Good question …

… ‘Global Temperature’ — Why Should We Trust A Statistic That Might Not Even Exist? – Issues & Insights.

“Discussions on global warming often refer to ‘global temperature.’ Yet the concept is thermodynamically as well as mathematically an impossibility,” says Science Daily, paraphrasing Bjarne Andresen, a professor at the University of Copenhagen’s Niels Bohr Institute, one of three authors of a paper questioning the “validity of a ‘global temperature.'”