PAST CHANGE

Records of direct climate observations using instruments extend back more than 200 years. These observations are generally more precise and globally extensive than records inferred from tree-, sediment-, ice- and coral cores. Sufficient data from thermometer measurements are available to document the evolving patterns of surface temperature during the past century.

A network of upper-air observatories was established during and after World War II. It provides daily upper-air temperature, wind, and humidity measurements from balloon-borne sensors. The late 1970s marked the advent of climate monitoring from satellite-borne instruments, which provide global coverage.

Ocean observations have been more limited. Ocean surface temperature is commonly measured in ships' water intake valves. Subsurface observations are made by sensors freely suspended in the water, dropped from the surface, or tethered to the bottom.

Systematic CO₂ Monitoring

Systematic measurements of atmospheric CO₂ date back to 1957, when the first long-term monitoring station was established at an observatory near the top of Mauna Loa, the highest mountain on the island of Hawaii. The initial plan was to make measurements at closely spaced intervals over an extended period of time in order to determine whether CO₂ concentrations were rising.

An upward trend soon became apparent. This trend, which continues today, is superimposed on a strong seasonal cycle caused by the uptake of CO₂ by plants during the Northern Hemisphere growing season. The CO₂ monitoring network has expanded to over 100 stations, and now also monitors concentrations of other greenhouses gases, such as methane.

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