It is a man-made shift. Here's the quick-and-dirty version of the reasoning: The earth's energy equation has three big components: sunlight in, reflected sunl

It is a man-made shift. Here's the quick-and-dirty version of the reasoning: The earth's energy equation has three big components: sunlight in, reflected sunlight out, and blackbody radiation out. Measurements of these three items indicate that we are not currently in equilibrium. Instead, there's an excess of 0.6 Watts/square meter. That means the planet is warming. How do we know it's us? We've increased the CO2 content of the atmosphere. CO2's absorption/emission spectrum happens to be quite strong precisely where earth's blackbody radiation is strong; it therefore reduces the energy out by blocking the blackbody radiation. A paper about this is [0]. I eyeball Figure 6a as indicating a theoretical radiative forcing of 2 Watts/square meter, given the amount of CO2 we've put into the air. [0] https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/wea.2072 https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/wea.20...