Ten Points
Today, there is both increased awareness and concern over rising sea levels. There is also considerable myth and misinformation. The following points and perspectives come from John Englander, accompanied by images from Horizon Rising / Rising Horizon and As Coastline is to Ocean. John is an oceanographer and author of High Tide on Main Street: Rising Sea Level & the Coming Coastal Crisis.
I
Contrary to popular belief, melting icebergs and sea ice have no effect on sea levels. Like floating ice in a glass, they are approximately 10% above the surface, due to the peculiar fact that ice has lower density than water.
II
The principal cause of change in global sea levels relates to the amount of ice on land, primarily our vast ice sheets and glaciers, 98% of which are on Antarctica and Greenland. These huge ice sheets are several miles high and would raise global sea levels some 65m if they were all to melt. Fortunately, though, this would take many centuries.
III
Over the last century, global sea levels have increased by approximately 20cm. In recent years, the rate of rise has rapidly increased. Most scientists believe that acceleration in the rise-rate will increase in a non-linear manner: this relates to the way ice sheets and glaciers go through phases of collapse which cannot be precisely predicted.
IV
A secondary factor of global sea rise is that seawater expands as it warms. In the last century thermal expansion of seawater has contributed around 8cm to ocean height.
V
In addition to global sea level variations, there are also local factors that influence change. One such factor is that land in different places moves either up or down: caused by tectonic shifts, the compaction of silts and organic matter, or pumping water or petroleum from the ground. Such local variations warrant evaluation both to better predict future sea levels and to design adaptations.
VI
Throughout Earth’s history sea levels have changed more than 100m vertically as the amount of ice on land has altered. The last low-water mark was approximately 22,000 years ago, when sea levels were around 120m below present. The last high-water mark was approximately 120,000 years ago, when sea levels were roughly 7m above present. These fluctuations were part of a natural repeating process that is commonly called the ice ages, a phenomenon that has been occurring naturally every 100,000 years, for almost 4 million years.
VII
We have now broken out of the natural ice age cycles of the last few million years and are in a period of abnormal warming. This correlates almost precisely with elevated levels of atmospheric carbon dioxide, which acts like a sheet of glass in a greenhouse: transmitting radiant sunlight but blocking the escape of warm air.
VIII
Today, the oceans are measured to be approximately 1°C warmer than they were a century ago. While the atmosphere is also getting warmer, more than 90% of the excess heat is stored in the oceans. Warmer oceans mean that the ice on land will melt until it reaches a new equilibrium.
IX
From geologic history it is clear that even 1°C of higher global temperature corresponds with metres of higher sea level. We are in the early stages of a transition that will continue throughout this century and beyond. Higher sea levels will gradually reshape coastlines all over the world. Even a few centimeters of change are already affecting the short-term flooding brought by storms, heavy rains, and extreme tides. Sea level is the baseline that raises all those temporary events.
X
Recognising this new reality, we need to do two things simultaneously: pursue all means to reduce the level of greenhouse gases in the atmosphere, with carbon dioxide from the combustion of fossil fuels as the primary factor that we can control. If done sufficiently at a global level, this can eventually slow the warming. Having already passed a tipping point, we need to start adapting to a new era with a rising sea. We have decades to adjust building codes, engineering, communities and infrastructure, but we have no time to waste.