Climate researchers use every possible direct and indirect measurement to study the full history of Earth's climate, from the latest satellite observations to samples of prehistoric ice extracted from glaciers. When they focus on changes of the past 100-150 years, they use observations made by modern scientific instruments. Although the mercury thermometer and barometer were both invented in the 1700s, they and other modern instruments were not widely used for weather observations till much later. In the United States, most weather stations were established in the late 1800s.
When scientists focus on climate from before the past 100-150 years, they use records from physical, chemical and biological materials preserved within the geologic record. Organisms (such as diatoms, forams and coral) can serve as useful climate proxies. Other proxies include ice cores, tree rings, and sediment cores. Chemical proxy records include isotope ratios, elemental analyses, biomarkers and biogenic silica. Taken together, these proxies extend our knowledge of past climate back hundreds of millions of years into the past.