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Friday, 30th April 2010

Climate change - Now the sea has also warmed up

With the help of colleagues from the Fire Services Department, the Observatory has been measuring the sea surface temperature at North Point for over 30 years. Recently I asked my colleagues to look at its trend and was amazed to find that it has risen by 0.5 degree since the 1970s.

Figure 1	  Annual mean sea surface temperature at North Point from 1975 to 2009.
Figure 1  Annual mean sea surface temperature at North Point from 1975 to 2009.
The linear fit is statistically significant at 5%.

The result agrees well with the global trend, given in the plot below.

Figure 2	  Annual anomalies (relative to the 1961 to 1990 mean)
Figure 2  Annual anomalies (relative to the 1961 to 1990 mean)
of global sea surface temperature from 1850 to 2005
(Source : Fourth Assessment Report, Intergovernmental Panel on Climate Change, 2007)

0.4 degree may not sound much to you. But not many people know that sea water is among one of the hardest to heat up. To compare how difficult (or easy) to heat things up, scientists use a measure called specific heat, which is the heat, or thermal energy, required to raise the temperature of, say, a gram of a substance by one degree. A substance having a high specific heat means that it is difficult to heat up. Below is a list of specific heats of common materials:

a list of specific heats of common materials

Here, the unit is Joule per gram per degree Kelvin (same as degree Celsius for the present purpose). Thus, a gram of sea water needs about 4 Joule of energy to warm up by one degree. As you can see from the above list, sea water's specific heat exceeds others' by a wide margin.

With the increase in the human-made greenhouse gases in the atmosphere, the entire Earth is accumulating heat. The 'extra heat' has been taken up by the land, air, ice/snow, and the ocean. Because of its high specific heat, ocean water is a tremendous reservoir storing up a huge amount of energy.

That the sea water warms up is not restricted to the surface. Take a look at the following diagram, which plots the warming in the upper 700 metres of ocean water.

Figure 3	  Time series of global ocean heat content[note] for the 0700 m layer.
Figure 3  Time series of global ocean heat content[note] for the 0-700 m layer.
(Source : NOAA National Oceanographic Data Centre, after Levitus et al. (2009)

A direct consequence of ocean warming is the associated rise in the sea level. Scientists estimated that thermal expansion of sea water accounts for a rise of about 18 mm from 1993 to 2003 (i.e. about 1.6 mm per year) and is expected to produce a further rise of over 0.4 m by the end of this century under a high greenhouse gas emission scenario (i.e. business as usual, no change whatsoever in lifestyle and use of energy).

In the blog of 2 November 2009, I said that with a raised sea, the storm surge brought by typhoons would be more severe.

Figure 4	  Flooding of coastal areas becomes worse under tropical cyclone situations.
Figure 4  Flooding of coastal areas becomes worse under tropical cyclone situations.

Recently my colleagues carried out some calculations, which indicate that a rise of 0.4 m in the sea level in Hong Kong would turn a 50-year storm surge into a 5-year event. That is, statistically, instead of once every 50 years on average, a 3.5 m storm surge will occur more often --- once every 5 years. We don't have to go very far back for such a storm surge. As recent as 2008, Typhoon Hagupit brought a 3.53 m storm surge, resulting in severe flooding in low-lying areas in Hong Kong. The picture of a reporter standing in chest-high water at Tai O is still vivid in many people's mind.

The above numbers tell us that under climate change, simple thermal expansion of sea water is enough to wreak havoc.

B Y Lee

Ocean heat content refers to the amount of heat stored in the Earth's oceans, the unit is in Joule.

References :
Bindoff, N.L., J. Willebrand, V. Artale, A, Cazenave, J. Gregory, S. Gulev, K. Hanawa, C. Le Quéré, S. Levitus, Y. Nojiri, C.K. Shum, L.D. Talley and A. Unnikrishnan, 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

Levitus, S., J. I. Antonov, T. P. Boyer, R. A. Locarnini, H. E. Garcia, and A. V. Mishonov (2009), Global ocean heat content 1955-2008 in light of recently revealed instrumentation problems, Geophys. Res. Lett., 36, L07608, doi:10.1029/2008GL037155.

Lee, B.Y., W.T. Wong, and W.C. Woo (2010), Sea-level rise and storm surge - Impacts of climate change on Hong Kong, The HKIE Civil Division Conference 2010 - Infrastructure Solutions for Tomorrow, 12-14 April 2010.

Last revision date: <17 Jan 2013>