It is very difficult to measure the main metric directly driving climate change, the amount of greenhouse gasses (GHGs) in earth’s atmosphere, because this varies at different altitudes, regionally and over short-time-frames as various sources (human-caused and natural) emit, and sinks absorb GHGs. However, we can measure human-caused emissions at their source pretty accurately, especially in the U.S. Furthermore, if you want to the most direct metric for GHG emitted when generating electric energy, the basic technology used for every kilowatt-hour of electric generation is more direct and accurately measured. Even though this varies seasonally, it is still easy to see long-term trends.
