The modern thermometer was invented in 1654, and global temperature records began in 1880. Climate researchers utilize a variety of direct and indirect measurements to investigate Earth's climate history comprehensively. Direct measurements include data from satellites in space, instruments on the International Space Station, aircraft, ships, buoys, and ground-based instruments.
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. The Earth holds climate clues dating back over three billion years, contained in rock layers, polar ice sheets, lake beds, and more.
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. Collectively, these proxies significantly extend our understanding of past climates, reaching far back into Earth's history.