Short answers, pulled from the story.
Scientists define global surface temperature as the average temperature of Earth's surface at a given time. It combines sea surface temperature with near-surface air temperature over land weighted by their respective areas.
A series of reliable temperature measurements in some regions began in the 1850 to 1880 time frame. This era marks the start of what scientists call the instrumental temperature record.
To estimate data in the distant past, proxy data can be used from tree rings, corals, and ice cores. Proxy reconstructions extending back 2,000 years have been performed but reconstructions for the last 1,000 years are supported by more independent data sets.
The global average and combined land and ocean surface temperature show a warming of 1.09 °C from 1850, 1900 to 2011, 2020. Land temperatures have warmed by 1.59 °C while sea surface temperatures have warmed by 0.88 °C over the same period.
Some of the temperature variations over this time period may also be due to ocean circulation patterns such as El Niño or La Niña. Major volcanic eruptions like Mount Pinatubo in 1991 produce aerosols that cause global cooling effects for one to three years after the eruption.