A simple analogy for this approach is your monthly check-book: by looking at your bank statement you know exactly how much money you have in your account on the 1st day of a new month. The challenge is to reconstruct the sum of income and expenses since the last month that matches your total exactly. For the global atmospheric carbon dioxide budget, this check book looks something like the figure on the left.
By measuring carbon dioxide frequently (the bold-faced sum that makes the total), and at many places on Earth, CarbonTracker can break down the budget even further, and thus keep track of uptake and release for many places on Earth. And just like your bank statement, the atmosphere never lies!