Extra heat causes glaciers to melt, and that water flows into the oceans. This matter is particularly worrying because it creates a positive feedback cycle which amplifies the effect. Continental ice on Greenland and west Antarctica is also warming at an accelerated pace, contributing to the rising levels. (~2 mm/year)
Oceans absorb 90% of the trapped heat in global warming and the ice absorbs roughly 3% of the heat, read more about that here.
Figure 1 below shows the changing levels of the oceans since 1993.
Figure 1. Sea level change since 1993. Global sea level change is not uniform, with some regions are more than 3x the global average. This is due to the variations in thermal structure of the top 1000 m of the ocean.
Rapidly rising sea levels rise have devastating consequences on coastal regions. As seawater reaches inland, it can cause destructive erosion and flooding, contamination of aquifers (called salt water intrusion), and loss of habitat for fish, plants, animals and humans.
When extreme weather events occur, higher sea levels mean more powerful storms near major cities. Moreover, hundreds of millions of people live in coastal regions that are becoming increasingly vulnerable to flooding. Potential damage as a result of rising sea levels has been studied in detail, and two categories have emerged in terms of which cities will be affected the most: highest overall cost of damage and overall cost as a percentage of GDP (in other words, cities that are poor or have many people, and would be devastated the most by an extreme event).
Future Sea Level Rise
Scientists have predicted possible contributions to sea level rise by the year 2100 of 16 cm - 54 cm from Greenland and 13 cm - 62 cm from Antartica. These values totalled are projected to contribute a sea level rise of approximately 56 cm by the end of the century. Adding in thermal expansion of the ocean, this figure is projected to increase to a range of 50 cm - 80 cm. Thermal expansion is projected to contribute more than half of the average rise, but land ice will lose mass rapidly as the century progresses. Sea level rise is set to continue at a faster rate than over the past 40 years.
Figure 1. A six-meter sea level rise shown on map of the Earth.
The 10 cities that are estimated to suffer the highest cost of damage in the event of such catastrophes are:
New York (USA)
New Orleans (USA)
The 10 cities that are most vulnerable, leading to the most devastation include:
New Orleans (USA)
Ho Chi Minh City (Vietnam)
Abidjan (Ivory Coast)
The video below by the World Bank discusses this sea level change and its consequences in detail.
↑ 1.01.1M. Melieres and C. Marechal, "The impact of warming on the ocean," in Climate Change: Past, Present and Future 1st ed., U.K.: Wiley, 2015, ch.27, pp. 273-283