The climate system is very complex, but when you take into account the ability of the ocean and atmosphere to absorb carbon and heat (and how that decreases as more is absorbed), the relationship between temperature change and accumulated CO2 is basically linear. Which means GHGs emitted at the start of the industrial revolution still matter. Those GHGs are still affecting how much the climate is warming. Let that soak in. The climate is a slow and laggy system, but it remembers.
So using this chart we can work out just how much carbon we as a planet can emit for a given amount of warming. This is our planet's carbon budget.
We have already 'spent' 531 billion tons of carbon (equivalent) since the industrial revolution. This is what you get when you calculate CO2 emissions from all of the trees and whale oil and coal and gasoline and natural gas we have burned and combusted to make our cars go and our iPhones tweet.
How much more we can 'spend' depends on how much warming we are comfortable with. For a warming scenario of less than 2C (3.6F) above temperatures in 1861-1880, we have a carbon budget of 1 trillion tons of carbon. Do the math and you'll see that leaves us with a 'safe to spend' amount of only 469 billion tons of carbon.
How long will it take to burn through our budget? Myles R. Allen, a scientist at the University of Oxford and one of the authors of the IPCC report, thinks we'll hit that trillionth ton around 2040. Our budget gets tighter if we look at non-CO2 GHGs like methane and CFCs. When we take those into account our budget shrinks to 800 billion tons of carbon.
To me, this is one of the most straight-forward ways of thinking about climate change and what we can do about it. Our policymakers may not be successful at balancing a budget, but at least the concept is easy to grasp - even if you don't understand the underlying climate science. How much warming we want is up to us. So forget Green Eggs and Ham. Perhaps our policymakers should be reading "Choose Your Own Adventure" books.