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 for scientific, commercial, and practi- altimeters, sensors that measure the 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. SPATIUM 46 2 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 imbalance is estimated to be around 1 W/m 2, a small quantity com- pared 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. SPATIUM 46 3 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 remain- ing 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 atmo sphere-ocean oscillation much of the tropics and subtropics in and low pressure in the eastern Pacific remain under study. SPATIUM 46 4 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.

Load more