Understanding climate requires long-term measurements
of Earths atmosphere. Direct measurements of global
temperatures have been recorded for only the past 140
years or so. To extend these records back in time, scientists
have learned that certain natural processes preserve indirect
evidence of past atmospheric conditions. Using data from
glacial ice cores, tree rings, lake-bottom sediments,
and ocean corals, they can estimate global temperatures
going back thousands of years.
The graph below shows reconstructed temperature data for
the Northern Hemisphere for the past 1000 years. Instead
of actual temperatures, the graph shows annual temperature
anomaliesdifferences from the average temperature
for each year. The gray lines are error bars, showing
the possible degree of error in each measurement.
! Examine the graph to interpret
how the temperature has changed over the last 1000 years.