rise increased from the 19th century to the 20th century, and there is “high confidence†the rate of sea level rise has increased during the last two centuries.
Satellite Altimeter Record (1993–2012)
High-precision satellite altimetry records started in 1992 and provide nearly global (±66° latitude) sea level measurements at 10-day intervals. Records from a newer satellite (Envisat), which observes to ±82° latitude, provide comparable estimates. Although there are slight differences at small time scales in the altimetry-based GMSL time series produced by different groups, there is “very good†agreement on the 20-year long GMSL trend. The different altimetry processing groups have agreed on a GMSL rate of 3.2 (2.8 to 3.6) mm/yr over 1993–2012. The current level of precision is derived from assessments of all sources of errors affecting the altimetric measurements and from tide gauge comparisons.
AR5 concludes that the GMSL trend since 1993 is “very likely†higher compared to the mean rates over the 20th century.
GMSLÂ budget
The GMSL budget summarises the direct observations of sea level change and assesses the degree to which the known factors contributing to sea level change accurately account for the total change. Most critical factors contributing to change could not be assessed rigorously before 1993, when satellite radar altimetry became available.
| Observed contributions to global mean sea level rise – (all data in mm/yr) | |||
| Source of contribution | 1901–1990 | 1971–2010 | 1993–2010 |
| Thermal expansion | – | 0.8 (0.5 to 1.1) | 1.1 (0.8 to 1.4) |
| Glaciers except in Greenland and Antarctica *a | 0.54 (0.47 to 0.61) | 0.62 (0.25 to 0.99) | 0.76 (0.39 to 1.13) |
| Glaciers in Greenland | 0.15 (0.10 to 0.19) | 0.06 (0.03 to 0.09) | 0.10 (0.07 to 0.13) *b |
| Greenland ice sheet | – | – | 0.33 (0.25 to 0.41) |
| Antarctic ice sheet | – | – | 0.27 (0.16 to 0.38) |
| Land water storage | –0.11 (–0.16 to –0.06) | 0.12 (0.03 to 0.22) | 0.38 (0.26 to 0.49) |
| Observed GMSL rise | 1.5 (1.3 to 1.7) | 2.0 (1.7 to 2.3) | 3.2 (2.8 to 3.6) |
| *a Data for all glaciers extend to 2009, not 2010. | |||
| *b This contribution not included in total as it is under Greenland ice sheet. | |||
| Above data from Table 13.1, chapter 13, AR5 | |||
AR5 states that for 1993–2010, allowing for uncertainties, the observed GMSL rise is consistent with the sum of the observed contributions (“high confidence”). The two largest sources are ocean thermal expansion (accounting for about 35% of the observed GMSL rise) and glacier mass loss (accounting for a further 25%, not including that from Greenland and Antarctica). Observations indicate an increased ice-sheet contribution over the last two decades.
AR5 concludes that the observational budget cannot be rigorously assessed for 1901–1990 or 1971–2010 because there is insufficient observational information to estimate ice-sheet contributions with high confidence before the 1990s, and ocean data sampling is too sparse to permit an estimate of global mean thermal expansion before the 1970s. For 1971–2010, the observed contributions from thermal expansion and mass loss from glaciers (not including those in Antarctica) alone explain about 75% of the observed GMSL (“high confidence”).
The fact that the sum of observed contributions to the ocean adds up the observational budget since 1993, within uncertainties, represents a significant advance since AR4 in the physical understanding of the causes of past GMSL change.
The Greenland and Antarctic Ice Sheets
Observations indicate that the contribution to GMSL rise from both Greenland and the Antarctic have increased significantly since 1992, as indicated in the following table.
| Contribution to GMSL | 1992 to 2001 | 2002 to 2011 | |
| Greenland | 0.09 (0.02 to 0.20) mm/yr | 0.59 (0.43 to 0.76) mm/yr | |
| Antarctica | 0.08 (-0.10 to 0.27) mm/yr | 0.40 (0.20 to 0.61) mm/yr | |
Regional variations in sea level
Regional sea level changes can differ substantially from a global average. These differences are responses to complex spatial patterns which result from ocean dynamical processes, movements of the sea floor, and changes in gravity due to water mass redistribution (land ice and other terrestrial water storage) in the climate system. As a result the oceans are simply not level. There are many major ocean currents such as the Gulf Stream in the Atlantic Ocean and the North and South Equatorial Currents, the West Wind Drift and the Peru (Humboldt) currents in the Pacific Ocean. These currents can actually reshape the ocean surface and cause it to tilt. The currents are driven in large measure by winds which in turn are subject to change as the planet heats and cools. These changes are not uniform and as global warming does not raise temperatures evenly around the planet, ocean warming varies regionally and locally. For example, trade winds on the Pacific Ocean drive currents and push warm surface water west (warm water has a larger volume than cold water) so that sea level can be around 0.5m higher in Indonesia than Ecuador. In el Nino years warm water is pushed over to the eastern Pacific and the sea level pattern can be reversed.
