The climate scientists record the air temps at many locations throughout the world and calculate an average global temp for the year. There's a graph in the link. Notice how the average global temp in the early 1900's was around 56.0 to 56.5 deg F. And now, its 58.0 to 58.5 deg F. A 2 deg F increase in a hundred years may not seem like much, but compared to natural warming cycles in the Earth's past, this increase is about 170 times faster that ever before. This 2 deg F increase converts to about 1.1 deg C. The climate scientists have designated a target of a 1.5 deg C increase that we need to stay under if we are to avert catastrophic consequences, but since climate warming is accelerating, we are on track to exceed this target in about 8-9 years. https://www.globalchange.gov/browse/multimedia/global-temperature-and-carbon-dioxide
https://www.carbonbrief.org/explainer-how-do-scientists-measure-global-temperature