Any rational response to climate change involves first knowing what the facts and evidence are. That is the province of science. For example, how do we know the ocean is warming? We now measure the increase in ocean heat content from an array of about 4,000 autonomous floats deployed throughout the world ocean under an international program called Argo. They are programmed to rise and sink by changing their volume. This is accomplished by pumping fluid into or out of a bladder on the float. The floats store the measurements, and then, when they are on the surface, they locate by GPS and transmit the stored data via satellites to scientists. The Argo floats have revolutionized our ability to observe the oceans. Argo data are available to everyone for free in near real time. The science is never complete. There is always more to learn. But the science that we have now is already good enough to help us make wise decisions. “Everybody is entitled to his own opinions, but not to his own facts.” Sound science can inform wise policy.

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.