Chapter 4 · Try it yourself
Breakdown Clock
You have probably seen a chart saying a plastic bottle “takes 450 years to decompose.” Nobody has watched one for 450 years. Here is what those numbers can and cannot tell you.
1. How long does it last?
Pick an item
Estimated time to “decompose”
450 years
Stage 0 of 4: whole
Plastic mostly doesn’t disappear. Sunlight and waves break it into smaller and smaller pieces. Below 5 mm they are called microplastics, and they keep going, far too small to scoop up.
2. The bathtub: residence time
Picture any part of the ocean as a bathtub with the tap on and the drain open. If water goes in as fast as it goes out, the level stays put, and the average time each drop spends in the tub is: residence time = amount in the tub ÷ rate flowing in.
Average time spent in the tub
Rough estimates. Item lifetimes are the widely shared “estimated decomposition times” from NOAA’s Marine Debris Program and Woods Hole Sea Grant (as compiled by Our World in Data): bottle 450, bag 20, foam cup 50, fishing line 600, disposable diaper 450, six-pack rings 400 and cigarette filter 5 years. NOAA itself cautions that nobody really knows how long plastic lasts; many scientists question these numbers, and time depends on sunlight, temperature and whether the item sinks or washes ashore. Stage timing in the animation is illustrative. Ocean water: about 1.335 billion km³ with about 430,000 km³ a year evaporating (and the same arriving as rain and rivers), giving roughly 3,100 years (LibreTexts, Our World Ocean 8.9). The plastic preset uses Lebreton and others’ (2018) estimate of about 79,000 tonnes in the patch with a made-up inflow; the real patch is not in balance and appears to be growing.