Today: Global Heating.
From 0 a.d. to 1880 temperatures were relatively stable. Afterwards they rose drastically to now almost 1,5°C above the average temperatures before.
We know the temperatures from before 1880 because of tree rings, corals, and ice cores.
The much more important temperature measurements we know because of something called a #thermometer.
Thermometers were not a single invention, but a development. Many however credit Hero of Alexandria (10 a.d. to 70 a.d.) with the invention.
In 1880 thermometers were already very precise and reliable.