About Sea Level Change
Why is Global Sea Level Rising?
Increased greenhouse gases lead to global sea level rise.
Why Does Sea Level Rise and Fall?Why Does Global Sea Level Have Temporary Ups and Downs?
Global mean sea level is the average height of the world’s oceans. It has been steadily increasing over the past century, primarily due to two major factors: the thermal expansion of ocean water as it warms and the addition of water from melting ice sheets and glaciers. This long-term rise is a clear indicator of ongoing climate change, driven by human activities that add greenhouse gases to the atmosphere.
Despite this clear upward trend, global mean sea level does not increase in a perfectly linear fashion. It exhibits temporary ups and downs due to various natural phenomena and seasonal variations. One that is very noticeable is an annual cycle that results from changes that happen in our climate over the course of a year. In each hemisphere the oceans warm and expand in summer, and cool and contract in the winter. This leads to sea levels being higher in summer and early autumn, and lower in winter and early spring in each hemisphere during those seasons. In addition, there is an increase of water stored on land in the Northern Hemisphere winter and thus less in the ocean, leading to a lower global average sea level at this time of year. While these annual changes happen at different times of the year in each hemisphere, when they are averaged together to create the global mean sea level, a noticeable shift up and down happens over the course of the year. In the global average, this shift is around 2 centimeters (1 inch). However for a specific location, the shift in sea level during this annual cycle can be much larger.
At time periods slightly longer than seasons, naturally occurring climate variations can also cause changes in global mean sea level. For instance, El Niño and La Niña events can temporarily raise or lower sea levels by affecting ocean temperatures and currents. During an El Niño event, the ocean tends to be warmer than normal temporarily and connected changes in weather patterns lead to more water raining down into the ocean rather than over land globally. This leads to higher global mean sea level. The opposite happens during a La Niña and global mean sea level can actually drop.
We can see the interplay between long-term sea level rise, the annual cycle and El Niño by looking at satellite altimeter measurements. The animation shows the rise in global mean sea level from 1993 to 2023 based on data from a series of five international satellites. The data over the course of each year in the record is displayed. The annual up-and-down is visible - global mean sea level is a little bit higher from August to October, for example - as are the temporary spikes in sea level due to El Niño and La Niña. The spikes in sea level from 2015 to 2016 and from 2022 to 2023 result from El Niño, while the noticeable drop from 2010 to 2011 is caused by La Niña. Despite these variations, the upward trend in global mean sea level is dominant and sea levels are roughly 4 inches higher today than they were in 1993.
Tracking the Melting IceTracking the Melting Ice
Melting ice, primarily from mountain glaciers and the vast ice sheets of Greenland and Antarctica, is a significant contributor to global sea level rise. As the planet warms due to human-induced climate change, these frozen reservoirs lose mass, adding substantial amounts of freshwater to the world's oceans.
To monitor and measure ice loss and its impact on sea levels, scientists use a variety of tools, one of the most significant being the GRACE (Gravity Recovery and Climate Experiment) and its successor, GRACE-FO (Follow-On) satellite missions. These satellites measure changes in Earth's gravity field, which are influenced by the distribution of water and ice on the planet's surface. As ice sheets lose mass, the gravitational pull in those regions weakens, allowing scientists to track the precise amount of ice loss over time. GRACE and GRACE-FO provide critical data for understanding the rate and extent of melting ice and have transformed our ability to quantify changes in ice sheet and glacier mass.
The amount of ice being lost is staggering and continues to accelerate. For example, Greenland alone is losing around 270 billion metric tons of ice per year, while Antarctica is shedding approximately 150 billion metric tons annually. To put this in perspective, each metric ton is equivalent to about 1,000 liters of water. The combined annual ice loss from Greenland and Antarctica is enough to raise global sea levels by nearly 1.2 millimeters each year. Over the past few decades, the cumulative effect has contributed significantly to the more than 20 centimeters of global sea level rise observed since 1900.
Taking the Temperature of the OceanTaking the Temperature of the Ocean
The oceans play a crucial role in regulating Earth’s climate, and their temperatures are a key indicator of the planet’s overall health. As the Earth warms due to the buildup of greenhouse gases, the oceans absorb over 90% of this excess heat, causing ocean temperatures to rise. This warming contributes significantly to sea level rise in two ways. First, as water warms, it expands—a process known as thermal expansion. This phenomenon has accounted for about half of the observed sea level rise in the 20th century. The graph below shows the amount of heat being accumulated in the ocean. For context, the total energy consumption around the world is about half a zettajoule every year. Since 1992, the ocean has had an average gain of about 7 zettajoules of heat every year.
In addition to thermal expansion, warmer ocean temperatures also accelerate the melting of ice sheets and glaciers. Warmer waters erode the edges of ice sheets from below, destabilizing them and speeding up ice loss. This feedback loop compounds the rate of sea level rise and highlights the interconnected nature of our climate system. By “taking the temperature of the ocean,” scientists can track these changes and improve models for future sea level projections.