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Any given week, there’s always plenty of news that reminds us how messed-up our environment is. But this week, let’s talk about something we thought we had started to fix.    Plastic.    You see it everywhere -- it’s all over your grocery store, and at the Target and the Walmart, and also on the roadways, and in the water.   And this is a problem, because plastic doesn’t biodegrade.    Biodegradation is nature’s recycling system - microorganisms eat waste and convert it into nutrients that other organisms can use.    For things like dead plant matter, that works really great.   But microorganisms can’t eat plastic, so those grocery bags and shampoo bottles that you throw away just hang out until they’re broken down by heat or the sun’s ultraviolet radiation.    And that can take centuries.    That’s why you’ve probably started to see more things made from biodegradable plastic -- it’s your usual, run of the mill plastic that’s treated with chemical additives that allow it to biodegrade faster.   Sounds great right?   Well.   This week, chemists from Michigan State University said they tested these biodegradable plastics, and found that … they don’t work.    The scientists studied the kinds of plastic that you and I use all the time -- like polyethylene, which is used to make grocery bags, and polyethylene terephthalate, which goes into soda bottles.   The chemists prepared these plastics with five different additives -- like compounds of iron, cobalt and nickel -- that have been said to speed up the degradation process, by breaking the bonds in plastics’ carbon-rich chains called polymers.    To put these additives to the test, the researchers put treated samples, along with additive-free plastic, in containers that simulated the different conditions that garbage is usually exposed to:    One was an oxygen-rich environment, like you find in a compost pile; another was an oxygen-poor environment, like a landfill, and the third was just in regular soil.     They then measured the daily output of methane and CO2, which microorganisms release when they digest matter.    After six months in compost, a year and a half in a landfill, and three years in soil, the amount of gas released from both kinds of plastic was the same.    And, under a microscope, both plastics showed almost no signs of physical degradation.   Interestingly enough, this research was paid for by companies that use so-called biodegradable plastics in their packaging, because they wanted to see if they were getting what they were paying for.    And now, it seems, they know.   Also in environment news, you’ve probably heard of the Gulf Stream -- that’s the neverending current of warm water from the Gulf of Mexico that helps keep North America’s east coast somewhat bearable during the winter, and also goes on to keep the British Isles nice and mild.   Well, that Gulf Stream is just one component of a bigger climate system known to scientists as the Atlantic Meridional Overturning Circulation, or AMOC.    It acts like a massive conveyor belt, pumping warm water from the equator up to the North Atlantic and back down again.   And the key to this cycle is the ocean water’s density. That might sound kinda weird, but cold water is denser than warm water, so after warm ocean currents run to the North Atlantic and release their heat, they sink and flow back south to warm up again.   So this cycle that’s been shaping our weather as we know it?   It’s been acting kind of strange lately.   A team of German scientists reports this week in Nature Climate Change that the AMOC appears to be drastically weakened.   For a while now, it turns out, the North Atlantic has been cooling. Which is weird, because the rest of the earth is getting warm, right?    But judging by more than a thousand years’ worth of temperature data that these scientists mapped out, the AMOC began weakening rapidly in the 1970s.    Then in 1990, it dipped to its lowest point in more than a millennium.    It’s rebounded slightly since then, but it’s still in decline. And the researchers think that it’s all due to the rapid melting of the Greenland Ice Sheet.    Greenland’s melting ice is dumping tons of fresh water into the ocean, which is making the salt water less dense.    That less-dense water is less likely to sink and flow south toward the equator like it has been, so it’s interrupting the circulation that’s been giving us the nice, temperate weather that we’re all so familiar with.    So if the ice sheet continues melting, the scientists say, the cycle will keep weakening for at least the next decade or two, and, things are gonna change.   Exactly how that change is going to happen is still mostly a matter of guessing. A previous report by the same scientists in 2009 said that less dense Atlantic waters could cause sea level rise in cities like New York and Boston, while a severely weakened cycle could lead to more extreme cold snaps and snowstorms on both sides of the Atlantic ... even as the planet’s average temperatures continue to climb.    At least, they say, the forecast doesn’t call for us to be drawn into some kind of modern-day Ice Age worthy of, like, casting Jake Gyllenhaal.   But still, I’m pretty happy with the way things are now. If the climate could just stop changing, that would be great.   If you’re happy with the way that SciShow is now, that's because of our Patrons on Patreon. This episode was brought to you by them. If you want to find out how you can help us continue making these cool episodes, you can go to Patreon.com/SciShow.