Sea level rise refers to the average increase in the water level of the Earth’s oceans. Over the last century sea level has increased (and continues to rise) due to global warming caused by the human activities that have increased greenhouse gases in the atmosphere. Forty percent of the world’s population lives in densely populated coastal regions. Thus, sea level rise has, and will continue to have, a major impact as it begins to displace coastal communities. Note that the rate (and sometimes the direction) of sea level changes can vary (see below).
A variety of human activities and Earth system processes affect sea level today, including:
- Human activities that cause global warming, primarily through the release of greenhouse gases via the burning of fossil fuels, agricultural activities, and deforestation. Warming increases sea level in two ways. First, increasing average global temperatures, and especially warming at the poles, causes glaciers and ice sheets on land to melt, which increases the amount of water in the oceans, thus increasing sea level. Second, the water in the oceans is expanding as it absorbs heat from the warming atmosphere, thus taking up more space, leading to sea level rise. This second phenomenon is called thermal expansion.
- Tectonic activity that causes the uplift or sinking of land at a particular location. This can be due to the major movement of land. For example, large earthquakes can sometimes cause meters of uplift. The uplift or sinking of land can also occur from more passive tectonic processes. For example, uplift is still happening where there were ice sheets during the last ice age. The weight of the ice caused the land to sink deeper into the mantle. As the ice melted, the weight of the continental crust was reduced, making it more buoyant. In these places the crust is still rising (called isostatic rebound), reducing the effective sea level rise. To learn more about the structure of the Earth see plate tectonics.
- Ocean circulation patterns, which can alter how water is distributed across ocean basins, causing sea level to rise more or less in certain locations.
In the past, sea levels have changed over thousands to millions of years due to a variety of Earth system processes and phenomena, including:
- Changes in Earth’s spin, tilt, and orbit, which increase or decrease the amount of sunlight absorbed by different areas of the Earth’s surface. This affects the climate, including regional temperatures and rain and snowfall, and thus the distribution and extent of snow and ice cover and the amount of water in the ocean.
- Increases in greenhouse gases that increase global temperatures, which reduce the extent of ice sheets, and cause thermal expansion of ocean water.
- The movement of tectonic plates, which determines the distribution, size, and depth of ocean basins, influencing how water is distributed around the continents.
Changes in sea level affect various Earth system processes and phenomena, including:
- Displacing human populations and other species due to flooding and also erosion.
- Increasing ground water salinity from coastal flooding, which reduces freshwater quality and availability and decreases soil quality. This, in turn, affects agricultural crops and livestock.
- The distribution of coastal habitats. For example, marshes and swamps protect coastlines by reducing coastal erosion and by promoting sedimentation. If there is space for these habitats migrate inland as sea level rises, this will decrease the vulnerability of coastal regions to sea level rise, especially storm surges.
- Species ranges, as sea level rise affects where organisms live, where they can move, and what other species they interact
Can you think of additional cause and effect relationships between sea level rise and other parts of the Earth system?