GW不是比较你底下理解的“平均温度”，而是比较一个叫做
anomaly的量。这个anomaly是某地某时刻的温度测量值与过去
30年平均值的差别。这反映的是时间趋势。把全球各个观测站
平均就是global趋势，正值说明变热，凡则反之。至于这个
量的合理性估计有大量专业文章，我就懒得看了。

如何给你的系统定义一个“平均温度”我想也是个有意思的
物理题目。如何定义应当跟你所要讨论的问题有关，也就是说
对你考虑的问题是否relevant。

https://www.climate.gov/news-features/understanding-climate/climate-change-global-temperature

“The concept of an average temperature for the entire globe may seem odd. After all, at this very moment, the highest and lowest temperatures on Earth are likely more than 100°F (55°C) apart. Temperatures vary from night to day and between seasonal extremes in the Northern and Southern Hemispheres. This means that some parts of Earth are quite cold while other parts are downright hot. To speak of the "average" temperature, then, may seem like nonsense. However, the concept of a global average temperature is convenient for detecting and tracking changes in Earth's condition over time.

To calculate a global average temperature, scientists begin with temperature measurements taken at locations around the globe. Because their goal is to track changes in temperature, measurements are converted from direct temperature readings to temperature anomalies—values that represent the difference between the observed temperature and the long-term average temperature for each location and date.”