We hadn't seen the Carbon Counter before, which is surprising, given it's a 70 foot billboard hovering outside Penn Station —and with a number currently at 3.64 Trillion!
The counter clocks the number of greenhouse gases in our atmosphere. The thinking behind it is that it's a visible way to draw attention to gasses that we can't actually see, and are therefore easy to forget (or ignore.)
It's sponsored by Deutsche Bank, and uses data developed by scientists at MIT. Check out the counter, and even download a carbon counter widget to your desktop here.