The best way to get the data would be to measure how much energy the planet absorbs vs how much it emits. The earth just wants to be in equilibrium so if these

The best way to get the data would be to measure how much energy the planet absorbs vs how much it emits. The earth just wants to be in equilibrium so if these amounts aren't equal the climate is changing and the difference can tell you to what degree (excuse the pun). The problem is that the earth transfers energy between many of its systems (think Ocean conveyor belt) and it's incredibly difficult to keep track of the energy. So any measurements of the earth directly that don't measure all the systems are likely to be inaccurate. Edit: yes there is a global average surface temperature but due to the reasons above it's not a great indicator.