One of the many effects of our changing climate is the rise in global sea level. Since the start of record-keeping (1880), the global sea level has risen 8″-9″… a third of that happening in the last 25 years.
The rise in global sea level, or the average height of the ocean around the world, is used in the discussion of climate change. Regional or local sea levels vary – some rising, others falling, some even staying stagnant. These more-focused areas are influenced by a variety of factors:
- ground settling
- upstream flood control
- regional ocean currents
- rebounding from weight of land ice
The map below shows regional trends in sea level based on 30 years of observations from 142 long-term water level stations. The arrows indicate the direction and magnitude of change.
What’s causing the rise?
Natural variability causes sea level to rise and fall year-to-year… but data shows that ultimately, a warmer climate is responsible for a pronounced rising trend in global sea level values. As greenhouse gas emissions continue to increase in our atmosphere, these heat-trapping gases cause our environment to warm. Warmer temperatures result in increased ice melt. It’s the combination of melting glaciers/ice sheets and thermal expansion of seawater (warm water taking up more space than cold water) forcing the sea level to change.
From the 1970s through the early 2000s, melting and heat expansion were thought to contribute to global sea level rise equally. But the melting of mountain glaciers and ice sheets has significantly accelerated in the past 10-15 years, contributing to nearly twice the amount of sea level rise compared to thermal expansion.
The facts: ice loss from the Greenland Ice Sheet increased seven-fold between 1992-2001 to 2012-2016… while Antarctic ice loss nearly quadrupled between 1992-2001 and 2012-2016.
Data shows the western Gulf of Mexico and mid-Atlantic (north of Virginia) are not only experiencing sea level rise above the global rate, but also, accelerated local sea level rise due to land subsidence.
Land Subsidence – the sinking of the ground because of underground material movement (water, oil, natural gas, mineral resources, etc.) by pumping, fracking, or mining activities
How is the rise measured?
Sea level is measured in two ways: (1) tide gauges and (2) satellite altimeters.
Tide gauges have been used around the world for more than a century. The gauges can be manual or automatic and are typically adjusted for seasonal differences.
Satellite altimeters have been around since in the early 1990s, used to measure the height of the sea surface by measuring the return speed and intensity of a radar pulse directed at the ocean. (This is similar to how our radars on land measure the distance and intensity of precipitation.)
To estimate thermal expansion, scientists record sea surface temperature using moored and drifting buoys, satellites and water samples collected by ships. The top half of the ocean is typically measured via aquatic robots whereas the bottom half is measured by instruments lowered by research ships.
Sea level rise projections
The most recent projections, analyzed by NOAA experts in 2017, show several possible outcomes of sea level rise given various greenhouse gas emission levels.
The projections indicate that it is very likely that global sea level will rise at least 12″ above 2000 levels by 2100, even if our greenhouse gas emissions were to stay at the lowest possible levels. Worst case scenario has global sea level rise upwards of 8 feet.
The graph below shows six possible future scenarios (colored lines) based on potential future rates of greenhouse gas emissions and differences in the plausible rates of glacier and ice sheet loss.
The 2017 projections come after a review by the U.S. Interagency Sea Level Rise Taskforce requested an upward revision from a previous 2012 study.
Why it matters?
In the United States, nearly 40% of the population lives in coastal areas. It’s also an area that provides jobs for over 54 million people and produces $7.6 trillion in goods and services each year.
Given higher sea levels, a significant number of people, places and industries will be more susceptible to:
- more inland flooding
- increased shoreline erosion
- greater threat of deadly & destructive storm surges
- more frequent high-tide flooding
- greater stress on coastal ecosystems which provide protection from storms
- disturbance of fish and wildlife habitats
- increased threat of contaminating freshwater aquifers
Understanding the current trend of sea level rise will help guide decision-makers on establishing safe building codes, restoring coastal habitats, building new infrastructure, evaluating floodplain maps and determining evacuation routes during disasters.