Land uplift counteracts sea level rise on the Finnish coast

The Baltic Sea is linked to oceans through the narrow and shallow Danish Straits. When sea levels rise in close by sea areas, the rise will be evident in the Baltic Sea, too. The isostatic land uplift counteracts sea level rise on the Finnish coast, especially in the Gulf of Bothnia.

Global sea level rise is uneven

The global average sea level is rising, because global warming is melting glaciers and ice sheets and heating up the oceans, which leads to thermal expansion of sea water. The current rate of sea level rise is about 3 mm per year. [1]Ocean levels do not rise as quickly everywhere, however. Thermal expansion of sea water varies in different sea areas. In addition, meltwater from glaciers and ice sheets is not evenly distributed in the oceans around the world.

The gravitation effect of a large mass of ice pulls sea water to it. When the mass melts away, gravitation weakens and sea water retreats from the thawing mass of ice. Therefore, according to model results, meltwater released from the Greenland ice sheet does not raise the sea level much in areas in the vicinity of the Baltic Sea, whereas possible melting of the Antarctic ice sheet would have a full influence here, too [2]. In total, the effect of sea level rise is estimated to be slightly lower in the Baltic Sea than globally [3].

Land is rising in the Baltic Sea area

In the Baltic Sea area, the isostatic land uplift is still ongoing after the ice age [4]. In the Vaasa region, land uplift totals 90 centimetres (cm) per century, and 40 cm in Helsinki. Land uplift has been stronger than sea level rise everywhere on the Finnish coast in the 20th century. Recently, and particularly in the future, the accelerating rate of sea level rise may change the situation. The race between land uplift and sea level rise determines to which direction the coastline will move in time. [3]

Sea level is about to rise in the Gulf of Finland

In light of the most recent estimates it seems that on the south coast of Finland, the average water level will rise in relation to land (Figure 1, Helsinki). In the Gulf of Finland, the best estimate predicts a 30 cm sea level rise in 2000–2100, whereas the worst case scenario would result in a 90 cm rise in the same time span. [3] Projections are uncertain particularly because the behaviour of the ice sheets under warming climate is not known well enough yet.

Land uplift reduces sea level rise in the Gulf of Bothnia

In the northern parts of the Gulf of Bothnia, land uplift can be expected to overcome the rise in sea level even in the future (Figure 1, Vaasa). New land would still emerge from under the sea, although more slowly than before. The most probable outcome in the Bothnian Bay is a sea level decline of 30 cm in 2000–2100, while the highest scenario predicts a 30 cm rise. [3]

More towards the south, in the Bothnian Sea, the land uplift and sea level rise are expected to roughly cancel each other during the 21st century. Should the highest scenarios come true, a sea level rise of about 65 cm would ensue. [3]

Average sea level in Helsinki and Vaasa © Ilmatieteen laitos

Figure 1. Average sea level in Helsinki and Vaasa. The points indicate observed annual averages, the unbroken line is the long-term average based on observations until 1999 and on the estimate of future average sea level from 2000. Dashed lines describe the degree of uncertainty of the estimates, due in particular to uncertainty in future behaviour of the ice sheets, not as much in greenhouse gas emissions.

Sea floods may intensify on Finland's coasts

In addition to average sea level, climate change may influence sea level variability. Changes in wind conditions and storms and the decreasing ice cover in winter affect short-term variations in sea level. Both the maximum sea levels and short-term variations have increased in the Baltic Sea in the past century, and the reason behind this phenomenon seems to be, at least partially, changes in wind conditions. [5] According to certain studies conducted using climate models, maximum levels may rise in the future, too. [6]

An important underlying factor to sea flooding is the total quantity of water in the Baltic Sea. If the quantity of water is low, not even a severe storm will suffice to cause record-high sea levels. Variation in the total quantity of water in the Baltic Sea is primarily determined by water flowing in and out through the Danish Straits. Factors like wind and atmospheric pressure conditions in the Danish Straits area influence the flow.

 

10.9.2013 (Updated)

References

  1. Bindoff, N. L., Willebrand, J., Artale, V., Cazenave, A., Gregory, J., Gulev, S., Hanawa, K., Le Quéré, C., Levitus, S., Nojiri, Y., Shum, C. K., Talley, L. D. & Unnikrishnan, A. 2007. Observations: Oceanic Climate Change and Sea Level. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch5.html
  2. Mitrovica, J. X., Tamisiea, M. E., Davis, J. L., & Milne, G. A. 2001. Recent mass balance of polar ice sheets inferred from patterns of global sea-level change. Nature, Volume 409, Number 6823: 1026–1029. http://dx.doi.org/10.1038/35059054
  3. Johansson, M. M., Pellikka, H., Kahma, K. K. & Ruosteenoja, K. 2012. Global sea level rise scenarios adapted to the Finnish coast. Journal of Marine Systems, in press. http://dx.doi.org/10.1016/j.jmarsys.2012.08.007
  4. The BACC Author Team. 2008. Assessment of Climate Change for the Baltic Sea Basin. Springer Verlag, Berlin, Heidelberg. 474 p. http://www.hzg.de/institute/coastal_research/projects/baltex/bacc_downloads/index.html.en
  5. Johansson, M., Boman, H., Kahma, K. K. & Launiainen, J. 2001. Trends in sea level variability in the Baltic Sea. Boreal Environment Research, Volume 6, Number 3: 159-179. http://www.borenv.net/BER/pdfs/ber6/ber6-159s.pdf
  6. Meier, H. E. M, Broman, B. & Kjellström, E. 2004. Simulated sea level in past and future climates of the Baltic Sea. Climate Research, Volume 27, Number 1: 59-75. http://dx.doi.org/ 10.3354/cr027059

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