Today Captain Moore gave the Crew a Rest Day. He set the Course, took the Helm, and did the cooking!
While sitting on deck looking over the great expanse of the North Pacific, we tried to develop a way to visualize the quantity of plastic distributed throughout the surface waters of the North Pacific Gyre. This is our calculation “done in the Field!” See the weather fax from NOAA for a look at what makes the Gyre, the North Pacific Subtropical High, which is seen with the "H" symbol on the Chart, and the 1036 millibar pressure (30.6 inches of mercury).
See if you can follow our reasoning, do your own calculations, and come up with a mental picture that illustrates the magnitude of the problem. We’ll give you our Formula, and some of our parameters and even enough numbers so you can check our math and do your calculations. So…. Get out your pencils and calculators…. It’s a great exercise, and we feel that it’s a great way to “wrap your mind around the great Gyre.”
1) Firstly we needed to calculate the surface area sampled by an average Trawl with the Manta-Trawl. Our “average trawl” goes for two hours at a speed of 3 nautical miles per hour. The “mouth” of the trawl is just about 3 feet wide (95cm to be exact). A nautical mile is 6076 linear feet. What is your calculation of the surface area sampled in our average trawl. The Crew came up with 109,368 square feet. Are we together so far?
2) After consultation with the Captain, based on ten years of accumulated experience sampling the Gyre, we estimated the amount of plastic in the average trawl to be 1/3 ounce (the range is between 4 ounces to less than one ounce).
3) The next step in our formula was to calculate the number of Trawls in a square mile of ocean surface. The number of square feet in a nautical square mile works out to 36,917,776 (thank you Kent!) square feet. We then divided this number by the surface area sampled in our average trawl, and concluded that a square mile of ocean surface is covered by 337.5 trawls.
4) How much plastic will be recovered in a 1 square mile surface trawl? Although we don’t weight our trawl contents in the field. The specific gravity of the plastic is very close to the specific gravity of the water, so for our calculation we considered 1/3 of an ounce to weigh 10 grams.
5) Now to calculate the area of the North Pacific Gyre. More specifically the area covered by the various exploratory voyages of ORV Alguita. This region lies between 130 and 170 degrees west longitude, and between 20 and 40 degrees north latitude. This is going to take a little work on a nautical chart. But it’s a good exercise in geophysics. The Crew’s calculation was rounded off to 2,500,000 nautical miles.
6) We now have the amount of plastic per trawl, the number of trawls per nautical square mile, and the number of square miles in the North Pacific Gyre. So what is your calculation of the weight of plastic in the Gyre? In order to work towards our goal of “visualizing” this number, we converted grams to pounds and then pounds to Tons.
7) After you get the number of Tons of plastic in the Gyre, use your imagination and tell the Crew how you can develop your mental image. We’ll post our estimate in the next blog as well as how each of us visualized this amount!
8) One thing to remember…. Don’t be too critical of the Crew; remember, we’re doing our calculations on the pitching deck of sailing boat... and it’s our day off!