3Gulf Stream Shutdown
In case you didn't know, the Gulf Stream is a sort of a "river" in the Atlantic ocean, a current of water that runs from Florida to western Europe. It has existed for thousands of years, and pumps warm water and air up to western and northern Europe, places that would otherwise be (more) frozen hell-holes.
You're welcome, Finland.
Scientists have noticed, however, that the overall temperature of the stream seems to be dropping, and has dropped a pants-shitting 30 percent since 1992. As you probably guessed, we again have climate change to thank. Before someone in the comments screams, "OH SO NOW GLOBAL WARMING MAKE PART OF THE OCEAN COOLER?!?!?!" keep in mind that when hot air melts a glacier, the hunks of ice break off and cool the water they melt into.
Originally, the fear was that if the Gulf Stream cooled too much, it would stop running, since it's those differences in temperature that keep it going. Then western and northern Europe would once again become home to woolly mammoths and fanged squirrels.
Artist's rendering from a U.N. report
However, studies have shown that if the Gulf Stream were to stop, there appear to be enough other factors warming Europe that it would have an effect, but wouldn't be the end of the world. So hell, why did we even bring it up?
The sudden drop off in inappropriate Speedo wearing in Europe would be the least of our concerns. According to measurements taken in 2005, the possible side effects of a "thermohaline shutdown" could include massive climate shifts, increases in major storms, greenhouse gases collecting in the upper ocean and phytoplankton dying off. In other words, Soylent Green would move from "Charlton Heston wailing" to "Grim predictor of the future."
In case you haven't seen this 37-year-old movie, Chuck discovered we were eating humans because we ran out of food, and we ran out of food because we killed the fucking ocean. So it would be like that, only with cataclysmic El Ninos.
So Are We Doomed?
Like with a lot of effects of climate change, we're kind of counting on uncertainty to save us (that is, we don't know exactly how this will work because we've never seen it happen before). Models show the current should keep running for another hundred years or so and they could still wind-up having to adjust those predictions--in either direction.
Global Dimming? What the hell is this, do we really need another Global ____ing crisis of some kind?
It's exactly what it sounds like, enough stuff gets into the atmosphere that it blocks the sunlight enough to shift temperatures. It's happened in the past due to the previously mentioned supervolcanoes, and it could be happening now in some parts of the world, due to pollution.
Angelinos will be unable to notice this phenomenon.
Whoa, wait a second. If it's making the world cooler, and the big problem is warming, then what the hell are we complaining about? It's less a natural disaster and more of an ironic superhero.
...Until you get to the side effects. Global dimming happens now because some pollutants act as cloud "seeds," that is water molecules in the air cling to them and form clouds. These pollutants form more and smaller collections of water, making the clouds denser and shinier, and therefore reflecting more light back into space.
These same pollutants are the ones responsible for acid rain, smog and a handful of other nasty shit. It also causes changes in weather patterns, leading to greater rainfall in some areas and droughts in others.
Some people believe that global dimming might have already lead to the massive droughts that ruined Africa back in the 70s and 80s, but the real shitstorm comes from the belief that if we cut back on the emissions that cause global dimming, global warming will suddenly accelerate and fuck us all, because that may be one of the mechanisms keeping global warming in check.
So Are We Doomed?
At the end of the day, most of the practices that cause the dimming also cause the warming. Going green fixes both problems in the long run. So while some people have actually suggested increasing the dimming to prevent the warming in the short term, this is kind of like killing a hooker and instead of just calling a lawyer to plea down to manslaughter, killing all of the witnesses in hopes that somehow you'll eventually kill enough people that the cops will stop investigating. It really doesn't work that way.
Or, on more scientific terms, Gavin Schmidt from NASA says:
"Ideas that we should increase aerosol emissions to counteract global warming have been described as a 'Faustian bargain' because that would imply an ever increasing amount of emissions in order to match the accumulated greenhouse gas in the atmosphere, with ever increasing monetary and health costs."
Of course, "monetary and health costs" would be the Vengeful Pimp in this analogy.