Skip to main content

Advertisement

SpringerLink
Log in

Storm surges in the Western Baltic Sea: the present and a possible future

  • Published:
Climate Dynamics Aims and scope Submit manuscript

Abstract

Globally-coupled climate models are generally capable of reproducing the observed trends in the globally averaged atmospheric temperature or mean sea level. However, the global models do not perform as well on regional/local scales. Here, we present results from four 100-year ocean model experiments for the Western Baltic Sea. In order to simulate storm surges in this region, we have used the General Estuarine Transport Model (GETM) as a high-resolution local model (spatial resolution ≈ 1 km), nested into a regional atmospheric and regional oceanic model in a fully baroclinic downscaling approach. The downscaling is based on the global model ECHAM5/MPI-OM. The projections are imbedded into two greenhouse-gas emission scenarios, A1B and B1, for the period 2000–2100, each with two realisations. Two control runs from 1960 to 2000 are used for validation. We use this modelling system to statistically reproduce the present distribution of surge extremes. The usage of the high-resolution local model leads to an improvement in surge heights of at least 10% compared to the driving model. To quantify uncertainties associated with climate projections, we investigate the impact of enhanced wind velocities and changes in mean sea levels. The analysis revealed a linear dependence of surge height and mean sea level, although the slope parameter is spatially varying. Furthermore, the modelling system is used to project possible changes within the next century. The results show that the sea level rise has greater potential to increase surge levels than does increased wind speed. The simulations further indicate that the changes in storm surge height in the scenarios can be consistently explained by the increase in mean sea level and variation in wind speed.

This is a preview of subscription content, log in via an institution to check access.

Access this article

Log in via an institution

Price excludes VAT (USA)
Tax calculation will be finalised during checkout.

Instant access to the full article PDF.

Institutional subscriptions

Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
Fig. 11
Fig. 12
Fig. 13

Similar content being viewed by others

References

  • Ådlandsvik B, Bentsen M (2007) Downscaling a twentieth century global climate simulation to the North Sea. Ocean Dyn 57(4):453–466

    Article  Google Scholar 

  • Alexandersson H, Tuomenvirta H, Smith H, Iden K (2000) Trends of storms in NW Europe derived from an updated pressure data set. Clim Res 14:71–73

    Article  Google Scholar 

  • An Y, Pandey M (2005) A comparison of methods of extreme wind speed estimation. J Wind Eng Ind Aerodyn 93(7):535–545

    Article  Google Scholar 

  • An Y, Pandey M (2007) The r-largest order statistics model for extreme wind speed estimation. J Wind Eng Ind Aerodyn 95(3):165–182

    Article  Google Scholar 

  • Anderson HC (2002) Influence of long-term regional and large-scale atmospheric circulation on the Baltic sea level. Tellus A 54(1):76–88

    Article  Google Scholar 

  • Baensch O (1875) Die Sturmflut an den Ostsee-Küsten des Preussischen Staates vom 12/13. November 1872. Zeitschrift für, Bauwesen

    Google Scholar 

  • Baerens C, Hupfer P (1999) Extremwasserstände an der deutschen Ostseeküste nach Beobachtungen und in einem Treibhausgasszenario. Die Küste 61

  • Barbosa SM (2008) Quantile trends in Baltic sea level. Geophys Res Lett 35(22):L22, 704

    Article  Google Scholar 

  • Beckley BD, Lemoine FG, Luthcke SB, Ray RD, Zelensky NP (2007) A reassessment of global and regional mean sea level trends from TOPEX and Jason-1 altimetry based on revised reference frame and orbits. Geophys Res Lett 34:L14, 608

    Article  Google Scholar 

  • Beckmann A, Döscher R (1997) A method for improved representation of dense water spreading over topography in geopotential-coordinate models. J Phys Oceanogr 27(4):581–591

    Article  Google Scholar 

  • Beniston M, Stephenson D, Christensen O, Ferro C, Frei C, Goyette S, Halsnaes K, Holt T, Jylhä K, Koffi B, Palutikof J, Schöll R, Semmler T, Woth K (2007) Future extreme events in European climate: an exploration of regional climate model projections. Clim Change 81:75–89

    Article  Google Scholar 

  • Bernier N, Thompson K, Ou J, Ritchie H (2007) Mapping the return periods of extreme sea levels: allowing for short sea level records, seasonality, and climate change. Glob Planet Change 57(1–2):139–150

    Article  Google Scholar 

  • Brown JM, Souza A, Wolf J (2010) Surge modelling in the eastern Irish Sea: present and future storm impact. Ocean Dyn 60(2):227–236

    Article  Google Scholar 

  • Burchard H, Bolding K (2001) Comparative analysis of four second-moment turbulence closure models for the oceanic mixed layer. J Phys Oceanogr 31(8):1943–1968

    Article  Google Scholar 

  • Burchard H, Bolding K (2002) GETM—a general estuarine transport model. Scientific documentation. Technical report EUR 20253 EN. Technical report, European Commission.

  • Burchard H, Rennau H (2008) Comparative quantification of physically and numerically induced mixing in ocean models. Ocean Model 20(3):293–311

    Article  Google Scholar 

  • Burchard H, Janssen F, Bolding K, Umlauf L, Rennau H (2009) Model simulations of dense bottom currents in the Western Baltic Sea. Cont Shelf Res 29(1):205–220

    Article  Google Scholar 

  • Butler A, Heffernan JE, Tawn JA, Flather RA, Horsburgh KJ (2007) Extreme value analysis of decadal variations in storm surge elevations. J Mar Syst 67(1–2):189–200

    Article  Google Scholar 

  • Canuto VM, Howard A, Cheng Y, Dubovikov MS (2001) Ocean turbulence. Part I: one-point closure model. Momentum and heat vertical diffusivities. J Phys Oceanogr 31:1413–1426

    Article  Google Scholar 

  • Carter DJT, Challenor PG (1981) Estimating return values of environmental parameters. Q J R Meteorol Soc 107(451):259–266

    Article  Google Scholar 

  • Cazenave A, Dominh K, Guinehut S, Berthier E, Llovel W, Ramillien G, Ablain M, Larnicol G (2009) Sea level budget over 2003–2008: a reevaluation from GRACE space gravimetry, satellite altimetry and argo. Glob Planet Change 65(1–2):83–88

    Article  Google Scholar 

  • Christensen JH, Christensen OB (2007) A summary of the PRUDENCE model projections of changes in European climate by the end of this century. Clim Change 81(0):7–30

    Article  Google Scholar 

  • Church J, White N, Aarup T, Wilson W, Woodworth P, Domingues C, Hunter J, Lambeck K (2008) Understanding global sea levels: past, present and future. Sustain Sci 3:9–22

    Article  Google Scholar 

  • Church JA, White NJ, Coleman R, Lambeck K, Mitrovica JX (2004) Estimates of the regional distribution of sea level rise over the 1950–2000 period. J Clim 17(13):2609–2625

    Article  Google Scholar 

  • CLM (2008) Climate limited-area modelling community. http://www.clm-community.eu

  • Coles S (2001) An introduction to statistical modeling of extreme values, 1st edn. Springer series in statistics. Springer, Berlin

  • Davison AC, Smith RL (1990) Models for exceedances over high thresholds. J R Stat Soc B 52(3):393–442

    Google Scholar 

  • Ekman M, Mäkinen J (1996) Mean sea surface topography in the Baltic Sea and its transition area to the North Sea: a geodetic solution and comparisons with oceanographic models. J Geophys Res 101(C5):11, 993

    Article  Google Scholar 

  • Feistel R, Nausch G, Wasmund N (2008) State and evolution of the Baltic Sea, chemistry 1952–2005: a detailed 50-year survey of meteorlogy and climate, phyiscs, biology and marine environment. Wiley, New York

    Google Scholar 

  • Fennel W, Sturm M (1992) Dynamics of the western Baltic. J Mar Syst 3(1–2):183–205

    Article  Google Scholar 

  • Flather RA (1976) A tidal model of the northwest European continental shelf. Memoires de la Societe Royale de Sciences de Liege 10(6):141–164

    Google Scholar 

  • Flather RA, Smith J, Richards J, Bell C, Blackman D (1998) Direct estimates of extreme storm surge elevations from a 40-year numerical model simulation and from observations. Glob Atmos Ocean Syst 6:165–176

    Google Scholar 

  • Gräwe U, Burchard H (2011) Regionalisation of climate Scenarios for the Western Baltic Sea. In: Global change and Baltic coastal zones, Springer, Dordrecht

  • Gregory JM, Church JA, Boer GJ, Dixon KW, Flato GM, Jackett DR, Lowe JA, O’Farrell SP, Roeckner E, Russell GL, Stouffer RJ, Winton M (2001) Comparison of results from several AOGCMs for global and regional sea-level change 1900–2100. Clim Dyn 18:225–240

    Article  Google Scholar 

  • Griffies SM, Pacanowski RC, Schmidt M, Balaji V (2001) Tracer conservation with an explicit free surface method for z-coordinate ocean models. Mon Weather Rev 129(5):1081–1098

    Article  Google Scholar 

  • Gustafsson B (1997) Interaction between Baltic Sea and North Sea. Ocean Dyn 49:165–183

    Google Scholar 

  • Haigh I, Nicholls RJ, Wells N (2010) Assessing changes in extreme sea levels: application to the English Channel, 1900–2006. Cont Shelf Res 30(9):1042–1055

    Article  Google Scholar 

  • Haigh ID, Nicholls R, Wells N (2010) A comparison of the main methods for estimating probabilities of extreme still water levels. Coast Eng 57(9):838–849

    Article  Google Scholar 

  • Hallegatte S, Ranger N, Mestre O, Dumas P, Corfee-Morlot J, Herweijer C, Wood R (2010) Assessing climate change impacts, sea level rise and storm surge risk in port cities: a case study on Copenhagen. Clim Change 1–25

  • Hanson H, Larson M (2008) Implications of extreme waves and water levels in the southern Baltic Sea. J Hydraul Res 46(2):292–302

    Article  Google Scholar 

  • HELCOM (1986) In: Proceedings of water balance of the Baltic Sea, 16, HELCOM, Baltic Sea environment

  • Heyen H, Zorita E, von Storch H (1996) Statistical downscaling of monthly mean North Atlantic air-pressure to sea level anomalies in the Baltic Sea. Tellus A 48(2):312–323

    Article  Google Scholar 

  • Holt J, Wakelin S, Lowe J, Tinker J (2010) The potential impacts of climate change on the hydrography of the northwest European continental shelf. Prog Oceanogr 86(3–4):361–379

    Article  Google Scholar 

  • Horsburgh KJ, Wilson C (2007) Tide-surge interaction and its role in the distribution of surge residuals in the North Sea. J Geophys Res 112(C08)

  • Hünicke B, Luterbacher J, Pauling A, Zorita E (2008) Regional differences in winter sea level variations in the Baltic Sea for the past 200 years. Tellus A 60(2):384–393

    Article  Google Scholar 

  • Hupfer P, Harff J, Sterr H, Stigge HJ (2003) Die Wasserstände an der Ostsee, Entwicklung–Sturmfluten–Klimawandel. Die Küste 66

  • IPCC (2007) Climate change 2007—The physical science basis: working group I contribution to the fourth assessment report of the IPCC. Cambridge University Press, Cambridge/New York

  • Jacob D, Bärring L, Christensen OB, Christensen JH, de Castro M, Déqué M, Giorgi F, Hagemann S, Hirschi M, Jones R, Kjellström E, Lenderink G, Rockel B, Sánchez E, Schär C, Seneviratne S, Somot S, van Ulden A, van den Hurk B (2007) An inter-comparison of regional climate models for Europe: model performance in present-day climate. Clim Change 81(0):31–52

    Article  Google Scholar 

  • Jevrejeva S, Moore JC, Woodworth PL, Grinsted A (2005) Influence of large-scale atmospheric circulation on European sea level: results based on the wavelet transform method. Tellus A 57:183–193

    Article  Google Scholar 

  • Johansson MM, Kahma KK, Boman H, Launiainen J (2004) Scenarios for sea level on the Finnish coast. Boreal Environ Res 9(2):153–166

    Google Scholar 

  • Jones J, Davies A (2007) Influence of non-linear effects upon surge elevations along the west coast of Britain. Ocean Dyn 57:401–416

    Article  Google Scholar 

  • Jones J, Davies A (2009) Storm surge computations in estuarine and near-coastal regions: the Mersey estuary and Irish Sea area. Ocean Dyn 59:1061–1076

    Article  Google Scholar 

  • Jönsson B, Döös K, Nycander J, Lundberg P (2008) Standing waves in the Gulf of Finland and their relationship to the basin-wide Baltic seiches. J Geophys Res 113(C3):C03,004

    Article  Google Scholar 

  • Jungclaus JH, Keenlyside N, Botzet M, Haak H, Luo JJ, Latif M, Marotzke J, Mikolajewicz U, Roeckner E (2006) Ocean circulation and tropical variability in the coupled model ECHAM5/MPI-OM. J Clim 19(16):3952–3972

    Article  Google Scholar 

  • Katz RW, Parlange MB, Naveau P (2002) Statistics of extremes in hydrology. Adv Water Resour 25(8–12):1287–1304

    Article  Google Scholar 

  • Kendall MG (1975) Rank correlation methods. Griffin, London

    Google Scholar 

  • Kjellström E (2004) Recent and future signatures of climate change in Europe. Ambio 33(4–5):193–198

    Google Scholar 

  • Kjellström E, Nikulin G, Hansson U, Strandberg G, Ullerstig A (2011) 21st century changes in the European climate: uncertainties derived from an ensemble of regional climate model simulations. Tellus A 63:24–40

    Article  Google Scholar 

  • Kowalewska-Kalkowska H, Wisniewski B (2009) Storm surges in the Odra mouth area during the 1997–2006 decade. Boreal Environ Res 14:183–192

    Google Scholar 

  • Landerer FW, Jungclaus JH, Marotzke J (2007) Regional dynamic and steric sea level change in response to the IPCC-A1B scenario. J Phys Oceanogr 37(2):296–312

    Article  Google Scholar 

  • Langenberg H, Pfizenmayer A, von Storch H, Sündermann J (1999) Storm-related sea level variations along the North Sea coast: natural variability and anthropogenic change. Cont Shelf Res 19(6):821–842

    Article  Google Scholar 

  • Lass HU, Mohrholz V (2003) On the dynamics and mixing of inflowing saltwater in the Arkona Sea. J Geophys Res 108(C2):1–15

    Article  Google Scholar 

  • Leadbetter MR (1991) On a basis for ’Peaks over Threshold’ modeling. Stat Probab Lett 12(4):357–362

    Article  Google Scholar 

  • Lehmann A, Getzlaff K, Jan Harlass (2011) Detailed assessment of climate variability of the Baltic Sea area for the period 1958–2009. Clim Res 46:185–196

    Article  Google Scholar 

  • Letetrel C, Marcos M, Martín-Míguez B, Woppelmann G (2010) Sea level extremes in Marseille (NW Mediterranean) during 1885–2008. Cont Shelf Res 30(12):1267–1274

    Article  Google Scholar 

  • Lowe JA, Gregory JM (2010) A sea of uncertainty. Nat Geosci (1004):42–43

  • Mann HB (1945) Nonparametric tests against trend. Econometrica 13:245–259

    Article  Google Scholar 

  • Matthäus W, Franck H (1992) Characteristics of major Baltic inflows—a statistical analysis. Cont Shelf Res 12(12):1375–1400

    Article  Google Scholar 

  • Meier HEM, Boman B, Kjellström E (2004) Simulated sea level in past and future climates of the Baltic Sea. Clim Res 27(1):59–75

    Article  Google Scholar 

  • Meier HEM, Kjellström E, Graham LP (2006) Estimating uncertainties of projected Baltic Sea salinity in the late 21st century. Geophys Res Lett 33:L15, 705

    Article  Google Scholar 

  • Meier M, Feistel R, Piechura J, Arneborg L, Burchard H, Fiekas V, Golenko N, Kuzmina N, Mohrholz V, Nohr C, Paka VT, Sellschopp J, Stips A, Zhurbas V (2006) Ventilation of the Baltic Sea deep water: a brief review of present knowledge from observations and models. Oceanologia 48:133–164

    Google Scholar 

  • Melsom A, Lien VS, Budgell WP (2009) Using the Regional Ocean Modeling System (ROMS) to improve the ocean circulation from a GCM 20th century simulation. Ocean Dyn 59(6):969–981

    Article  Google Scholar 

  • Mínguez R, Menéndez M, Méndez F, Losada I (2010) Sensitivity analysis of time-dependent generalized extreme value models for ocean climate variables. Adv Water Resour 33(8):833–845

    Article  Google Scholar 

  • Munich Re (2008) Highs and lows—weather risks in central Europe, Munich Re, Knowledge Series, p 56

  • Munk W (2002) Twentieth century sea level: an enigma. Proc Natl Acad Sci USA 99(10):6550–6555

    Google Scholar 

  • Naess A, Clausen PH (2002) The impact of data accuracy on the POT estimates of long return period design values. J Offshore Mech Arctic Eng 124(1):53–58

    Article  Google Scholar 

  • Neumann G (1941) Eigenschwingungen der Ostsee. Deutsche Seewarte, Berlin

    Google Scholar 

  • Neumann T (2010) Climate-change effects on the Baltic Sea ecosystem: a amodel study. J Mar Syst 81(3):213–224

    Article  Google Scholar 

  • Nikulin G, Kjellström E, Hansson U, Strandberg G, Ullerstig A (2011) Evaluation and future projections of temperature, precipitation and wind extremes over Europe in an ensemble of regional climate simulations. Tellus A 63(1):41–55

    Article  Google Scholar 

  • Omstedt A, Pettersen C, Rodhe J, Winsor P (2004) Baltic Sea climate: 200 years of data on air temperature, sea level variation, ice cover, and atmospheric circulation. Clim Res 25(3):205–216

    Article  Google Scholar 

  • Pawlowicz R, Beardsley B, Lentz S (2002) Classical tidal harmonic analysis including error estimates in MATLAB using T_TIDE. Comput Geosci 28:929–937

    Article  Google Scholar 

  • Pickands J (1975) Statistical inference using extreme order statistics. Ann Stat 3(1):119–131

    Article  Google Scholar 

  • Prandle D, Wolf J (1978) The interaction of surge and tide in the North Sea and River Thames. Geophys J R Astron Soc 55(1):203–216

    Article  Google Scholar 

  • Radic V, Hock R (2011) Regionally differentiated contribution of mountain glaciers and ice caps to future sea-level rise. Nat Geosci 4(2):91–94

    Article  Google Scholar 

  • Rahmstorf S (2007) A semi-empirical approach to projecting future sea-level rise. Science 315(5810):368–370

    Article  Google Scholar 

  • Räisänen J, Hansson U, Ullerstig A, Döscher R, Graham L, Jones C, Meier HEM, Samuelsson P, Willén U (2004) European climate in the late twenty-first century: regional simulations with two driving global models and two forcing scenarios. Clim Dyn 22:13–31

    Article  Google Scholar 

  • Rosenhagen G, Bork I (2009) Rekonstruktion der Sturmflutwetterlage vom 13. November 1872. Die Küste 75:51–70

    Google Scholar 

  • Samuelsson M, Stigebrandt A (1996) Main characteristics of the long-term sea level variability in the Baltic sea. Tellus A 48(5):672–683

    Article  Google Scholar 

  • Schinke H (1993) On the occurence of deep cyclones over Europe and the North Atlantic in the period 1930–1991. Beiträge zur Physik der Atmosphäre 66:223–237

    Google Scholar 

  • Schmidt M, Seifert T, Lass H, Fennel W (1998) Patterns of salt propagation in the Southwestern Baltic Sea. Ocean Dyn 50:345–364

    Google Scholar 

  • Schmith T, Kaas E, Li TS (1998) Northeast Atlantic winter storminess 1875–1995 re-analysed. Clim Dyn 14:529–536

    Article  Google Scholar 

  • Shane RM, Lynn WR (1964) Mathematical model for flood risk evaluation. J Hydraul Eng 90:1–20

    Google Scholar 

  • Smith RL (1986) Extreme value theory based on the r largest annual events. J Hydrol 86(1–2):27–43

    Article  Google Scholar 

  • Soares CG, Scotto MG (2004) Application of the r largest-order statistics for long-term predictions of significant wave height. Coast Eng 51(5-6):387–394

    Article  Google Scholar 

  • Sobey RJ, Orloff LS (1995) Triple annual maximum series in wave climate analyses. Coast Eng 26(3-4):135–151

    Article  Google Scholar 

  • Stigge HJ (1994) Die Wasserstände an der Küste Mecklenburg-Vorpommerns. Die Küste 56

  • Suursaar Ü, Kullas T, Otsmann M, Kuts T (2003) Extreme sea level events in the coastal waters of western Estonia. J Sea Res 49(4):295–303

    Article  Google Scholar 

  • Tawn JA (1988) An extreme-value theory model for dependent observations. J Hydrol 101(1-4):227–250

    Article  Google Scholar 

  • Todorovic P, Zelenhasic E (1970) A stochastic model for flood analysis. Water Resour Res 6:1641–1648

    Article  Google Scholar 

  • Tol RSJ, Klein RJT, Nicholls RJ (2008) Towards successful adaptation to sea-level rise along Europe’s coasts. J Coast Res 242:432–442

    Article  Google Scholar 

  • Umlauf L, Burchard H, Bolding K (2006) General ocean turbulence model. Source code documentation. Technical Report 63. Technical report, Baltic Sea Research Institute Warnemünde, Warnemünde, Germany

  • van den Brink H, Können G, Opsteegh J (2005) Uncertainties in extreme surge level estimates from observational records. Philos Transact A Math Phys Eng Sci 363(1831):1377–1386

    Article  Google Scholar 

  • Vilibic I, Sepic J (2010) Long-term variability and trends of sea level storminess and extremes in European Seas. Glob Planet Change 71(1–2):1–12

    Article  Google Scholar 

  • Weisse R, von Storch H, Callies U, Chrastansky A, Feser F, Grabemann I, Günther H, Winterfeldt J, Woth K, Pluess A, Stoye T, Tellkamp J (2009) Regional meteorological—marine reanalyses and climate change projections. Bull Am Meteorol Soc 90(6):849–860

    Article  Google Scholar 

  • Weissman J (1978) Estimation of parameters and larger quantiles based on the k largest observations. J Am Stat Assoc 73(364)

  • West J, Small M, Dowlatabadi H (2001) Storms, investor decisions, and the economic impacts of sea level rise. Clim Change 48:317–342

    Article  Google Scholar 

  • Wiśniewski B, Wolski T (2011) Physical aspects of extreme storm surges and falls on the Polish coast. Oceanologia 53(1):373–390

    Article  Google Scholar 

  • Woodworth PL (2006) Some important issues to do with long-term sea level change. Philos Transact A Math Phys Eng Sci 364(1841):787–803

    Article  Google Scholar 

  • Woodworth PL, Blackman DL (2004) Evidence for systematic changes in extreme high waters since the mid-1970s. J Clim 17(6):1190–1197

    Article  Google Scholar 

  • Woth K, Weisse R, von Storch H (2006) Climate change and North Sea storm surge extremes: an ensemble study of storm surge extremes expected in a changed climate projected by four different regional climate models. Ocean Dyn 56:3–15

    Article  Google Scholar 

  • Wyrtki K (1954) Schwankungen im Wasserhaushalt der Ostsee. Ocean Dyn 7:91–129

    Google Scholar 

Download references

Acknowledgments

Gauge data were kindly provided by the German Federal Maritime and Hydrographic Agency (BSH), the Danish Maritime Safety Administration (DaMSA) and the Swedish Meteorological and Hydrological Institute (SMHI). Supercomputing power was provided by HLRN (Norddeutscher Verbund für Hoch-und Höchstleistungsrechnen). We are grateful to Karsten Bolding (Asperup, Denmark) for the code maintenance of GETM.

Author information

Authors and Affiliations

  1. Leibniz Institute for Baltic Sea Research, Warnemünde, Germany

    Ulf Gräwe & Hans Burchard

Authors
  1. Ulf Gräwe
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Hans Burchard
    View author publications

    You can also search for this author in PubMed Google Scholar

Corresponding author

Correspondence to Ulf Gräwe.

Rights and permissions

Reprints and permissions

About this article

Cite this article

Gräwe, U., Burchard, H. Storm surges in the Western Baltic Sea: the present and a possible future. Clim Dyn 39, 165–183 (2012). https://doi.org/10.1007/s00382-011-1185-z

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1007/s00382-011-1185-z

Keywords

Search

Navigation