Climate Change and Sea Level Rise

INTERNATIONAL SPACE SCIENCE INSTITUTE SPATIUM Published by the Association Pro ISSI No. 46, September 2020

Climate Change

and

Sea Level Rise

2002155_[202015]_Spatium_46_2020_(001_016).indd 1 22.09.20 14:46 Editorial

The climate on our planet Earth is the ground below their path. Be- changing, recently at an acceler- tween the years 1900 and the early Impressum ated pace. As more than 70 % of 1990s, the sea level rose by about our planet is covered by water, 15 cm. Recent satellite altimeter surely one of the most sensitive data reveal an accelerating rise of ISSN 2297–5888 (Print) indicators of this change is the state 9.5 cm from 1993 to 2020. This ISSN 2297–590X (Online) of the global ocean. acceleration is due to an increased heat content in the global ocean Spatium The sea level, for example, varies driving thermal expansion of the Published by the on a large number of temporal and seawater and to the melting of Association Pro ISSI spatial scales, driven by forces due land-based ice sheets and glaciers. to wind, tides, atmospheric pres- Estimating the future develop- sure differences, temperature, sa- ments is not easy, for example in linity, gravitational differences and 2007, the Intergovernmental Panel so forth. on Climate Change (IPCC) pro- Association Pro ISSI jected a maximum rise of 60 cm Hallerstrasse 6, CH-3012 Bern In order to quantify and distin- until 2099, their report of 2013 al- Phone +41 (0)31 631 48 96 guish between these variations, ready projected between 70 cm and see precise measurements are neces- just below 100 cm. www.issibern.ch/pro-issi.html sary. Investigating changes of the for the whole Spatium series sea level requires a reference, and Anny Cazenave, the author of this a mean sea level can be defined as edition of Spatium, is an expert in President an average over a long time series the field of geophysics and in par- Prof. Dr. Adrian Jäggi, of measurements, such that changes ticular in space geodesy. She has University of Bern due to tides and wind effects are pioneered the use of satellite-borne eliminated. The global mean sea instrumentation to investigate the Editor level refers to a spatial average over Earth’s mantle and crust and very Dr. Anuschka Pauluhn the entire ocean. In the absence of early on developed methods to CH-5237 Mönthal any external forces, the mean sea compute ocean topography using level would follow an equipoten- satellite altimetry. In a fascinating Printing tial surface of the Earth’s gravita- talk within the ISSI lecture series Stämpf li AG tional field. This surface is called on 30th October 2019, she shared CH-3001 Bern the geoid (e.g. see Spatium 31) her recent results on sea level rise which does not resemble a simple and its role within climate sphere or ellipsoid but varies sig- research. nificantly from one place to another due to the Earth’s uneven Anuschka Pauluhn mass distribution. The difference Mönthal, September 2020 Title Caption between mean sea level and the Artist’s impression of the Jason-3 geoid is referred to as the ocean satellite (“Joint Altimetry Satellite surface topography. Precise meas- Oceanography Network 3”) which is a collaboration project of several urements of the ocean surface to- European and US American partners pography have been possible since (EUMETSAT, CNES, NASA, the launch of advanced satellite NOAA). Its mission is to supply data altimeters, sensors that measure the for scientific, commercial, and practi- cal applications to sea level rise, sea sea surface height via the return surface temperature, ocean circula- travel time of a radar signal sent to tion, and climate change.

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2002155_[202015]_Spatium_46_2020_(001_016).indd 2 22.09.20 14:46 Climate Change and Sea Level Rise

Anny Cazenave, Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, Toulouse, France and ISSI, Bern, Switzerland

1. Introduction

Planet Earth is currently in a state of energy imbalance: it reemits less energy into space than it receives from the Sun. This energy im balance is estimated to be around 1 W/m 2, a small quantity compared to the 342 W/m2 received from the Sun1, but significant enough to cause important changes to the global climate. The cause of this energy imbalance is well known and attributed to so-called Figure 1: Total GHG emissions (in Gt/year equivalent CO2) since 1960 greenhouse gases (GHG), mostly (source: The Global Carbon Budget project, 2019). carbon dioxide (CO2), emitted by human activities as a result of fossil fuel combustion and of land trous consequences on marine life million) in 2019, a value 40 % use change, mostly deforestation. and entire marine ecosystems. The higher than the highest concentra-

GHG absorb part of the infrared CO2 concentration in the atmo tions encountered by planet Earth radiation re-emitted by the Earth sphere reached 410 ppm (parts per during the last 800 000 years. causing an additional heating effect that superimposes on the nat- Figure 2: GHG emissions (in Gt/year equivalent CO2) per region since 1960 ural greenhouse effect. Figure 1 and (source: The Global Carbon Budget project, 2019). Figure 2 show the global GHG emissions since 1960 and the emissions by regions.

Over the decade 2009 to 2018,

emissions in CO2 equivalent from fossil fuels and deforestation amount to 37.5 and 5.5 Gt/year, respectively. Only 44 % have stayed in the atmosphere because of two important sinks: the vegetation and the oceans that absorbed 29 % and

23 % of emitted CO2, respectively. However, a negative side effect of the ocean sink is acidification of sea water, with potentially disas-

1 Of the 342 W/m² of solar radiation received on average by the Earth, about 30 % is directly reflected back to space by the atmosphere and the surface, in particular Arctic sea ice, leaving approximately 240 W/m² of solar energy available for warming the atmosphere, land surfaces and the oceans.

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2002155_[202015]_Spatium_46_2020_(001_016).indd 3 22.09.20 14:46 The ocean has the largest heat capacity (and thus thermal inertia) of all compartments of the climate system. Over the past 50 years, about 93 % of the energy excess due to human activities has been stored as heat in the ocean. The remaining 7 % have been equally shared to warm the atmosphere and the continents, and to melt sea ice and land ice.

Figure 3 shows the evolution of the global mean Earth temperature since 1850. The last decade (2010 to 2019) has been the warmest since Figure 3: Evolution of the Earth’s global mean temperature since 1850 the beginning of the record. (source: WMO “State of the global climate 2019”).

Warming of the ocean and melting of land ice (glaciers, and Green- ridges expanding). Growth and small sea level variations on inter- land and Antarctica ice sheets) have decay of polar ice sheets during the annual and decadal timescales another consequence: sea level rise. Quaternary glacial ages, driven by (see below). Over the last few changes of the Earth’s obliquity decades, sea level has also displayed During the Earth’s history, sea and orbit around the Sun, in par- a long-term increase caused by level has varied over a broad range ticular its eccentricity, have also human-induced global warming. of temporal scales. On geological caused sea level variations of ap- In the following, we essentially timescales (about 5 to 100 million proximately one hundred metres. discuss the latter two effects. years) large-amplitude (> 100 m) Other natural factors, e. g. change sea level changes primarily resulted in solar irradiance and vo lcanic from tectonics processes (e. g. eruptions, can cause sea level fluc- large-scale change in the shape of tuations of a metre or less. Internal ocean basins associated with sea- climate variability (e. g. related to floor spreading and mid-ocean El Niño events) is responsible for

El Niño: Span. for “the child” is the the Pacific Ocean. The El Niño phase and vice versa for the cold phase. Ex- name given to a climate phenomenon of the ENSO cycle corresponds to a treme events of this climate pattern observed mainly in the Pacific Ocean. mass of warm water transported from can have strong effects on the weather, In fact, the El Niño/Southern Oscil- the western tropical Pacific Ocean such as floods and droughts, in central lation (ENSO) cycle has causes and eastward, superposing the cold nutri- and south America, as well as in other consequences on a rather global scale. ent-rich upwelling water along the regions of the world. Developing It is a rather irregularly appearing, South American west coast. The cool- countries dependent upon agriculture however quasi-periodic, variation ing phase of the cycle subsequent to and fishing, particularly those border- (observed to happen on timescales be- El Niño is referred to as “La Niña”. The ing the Pacific Ocean, are the most tween two and seven years) in winds warm phase is accompanied by high air affected. Mechanisms that cause this and sea surface temperatures affecting surface pressure in the western Pacific coupled atmosphere-ocean oscillation much of the tropics and subtropics in and low pressure in the eastern Pacific remain under study.

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2002155_[202015]_Spatium_46_2020_(001_016).indd 4 22.09.20 14:46 2. Measuring sea records is the Permanent Service last deglaciation, also called “Gla- for Mean Sea Level (PSMSL, cial Isostatic Adjustment” (GIA) is level changes www.psmsl.org) which contains another process that gives rise to data for the 20th century from vertical land movements.2 roughly 2000 sites. However, these records are often inhomogeneous After the approximately 130 m sea 2.1 Tide gauges in terms of data length and qual- level rise associated with the last ity. For long-term sea level studies, deglaciation that started about On paleo timescales, spanning the only about 10 % of this data set is 20 000 years ago, geological, geo- last two millennia, sea level change usable (see Figure 4), due to data chemical and archeological obser- is known indirectly from proxy gaps and limited tide gauge distri- vations indicate that the mean sea indicators, such as records from bution in the past. level remained almost stable dur- sediments on the ocean floor, up- ing the last two to three millennia, lifted marine terraces or ancient Tide gauges measure sea level rel- then started to rise about two cen- corals. ative to the ground, and as they are turies ago. A large number of attached to the coast, thus also reg- studies have analysed tide gauge More recent (i. e. roughly since the ister the vertical ground motions, data from the PSMSL to provide a beginning of the 20th century) in addition to the climate-related historical mean sea level time se- data on sea level variations have sea level change. These vertical ries. The most recent ones report come from direct in-situ measure- ground motions can result from a a mean sea level increase of 15 cm ments by tide gauges, instruments variety of factors caused by tectonic over the 20th century, however located along continental coast- and volcanic processes, as well as with a large range of different val- lines and islands. Prior to the be- other natural factors (e. g. sediment ues (from 12 cm to 19 cm). This ginning of the 20th century, data loading in river deltas) or human large uncertainty results from are very sparse and based only on activities (ground water pumping many factors, e. g. uneven and re- a few long records at sites mostly and oil/gas extraction) causing stricted distribution of tide gauges, located in Western Europe. The ground subsidence. Post-glacial re- treatment of data gaps in the re- largest tide gauge database of bound, the viscoelastic re sponse of cords, geographical averaging pro- monthly and annual mean sea level the Earth’s crust and mantle to the cedure, impact of vertical crustal motions and methods to correct them. Figure 4: Tide gauge network (from the PSMSL) with more 2.2 Satellite altimetry than 40-years of sea level records. Since the early 1990s, satellite altimetry has become the main tool for precisely and continuously measuring sea level with quasi- global coverage and a few days

2 During the last glacial period about 20 000 years ago much of northern Europe, Asia, North America, Greenland and Antarctica was covered by ice sheets of up to three kilometres thickness during the glacial maximum. The enormous weight of this ice caused the surface of the Earth’s crust to deform and warp downward, forcing the viscoelastic mantle material to flow away from the loaded region. When the glaciers retreated, the removal of this weight led to slow (and still ongoing) uplift or rebound of the land and the return flow of mantle material back under the deglaciated area. Due to the extreme viscosity of the mantle, it will take many thousands of years for the land to reach an equilibrium level.

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2002155_[202015]_Spatium_46_2020_(001_016).indd 5 22.09.20 14:46 physical effects, such as the sea state Figure 5: The princi- bias (the sea is seldom calm), and ple of nadir the tides of the solid Earth and satellite the ocean are applied as well. The altimetry. final result of a coherent time se ries of the sea surface height has to take instrumental drifts and bias between successive altimetry missions into account.

High-precision satellite altimetry started in 1991 and 1992 with the launch of ERS-1 by the European Space Agency (ESA), and of the joint NASA (National Aeronaut- ics and Space Administration) - CNES (Centre National d’Etudes Spatiales) satellite Topex/Poseidon (T/P), respectively. Since then, several high-precision altimetry revisit time (called the “orbital lipsoid or a mean sea surface). It is missions have followed: Jason-1 cycle”). Compared to tide gauges obtained by the difference between (2001), Jason-2 (2008) and Jason-3 that provide sea level relative to the height of the satellite above the (2016), the successors of T/P with the ground, satellite altimetry reference (deduced from precise similar orbital characteristics. ESA measures “absolute” sea level var- orbitography3) and the range also developed ERS-2 (1995), iations. The concept of the satellite measurement. A description of the Envisat (2002), CryoSat (2010) altimetry measurement is simple reference surfaces like the geoid and Sentinel-3A/3B (2016/2018). (see Figure 5): the on-board radar can be found for example in Spa- Cryosat and Sentinel-3A use new altimeter transmits microwave tium 31 (Earth Gravity from Space technology, i. e. Synthetic Aper- radiation towards the sea surface, by R. Rummel). ture Radar (SAR) altimetry.4 The which partly reflects back to the Sentinel-3 missions contribute to satellite. Measurement of the Every single measurement of the the COPERNICUS operational round-trip travel time of the sig- signal travel time and thus the programme of the European nal provides the height of the sat- height needs to be corrected for Union. SARAL/AltiKa (2013), a ellite above the sea surface (called various factors, such as atmospheric joint Indian-French mission, oper- “range”). The quantity of interest (ionospheric and tropospheric) de- ates in the Ka-band (≈ 35 GHz), in oceanography and sea level stud- lays generated by varying electron allowing a smaller radar footprint ies is the sea surface height above a content and by the amount of wa ter on the ground than other missions fixed surface as a reference (typi- and dry gases in the atmosphere. (T/P, the Jason series, ERS and cally a conventional reference el- Other corrections due to geo Envisat being equipped with Ku-

3 Also called Precise Orbit Determination, POD. The determination of precise satellite orbits by means of geodesy techniques like GNSS (Global Navigation Satellite Systems, e. g., GPS, GLONASS or Galileo), DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite, where ground-based radio beacons emit signals that are received by the monitored satellite), or Satellite Laser Ranging. 4 In the Synthetic Aperture or delay-Doppler processing method, additionally the Doppler shifts of the return echo of the radar signal from a little in front and behind of the nadir point are recorded. This enhances the resolution.

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2002155_[202015]_Spatium_46_2020_(001_016).indd 6 22.09.20 14:46 Figure 6: Constella- tion of modern radar alti metry missions.

band (13 GHz to 17 GHz) radars). Waters Ocean Topography) able oped by NASA and CNES for a A wide-swath interferometric alti to study the mesoscale ocean cir- launch in 2022. Altimeter satellites metry mission (SWOT- Surface culation is currently being devel- have also been launched by the Chinese Space Agency (HY2A and HY2B). The T/P and Jason series Figure 7: Geographical coverage of satellite altimetry missions. have an orbital cycle of 10 days but a large spacing between satellite tracks (roughly 300 km at the equator). They cover the 66°S to 66°N latitude domain. The orbital cycle of the ESA missions and SARAL/AltiKa is 35 days, but the spacing between tracks is smaller and the latitudinal coverage reaches up to 82°, allowing a large portion of the Arctic Ocean to be covered.

Figure 6 shows the status of the con- stellation of high-precision altim-

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2002155_[202015]_Spatium_46_2020_(001_016).indd 7 22.09.20 14:46 eter satellites while Figure 7 shows the geographical coverage of the altimetry missions.

In altimetry, the global mean sea level (GMSL) is estimated by geographically averaging all sea surface height measurements per- formed by the satellite during an orbital cycle. The GMSL evolution is deduced from successive orbital cycles. During the past 27 years, at least two altimeter satellites have been operating simultaneously, and during some periods, even more than two. Such data are combined to compute the GMSL change over time. This is shown in Figure 8.

The curve shown in Figure 8 shows Figure 8: Evolution of the global mean sea level from January 1993 to May 2020 that the GMSL is rising and that based on satellite altimetry data (source: LEGOS). the rise is even accelerating. Over the period 1993 to 2020, the mean rate of rise is (3.25 +/- 0.3) mm/ a single sea surface height measure- of the mean rate of rise to reach year. The acceleration is estimated ment has now reached the 1 cm to about 0.3 mm/year. This level of to 0.1 mm/year². The precision of 2 cm level allowing the precision precision is also confirmed by comparison with tide gauges and assessments of all sources of errors Figure 9: Regional trends in sea level over 1993 to 2019 from satellite altimetry affecting the altimetry system. (source: LEGOS). Owing to its quasi-global coverage of the oceans, satellite alti metry also allows mapping of sea level change on a regional scale. This is shown in Figure 9. In some regions, sea level trends are two to three times larger than the global mean rise. This is particularly clear in the northern and western tropical Pacific, as well as in the southern ocean.

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2002155_[202015]_Spatium_46_2020_(001_016).indd 8 22.09.20 14:46 3. Understanding larly, ice mass loss from the ice though the coverage was improved sheets causes sea level rise. Modi- compared to the previous decades, sea level fication of the land hydrological large regions remain un-sampled, cycle and associated land water in particular in the southern hem- change storage due to combined natural isphere and in the Arctic. Besides, and man-made climate variability salinity measurements were very and change also lead to sea level few. The Argo project has totally 3.1 Causes of present-day change. revolutionized the situation, with global mean sea level rise its 4000 automatic floats providing systematic temperature and sa linity In terms of global mean, sea level Ocean warming measurements down to a depth of change mostly results from ocean 2000 m at 10 day intervals, with thermal expansion and ocean mass Since the middle of the 20th cen- quasi global coverage (though not increase due to melting of glaciers tury, in-situ ocean temperature yet over the Arctic Ocean). Full and ice mass loss from the Green- (and to a lesser extent salinity) data deployment of the Argo floats was land and Antarctica ice sheets. A has been collected by various de- effective in around 2005. Figure 10 small contribution comes from vices, e. g. expendable bathyther- shows the Argo floats coverage at changes in land water storage and mographs (XBT) from ships, buoys the time of writing. atmospheric water content. As the and moorings, and since the begin- oceans warm in response to an- ning of the 21st century by the The ocean temperature data col- thropogenic forcing, sea waters automatic profiling floats from the lected over a few decades have expand, thus sea level rises. When Argo system5. During the 1990s, clearly shown that the oceans are mountain glaciers melt in response temperature measurements of the warming, as illustrated in Figure 11 to increasing air temperature, sea upper ocean were collected down by the ocean heat content level rises because of fresh water to a depth of about 700 m along increase. mass input into the oceans. Simi- commercial shipping routes. Al-

Figure 10: Argo floats coverage. Figure 11: Ocean heat content since 1955 (source: WMO State of the global climate 2019).

5 Argo is an international programme that uses profiling floats to measure temperature, salinity, surface currents, and, recently, bio-optical properties in the Earth’s oceans. The fleet consists of about 4000 drifting floats deployed worldwide.

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2002155_[202015]_Spatium_46_2020_(001_016).indd 9 22.09.20 14:46 The main land ice contributions to sea level come from glacier melting and ice mass loss from the Greenland and Antarctica ice sheets.

Ice sheets

If totally melted, Greenland and West Antarctica (the unstable part of the continent) would raise sea level by about 7 m and 5 m, respectively. Clearly, even a small amount of ice mass loss from the ice sheets can produce substantial sea level rise, with adverse impacts on vul- Figure 12: Ice mass loss from the Greenland ice sheet (source: NASA). nerable low-lying coastal regions.

Since the early 1990s, different re- proximately 300 km at monthly and post-glacial rebound, the mote sensing observations have intervals. On timescales from a few so-called GIA described before. provided important observations months to several years/decades, Over Greenland and Antarctica, of the mass balance of the ice changes in the gravity field mostly GRACE directly provides the sheets. These in clude airborne, sat- result from mass redistributions in changes of mass of the ice sheets. ellite-based la ser and radar alti the Earth surface fluid envelopes This is illustrated in Figure2 1 and metry, synthetic aperture radar in- related to changes of land hydro Figure 13, showing that Greenland terferometry –InSAR, and since logy and of the cryosphere. How- and West Antarctica are losing 2002, space gravimetry from ever, GRACE also measures solid mass at an accelerated rate. GRACE (gravity recovery and cli- Earth processes such as earth- mate experiment). quake-related crustal deformations

GRACE is a space mission developed by NASA and the German Figure 13: Ice mass loss from the Antarctica ice sheet (credit: IMBIE 2 Project). Space Agency. It consists of two twin satellites flying on the same orbit and measuring their mutual distance using a microwave device with 1 micrometre accuracy (cf. also Spatium 31).

The first GRACE mission covered the period 2002 to 2017. A GRACE follow-on mission with similar characteristics was launched in 2018. GRACE measures temporal changes of the Earth’s gravity field with a ground resolution of ap-

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2002155_[202015]_Spatium_46_2020_(001_016).indd 10 22.09.20 14:47 It is now well established that the short. For a few years, remote sens- crease or decrease of ocean mass). recent ice sheet mass loss partly re- ing techniques have also been used Because the atmospheric reservoir sults from accelerated glacier flow to estimate glacier mass changes. cannot contain an important along some coastal margins of the amount of water mass over a multi- ice sheets and further iceberg dis- year timescale, change in ocean charge into the surrounding ocean. Terrestrial waters mass is nearly compensated by This results from short-term dy- change in land water storage in namical instabilities occurring in Change in land water storage due tropical river basins and vice versa. regions where coastal glaciers are to natural climate variability has grounded below sea level (this is contributed little to the GMSL Human activities (i. e. anthropo- especially the case in West Antarc- long-term trend during the past genic changes in the amount of tica). Thinning and subsequent few decades. On the other hand, water stored in soils, reservoirs and break-up of floating ice tongues or on an interannual timescale, in aquifers as a result of dam building ice shelves that buttressed the gla- particular during ENSO (El Niño- along rivers, underground water ciers result in rapid grounding-line Southern Oscillation) events, fluc- mining, irrigation, urbanisation, retreat and accelerated glacier flow. tuations in water storage on land deforestation, etc.) represent an- Several recent observations have cause temporary anomalies of the other potential contribution of shown that warming of subsurface global mean sea level of several terrestrial waters to sea level ocean waters triggers these dy- millimetres (Figure 14). change. The dominant contribu- namical instabilities. For Antarc- tions come from two factors of op- tica, this process explains almost all During an El Niño or a La Niña posite sign: dam building, causing ice mass loss, while for Greenland event, the GMSL presents positive sea level drop, and ground water it only accounts for about half, the or negative anomalies of one to depletion, causing sea level rise. other half being due to surface two years duration due to an ex- For the last few decades, ground melting. cess or deficit of precipitation over water pumping has considerably the tropical Pacific (causing an in- increased while the building of

Glaciers Figure 14: Detrended global mean sea level. The largest anomalies are driven Being very sensitive to global by ENSO events. This is the case for the large negative anomaly in 2011 due to a La Niña event (source: LEGOS). warming, mountain glaciers and small ice caps have retreated worldwide during the 20th century, with significant acceleration since the early 1990s. Volume and mass balance studies of a significant number of glaciers, based on various in-situ observing methods and modelling, have been proposed, and estimates of the contribution of glacier melt to sea level rise have been deduced. Because of high year-to-year variability in glacier melting, in-situ based mass bal- ances are generally provided as 5-year averages, a strong limitation when the period of investigation is

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2002155_[202015]_Spatium_46_2020_(001_016).indd 11 22.09.20 14:47 dams has decreased, so that there the altimetry era suggests that any with the ocean circulation. In- is a net slightly positive effect on non-assessed component (i. e. deep crease in wind stress and associated the global mean sea level. Its exact ocean heat content below 2000 m) deepening of the thermocline ex- contribution, however, remains does not yet significantly contrib- plain the strong sea level trends highly uncertain. ute to the GMSL rise. Besides, this observed in the western tropical indicates that no systematic errors Pacific during the altimetry era. affect the different observing sys- This is illustrated by Figure 16 which 3.2 The sea level budget tems used to quantify the compo- compares spatial trend patterns in nents of the sea level budget. altimetry-based observations (left A large number of studies have panel) and thermal expansion (right tried to close the sea level budget, panel) during the period 1993 to i. e. to compare the observed rate 3.3 Regional scale 2015 over the western tropical of sea level rise with the sum of Pacific (global mean trend re- contributions estimated indepen- The regional variability in sea level moved in both cases). Altimetry- dently. Studying closure (or non- trends is mainly due to large-scale based sea level and thermal expan- closure) of the sea level budget is changes in temperature and salin- sion trends show very good important as it provides constraints ity-related density structure of the agreement. on missing or poorly-known con- oceans, in response to forcing fac- tributions such as the deep ocean tors (e. g. heat and fresh water ex- While in most regions regional under-sampled by current observ- change at the sea-air interface and changes in sea level trends essen- ing systems, or still uncertain wind stress) and their interaction tially result from ocean tempera- changes in water storage on land due to human activities (e. g. groundwater depletion in aquifers). Figure 15: Sea level budget from 1993 to 2016. The individual contributions are shown at the bottom of the panel. The altimetry-based sea level and sum of Global mean sea level corrected contributions are shown by the black and red curves, respectively for ocean mass change allows in- (source: ESA Sea Level Budget Closure Project 2019). dependent estimates of changes in total ocean heat content over time, from which the Earth’s energy imbalance can be deduced.

Figure 15 shows time series of individual components contributing to the GMSL as well as the sum of the components compared to the observed global mean sea level from 1993 to present. It is obvious that the sea level budget is almost closed over the study period within the respective uncertainties. Annual residuals remain below the 2 mm level. In terms of trends, the sea level budget since 2005 is closed to better than 0.3 mm/year, an order of magnitude similar to the uncertainty of the mean sea level rise. Quasi closure of the budget over

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2002155_[202015]_Spatium_46_2020_(001_016).indd 12 22.09.20 14:47 Figure 16: Spatial trend patterns in sea level over 1993-2015. Left panel: from altimetry; right panel: from thermal expansion (thermal expansion is estimated by vertically integrating in-situ temperature data from depth to the surface).

ture change, in a few others (e.g. level drop in the immediate vicin- 4. Changes and in the Arctic) the change in salinity of the melting bodies but sea ity can also be important. level rise in the far field (e. g., along impacts in the North East coast of America In addition to changes in sea water and in the tropics). The GIA effect coastal regions density due to changes in depends on Earth’s mantle viscos- temperature and salinity, other ity and deglaciation history while processes can cause regional trends the response of the solid Earth to At local scales, in particular in in sea level, e. g. atmospheric load- on-going land ice melt essentially coastal areas, additional small-scale ing,6 and solid Earth deforma- depends on lithosphere elasticity as processes superimpose on the tions and associated gravitational well as on the amount and location global mean and regional sea level changes in response to mass redis- of the current ice mass loss. Re- components and can make coastal tributions caused by land ice melt gional effects of both GIA and sea level substantially different and land water storage changes. present-day ice melting are small from open-ocean sea level rise. For The land ice melt related factor has compared to ocean thermal ex example, changes in small-scale two components: one associated pansion and still hardly detectable currents, as well as freshwater in- with the last deglaciation (the GIA in the satellite altimetry put in river estuaries, can modify phenomenon), and the other due obser vations. the density structure of sea waters, to on-going land ice melt. These hence the coastal sea level. Changes factors - mostly known from mod- in wind, waves, shelf bathymetry, elling - give rise to complex re- along-shore and cross-shore sedi- gional patterns in sea level change ment transport, vertical land mo- (often called “fingerprints”): sea tions, land use change, and urbanisation are additional forcing factors able to modify sea level variations in the coastal zone. Unlike 6 Changes of the weight of the column of atmosphere due to variations of global mean and regional sea level pressure result in vertical changes of the sea surface. measured by satellite altimetry

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2002155_[202015]_Spatium_46_2020_(001_016).indd 13 22.09.20 14:47 missions, coastal sea level changes At the coast, in addition to cli- winter storms and urbanisation. By remain poorly known. Coastal mate-related sea level rise, another the end of the 21st century and zones are indeed highly under- non climatic factor is vertical beyond, however, the expected sea sampled by tide gauges and ground motion. Withdrawal of level rise will make it the domi- currently un-surveyed (within water from aquifers can produce nant forcing factor of future coastal 15 kilometres of the coast) by con- dramatic ground subsidence, and system changes. ventional altimetry missions be- hence relative sea level rise, espe- cause of land contamination on the cially in coastal megacities. Dur- radar signal. However, dedicated ing the second half of the 20th cen- reprocessing of the data from these tury, Tokyo, Shanghai, Bangkok 5. Future missions now helps to estimate sea and Jakarta subsided by several me- level change very close to the coast. ters because of groundwater with- changes In the near future, systematic use drawal. Ground subsidence is also of new SAR technology imple- widely observed in highly-popu- mented in recent ESA missions lated deltaic areas, in particular in In the recently published (Septem- (e. g. CryoSat-2 and Sentinel- Asia. Ground subsidence amplifies ber 2019) special report on the 3A/B) will also allow estimating the climate-related sea level rise oceans and the cryosphere of sea level changes in the coastal and its negative impacts. (SROCC) from the Intergovern- areas. mental Panel on Climate Change Sea level rise is generally consid- It is now well established that at a (IPCC), projections of future sea ered as a major threat of current local scale, the coastal response to level rise based on ensemble means global warming for the world’s sea level rise depends on several of process-based climate models low-lying coastal regions. Coastal non-linearly-related factors such as indicate that at the end of the zones are indeed the most densely coastal morphology, near-shore 21st century, global mean sea level populated and economically active bathymetry, sediment supply from would have increased relative to areas of the world. They concen- rivers, changing waves and cur- today by 40 cm to 85 cm, depend- trate important infrastructures rents, etc. For the 20th century, ing on the radiative forcing sce- such as harbours and industries. shoreline erosion and retreat mostly nario (Figure7 1 ). The moderate Today, about 600 million people resulted from multiple drivers, in forecast of 40 cm corresponds to a live near the sea, at an altitude less particular extreme events like climate scenario that fulfills the than 10 m above sea level and mostly concentrated in several of the world’s largest megacities. This Figure 17: Projections of the global mean sea level rise (in m) until 2100 and number is expected to double by 2300 for two global warming scenarios expressed in terms of representative 2060. concentration pathways (RCP). (Source: SCROCC, IPCC, 2019).

Compared to extreme events, sea level rise is a slow process but it has long-term consequences. Adverse effects of sea level rise in coastal areas include permanent and temporary inundation, shoreline erosion, increased flooding during storm surges, salinisation of wet- lands and aquifers, and loss of coastal ecosystems.

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2002155_[202015]_Spatium_46_2020_(001_016).indd 14 22.09.20 14:47 2015 Paris Agreement, i. e. a 2 °C 6. Conclusion aptation and mitigation strategies target for the global mean Earth to face environmental risks asso temperature in 2100 (compared ciated with climate change and to pre-industrial). This scenario human activities. would require drastic reductions of Today, sea level rise is one of the GHG emissions in the coming two best indicators of climate change. to three decades and zero emission This is due to the fact that it inte- around 2070. The forecast of 85 cm grates changes occurring in the corresponds to a high warming Earth’s climate system in response scenario, more or less pursued by to system-inherent climate varia- the current GHG emissions. In this bility, as well as external forcing scenario, a global mean sea level factors of natural and anthropo- elevation of +1 m compared to the genic origin. These changes in- Further reading/literature year 2000 is not excluded. clude ocean warming, land ice Spatium No. 31, Rummel, R.; melt, and changes in water storage Earth Gravity from Space, May 2013. In the high warming scenario in continental river basins. Bengtsson, L.; Koumoutsaris, S.; (RCP 8.5), ocean warming, gla- Bonnet, R.-M.; Herland, E.-A.; ciers melting, and Greenland and Measuring sea level rise and under- Huybrechts, P.; Johannessen, O.M.; Antarctica ice sheet mass loss standing its causes has considerably Milne, G.; Oerlemans, J.; Ohmura, A.; Ramstein, G.; Woodworth, P. would contribute by about 37 %, improved in the recent years, es- (eds.), The Earth Cryosphere and Sea 23 %, 18 % and 17 %, respectively, sentially because new in-situ and Level Change, SSSI Vol. 40, the remaining 5 % being attributed remote sensing observations have ISBN 978-94-007-2062-6, 2011. to land water storage change. Sum- become available. Sea level is pres- Cazenave, A.; Champollion, N.; Paul, ming the contributions of glaciers ently rising at a sustained rate of F.; Benveniste, J. (eds.); Integrative Study of Mean Sea Level, SSSI Vol. and ice sheets shows that land ice more than 3 mm/year and its rise 58, ISBN 978- 3-319-56490-6, 2017. melt would contribute with nearly is accelerating. These observations Oppenheimer, M.; B.C. Glavovic; 60 %. Recent model calculations are of major importance for pro- J. Hinkel; R. van de Wal; A. K. Mag- suggested the existence of a new cess understanding and validation nan; A. Abd-Elgawad; R. Cai; type of ice-sheet instability in Ant- of climate models developed to M. Cifuentes-Jara; R. M. DeConto; T. Ghosh; J. Hay; F. Isla; B. Marzeion; arctica (as yet unobserved), which forecast future changes. It is indeed B. Meyssignac; and Z. Sebesvari; would lead to approximately 1 m almost certain that sea level rise 2019: Sea Level Rise and Implications of global mean sea level rise by will continue in the future decades for Low-Lying Islands, Coasts and 2100 for Antarctica alone. But such and centuries because of expected Communities. In: IPCC Special Re- port on the Ocean and Cryosphere in a possibility is still controversial. continuing global warming, a Changing Climate. In press. thereby affecting a large number of Ponte, R.; Meyssignac, B.; In addition, as is already the case world coastal regions. Sustained Domingues, C.; Stammer, D.; Caze- today, future sea level rise will not observations of sea level changes, nave, A.; Lopez, T. (eds.), Under- be uniform, deviation from the as well as observations of the vari- standing the Relationship between Coastal Sea Level and Large-Scale global mean being expected in sev- ous factors that cause these changes, Ocean Circulation, SSSI 75, 2020. eral oceanic regions, in particular are of high priority in climate Stammer, D. and Cazenave, A. (eds.), in the tropics where an amplifica- change research in order to provide Applications of satellite altimetry over tion of the global mean rise of improved sea level predictions oceans and land surfaces, CRC Press, 20 % to 30 % is expected. from global to local scales. In the Taylor and Francis, 617 pages, coastal zones, sea level observations ISBN 978-1-4987-4345-7, 2018. and predictions will provide inval- WMO Statement on the State of the Global Climate in 2019, World Mete- uable information to decision- orological Organization, Report 1248, makers in charge of developing ad- 2020.

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The Author

space. She first focused on the dy- don, Jason-1, and the Ocean Sur- namics of the oceanic crust and the face Topography Mission, Anny mechanically strong layer of the has contributed to a greater under- uppermost mantle below it. Among standing of this sea level rise other things, she used early space- and its dependence on global borne radars to show that the ocean warming. surface is not flat, but follows the topography of the ocean floor. In Besides a large number of publica- other early work, she addressed tions, Anny was lead author of the questions about the rotation of sea level sections of the Inter Venus and the origins of the Mars governmental Panel on Climate moons, Phobos and Deimos. Change’s most recent full reports, in 2007 and 2014. Towards the end of last century, European and American space Since 2013, she has been director agencies launched a new series of of Earth sciences at the Inter satellite radar altimeters capable of national Space Science Institute. monitoring sea level ev erywhere in the world oceans in more or In 2020, she received the prestig- Anny Cazenave received her Ph.D. less real time. By the early part of ious Vetlesen Prize, often referred in geophysics in 1975 from the the 21st century, it had been to as the Nobel Prize in geophys- University of Toulouse. Subse- determined that global sea level ics, for her pioneering work in us- quently, working at the French was rising by at least about three ing satellite data to chart and quan- space agency CNES, she went into millimetres a year. As one of tify rises in the surface of the space geodesy, the use of satellites the leading scientists in the joint oceans, and related changes in ice to track changes in Earth’s surface, French/American satellite alti sheets, landmasses and freshwater gravity field and orientation in metry missions, TOPEX/Posei- bodies.

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