The measurements on which we base our estimates of the Earth’s global average surface temperature are contaminated by several serious sources of error. One of them is sampling error: We have not made measurements uniformly in space and time with identical instruments at identical locations on Earth over the whole 100-plus years of record. About 70% of the Earth is covered with ocean, and the Southern Hemisphere is nearly all ocean. For many years, few measurements were made at sea because people do not live there. And even within the remaining 30% that is land, there are vast areas (imagine the great ice-covered areas of Antarctica and Greenland) where very few people live and very few measurements were made until relatively recently. Satellite measurements and data from the Argo program, discussed earlier in this book, provide greatly improved ocean data. In general, the atmosphere over the land has warmed more than the atmosphere over the ocean, and largely because of this fact and the uneven distribution of land and ocean, the warming has not been the same in the two hemispheres.