Sunday, February 3, 2008

Visualizing the quantity of plastic.

Dr. Herb Machleder here, ship’s physician.

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!

Aloha from ORV Alguita.

11 comments:

Jeff's Dad Kent said...

Okay Doc,
You either made a typo or were trying to see if we were paying attention. You posted the sq.ft. of a nautical mile to be 3,691,776 instead of 36,917,776. My answer is approximately 25,350 metric tons of plastic in the area you defined. Do I win a prize?

tinablue87 said...

Dan sent this blog to me and now I follow it everyday. Until now I was unaware of the magnitude of pollution that exists in the Gyre. Thank you for informing us and teaching us at the same time.

-Sara Machleder

Paul S. said...

Have I got this right -- you trawl an area approximately 3 feet wide and 6 nautical miles long, and come up with only one-third of an ounce of plastic?

ORV Alguita said...

Herb and Charlie agree on changes for calculation proposed by Kent, Jeff's dad, you can go ahead and change (typo), and credit him!
-Anna-

LifeLessPlastic said...

That's almost 51 million pounds, right? Wow, that's a lot of plastic. How sad...

Nautical Information said...

Great blog and thanks for the good information. We have seen lots of plastic on our run from Oregon to Honolulu. Gary R/V Wecoma
garysnauticalinformation

Brennan said...

What is the volume of plastic in the gyre? Any idea? It might be easier to visualize volume rather than weight...

Eric said...

I came up with 8,125,000 kg (or 8,125 metric tons)

An incredible amount.

Ecogreen said...

This is a very helpful blog. Hope we can cooperate in some way educating ourselves and the rest. We need leaders to change for a first step to a greener life. Actually existing options to substitute plastic (biotechnology)are available. It is time to change. I personally work in a company dedicated to environmental packaging, and even everybody says the that the technology is amazing and good for the Earth, is hard to get executives to turn into change.
Also governments have to show attitude for this change.

Amigos de la Tierra said...

I've recently read about this amazing discover.
Moreover, I find very valuable your research and this blog is quite a good idea to share and get people involved.
I write in representation of Friends of Earth Spain; we are an NGO and we've recently published a post about this news.
From Spain we appreciate your contribution to the environment and encourage you to go on with your research

Lajuana said...

Keep up the good work.