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Sea level (around Japan)

Updated on 15 Feb. 2019
Global Environment and Marine Department

A trend of sea level rise has been observed in Japanese coastal areas since the 1980s, but no long-term trend of rise is seen for the period from 1906 to 2018. Variations with 10- to 20-year periods (near-10-year variations) are seen for the period from 1906 to 2018.

Sea surface water level change

Time-series representation of annual mean sea level values (1906 – 2018)
The 1981 – 2010 average is used as the normal.
Annual sea level anomaly time series (comma-separated value file: 3 KB)

The graph indicates annual mean sea level anomalies for each year averaged among the four tide gauge stations shown in the map on the left below for the period from 1906 to 1959, and among the four regions shown in the map on the right below for the period from 1960 onward. The solid blue line represents the five-year running mean of annual sea level anomalies averaged among the four stations, while the solid red line represents the corresponding value for the four regions. The dashed blue line represents the value at the four stations for the same period shown by the solid red line (from 1960 onward) for reference.


tide-station1 tide-station2

Locations of tide gauge stations

Tide gauge stations assessed as being affected to a lesser extent by crustal movement are selected for monitoring of sea levels. The four stations shown on the map on the left were selected for the period from 1906 to 1959, and the 16 on the right are for the period from 1960 onward. These 16 stations are divided into four regions (I – IV) for the latter period.
The stations at Oshoro, Kashiwazaki, Wajima and Hosojima belong to the Geospatial Information Authority of Japan.
Sea level data for the Tokyo station are available from 1968 onward.
Sea level data from 2011 to 2018 at Hakodate, Fukaura, Kashiwazaki, Tokyo and Hachinohe are not used due to the influence of the 2011 Great East Japan Earthquake.


Annual mean sea level anomalies (1906 - 2018) for the for regions

Time-series representation of annual mean sea level anomalies (1960 – 2018) for the four regions
Four-region annual sea level anomaly time series (comma-separated value file: 2 KB)


Rate of sea level rise in each region (mm/year)

  I II III IV Average Global mean
(IPCC AR5)
1960 – 2018 1.2 [0.9 – 1.4] * 1.1 [0.7 – 1.5] 2.4 [2.1 – 2.7] 1.3 [1.0 – 1.6]
1971 – 2010 1.4 [1.0 – 1.9] * * 2.4 [1.9 – 2.9] 1.1 [0.6 – 1.6] 2.0 [1.7 – 2.3]
1993 – 2010 2.2 [0.8 – 3.7] 3.5 [1.2 – 5.7] * 3.8 [2.5 – 5.1] 2.8 [1.3 – 4.3] 3.2 [2.8 – 3.6]

Asterisks indicate an absence of statistical significance.

Commentary

A trend of sea level rise has been observed in Japanese coastal areas since the 1980s, but no long-term trend of rise is seen for the period from 1906 to 2018. Variations with 10- to 20-year periods (near-10-year variations) are seen for the period from 1906 to 2018. The annual mean sea level around Japan in 2018 was 44 mm higher than the normal (i.e., the 1981 – 2010 average), making it the highest since 1960.

From 1960 to 2018, the trend of sea level increase in Region IV was larger than that of the other regions.

Analysis has shown that the near-10-year variations observed are caused by changes in the strength and meridional movement of westerlies over the North Pacific. Sea level variation data with the effects of crustal movement removed correlate well with those showing changes in subsurface seawater temperatures, especially around the Nansei Islands.

The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2013) concluded: "Over the period 1901 to 2010, global mean sea level rose by 0.19 [0.17 to 0.21] m. It is very likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to 1.9] mm/year between 1901 and 2010, 2.0 [1.7 to 2.3] mm/year between 1971 and 2010, and 3.2 [2.8 to 3.6] mm/year between 1993 and 2010." In contrast to the globally averaged rate noted in the IPCC report, sea levels in Japanese coastal areas showed no significant rise from 1906 to 2010; the rate of rise around the country was 1.1 [0.6 to 1.6] mm/year from 1971 to 2010, and 2.8 [1.3 to 4.3] from 1993 to 2010. These figures are comparable to those observed for the global average in recent years.

However, the extent to which global warming has contributed to sea level change along the Japanese coast remains unclear due to the involvement of various other factors such as near-10-year variations. In order to determine the long-term trend of sea level rise caused by global warming, continuous monitoring is required.

The values in square brackets show the 90% uncertainty range.

Reference