and other gases help the atmosphere retain more heat from the sun warming the planet This concept reached broad acceptance in the scientific community relatively recently Throughout the 1900s scientists wondered whether increased levels of carbon dioxide and other gases could actually raise global temperatures They debated whether humans could really change the climate and if so how significantly What Is the Hockey Stick Graph In 1998 Michael Mann of the University of Massachusetts decided to follow the evidence Along with Raymond S Bradley also of the University of Massachusetts and Malcolm K Hughes of the University of Arizona he analyzed data for ancient climates in the Northern Hemisphere The scientists used data from fossil tree rings ice cores and ancient corals These objects can be studied measured and analyzed to determine the approximate temperatures during the time they formed They combined this ancient data with modern thermometer readings collected since the mid-1800s The result was a continuous reconstruction of Northern Hemisphere temperatures going back 1,000 years Mann and his colleagues published their report in the scientific journal Geophysical Research Letters One key part of the report was a graph showing average Northern Hemisphere temperatures over the last millennium It soon became known as the hockey stick graph Temperatures were relatively flat for the first 900 years This made up the long handle of the hockey stick In the last century and especially in the last several 8
9 decades the graph spiked sharply upward creating the stick’s blade The graph was featured in the 2001 report of the Intergovernmental Panel on Climate Change IPCC a scientific body set up by the members of the United Nations It became an iconic illustration of human influence on climate Mann summarized its significance The graph told a simple story that a sharp and highly unusual rise in atmospheric Mann has become a major public face for the issues surrounding climate change
10 warming was occurring on Earth Furthermore that rise seemed to coincide with human-caused increases in greenhouse gas levels due to the burning of fossil fuels 3 Proxy Data What and Why To understand how the climate operates scientists must understand that changing climates have existed throughout Earth’s history But while Earth is 4 54 billion years old humans have been directly collecting weather data for less than 200 years 4 Data from ice cores can be used to infer temperatures from the distant past
11 The only way scientists can understand older climates is to study objects that remain from ancient times and that have recorded some aspect of the climate These natural recorders provide proxy data In general a proxy is an indirect measurement that serves as a substitute or representative Here the proxy data stands in for thermometer data Mann’s graph was based on proxy data from the past 1,000 years combined with actual temperature measurements when available Much of the proxy data consisted of tree ring data from various locations and ice cores from Greenland and Peru Mann’s first graph created in 1998 went back 600 years and included more than 100 data sets A revised version made in 1999 extended back a full millennium However only 12 tree ring and two ice core data sets were available for the years before 1400 5 Piecing together information on ancient climates is difficult Some data sets are sparse and data becomes less available the farther back in time one goes The authors carefully described the graph’s limitations and uncertainties They also validated the proxy data by comparing proxy data with direct thermometer data wherever possible WHAT PROXY DATA TELLS US Each type of proxy data leads to temperature estimates in a different way Annual tree rings vary in width and density according to temperature and precipitation Trees may live for hundreds or thousands of years Ocean and lake sediments build up in layers the deepest layers are the oldest Sediment cores are analyzed for pollen grains shells and chemicals Coral skeletons are composed of calcium carbonate Oxygen isotopes in the calcium carbonate vary depending on the water temperature in which the coral grew Ice cores from the Greenland and Antarctic ice sheets date back hundreds of thousands of years The temperature estimates are determined by studying pockets of air trapped within the cores and analyzing the oxygen isotopes in each layer
