 This figure from NASA, from the AERS team, shows the concentration of CO2 in the atmosphere in May of 2013. This is after a winter of lots of fossil fuel burning in the north and lots of decay of plants, and just before the plants really start growing in the spring and taking out CO2 from the air. And so what you see in this broadband up here is a relatively high CO2 from the fossil fuel burning in the plant decay, whereas down here where plants have been growing really rapidly, the CO2 is a little on the low side. If you were to come back a few months later, these would have reversed a little bit. Now these variations are very small. 402 is the highest, 391 is the lowest. It's barely more than plus or minus 1% about the mean, because there's a lot of mixing around the planet. CO2 goes up and down with winter and summer, and CO2 goes up and up with fossil fuel burning. What's useful here is that these images plus measurements that are made on the ground plus economic data tell us who is burning fossil fuels and adding it to the atmosphere as well as what's going on naturally. And so if we were to have a treaty that limited fossil fuel burning, it could be verified by using available data. You can't cheat on fossil fuel burning and releasing the CO2 because the CO2 is measurable in the air.