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On 16 May 2018, the House Committee on Science, Space, and Technology held a hearing ostensibly to discuss the ways in which technology could be deployed for climate change adaptation. As this committee’s hearings are wont to do, however, some committee members used their time to interrogate the expert witness about the veracity of basics of climate science. In this case, that witness was Philip Duffy, the president of the Woods Hole Research Center in Massachusetts. The most infamous exchange occurred between Duffy and Representative Mo Brooks, a Republican from Alabama. Philip Bump at the Washington Post described it in a gripping play-by-play: Brooks is accurate when he says that tons of material — silt, sand, dissolved solids, maybe some rocks from the cliffs of Dover, etc. — are constantly flowing into the world’s ocean basins. According to the National Oceanic and Atmospheric Administration: Though his question makes sense from an elementary physics standpoint, Brooks is inaccurate in suggesting that this contributes in any meaningful way to the sea level rise attributed to anthropogenic global warming. Certainly, when you put something into water, that water is displaced and the water rises relative to the volume of that object — we can thank Archimedes for that one. On human timescales, however, the amount of water displaced by this sediment flow is essentially undetectable. We asked Phil Woodworth, the former director of the Permanent Service for Mean Sea Level (PSMSL), an international organization that maintains tide gauge records worldwide, about Brooks’ question. He told us, As large as these river transports of sediments are, the ocean volume is so enormous that they don’t result in a significant change in global-average sea level. According to the IPCC’s Fifth Assessment Report, which is a consensus statement produced with the input of thousands of international scientists and overseen by the United Nations, sea levels overall since 1900 have gone up on average 1.7 millimeters per year with an uncertainty of 0.2 millimeters in either direction. (Satellite data from 1993 onward indicate that this rate has been accelerating in recent years and is now thought to be around 3.3 millimeters per year). That IPCC report also estimated that the annual contribution of sea level rise caused by sediment dumped into ocean basins is about 0.01 millimeters per year — a meaningless number compared to the uncertainty associated with historical sea level data. The Post, using the 3.3 millimeter per year rate estimate relevant to more recent sea level rise, described how much material would need to be carried into the ocean every year: Of course, on geologic time scales (tens of millions of years) such an amount material added to the oceans would, in theory, become meaningful. That would be true if not for the fact that — also on geologic time scales — the ocean floor actively recycles old seafloor at rates comparable to the mass added by sediment each year. On these timescales the major factors driving global sea level variability are much larger-scale processes like the subduction of continents, the rise and fall of ocean ridges, and the volcanic processes that create new rocks. Tim Herbert, a professor of oceanography at Brown University, told us that Brooks’ argument — if looked at on these timescales — is akin to focusing on one side of a cyclic process while ignoring the processes that go the other direction. On relevant timescales, Herbert told us, the most important point is that current rate of sea level rise is substantially higher than what geologists believe to be a normal background rate: While Brooks is correct both in his understanding of Archimedes' principle and in his statement that tons of sediment flow into the oceans each year, his assertion that this sediment flow is in any way a measurable contributor to the sea level rise scientists are discussing in the context of anthropogenic global warming is false.
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