WW-I-7: Water temperature in the sea

The picture shows a slightly moving sea surface in which the pink clouds are reflected. Click to enlarge
Water temperatures in the North Sea are rising
Source: Photograph: © Peter Löwe/BSH

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents


WW-I-7: Water temperature in the sea

The mean surface temperature of the North Sea has increased in the period from 1969 until 2017. The abrupt temperature jump in 1987/88 manifested in a regime shift and mean value shift by 0.8°C.

The line graph shows the averaged annual surface temperature of the North Sea from 1969 onwards. The trend is increasing over the entire time series. In 1987/88 a clear regime shift with a delta of 0.8°C took place.
WW-I-7: Water temperature in the sea

The line graph shows the averaged annual surface temperature of the North Sea from 1969 onwards. The trend is increasing over the entire time series. In 1987/88 a clear regime shift with a delta of 0.8°C took place.

Source: BSH

The North Sea is warming

The heatwave of summer 2018 was not limited to Germany; in fact, it extended to large parts of the land mass in the northern hemisphere. On German coasts water temperatures of 25°C reached Mediterranean proportions. In July 2018 the mean surface temperature of the North Sea missed the record temperature of 17.4°C of July 2014 by just 0.1°C. The ‘marine heatwave’ also extended over the entire Baltic Sea where the mean temperature for July amounted to 20.0°C thus exceeding the previous record temperature, measured in 2014, by another 0.5°C. It seemed obvious that the cause was an almost stationary distribution of high and low pressure areas – a prolonged stagnation period which was favoured by the diminishing temperature contrast between polar and mid-latitudes11.

More than 90% of excess heat caused by the anthropogenic greenhouse effect is stored in the ocean. Between 1960 and 2015 the heat energy contained in the upper 2,000m of the earth’s ocean increased by 304 sextillion Joules (304 x 1021 J). For the entire earth’s surface the warming rate mean calculated amounted to 0.33 Watt per square metre (W/m2), which latterly (1992–2015) almost doubled reaching 0.61 W/m212. Consequently, the heat energy absorbed annually by the ocean currently amounts to 9.8 x 1021 J which is 17 times higher than the world energy demand in 2017 (equivalent to 13,511.2 billion tonnes of oil)13. If you imagine this amount of heat energy distributed exclusively over the upper 5m of ocean, this would result in a temperature rise of 1.3°C per annum. In fact, between 2000 and 2015 the ocean’s surface temperature rose at a rate of 0.013°C per annum14. Accordingly, 99% of the energy increase must be ‘concealed’ in lower layers of the ocean. It therefore follows that to measure global warming primarily in terms of the globally calculated surface temperature (GMST) is tantamount to a 100-fold underestimation and misperception of this complex issue. In fact, global warming came into question when the GMST more or less stagnated (global warming hiatus) between 1998 and 2013. Climate change indeed manifested just superficially in terms of GMST. A hiatus in global warming is, however, out of the question, because the heat content of the ocean (and also the sea level) did increase faster in the so-called hiatus phase.

A direct consequence of heat storage in the ocean is the expansion (increase in volume) of seawater – one of the crucial causes of sea level rise. In 2017 the global sea level was 77mm above the 1993 level (when satellite measuring began) thus reaching a record level15. Just under 40% of this increase is due to the thermal expansion of sea water, most of the rest of the increase in mass is due to melt water inflows.

Evidence for the warming of North Sea and Baltic Sea is found in the large-scale analyses of surface temperatures which have been carried out in the North Sea for over 50 years by the Bundesamt für Seeschifffahrt und Hydrographie (BSH/Federal Maritime and Hydrographic Agency). Annual mean temperatures were calculated on the basis of aggregated measurements. Formally, a significant linear trend of 1.3 ± 0.6°C (95% confidence interval) is stated for the entire period in question. The slow and gradual rise in temperature suggested does not, however, adequately describe the regime character of the historic development. This development is characterised by a cold regime which lasted until 1987; it ended abruptly with the temperature jump in 1987/88 and was replaced by the current warm regime. The regime change is manifested in starkly contrasting long-term means of 9.7°C until 1987 and 10.5°C from 1988 onwards. This development shows similarities with the hiatus phases of the GMST cold regimes (–0.5±1.0°C) and of the warm regimes (until 2013: 0.3±0.7°C) trend-free. Whether the temperature jump which occurred in 2013/14 initiated a more extreme warm regime remains to be seen. In terms of quality, the time series of air temperature in Germany shows a similar development (see Figure 1, p. 19).

The highest annual mean temperatures of the North Sea, typically resulting from extreme warming in the summer months, amounted to 11.0°C (2003, 2006, 2016) and above (11.4°C, 2014) The cumulative occurrence of such events during the warm regime does not come as a surprise. The regime change was not observed for North and Baltic Sea alone; in fact it was observed globally in a multitude of variables16,17. The ecological consequences of warming in both North and Baltic Sea were documented for instance by Beaugrand (2004)18 and Alheit et al. (2005)19.

The increasing sea temperatures have far-reaching impacts on the entire marine ecosystem. Species either adapt their range of distribution or become extinct (either locally or regionally) while other species come in to occupy these ecological niches. Indirect side effects of climate change such as lack of oxygen and the acidification of the seas contribute to changes in the diversity, composition and distribution of species thus changing the entire food web prevailing in marine habitats. Likewise, the economic consequences for marine fisheries are difficult to assess. Along German coastlines high seawater temperatures have made headlines in recent years, whenever the bathing tourism was affected by blue-green algal bloom; such processes are provoked additionally by the over-fertilisation of the seas.

11 Masters J. 2014: The Jet Stream is Getting Weird. Scientific American, 311: 68–75.
DOI: 10.1038/scientificamerican1214-68
12 Cheng L., Trenberth K. E., Fasullo J., Boyer T., Schuckmann K., Zhu J. 2017: Taking the Pulse of the Planet. Eos Transactions American Geophysical Union, 98.
13 BP Statistical Review of World Energy 2018: BP Statistical Review of World Energy – June 2018. 67th edition. London, 53 pp.
14 Huang B., Thorne P.W., Banzon V.F., Boyer T., Chepurin G., Lawrimore J.H., Menne M.J., Smith T.M., Vose R.S., Zhang H. 2017: Extended Reconstructed Sea Surface Temperature, Version 5 (ERSSTv5): Upgrades, Validations, and Intercomparisons. J. Climate, 30: 8179–8205
DOI: 10.1175/JCLI-D-16-0836.1
15 Thompson P.R., Merrifield M.A., Leuliette E., Sweet W., Chambers D.P., Hamlington B.D., Jevrejeva S., Marra J.J., Mitchum G.T., Nerem R.S., Widlansky M.J. 2018: Sea Level Variability and Change. In: State of the Climate in 2017. Bulletin of the American Meteorological Society, 99(8): 84–87.
16 Loewe P., Frohse F., Schulz A. 2009: Temperatur. In: Loewe P. (Hrsg.): System Nordsee – Zustand 2005 im Kontext langzeitlicher Entwicklungen. Berichte des BSH (Bundesamt für Seeschifffahrt und Hydrographie, Hamburg und Rostock), 44: 111–134.
17 Reid P.C., Hari R.E., Beaugrand G., Livingstone D. M., Marty C., Straile D., Barichivich J., Goberville E., Adrian R., Aono Y., Brown R., Foster J., Groisman P., Hélaouët P., Hsu H., Kirby R., Knight J., Kraberg A., Li J., Lo T.-T., Myneni R.B., North R.P., Pounds J. A., Sparks T., Stübi R., Tian Y., Wiltshire K.H., Xiao D., Zhu Z. 2016: Global Impacts of the 1980s Regime Shift. Global Change Biology, 22: 682–703.
DOI: 10.1111/gcb.13106.
18 Beaugrand G. 2004: The North Sea Regime Shift: Evidence, Causes, Mechanisms and Consequences. Progress in Oceanography, 60: 245–262.
DOI: 10.1016/j.pocean.2004.02.018.
19 Alheit J., Möllmann C., Dutz J., Kornilovs G., Loewe P., Mohrholz V., Wasmund N. 2005: Synchronous ecological regime shifts in the central Baltic and the North Sea in the late 1980s. ICES Journal of Marine Science, 62:1205–1215
DOI: 10.1016/j.icesjms.2005.04.024.




FI-I-1: Distribuion of thermophilic marine species

TOU-I-1: Coastal bathing temperatures



Limitation of all factors which lead to warming and acidification (DAS, ch. 3.2.3).

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