Early weather stations only covered 20 per cent of the globe, causing them to underestimate how much our greenhouse gas emissions have warmed the climate
Source:
https://www.newscientist.com/article/2208726-climate-mistake-reveals-earth-warmed-more-than-we-thought-last-century/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home