Do falling rocks cause sea level rise? Science weighs in

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Don Lincoln is a physics researcher. He is the author of "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind" and he produces a series of science education videos. Follow him on Facebook. The opinions expressed in this commentary are his.

(CNN)As any teacher or parent knows, occasionally a public figure says something that has "teachable moment" written all over it. US Rep. Mo Brooks, R-Alabama, provided such an opportunity last week.

At a hearing held by the House Science, Space and Technology Committee to explore how technology could be used to mitigate effects due to climate change, he made an unorthodox assertion connecting sea level rise and erosion.
Brooks said, "Every time you have that soil or rock or whatever it is that is deposited into the seas, that forces the sea levels to rise, because now you have less space in those oceans, because the bottom is moving up." In other words, Brooks argued that the rise in sea level could be caused by rocks falling off cliffs or mud from the Earth's surface being washed into the sea.
Don Lincoln
The debate over climate change and global sea level change is a familiar one, with political voices often being louder than scientific ones. In my not very humble opinion, this is a shame, as this is clearly a question with a quantifiable answer. The planet is warming, or it is not. Sea level is rising or falling, or it is staying the same. Answering questions like these is what science does.
    If a scientific consensus arises (as it has), then deciding what to do is a political question. Policy makers need to consider a broad range of issues in addition to the scientific input. These issues include human impact, economics, military considerations, and global and geopolitical concerns, and then they need to present proposed solutions that should be subjected to vigorous public debate.
    But the question starts with science and the best possible data.
    So, returning to Brooks' claim -- it's true that rocks fall from cliffs, silt is deposited by rivers and all manners of terrestrial debris enter the oceans and fall to the bottom. But could this be the cause?
    On the face of it, this isn't so completely outrageous. After all, anyone who has taken a bath has seen the water level in the bathtub rise when they got into the tub. It's an idea that needs to be evaluated to see if it hangs together. But it turns out to not be the explanation.
    How do I know? I did the math.
    The National Oceanic and Atmospheric Administration, a government agency tasked with monitoring the air and the seas, notes that the global sea level is rising at an average rate of about an eighth of an inch (3.3 mm) per year. In very rough numbers, the surface of the Earth is 200 million square miles. About 71% of the Earth is covered by water, so a global rise of sea level of an eighth of an inch is the same volume as a cube about 6 miles on a side. To give a sense of scale, the US has an area of about 4 million square miles. To get the necessary volume, you'd have to scrape off the top four inches of soil from the entire country. That's a lot of dirt.
    And the world's oceans don't experience anywhere near that kind of sedimentation. The amount of sediment carried to the sea by the world's rivers is about 14 billion tons a year, which works out to be about 1.2 cubic miles (five cubic kilometers) of rock, or 0.5 percent of the amount required to raise the sea level by the observed eighth of an inch (3.3 mm) per year.
    So, rocks falling into the ocean and silt from rivers isn't the cause of sea level rise.
    Now this isn't intended to shame anyone who believed that sea level rise and sedimentation were somehow connected. It's a reasonable conjecture and worth evaluating. But it's not true. That's the beauty of using science. You can test ideas and see if they're right or not.
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    If sea level rise isn't caused by sedimentation, what is the cause? Well, there's the oft-mentioned issue of ice melt from Antarctica and Greenland, but there is another unappreciated effect, which is the fact that things get bigger when they warm up. As the temperature of the ocean's water increases, water literally expands. One third of sea level rise is just due to this expansion. Knowing the real cause will guide the public debate in the right direction.
    The international conversation about the Earth's climate will continue in the years ahead, with many voices and many agendas -- and the outcome is far from certain. But it is very important that the scientific community speak up when incorrect statements are made. We can all hope that when policy makers are informed of and then embrace the facts, that they will eventually make the right decisions.