*1 Director, Institute for Global Change Adaptation Science (ICAS), Ibaraki University, Ibaraki, Japan.
Find articles by Nobuo MIMURA Editor: Kiyoshi HORIKAWA*1 Director, Institute for Global Change Adaptation Science (ICAS), Ibaraki University, Ibaraki, Japan.
† Correspondence should be addressed: N. Mimura, Institute for Global Change Adaptation Science (ICAS), Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Ibaraki 316-8511, Japan (e-mail: pj.ca.ikarabi.xm@arumim).
(Communicated by Kiyoshi HORIKAWA, M.J.A.) Received 2013 Jan 15; Accepted 2013 May 10. Copyright © 2013 The Japan AcademyThis is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Sea-level rise is a major effect of climate change. It has drawn international attention, because higher sea levels in the future would cause serious impacts in various parts of the world. There are questions associated with sea-level rise which science needs to answer. To what extent did climate change contribute to sea-level rise in the past? How much will global mean sea level increase in the future? How serious are the impacts of the anticipated sea-level rise likely to be, and can human society respond to them? This paper aims to answer these questions through a comprehensive review of the relevant literature. First, the present status of observed sea-level rise, analyses of its causes, and future projections are summarized. Then the impacts are examined along with other consequences of climate change, from both global and Japanese perspectives. Finally, responses to adverse impacts will be discussed in order to clarify the implications of the sea-level rise issue for human society.
Keywords: sea-level rise, climate change, ice sheet melting, future projections, coastal impacts, adaptation
Sea-level rise is one of the most significant effects of climate change. High projected rates of future sea-level rise have captured the attention of the world. Particularly, countries which are located in low-lying areas as well as small islands are concerned that their land areas would be decreased due to inundation and coastal erosion and, at worst, a large proportion of their population may be forced to migrate to other countries. Therefore, this issue has resulted in heightened attention internationally, as the effects of climate change become apparent.
The level of the sea varies with time and space due to physical processes, such as tide and waves. Mean sea level at a given position is defined as the height of the sea surface averaged over a period of time, such as a month or a year, long enough that fluctuations caused by tide and waves are largely removed. 1) Mean sea level has also spatial distribution in the world. The mean sea level averaged over the global oceans is called global mean sea level (GMSL). The changes in local mean sea level usually differ from that of GMSL, because phenomena dominating in regional and local scales modify the global mean change. When we refer to sea-level change (rise) in this paper, it means the change (increase) in mean sea level, including both global and local, in a general sense. A specific word such as “global (local) sea-level rise” is used to denote the increase in global (local) mean sea level, where it is necessary to distinguish the two terms.
The causes of changes in sea level are not limited to those related to climate change. It is well known that the mean sea level has repeatedly had a large fluctuation due to the alternation of glacial and interglacial periods for the past several hundred thousand years in the Holocene. This fluctuation of mean sea level reached about 120 m. There are also much shorter-term fluctuations in sea level, such as tide, waves and tsunamis. Among such a wide range of fluctuations, climate change-related sea-level change has its own unique characteristics. Given its potential significant effects, there are many questions which scientific research is expected to answer. They include: To what extent did climate change contribute to GMSL rise in the past? How much will GMSL increase in the future due to climate change? How serious are the impacts of the expected sea-level rise, and can human society respond properly to them?
This paper aims to give answers to these questions, based on a review of recent research in the relevant areas. First, the present status of observed sea-level rise, analyses of its causes, and future projections are summarized. Then this paper will examine the impacts of sea-level rise along with other factors of climate change, from both global and Japanese perspectives. Finally, planned responses to the adverse impacts will be discussed.
The latest scientific understanding has been assessed in a systematic way by the Intergovernmental Panel on Climate Change (IPCC). The IPCC has published four major assessment reports since its establishment in 1988. The future sea-level rises by 2100 projected in the First to Fourth Assessment Reports are 31–110 cm (Business as usual scenario), 13–94 cm, 9–88 cm and 18–59 cm, respectively. 2–5) In each of the assessments, the IPCC used different climate models and scenarios for greenhouse gas (GHG) emissions. This is the major reason for the changes in the projected rise in mean sea level. In particular, the values given in the Fourth Assessment Report (AR4), published in 2007, are relatively low. This is because the assessment did not take account of the contribution of the Greenland Ice Sheet (GIS) and West Antarctica Ice Sheet (WAIS), as there was a lack of reliable understanding of the dynamics of these ice sheets, including collapse and outflow. As a result, some researchers considered that these estimates might be the low end of the future rise in sea level.
After the IPCC AR4 was published, progress has been made in various areas related to the sea-level rise issue. For the past changes in GMSL from 19th to 21st Century, by the improvement of analysis techniques applied to satellite altimeter data together with tide gauge data, more detailed and reliable sea-level change data became available (Cazenave and Llovel, 2010; Church and White, 2011; Ray and Douglas, 2011). 6–8) Increase in ocean heat content is caused by global warming, which in turn becomes a major factor of mean sea-level rise through thermal expansion of the sea water. Various techniques were also developed for correction of the past data observed by expendable bathythermograph (XBT) to produce a more reliable estimate of the changes in ocean heat content. Regarding the ice sheets in Greenland and Antarctica, although accurate quantitative assessments are still difficult, understanding of the mechanisms dominating the increase/reduction of the ice sheet mass has progressed. New projections of future sea-level rise, for both 21st century and a longer-term beyond this century have been made extensively based on the development of process-based models including atmospheric and oceanic general circulation models (AOGCMs). In parallel with the process-based models, semi-empirical models are developed for the projection by correlating the past data on global mean temperature and mean sea-level, although the empirical relation has no physical scientific basis. In the field of impact assessment, many have been made in order to understand the potential consequences of sea-level rise. In Japan, several projects for climate change impacts, including those for the coastal zone, have been implemented. Scientific papers and reports have been published from these projects, such as Project Team for Comprehensive Projection of Climate Change Impacts (2008, 2009). 9,10) The present paper provides an overview of the sea-level rise issue based on a review of these and other relevant publications.
We will first examine observed sea-level changes during the past 100 years. The observations have been made in two ways; one is tide gauge observation, and the other is by radar altimeters installed in satellites.
It is not easy to estimate the height of global mean sea level directly, because dense and uniform observations are needed for entire oceans. The data obtained by tide gauges has been compiled by the Permanent Service for Mean Sea Level (PSMSL) managed by Proudman Oceanographic Laboratory, Bidston Observatory in the UK. Today, data from about 1900 tide gauges in nearly 200 countries are sent to PSMSL. 11) Although the number is large, tide gauges are disproportionately located in the Northern Hemisphere. As uncorrected tide gauge data also reflect land movement at the observing location, selected tide gauges, which are located on stable ground, are used for the analysis for a long-term change in the global mean sea level. Further, the earth’s crust has been continuing a rebound (visco-elastic response) to the melt of large glaciers during the present interglacial period. This is known as Glacier Isostatic Adjustment (GIA, see 3.4 for further discussion). Therefore, correction of the GIA land movement is needed to estimate the mean sea-level. Lemke et al. (2007) 12) indicated that the GIA correction ranges from about 1 mm/year (or more) near to former ice sheets to a few tenths of a millimeter per year in the far field; the error in tide-gauge based GMSL change resulting from GIA is assessed as 0.15 mm/year.
Observations by satellite started with Topex/Poseidon, a satellite altimeter launched in 1992. This observation continues, using subsequent satellites such as Jason-1 and Jason-2. The satellite altimeter gives the distance between a fixed reference surface (typically a conventional reference ellipsoid) and the sea surface. Since the early 1990s satellite altimetry has become the main tool for precisely and continuously measuring sea level. This is due to its advantages over tide gauges, such as lack of influence of land movement, and a regular global coverage with frequent revisit cycles. At the beginning, the Topex/Poseidon altimeter had relatively large errors, but the precision of the estimate has greatly improved due to adjustment of the altimetry system and data processing.
There are similar studies to summarize the contemporary GMSL change such as Cazenave and Llovel (2010) 6) and Church and White (2011). 7) As they show close results, the result obtained by Church and White (2011) 7) is introduced here. Figure Figure1 1 is a summary of the long-term trend of GMSL from 1860 to 2010 obtained by superposing the two sources of observed data. From Fig. Fig.1 1 we can see that the rate of increase in GMSL was not large until 1930, but since then it has increased at an accelerating rate. Church and White (2011) 7) indicated that the increase of GMSL from 1880 to 2009 was 21 cm, and that the rate of increase almost doubled to 3.2 ± 0.4 mm/year for 1993 to 2009, relative to 1900 to 2009 when the rate was 1.7 ± 0.2 mm/year.
Global mean sea level from 1860 to 2009. 7) Blue line: estimated from coastal and island sea-level data with one standard deviation error (shading). Red line: estimated by Church and White (2006) 13) for 1870–2001 (solid line) with one standard deviation error (dashed lines). Black line: satellite altimeter data since 1993.
Figure Figure2 2 shows the change of GMSL for 1993 to 2008 obtained by satellite altimeters, 6) which corresponds to the latter part of Fig. Fig.1. 1 . This figure indicates a sudden increase in GMSL from 1997 to 1999, and a decrease in 2008. These irregular changes are considered to be related to an intense El Niño during 1997 to 1998, and La Niña in 2007. Cazenave and Llovel (2010) 6) also identified the influence of North Atlantic Oscillation (NAO) and Pacific Decadal Oscillation (PDO). These are marked longer-term fluctuations, with a period of 10 to 20 years. It is apparent that the fluctuation of the GMSL is a combination of anthropogenic climate change and such natural variations. It is also noted that since the start of the altimeter record in 1993, the rate of increase in GMSL was at the upper end of the sea level projections of the Third and Fourth Assessment Reports of IPCC.
Global mean sea level from satellite altimetry for 1993 to 2008. 6) Blue dots are raw 10-day data. The red line corresponds to a 90-day smoothing of the raw data. The −0.3 mm/year GIA correction has been applied.
The next question is what factors cause long-term changes in mean sea level. To answer this question we will examine the role of the various factors affecting mean sea level. The two basic factors are change of total volume of sea water, and movements of ground and ocean bottom that result in changes to the size and shape of the ocean basins. In addition to these, dynamic factors such as winds, atmospheric pressures, ocean currents, and waves, play a role. Thus there are many factors, with various temporal and spatial scales as shown below. 6,14)
