temperatures
thermometers are a few hundred years old. they give us the instantaneous temperature. average temperatures from before thermometers can be estimated from things like tree rings and ice cores. tree rings give us a sort of average annual temperature. the tree ring record goes back thousands of years. ice cores give us the average temperature over in the range of decades to centuries. the ice core record goes back some 600,000 years. now if you put all of those measurements on one graph you get the infamous hockey stick. the error of course is there could have been a temperature spike bigger than the current one say 200,000 years ago. but it would have been smeared out in the average. does this mean we can safely ignore the current temperature spike? well no. that conclusion would be just as incorrect. and obviously potentially much more deadly.