Global Warming and Sea Levels

The Maldives are a low lying archipelago vulnerable to rising sea levels.
The Maldives are a low lying archipelago vulnerable to rising sea levels. Sakis Papadopoulos/Photodisc/Getty Images

We know that burning fossil fuels, clearing land, manufacturing fertilizers, and making cement all release greenhouse gases. We know that these greenhouse gases have been retaining heat in our atmosphere for decades now. As a result, average air and ocean temperatures have been increasing over much of the planet – a process often referred to as global warming, but more accurately called global climate change.

This leads to the melting of water locked up in glaciers, ice sheets, and snow. This water ends up in the oceans, contributing to the rise of sea levels. The Intergovernmental Panel on Climate Change (IPCC) has recently issued its 5th Assessment Report, with a chapter devoted to sea level rises. The report’s important findings include:

Sea Levels in the Past

  • There is solid evidence that in the last 5 million years, sea levels have reached 5 meters (15 feet) above current levels for long periods of time.
  • The rates of sea level rise have been very slow for the last 2,000 years, and picked up speed towards the beginning of the 20th century. Between 1901 and 2010, sea levels have gone up by 19 cm, or about 7.5 inches. This rate of rise has accelerated significantly since 1993.

Contemporary Sea Levels

  • Thermal expansion is the property that makes materials take up more space when heated. It is the reason for the bumpy expansion joints we feel when driving on a bridge; they allow expansion of the bridge sections, avoiding buckling. Water expands too when it is heated. Currently, about 39% of sea level change can be explained by thermal expansion of water.
  • The rest of the change is mostly due to the melting of glaciers (31%), the Greenland ice sheet (12%), and the Antarctic ice sheet (10%).

Projected Sea Level Rise

  • Predicted climate change consequences are based on model scenarios. This is because any future changes will depend on whether greenhouse gas emissions are reduced, and mitigation strategies put in place.
  • Regardless of the scenario contemplated, the rate of sea level rise will accelerate during the 21st century.
  • In the most optimistic scenario, in which we would quickly deploy carbon capture and storage technology, and switch much of our fossil fuel use to renewable energy, a sea level rise of about 40 cm (16 inches) would be expected by 2100.
  • In a scenario without coordinated climate policies among countries, but with a significant increase in renewable energy use, we would see a 48 cm (19 inches) rise by 2100. 
  • In the most pessimistic scenario, with a continued focus on fossil fuel and no carbon capture and storage efforts, the model forecasts 63 cm (25 inches) of sea level elevation. Incredibly, more recent estimates put the rise for this upper-end, worst-case scenario at 6.6 feet. Close to 5 million people live within 4 feet of the high tide line in the United States.
  • Any increase in sea levels will not be felt the same way around the world. Some areas will see only a very modest change, while others will experience a sea rise more than twice the predicted global average.
  • We will continue to see an increase in the frequency of sea level extremes (for example, very high storm surges) for many locations.
  • Looking at longer term global warming (over 1000 years or more), some evidence suggests that the complete loss of the Greenland ice sheet will occur, leading to a 7 m (23 feet) sea level rise.

Confidence in observed sea level rises has increased since the last report, and the projection scenarios are much refined. In particular, we understand much better the relationships between sea water temperatures and glacier ice.  However, much uncertainty remains about some important drivers of sea level change. In particular, scientists do not have a high degree of confidence about the contribution of melting ice sheets (particularly in Greenland and Antarctica) to sea level rises.


IPCC, Fifth Assessment Report. 2013. Sea Level Change.

National Climate Assessment Report. 2014. Sea Level Rise.