Global warming
Before 1850, there were not enough temperature measurements for us to know how warm or cold it was. Climatologists measure other things to try to figure out past temperatures before there were thermometers. This means measuring things that change when it gets colder or warmer. One way is to cut into a tree and measure how far apart the growth rings are. Trees that live a long time can give us an idea of how temperature and rain changed while they were alive.