Science Behind Sea Level Rise

There are two primary reasons that sea level is rising.

1. Ice is melting and flowing into the world’s oceans.

Artic Surface Trends - 1956-2008. Image by NASA.

As temperatures on Earth rise, water is released from polar ice caps and glaciers. After melting, the water makes it way to the oceans and causes sea levels to rise. Recent estimates are that 30% of sea level rise is from glaciers (Science, May 17, 2013). While the change is seasonal and some the of water refreezes during the winter months, the overall trend is more ice melting each year than refreezing.

In the graph you looked at earlier, the lower estimates of how much sea level will rise didn't take into account ice melt.




2. Water in the oceans is expanding.

 

Many substances expand when heated, including water. As temperature increases atoms and molecules move faster, spread out, and occupy more space. The change is small but important. Since the oceans hold vast amounts of water these small changes add up.



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Other Factors:

  • Sinking land: Sea level rise is not the same everywhere. On the U.S. Atlantic coast, northern land areas (e.g. Maine) are lifting up as they “bounce back” from being covered by an ice sheet. In these areas, sea level rise is less severe. By contrast, Mid-Atlantic lands (e.g., New Jersey, Maryland, Delaware) that were once on the edge of an ice sheet, are sinking down (or subsiding). In these areas, sea level rise is more severe.
  • Ocean currents: Ocean currents are also projected to contribute to sea level rise, although the effect will vary with geographic location.
  • Gravitational pull of large ice sheets: As ice sheets melt and lose mass, their pull on the surrounding ocean will decrease.