APPLES, ORANGES, AND ERRORS Up to this point I have assumed that the global temperature estimates in Fig. 1 are free from errors. But there has been considerable debate over the accuracy of all methods of monitoring temperatures: proxies, thermometers, and satellites. No physical measurement is free of errors, and estimates of global average temperatures are no different. Some scientists have even claimed that there is no such thing as a global average temperature, and that even if there were it would be irrelevant for climate anyway. I disagree. While scientists might never agree on exactly what temperatures would go into such an average, the fact remains that the global distribution of atmospheric and surface temperatures is the largest single influence on how fast the Earth continuously loses radiant energy to outer space in the face of its continuous absorption of energy from the sun. The temperature proxy data have been the most controversial because they are indirect, based on such things as sea sediments and stalagmites in caves, mostly in the Northern Hemisphere. There is simply no way to determine how accurate past temperature reconstructions based on proxies are. That would require many centuries of accurate thermometer measurements, and those do not exist. Even if the proxies provided totally accurate temperature estimates, the low time resolution of the proxy estimates in Fig. 1 (thirty-year averages) must be considered before jumping to conclusions about record warm years. For instance, 1998 is generally regarded as the warmest year for global average temperatures in at least the last 150 years. A few scientists have even proclaimed 1998 to be the warmest in the last 2,000 years, if not longer. But I consider any such statements to be meaningless, like comparing apples to oranges. The proxy data are not good enough to tell us just how warm individual years were, say, during the Medieval Warm Period. So, for example, there is no way to know how much warmer or cooler the year 855 A.D. was compared with the year 854 A. D. If those individual years are embedded in a very warm thirty-year period, it is entirely possible that one or more of them was considerably warmer than the “record” year of 1998. We had daily global measurements from multiple Earth-orbiting satellites in that year, and therefore we have a very good estimate of how much warmer 1998 was than 1997, probably to a precision approaching 0.01 deg. C. But there is no way to know with confidence whether 855 A.D. was warmer than 854 A.D. It is entirely reasonable to suppose–but impossible to prove–that one or more years in the Medieval Warm Period were warmer than 1998. It is easy for scientists to make grand claims when there is no way to prove them wrong. In fact, the time scale of the temperature proxies in Fig. 1, thirty years, is exactly the same as that used by the National Weather Service to determine climatological averages, or “normals.” So, what is regarded as the highest time resolution in the proxy data is the same as the time resolution used to define climatological normal temperatures in the modern instrumental period of record. This further illustrates the absurdity of comparing the warmth of recent years with past centuries when we did not have sufficient measurements to compute accurate global averages on a yearly basis.