Transcript of Sea Level & Climate Change

[music] You’ll remember   from the lesson on Earth’s formation, seafloor  sediments are extremely important records of   Earth’s past climate. How? The species of diatoms,  coccolithophores, radiolarian, and foraminifera   will change as seawater temperatures change. In  addition, the type of sediment that is deposited   will change. Increased glacial activity on land  during an ice age will bring more sediment to the   margins, and in areas closer to the poles, these  sediments will consist of large angular immature   material carried by glaciers and then sea ice. In  the mid latitudes, where glacial ice was present   only in the mountains, rivers have a higher  amount of water and sediment. Sea level drops,   as water is trapped on land in glaciers. The  new coastline moves out towards the edge of the   continental shelf, or break. The area exposed can  be covered by huge amounts of delta sediments and   sands. What’s the deepest part of the oceans that  can be exposed during an ice age? The continental   break. Do you remember how deep the depth is at  the break (mentioned in previous lectures)? 125   meters or about 375 feet. That’s the deepest  sea level will drop during an Ice Age. What’s   the average depth of the oceans? 4000 meters.  So most of the ocean is still covered! Since   the earth has experienced multiple episodes of  ice ages and interglacials during the past 1.6   million years of the Pleistocene, what story do  our shelf sediments record? Let’s imagine we take   a research cruise to the outer shelf, near the  break, and drill through the layers of sediment   that have collected. What will we find? What kind  of sediment is collecting there today? Assuming   it’s a wide shelf, we would expect mostly muds  that far offshore – maybe some fine sands. What’s   under them? The sediment that collected during  the last ice age, when that spot was the beach.   What kinds of sediments collect along a beach?  Gravels and sands! So we should see alternating   sequences of sediment representing different  levels of the ocean. In fact, if we wanted to   study fishing villages of earlier cultures that  lived 15,000 years ago during the last ice age,   we would have to go to the edge of the shelf and  drill through many layers of sediment that have   deposited since, burying those records. What’s  the highest sea level could be if we melted all   the current glaciers, a direction the planet  is currently heading, as it has many times   in the past 1.6 million years of glacials and  interglacials? 70 meters above current sea level.   Are there other ways to cause sea level to change  besides an ice age? What happens if we simply   cool water, without producing glaciers? What  happens to water as we cool it? Or the reverse:   what happens to water as we warm it? To answer  that, let’s watch what happens to a marshmallow in   a microwave. This peep is cold – compact and well  preserved. Want to see what it looks like after   we heat it? What would we expect? The molecules of  marshmallow will pick up energy, moving faster and   spreading themselves further and further apart –  expanding. The marshmallow should grow. And here’s   the result. So warming up water should cause it to  expand, much like what happens during convection   – the expanded warmer water rises up because it’s  less dense, while the colder denser material sinks   to get heated up itself. In this case, we warm up  all the oceans, so all of it expands. Sea level   would rise. And if the water then cools,  it would contract. Sea level would drop.   One more way to change sea level is to change the  size of the ocean basins. If we make the bathtub   bigger, so to speak, the water that fills it will  have more room – sea level will drop. The reverse,   shrinking the bathtub, would make sea level rise.  As seafloor spreading rates change in the world’s   oceans, so too will the size of the oceans change  and correspondingly, sea level. What’s the lowest   sea level has been in the past few million years  of earth’s history as a result of cooling oceans,   ice ages, or larger ocean basins? About 130 m  below its present position. What’s the highest   sea level has been? About 170 m (and that was  during the Cretaceous, over 100 million years ago,   when average surface temperatures were 10  degrees Celsius warmer than they are today).   Remember that the average elevation of the land  is 800 m, so 170 m wouldn’t cover much – however,   it would cover about 85% of the world’s  population, as that’s where the majority of our   population centers and associated infrastructure  are located. This fact is probably one of the   most important you will learn in this class this  semester – sea level is rising and will have a   huge financial impact on the future of human  civilization. An intelligent and sustainable   plan for future development should include a  good understanding of this regular and natural   process of sea level fluctuation. Temporary and  transportable structures are logical for building   within 100 m of sea level. Permanent structures  will require constant financial input to maintain   in the face of rising sea level. Pause now. [music] [music]