Several independent types of evidence show that the Earth is warming.Temperature records: reliable thermometer readings began in the mid-19th century (from about 1850). Records from weather stations, ships and, since the 1970s, satellites show the mean global temperature has risen by about 1.1 °C, with most of the rise since 1970. These records only go back about 170 years, so to look further back scientists use indirect (proxy) evidence.Dendrochronology is the use of tree rings to date events and study past climate. Each year a tree lays down one growth ring; in warm, wet years the ring is wider. Counting back from the bark dates each ring, and its width indicates the conditions in that year. Matching overlapping ring patterns in living trees and old timber extends the record back thousands of years.Pollen in peat bogs: pollen grains are tough, have a shape characteristic of each plant species, and are preserved in the acidic, anaerobic conditions of peat. Peat builds up in layers, so deeper layers are older, and they can be dated by radiocarbon dating. Identifying the pollen in each layer shows which plants grew at that time. Each species grows only in a certain climate, so a change from cold-tolerant birch and pine pollen to warmth-loving oak and lime pollen indicates warming.
Ice cores, covered in the next lesson, provide a still longer record. Confidence that the Earth is warming is high because these independent sources agree.
Ice cores, covered in the next lesson, provide a still longer record. Confidence that the Earth is warming is high because these independent sources agree.