Weekend Reads: NOAA's New, Narrower Sea Level Rise Predictions

by Kevin Schofield

This weekend's read is an updated report from the National Oceanic and Atmospheric Administration (NOAA) on their latest predictions for sea level rise through the rest of this century due to climate change. If that sounds, well, dry, I can assure you that it has very wet implications for the coastal United States, including here in Washington. But this new report is also interesting because, according to NOAA, both the current empirical measurements of actual sea level rise and the computer models of future rise have converged, resulting in a much narrower range of predictions than the last report five years ago.

Several factors must be considered in predicting sea level rise. Those include greenhouse-gas-driven temperature and chemistry changes to both the atmosphere and the oceans (which absorb some carbon dioxide in addition to solar heat); thermal expansion of the oceans (as water warms, it expands in volume a bit); the melting of glaciers, ice sheets, and polar ice caps; extreme tides; and storm surges (and the frequency of large storms).

There is, however, one more factor incorporated into NOAA's new calculations that has received greater attention in recent years: vertical land motion (or VLM). Recall that the tectonic plates that the Earth's continents sit on are themselves floating: not on water, but on a layer of molten magma. As the planet's ice sheets and glaciers that are currently sitting on land melt away, they put more water into the ocean, but they also reduce the weight of the land mass — which can then float up higher. What this means is that even if the oceans rose uniformly around the world (which they don't: tides, the Earth's spin, and ocean currents create bulges in some places and troughs in others), not all coastlines will see the same level of rise around the world — or even in just the United States.

On top of this, scientists now have less faith in their ability to accurately model the melting of large ice sheets near the poles. They know they are melting and the current rate of melt, but they have become less confident in their understanding of what triggers the sudden collapse of an ice sheet. So in this latest report, they took a conservative path and made estimates that leave out that factor, noting that the real sea level rise may be higher.

According to their calculations, over the next thirty years the average sea level rise along continental U.S. coastlines will be at least 25 to 30 centimeters, or about one foot. That would be as much sea level rise in the next thirty years as we have seen in the past 100 years. It will be higher along the East and Gulf of Mexico coasts (25—30 centimeters) and lower along the West coast including Seattle (10—20 centimeters). Alaska, they predict, will be the least affected, due in large part to the upward land movement as its ice sheets melt. But across the United States this means more saltwater seeping into groundwater (potentially affecting water supplies) and backflowing through sewer and storm drain systems in coastal communities.

But sea level rise, they point out, isn't just an issue of an average level: We need to factor in extreme high tides, as well as the effect of severe weather events such as tropical storms and hurricanes. Whereas by 2020 the U.S. typically saw about three minor high tide flood events per year one moderate event every three years and one major high tide flood every 25 years, by 2050 they predict over 10 minor high tide floods per year, four moderate events per year, and one major event every five years. The average sea-level rise means that we'll lose some shoreline and see more nuisance flooding, especially in low-lying areas such as southern Florida, New Orleans, and Galveston; but flooding from extreme high tides and severe storms will take a much more serious toll on coastal communities thirty years from now — and much farther inland than the beaches.

These predictions assume that Earth continues to pour greenhouse gasses into our atmosphere and that there are no significant changes to that activity. To date, we have seen atmospheric temperatures rise one degree Celsius compared to pre-industrial times. On our current trajectory, NOAA predicts that if that reaches two degrees by 2100, the U.S. has a 50% chance of seeing 70 centimeters of sea level rise — more than two feet. If the temperature rises by 3—5 degrees, the probability goes up to 80-99%.

And again, this is optimistic: NOAA left out the impact of ice-sheet collapse because its scientists don't yet feel confident in their modeling of that phenomenon.

Climate change is here, and NOAA's latest predictions argue that thirty years from now the coastal United States is in for some big changes. We need to get serious about greenhouse gasses now, and we also need to start preparing for what looks to be inevitable sea level rise and the impacts that it will have on coastal communities.

Global And Regional Sea Level Rise Scenarios for the United States

Featured Image: Photo by Munimara/Shutterstock.com.