Climate indicators

The climate system is complex, and a complete description of its state would require huge amounts of data. However, it is possible to keep track of its conditions through summary statistics. There are some nice resources which give an overview of a number for climate indicators. Some examples include NASA and The Climate Reality Project. The most common indicator is the atmospheric background CO2 concentration, the global mean temperature, the global mean sea level, and the area with snow or Arctic sea ice. Other indicators include rainfall statistics, drought indices, or other hydrological aspects. The EPA provides some examples. One challenge has been that the state of the hydrological cycle is not as easily summarised