Guest essay by Rich Taylor

Abstract

Human population is becoming increasingly urban, and most of the world’s largest and fastest-growing cities border tidewater. This note presents charts of annual-value (AV) tide-gauge records in or near major coastal cities to illustrate the sea-level change these cities have observed recently, and fits linear trends to the records. Trends range from -1.5 mm per year (mm/y) to 18 mm/y. Tectonic uplift can explain the lowest trends, and cities growing rapidly on unconsolidated sediments (perhaps dredged) have the highest trends due to land subsidence. Urban areas that encompass ground of variable stability observe variable sea-level change. Where the ground is stable, typical change appears to be a rise of 1- to 2-mm/y. Rates above 3 mm/y seem to have a substantial component of natural and/or anthropogenic subsidence. Rates above 10 mm/y appear to be a primarily a consequence of human activity, which implies they should be manageable to some degree.

All records in this review are from the website www.psmsl.org of the Permanent Service for Mean Sea Level. Profound thanks are due to the Service and its supporters; the website makes it easy to find and download data of apparent fidelity. All geological information is from the website http://ift.tt/2ofPFl5 of the US Geological Survey. The website presents world geology compiled by the Geological Survey of Canada (Open File 2915) as an interactive map that is easy to navigate and interrogate.

Trends in long records

A few major coastal cities have tide-gauge records that exceed 100 years in length. Records that are sufficiently long and accurate show the transition from stable sea-level that prevailed during the 1800s to the general rise that has been characteristic since about 1900. AVs from the gauge at the small city of Brest show this history clearly.

Brest is on terrane that is mainly sedimentary, which rests on older metamorphic and plutonic rocks that outcrop within 20 km to the north and south. Sedimentary rocks can be porous, and they decompose more readily than plutonic and metamorphic rocks into unconsolidated sediments. Plutonic and metamorphic rocks are typically non-porous. Unconsolidated and consolidated sedimentary terranes are more prone to land subsidence, especially when pore-fluid such as groundwater or natural gas is extracted for some combination of civic, industrial or agricultural use. Volcanic rocks have variable porosity and durability.

Brest has the longest record in the regular PSMSL database, and the record has good continuity and quality. From 1807 to 1900, AVs at Brest suggest sea-level was essentially stable. The trend for the last 100 years has been 1.5 mm/y, likely due to thermal expansion of sea water and the net transfer of water from continental aquifers to the ocean.

Accordingly for major cities with long records, AVs are used that provide a trend as close as possible of 100-years-to-the-present, and the rest of the AVs are presented but not trended.

An alphabetical review follows of the most populous coastal cities.

Bangkok harbor is the site of the Fort Phrachula Chomklao gauge. It is in the delta of the Chao Phraya River, which rests on mainly sedimentary terrane. From 1940 to 1959 (B59) its trend was 2.7 mm/y. Since 1962 (B62) it has been 18 mm/y (i.e. 18 cm/decade). The gauge has data-quality cautions (QCFLAGs) for the sharp increase in trend from 1962 and an apparent datum shift from 2003.

Sixty km to the south-southeast in the Gulf of Thailand is sparsely populated Ko Sichang Island and its gauge. The island is near the boundary where sedimentary bedrock rests on older plutonic terrane. From 1940 to 2002, the trend of the gauge (KS) was 0.8 mm/y. The Ko Sichang trend suggests that most of the apparent sea-level rise at Bangkok to 1959 is due to land subsidence, and that urban activity since 1962 has made the rise about 7-times more rapid than before and about 20-times more rapid than on Ko Sichang.

Unconsolidated sediments, such as in Bangkok harbor, are prone to subsidence but gauges in the Netherlands show that stability can result from planning and management. The Maassluis gauge has the longest record; it sits about 15 km from the North Sea on the Maas channel that takes most of the flow through the Rhine (etc.) delta. Its 1.8-mm/y trend is also the average 100-year trend of the six long-standing gauges (Vlissingen, Maassluis, Hoek van Holland, Ijmuiden, Den Helder, Harlingen and Delfzijl) that monitor sea level for the Netherlands.

Buenos Aires is on mainly sedimentary terrane bordering the Rio de la Plata estuary. Its Buenos Aires gauge provided AVs from 1905 to 1987, and the nearby Palermo gauge provides AVs to the present. Both have trends of 1.6 mm/y.

Chennai is on mostly sedimentary terrane, near the surface contact with underlying metamorphic/plutonic terrane. The trend at the Chennai / Madras gauge from 1916 to 2010 was 0.6 mm/y.

Guangzhou, Dongguan, Shenzhen, Hong Kong (HK), Macau and Zhongshan encircle the Pearl River Estuary (Shiziyuan). This area is on mainly sedimentary bedrock, but underlying plutonic terrane outcrops in the northern part of Guangzhou and in Macau. There are nine gauges in HK and in one in Macau that have operated during the last 100 years, which allow an insight into intra-urban variability. In order of initial AV, the following table and chart summarize information provided by these gauges.

Gauge	Span of AVs	Trend mm/y	AVs / Span	Chart Legend
Macau	1925-1982	0.2	58 / 58	M
North Point	1950-1985	-1.2	35 / 36	N
Chi Ma Wan	1961-1989	1.8	15 / 29	C
Tai Po Kau	1963-2016	3.1	50 / 50	P
Tsim Bei Tsui	1975-2016	0.6	27 / 42	T
Loc On Pai	1986-1998	-1.1	10 / 13	L
Quarry Bay	1986-2016	2.9	31 / 31	Q
Waglan Island	1995-2015	4.0	13 / 21	W
Shek Pik	1998-2016	0.1	17 / 19	S
Tai Miu Wan	1998-2016	2.9	16 / 19	MW

In this close cluster of gauges, diversity remains in some trends that span similar intervals. A trend of 1.3 mm/y for this urban area can be obtained by averaging the trends for the gauges, where each trend is weighted by the number of years spanned by the gauge.

Hangzhou is at the south end of the Grand Canal of China in the south-central part of the Yangtze River Delta, and is underlain by sedimentary and volcanic bedrock. It has no gauge in its urban area; the Kanmen and Lusi (discussed with Shanghai) gauges are each about 300 km away. The Kanmen gauge is on volcanic terrane; its trend since 1959 is 5.6 mm/y.

Istanbul is on mixed sedimentary and volcanic bedrock. The nearest indicative gauge might be at Alexandroupolis about 400 km to the east, on mainly sedimentary bedrock. From 1969 to 2014, the Alexandroupolis trend has been 2.6 mm/y.

Jakarta is on mainly sedimentary terrane. It has no gauge in its urban area (and no gauge in Indonesia has more than 8 AVs in its record). Jakarta sits over a sea-floor subduction zone; Lima (q.v.) is in a similar tectonic situation and has a gauge in its urban area.

Karachi is on mainly sedimentary terrane. Intermittent measurements at its gauge from 1916 to 2014 provide a trend of 1.9 mm/y.

Kolkata is in the western Ganges Delta on mainly sedimentary terrane. Its Calcutta gauge has a QCFLAG for an apparent datum shift starting in 1976; its trend since 1932 is 6.9 mm/y. The Diamond Harbour gauge is 40 km further south on the delta; its trend since 1948 is 4 mm/y.

Lagos is on mainly sedimentary terrane. It has no gauge but the Takoradi and Tema gauges, respectively 700 km and 500 km to the west on the same terrane, might provide some indication of sea-level change there. The Takoradi trend from 1930 to 2008 was 2.8 mm/y, excluding AVs from 1972 and 1991 with a QCFLAG for irregular appearance. The Tema trend from 1963 to 1981 was 1 mm/y.

Lima sits on mainly volcanic terrane above the subduction of Pacific sea-floor under South America. Its harbor gauge, Callao 2, has a QCFLAG for many ad hoc datum adjustments made to original data. The trend of the adjusted AVs at Callao 2 since 1970 is -0.3 mm/y. The La Libertad II gauge in Ecuador and the Antofagasta II gauge in Chile sit above the same subduction, have longer records than Callao 2 and neither has a QCFLAG. Their trends, respectively, are -1.3 mm/y for 1950 to 2002 and -0.8 mm/y for 1946 to 2015.

London and area are on mainly sedimentary terrane. The Tower Pier gauge provided urban data from 1929 to 1982 with a trend of 1.7 mm/y. The Southend gauge is 50 km east in the Thames Estuary, and its trend from 1933 to the present is 1.3 mm/y.

The Los Angeles gauge is on mainly sedimentary terrane, as are the Santa Monica. Alamitos Bay Entrance and Newport Bay gauges in the Los Angeles urban area. Santa Monica and Newport Bay are near the outcrop of underlying metamorphic and/or plutonic terrain. The trends in mm/y of the gauges are, respectively, 1, 1.5, 1.6 and 8, and the span-weighted average is 1.7.

Manila is on sedimentary and volcanic terrane. The Manila gauge has QCFLAGs for river discharges and land reclamation. The gauge was moved in 2002. The trend (M62) from 1902 to 1962 was 1.6 mm/y. Subsequently the trend (M63) increased abruptly and has continued to the present at 15 mm/y. The Cebu gauge, 600 km to the south-southeast, is on similar terrane, has a record of comparable length and no noted adjustments or disturbances. Its trend since 1936 has been 1.2 mm/y.

Mumbai is on Deccan basalt, a volcanic rock that typically has low porosity. The trend of the Mumbai / Bombay gauge from 1911 to 2010 was 0.9 mm/y.

Nagoya is on mainly volcanic terrane. There is a non-specific QCFLAG for the Nagoya gauge, but the pattern seen in the combined AVs for Nagoya and Nagoya II is similar to that seen at the Onisaki gauge, on the same terrane 20 km to the south. Tectonic movement is a likely cause of the pattern. Since 1963, the Onisaki trend is -1.5 mm/y.

The New York gauge is on mixed sedimentary-volcanic terrane, as are the Bergen Point gauge on Staten Island and the New Rochelle gauge north of the Bronx. Gauges on the mainly sedimentary terrane are Willets Point, Kings Point, Port Jefferson, Montauk and Plum Island on/by Long Island and Sandy Point off the south shore of New York Bay. USGS Fact Sheet-165-00 mentions subsidence at New York Bay.

In order of initial AV, the following table and chart summarize information provided by these gauges.

Gauge	Span of AVs	Trend mm/y	AVs / Span	Chart Legend
New York	1917-2016	3.1	97 / 100	NY
Willets Point	1932-1999	2.4	65 / 68	WP
Sandy Point	1933-2016	4.1	80 / 84	SP
Montauk	1948-2016	3.1	58 / 69	M
Plum Island	1958-1967	-4.4	8 / 10	PI
New Rochelle	1958-1981	0.6	21 / 24	NR
Port Jefferson	1958-1990	2.2	31 / 33	PJ
Bergen Point	1985-2016	4.8	25 / 32	BP
Kings Point	1999-2016	5.3	18 / 18	KP

The span-weighted average of the trends is 3.0 mm/y. Given the diversity of trends among gauges and changes in gauge activity, the long-established New York and Montauk gauges, respectively at the southern tip of Manhattan and the eastern end of Long Island, appear to be good indicators for this urban area.

Osaka is in the delta of the Yodo River, which is underlain by volcanic terrane. In its urban area are the Osaka, Kobe and Kobe II gauges, and each has a QCFLAG for subsidence. The trend of the Osaka gauge since 1965 is 5.2 mm/y.

Qingdao has no gauge in its urban area; its coastal portion is on metamorphic and/or plutonic terrane. The Shijiusho gauge, 100 km to the southwest, is on the same terrane. The Yantai gauge is 200 km to the northwest, on plutonic terrane and has a QCFLAG for possible datum shifts. From 1954 to 1994, the Yantai trend was -0.2 mm/y and the Shijiusho trend from 1975 to 1994 was 1.7 mm/y.

Rio de Janeiro is on metamorphic and/or plutonic terrane. The Rio de Janeiro gauge provided 13 AVs from 1950 to 1967, with a trend of 3.7 mm/y. Since 1965, the trend for the Ilha Fiscal gauge has been 1.8 mm/y. The span-weighted average is 2.3 mm/y.

São Paulo has no gauge in its urban area. The closest gauge is Cananeia, 200 km west-southwest, which has QCFLAGs for its anomalous trend of 3.8 mm/y. São Paulo and the gauges at Rio de Janeiro 350 km east-northeast are on the same metamorphic and/or plutonic terrane so the Rio de Janeiro average of 2.3 mm/y might be also indicative for São Paulo.

Seoul is on metamorphic and/or plutonic terrane. Its urban area extends to the coast at Incheon, where the trend of that gauge since 1960 is 1.3 mm/y.

Shanghai is in the north-central Yangtze River Delta, as is the Luci gauge 100 km north of the city centre. Both are underlain by mainly sedimentary terrane. There is a non-specific QCFLAG for the gauge, where the trend since 1969 is 5.6 mm/y.

Shantou has no gauge in its urban area, but it and the Xiamen gauge 200 km to the northeast are both on plutonic terrane. The trend at Xiamen from 1954 to 2003 was 1.1 mm/y.

Tianjin hosts the Grand Canal of China, in the Hai River delta on mainly sedimentary terrane. The trend of its Tanggu gauge from 1975 to 1994 was 5.6 mm/y.

Tokyo is on mainly sedimentary terrane where the Sumida and Tama Rivers reach tidewater. The Tokyo I gauge provided a few scattered AVs from 1958 to 1962. The Sibaura and Tokyo III gauges in combination provide AVs from 1961 to the present, and the trend of both gauges is 1.6 mm/y.

Summary

Most of the world’s largest coastal cities border the Pacific Ocean. In recent decades, apparent sea-level has dropped at Nagoya, Lima and perhaps Jakarta. Sea level has likely risen 1- to 2-mm/y at Qingdao, Shantou, Guangzhou-Shenzhen-HK, Seoul, Tokyo and Los Angeles. It has apparently risen 5- to 6-mm/y at the delta cities of Osaka, Tianjin and Shanghai-Hangzhou. The effect of urban activity is clear in apparent rises of 15 mm/y at Manila and 18 mm/y at Bangkok.

For the Atlantic Basin, sea level has likely risen about 2-mm/y at Buenos Aires, London and Rio de Janeiro. Perhaps any change at São Paulo or Lagos has been similar. The apparent rise at Istanbul might be more than 2 mm/y, and apparent rise of 3 mm/y at New York might be due in part to subsidence.

For the Indian Ocean, sea level has likely risen 0.5- to 2-mm/y at Chennai, Mumbai and Karachi. The delta city of Kolkata has seen an apparent rise of 7 mm/y.

Delta cities and others on unconsolidated sediments have higher apparent rises. However, gauges in the Netherlands show that sea-level change in highly developed regions on unconsolidated sediments can be kept close to change seen generally around the world.