Yesterday, Saturday January 24th, my wife Doris and I attended the town hall meeting at Sea Isle and wandered around the various booths reading the comments posted for each group. At the Environment Booth I was struck by one comment in particular that read “Stop Erosion”, which provoked me to sit down and write up this summary of the problem. Stopping beach erosion is about as complicated as stopping volcanoes from erupting and earthquakes from quaking and the sooner we accept this reality the sooner we will get on with the business of sustainable development in Galveston. I welcome questions and comments and am happy to provide scientific documentation of the statements I have made in this summary.

1. For those who would question the importance of sea-level rise in controlling coastal retreat (erosion), consider that in the past 17,000 years, since the last major glaciation, sea-level has risen approximately 100 meters causing the shoreline to retreat approximately 80 miles from it previous location at the edge of the continental shelf to its current location.

   
   

2. The current rate of shoreline retreat, which averages 3 feet/year along the west end of the island but in places is as much as 10 feet/year, is slow compared to times in the past when sea level was rising at a more rapid rate, at times as much as 50 feet/yr.
3. The rate of rise in any given location is combination of eustasy (global sea level rise) and subsidence. This is called relative sea-level rise. Eustasy is fairly constant across the northern Gulf of Mexico but subsidence is highly variable. In western Louisiana the rate of subsidence is in places as much as 30 times faster than the rate of eustatic rise. Our neighbors to the east are already contending with problems that we will face by the end of this century. The rate of subsidence in Texas is significantly lower but locally is still a major contributor to shoreline change. Unfortunately, we have relatively few locations where elevation changes are being monitored and those are not all in the most ideal locations. There are ways to acquire long-term subsidence measurements but little work has been done in the field. Remember, we spend all our money in Texas dumping sand on the beach and pitifully little goes to address the real issues of why the coast is changing. If we don’t know why, we can’t begin address the problem.
4. Sediment supply is as important as sea-level rise in controlling coastal change. If sediment is added to the coast fast enough to keep up with the rate of sea-level rise the shoreline will remain stable and even grow. This is how Galveston Island formed at a time when sea level was rising and why beaches adjacent to the north and south jetties have grown since the jetties were constructed. Sediment supply to the upper Texas coast is minimal. So, even a slow rise in sea level results in shoreline retreat. We still do not have an accurate sediment budget for the upper Texas coast. The recent study by the Corps of Engineers was a poor attempt to quantify the sediment budget and until the budget is improved it will not be possible to predict the response of the Gulf and Bay shorelines to accelerated sea-level rise. I have a PhD student, Davin Wallace, who is currently working on a more accurate sediment budget for the Texas coast.
5. Many of the current maps that show coastal inundation through time are highly inaccurate because they simply flood the landscape with different scenarios for sea level rise. They do not account for subsidence or sediment supply. For the most part, they are best-case scenarios.
6. The current Galveston Island Geohazard Map is a more accurate approach to predicting coastal change because it relies on historical rates of shoreline change to predict future change. However, it assumes a constant rate of sea level rise and does not take into account changes in sediment supply through time. Neither does it account for the impact of hurricanes. So, it is a best-case scenario and should be immediately adopted by the city until we have a better handle on subsidence and the sediment budget.
7. Eustasy is controlled by the temperature of the oceans (remember water expands when heated) and the contribution from glaciers and ice sheets. There is no question but that the oceans are getting warmer as the atmosphere warms, and this is the main contributor to the current acceleration in sea-level rise. In addition, 80% of the glaciers on Earth are melting. The net result is that the rate of sea-level rise has nearly doubled (from an average of 1.8 mm/yr to nearly 3.0 mm/yr) this century. The contribution of ice sheets to sea-level rise is the greatest uncertainty when it comes to predicting future rise, but it is by far the most important in terms of magnitude. If the Antarctic Ice Sheet were to melt global sea level would rise 60 meters (~200 feet). While this is not going to happen in the next few tens of millions of years, the most current scientific evidence suggests that the West Antarctic and Greenland ice sheets are going to contribute to sea-level rise this century. At the most recent meeting of the American Geophysical Union in San Francisco one of the keynote speakers at the meeting (Dr. Eric Rignot) summarized the current scientific information on ice sheet mass balance and concluded that global sea-level rise will likely reach 5 mm/yr by the end of this century. Dr. Rignot is one of the best, most respected scientists in the field and I see no scientific justification to doubt his predictions.
8. Let’s put Dr. Rignot’s sea-level prediction into perspective. If we want to view the world of coastal change at a time when sea level was last rising at a rate of 5 mm/yr we have to turn back our geological clocks 7,000 years. I have spent the past three decades studying the evolution of the Texas coast before and after 7,000 years. Take it from me, this was a time when our coast, including estuaries, was changing at a rate most people cannot (don’t want to) imagine. This is not science fiction, this scientific consensus based on the peer-reviewed literature. This is why we have to fight to educate people about the seriousness of the situation. We can no longer tolerate ignorance, indifference and greed on the part of those who control the destiny of our coast. We owe it to our children and grandchildren to fight for a more sustainable coast.

For additional reading consult Dr. Eric Rignot’s research.

John Anderson

Maurice Ewing Professor of Oceanography

Department of Earth Sciences

Rice University

Houston, Texas 77251-1892