Global sea level on the rise
Around the world, sea level is rising. From 1901 to 2018, global mean sea level rose by 20 centimetres – and in the future, it will continue to do so, and more quickly. This is chiefly due to anthropogenic climate change. The warmer atmosphere is heating the ocean, causing seawater to expand. In addition, glaciers around the globe and the ice sheets of Greenland and Antarctica are melting. The additional melt water ends up in the ocean, contributing to the problem. This poses a risk for the nearly 600 million people living in coastal regions, while it represents an existential threat to small island nations.
The facts
The Intergovernmental Panel on Climate Change (IPCC) regularly provides the public with information on the causes and impacts of climate change via its Assessment Reports and Special Reports. In its sixth and latest Assessment Report, the IPCC collects hard facts. Between 1901 and 2018, mean sea level rose by 20 centimetres. Further, with time this rise has gathered momentum: from 1.3 millimetres per year between 1901 and 1971, to 1.9 millimetres (1971 – 2006), to 3.7 millimetres per year from 2006 to 2018. The sea level is rising faster now than at any time in the past 3,000 years. In the 20th century, thermal expansion of seawater, in response to higher atmospheric temperatures and more heat absorption, was the main cause. According to the IPCC, the acceleration observed in recent years is due to growing ice loss in Greenland and the Antarctic. Today, the loss of ice (in glaciers and ice sheets) has replaced thermal expansion as the main cause. What the future looks like will above all depend on anthropogenic greenhouse-gas emissions. According to the IPCC’s climate projections, if emissions can be cut rapidly, the sea level will still rise by another 28 – 55 centimetres by the end of the century; if they remain high, in the worst-case scenario it could climb more than a metre by 2100. And after that, the ice sheets, given their slow responses to climatic changes, would continue adding to sea-level rise for centuries to come.
Text Ice sheets as a risk factor
Ice sheets as a risk factor
With an area of 1.7 million square kilometres and measuring up to 3,000 metres thick, the Greenland Ice Sheet is the largest contiguous inland ice mass in the northern Hemisphere. The enormous ice sheet contains roughly 10 percent of global freshwater. If it melted completely, the global sea level would rise by ca. 7 metres. Record high temperatures, which have become increasingly frequent due to climate change, have also affected Greenland’s ice sheet: meltwater ponds form on the ice and the glaciers flow out to sea faster and faster. Today, the colossus is typically losing more than 250 billion metric tons of ice to the ocean every year – enough water to fill Lake Constance five times over. The Antarctic Ice Sheet, with an area of 12.3 million square kilometres, is considerably larger than its counterpart on Greenland. If it melted completely, the resulting sea-level rise would theoretically be roughly 58 metres. For many years, the Antarctic Ice Sheet was assumed to be largely stable. But in recent years, researchers have also detected considerable ice loss caused by accelerating glacier flow – especially in West Antarctica. The Antarctic Ice Sheet is now losing more than 150 million metric tons of ice per year.
Facts and Figures
130
meters
At the peak of the last ice age, 20,000 years ago, when there was much more land ice, the global sea level was 130 metres lower than today.
3.7
millimetres
The sea-level rose by 3.7 millimetres between 2006 and 2018, almost twice as fast as between 1971 and 2006.
94
minutes
It takes 94 minutes for the two Earth observation satellites of the GRACE-FO mission to complete a single orbit of the planet at an altitude of 500 kilometres.
FAQ
Will the ice sheets soon disappear entirely?
It would take several centuries or millennia for the Greenland Ice Sheet to melt entirely. Consequently, a sudden sea level rise of 7 metres is highly unlikely. Nevertheless, the ice sheets will contribute even more to sea-level rise in the future; according to the IPCC, the melting has accelerated. In comparison to the period 1997 – 2006, the loss of ice mass on Greenland doubled between 2007 and 2016. And the ice-mass loss in Antarctica tripled in the same timeframe. In the IPCC’s view, this accelerating melting could mark the beginning of irrevocable ice loss. If this projected instability becomes a reality, the ice sheets will constantly melt, gaining speed as they do, and lose major percentages of their mass in just a few centuries. Nevertheless, their complete disappearance is considered unlikely.
How is ice loss measured?
In order to calculate and monitor ice-mass loss, e.g. on Greenland, AWI experts chiefly rely on three methods. The two satellites from the German-American GRACE-FO (Gravity Recovery and Climate Experiment – Follow on) mission continually monitor Earth’s gravitational field. When ice mass is lost in Greenland, it affects the gravitational force on the satellite, making it detectable in the measurements of the satellite orbits. In contrast, the ESA satellite CryoSat-2 measures the height of the ice cover. If there is a noticeable drop in that height, it indicates the ice is dwindling. As a third source of information, the AWI experts rely on mathematical computer models, which use meteorological data to calculate how much snow and ice forms in the polar regions and how much melts. By combining this with measurements of outlet glaciers’ flow speed, taken by radar satellites, they can then determine the loss of mass in the ice sheets. Though each of the three methods has its strengths and weaknesses, taken together they offer a good representation of the reality.
Is sea-level rise the same everywhere?
The statements on sea-level rise made e.g. by the IPCC are predominantly global averages. In reality, there are substantial regional differences. For example, the major ocean currents, the tides, and in particular prevailing wind systems influence the worldwide distribution of our oceans’ waters. The trade winds, for example, transport large amounts of water westward over the Pacific; as a result, the sea level is half a metre higher in Southeast Asia than on the western coast of South America. Postglacial rebound is another important factor. After the last ice age, Scandinavia’s massive ice sheets melted. Because of the weight lifted off it, to this day the land continues to rise by several millimetres per year. As a result, paradoxically the sea level will actually drop by a few centimetres in the northern Baltic by 2100, as the rebound exceeds the sea-level rise. But these are regional exceptions to the rule; in most coastal regions, the amount of sea-level rise will be similar to the global mean.
What’s going on in the North Sea?
There are also regional differences in sea-level rise in the North Sea. Over the past 100 years, it rose by 14 centimetres off Norderney, by 18 centimetres in Cuxhaven and by 20 centimetres in Husum; the global mean for the same timeframe is 18 centimetres. The differences are chiefly due to ocean currents and wind patterns. Today, most of Germany’s coastal regions are well protected by levees. Further, the tidal flats of the North Sea can to some extent rise in step with the sea level, as high and low tide regularly add new sediment. However, if sea-level rise exceeds a certain point – which is a very real possibility in the future – many intertidal areas that have regular dry periods could be transformed into lagoons permanently covered with water. If that happens, many animals living in the tidal flats and salt marshes – including countless seabirds and migratory birds – could lose their habitat and feeding grounds.
What coastal protection measures have been ratified in response?
Over the next several years, the levees in the German North Sea are to be transformed into “climate levees”, in the course of which their height will gain a metre or more and they’ll be widened. In addition, the emergency plans – e.g. for the fire brigade or the THW (Federal Agency for Technical Relief) – are to be modified. After all, levees and coastal protection measures focus less on the “normal” sea level, and more on protecting the hinterland when the level is especially high – like during storm surges, when prolonged winds and large pressure systems in the North Atlantic drive large amounts of water toward the coast. Due to the higher sea level, these surges will become more frequent and severe over the next few decades. Generally speaking, climate change will mean greater risks for Germany’s coastal zone.
What’s the situation in other regions?
Rich countries can more easily adapt to rising sea levels by implementing additional coastal protection measures. In poorer regions – like Southeast Asia – this will only be possible to a limited extent. Here, storm surges and floods will increasingly strike densely populated coastal regions – with devastating consequences for the citizenry. The outlook is especially bleak for small island nations and atolls. Here, not only is the money for coastal protection lacking; they don’t have the materials or the room, either. Accordingly, the flat land is at risk of gradually disappearing beneath the waves. In addition, increasingly frequent floods are threatening islands’ and atolls’ precious freshwater supplies. As a result, in the long term many people will be forced to leave, looking for new homes as climate refugees.
