Our current rising sea level is often given as evidence of anthropogenic influence on the oceans, though usually when data from only the past decade or two are taken into consideration. An example is frequent comments by the UK government’s Chief Scientific Adviser, Professor Sir John Beddington, who has said that global sea level has increased by about 10 cm in the last 50 years and that is evidence of mankind’s influence. At first sight it is a dramatic and compelling statement for mankind’s effects on this parameter. Beddington also said, “the general issues on overall global temperature, on sea level and so on, are all pretty unequivocal”.
To get an idea of what is happening to sea levels it is necessary to take a view that is longer than 50 years. In its influential report the IPCC assumed that global sea level change during the past two thousand years up to the middle of the nineteenth century, was zero.
To my mind this is a puzzling statement. Few parameters of our complex, changing world are actually zero. If it is zero then it is in stark contrast to today’s rising seas, and if one was cynical one could deduce a political motive behind the IPCC’s sweeping statement. Whatever the motivation it is obvious that looking at sea level change over the past two thousand years is essential to put today’s changes into context.
A Complex Mixture
Sea level, at any given location, is a complex mixture of local, regional, and global processes. Information about sea levels prior to the tidal gauge and satellite era is limited although the substantial evidence that glaciers and ice sheets have altered significantly in size over the past two millennia makes the thought that sea levels have remained static a bit strange.
Curiously, the rebound of the Earth following the release of ice overlay after the end of the last glaciation, has caused sea level globally to fall at about 0.3 mm per year.
A crucial point is the effect of global ice melt in the past two millennia is unknown, but is thought by some to be close to zero. The IPCC assumes it is 0.0 – 0.2 mm per year.
However, there is also evidence that Northern hemisphere glaciers grew until the end of the Little Ice Age (1550 – 1850.) Antarctic ice surges have been estimated to be 2,000 and 700 years ago. Ice advances are not consistent, thinning of the Marie Byrd Land in West Antarctica thinning may have contributed 20 – 30 cm to global sea level rise.
In geologically stable areas of the world one can deduce sea levels using proxies such as; salt marsh sediments from Atlantic Canada that show stable sea levels between 0 and 1000 AD, then falling levels until to the mid-19th century, Mollusc distribution, and corals microatols (that occur close to average low water) also show falling levels until the past 200 years or so.
Archeological evidence, especially from the Mediterranean, suggest sea level was at its current level 2000 years ago, possibly higher between 300 and 600 and lower between the 13th century and the 19th, and rising since.
The important point is that all records show a falling sea level in the past thousand years. Another important point is what happened about 1850 when sea levels started to rise consistently?
The IPCC is therefore wrong in saying that prior to 1850 or so global sea levels were unchanging. Adding what we know casts a very different light on the changes we see today. It seems that sea level was rising and falling, due to millennia cycles perhaps, up until about 1850 when it started rising linearly. There was a change of gradient around 1910 when the rate of rise increases, and it has been constant ever since. Claims that sea level is accelerating, and, more recently, that it is slowing down are not statistically significant.
Just like the long term decline in Arctic sea ice we recently discussed, it is obvious that the current observed trend in sea level has its origins in the mid-nineteenth century before man’s influence on the climate became apparent (according IPCC estimates.) the fact that it is a straight line for the past century is also significant as it betrays no imprint of recent warming..
So, the statement by Professor Sir John Beddington, who has said that global sea level has increased by about 10 cm in the last 50 years (and so man must be to blame, unequivocally) is highly misleading, and a partial representation of the data. Whilst it is true that the sea level has increased by 10 cm in the past 50 years (coincident with a period of global warming), it also increased by 10 cm in the previous 50 years when man could not have been to blame!