ENVIRONMENTAL LITERACY TEACHER GUIDE SERIES Changing Climate A Guide for Teaching Climate Change in Grades 3 to 8 A Look at Climate in the Past 4 by Nicole D LaDue and Lindsey Mohan limate scientists commonly as tree rings, ice core, pollen or coral Global Temperature show graphs that depict Earth’s samples, and so on, that provide Records C temperature thousands of years scientists with a window into the Anyone who examines instrumental ago. Students may wonder how scientists past. While humans may not have climate records will notice that the can possibly know what Earth’s climate been around, or may not have had earliest records of temperature are was like in the past when no instruments sophisticated instruments to measure from the mid- to late-1800s. Before were available to measure it. climate, living organisms have retained that time, very few reliable scientific Processes such as annual growth their own records that we can study. instruments existed. In order to extend in trees and accumulated layers of In this chapter we take a closer our understanding of climate any snow have preserved natural records look at one of the most controversial farther into the past we must look of climate conditions throughout reconstructions of past climate. This at natural processes that recorded Earth’s history. This chapter is about graph is now known as the hockey stick. changing climate conditions. Scientists how scientists read and interpret After discussing both instrumental refer to this data as proxy data. A proxy historical records of climate and how records and reconstructions such as is something that can be measured they reconstruct these records to the hockey stick, we take a closer look that fluctuates with the changing communicate about our past. These at two types of proxy data collected temperature and is used to understand reconstructions involve collecting by scientists—ice-core and tree-ring climate variations in the absence of samples from the present day, such proxy data. actual temperature data. 58 A Look at Climate in the Past GRADE STANDARD EEI UNIT from natural records are called proxy data. The word proxy means “something Grade 3 3 3 d Living Things in Changing Environments that stands in for another thing or serves 3 5 as a substitute.” By comparing records Grade 4 4 6 produced by several processes, scientists have reconstructed the climate history Grade 5 5 2 f for many different regions, and for Earth 5 6 as a whole. 5 1 1 Grade 6 6 1 3 The Hockey Stick 6 7 One of the most notable representations from the climate science field is known Grade 7 7 4 e as the hockey-stick graph. The hockey- 7 4 g Extinction: Past and Present 7 7 stick graph has been used to show that our recent warming is happening and Grade 8 8 9 that it is caused by human activities. In 1999, a scientific paper came out that led Perhaps the easiest process to rings that were produced under known to one of the biggest modern scientific understand how nature can form a record conditions, scientists can determine the controversies. Michael Mann, Raymond of conditions is the growth of annual climate conditions that the trees lived Bradley, and Malcolm Hughes published tree rings. During years that conditions through. a paper that gathered several sets of of temperature and precipitation are Several other natural processes proxy data for global temperature and conducive for tree growth, trees produce fluctuate with changes in temperature demonstrated that temperatures started relatively wide annual rings. Conversely, and preserve those records within rising at a faster rate in the 20th century when the combination of temperature physical structures. Scientists have (Mann et al. 1998, 1999; for summary see and precipitation is too high or too low, learned to unravel clues recorded in Appell 2005). This line graph has a gentle trees will produce a narrower annual layers of ice, corals, and stalactites in slope, indicating a slowly declining trend growth ring. By examining ring widths caves to determine past temperatures. in temperatures from 1000 through 1900. from many trees and comparing them to Temperature data that are determined The graph takes a sharp upward turn CHAPTER OVERVIEW Climate scientists use data from tree rings, layers of ice, and other sources Student Thinking: to reconstruct past climates. While we have no instrumental records of this The Mauna Loa Graph 62 time, natural processes that record climate conditions can provide us with Pictures of Practice: information about our past. Mauna Loa Carbon-Dioxide Layers of snow that fell during winters and did not melt away during Records 63 summers accumulated into thick ice sheets on Greenland and Antarctica. Case Study: When scientists drill into these ice sheets and retrieve cores of ice, Ice-Core Proxy Data 64 they analyze water molecules from the layers to determine the global Case Study: temperature when that snow fell. Layers of ice also contain tiny air bubbles Tree-Ring Proxy Data 66 that were trapped in the snow. Analyses of these air samples reveal the relative abundance of gases in the atmosphere when the snow fell. Like ice Pictures of Practice: Evidence of Past cores, tree rings also show a record of annual climate cycles. The thickness Climate Change 68 of tree rings can be affected by temperature, rainfall, availability of sunlight, carbon dioxide, soil pH, wind, cloud cover, and nutrition. Scientists use this information to help reconstruct the variations in climate that the tree has lived through during its lifetime. A Look at Climate in the Past 59 with a steep slope indicating increasing accusations by members of U.S. Congress as gray ranges above and below the blue temperatures for the past 100 years. The that the scientists may have had ulterior reconstruction line. upward turn occurs just a bit after the motives for financial gain by publishing More Certainty With beginning of the Industrial Revolution, their data. The congressmen asked the Instrumental Records. While when humans started adding large National Academies of Sciences to create controversy around the hockey- amounts of CO₂ to the atmosphere by a committee to investigate the statistical stick graph centered on proxy data, burning fossil fuels. The graph is called methods that the scientists had used. The instrumental temperature records the hockey-stick diagram because it committee did find some errors in the in present day indicate that global resembles the shape of a hockey stick. statistical analysis but, with corrections, temperature has risen by 0.74 The graph appeared in the 2001 confirmed all of the major findings of degrees Celsius in the last 100 years. Third Assessment Report from the UN the paper. Nevertheless, the controversy Multiple proxies have shown that Intergovernmental Panel on Climate has continued as statisticians, climate the global temperature was cooling Change (IPCC). The IPCC report scientists, and the public have picked at until the Industrial Revolution. Also, represents the scientific consensus about details of the findings. instrumental measurements of carbon climate change. Since some people Because the graph includes dioxide at the Mauna Loa Observatory expect significant economic impacts temperature estimations from proxy data, on Hawaii since the late 1950s show a 30 from regulating CO₂ production as we it involved a known range of possible percent increase in the concentration of decrease our use of fossil fuels, policy error. Proxy data allows us to estimate atmospheric CO₂ since that time. makers and others became greatly temperature changes, but problems with When interpreting climate-science concerned that climate change had been data collection and incomplete records graphs, students may be confused connected to human activity. Many can lead to imperfect data. Scientists have on a couple of points. Even given people, including some scientists, began attempted to take out each set of data to instrumental records, not all scientific questioning Mann, Bradley, and Hughes see if one set was unreasonably changing studies that make estimates of global about their data and even their personal the result and have found that the overall temperature produce the same results. motives for presenting their findings. trends on the graph have remained As the graph Instrumental Temperature The Controversy. The controversy consistent. The possible range of error Records shows, estimates of global surrounding the report included in the hockey-stick diagram shows up temperature do not match exactly across THE HOCKEY STICK The hockey-stick graph includes more recent instrumental temperature data, as well as reconstruction using proxy data. 60 A Look at Climate in the Past INSTRUMENTAL TEMPERATURE RECORDS science studies are pulled together, however, we see a very similar trend, or pattern, among the different studies. Scientists also estimate uncertainty, and many climate-change graphs depict this uncertainty. Oftentimes students take a scientific observation as fact, but the range of estimates on the Instrumental Temperature Records graph shows that observations can be interpreted in different ways. Though each estimate of global temperature is slightly different in the details of how it was calculated, comparing the full range of values across studies reveals patterns that hold true across all the data. The fuzzy gray range on the hockey-stick graph shows more uncertainty regarding This graphs shows several scientific studies of global average temperature temperature prior to the 20th century, during the past 150 years of instrumental records. From IPCC (2007) AR4. with scientists becoming even more certain as instrumental records improved all the research groups who publish such of this variation is due to the different in accuracy. results. During the 1800s, the variation algorithms that research groups use Using instrumental records and proxy between different group’s estimates was to fill in (or omit) missing values over data helps scientists construct models for much greater but even after 1900 results regions where temperature records are climate change.