Temperature records are information about air and water temperatures at specific times and places. Without such records, little could be said about the history of Earth’s climate.
Most temperature records are either proxy records or instrumental records. A proxy is something that stands for something else: Proxy records are natural deposits or features that record information about temperature at the time that they were formed. Past temperatures are not directly recorded by proxies, but can be deduced from them. For example, the amounts of different isotopes of oxygen and hydrogen in snow that fell thousands of years ago in Antarctica and Greenland record the temperature of the air at the time the snow fell, since heavier isotopes fall in snow at a higher rate when the air is colder. By studying ancient snow layers, climatologists can gather information about ancient temperatures.
Instrumental records are readings from instruments that have been written down. Since about 1880, instrumental records from thermometers have been kept in many parts of the world. Since the 1970s, the instrumental record includes satellite observations of the entire planet.
Historical Background and Scientific Foundations
The temperature at any one place on Earth’s surface at any given time is the result of many causes, including short-term weather events and long-term climate trends. Earth’s average, long-term surface temperature depends on how much energy the planet receives from the sun and how much it radiates back out into space. Since the mid-1800s, human beings have been altering this energy balance, mainly by adding greenhouse gases (carbon dioxide, methane, and others) to the atmosphere and by destroying forests. This has caused Earth’s average temperature to rise, the phenomenon known as global warming.
Global warming is known through a combination of proxy temperature records and instrumental records. Instrumental records reveal Earth’s average temperature with high precision over recent decades. For years prior to about 1850, historical non-instrumental records of temperature-related events such as freezing rivers are also a source of information about temperatures. The main source of pre-1850 temperature information, however, is proxy records. These include tree rings, layers of sediment (muck) on ocean and lake bottoms, snow layers preserved in ice sheets, and others. By far the longest continuous proxy records are found in ice cores, cylindrical samples of ice drilled from the depths of the ice sheets of Greenland and Antarctica, which are several miles deep in places. Each year’s snowfall has been preserved as a thin, distinct layer of ice in these ice sheets. In 2004, a record of snowfall layers was recovered from Antarctica that goes back 800,000 years.
Proxies must be interpreted, and interpretation is not perfect. Therefore, temperature records derived from different proxies do not always agree exactly. However, temperature records derived from various proxies all agree on the general temperature history of Earth over the last millennium or two: The warming seen in the last century is unprecedented, with high probability. An earlier warming period around the year 1000, called the Medieval Warm Period, was neither as sudden nor as intense as today’s warming.
The temperature record over the last one or two thousand years, if plotted with temperature on the vertical axis and time on the horizontal axis, is a wiggling but more or less level line from at least 1,300 years ago until about 100 years ago, with a sudden, unprecedented rise
WORDS TO KNOW
HOCKEY STICK GRAPH: A chart of global average air temperatures indicating that recent climate warming is unprecedented for at least the last 1100 years or so. Critics argued the graph was flawed and misleading, but in 2006, after a careful review of the evidence, the US National Academy of Sciences affirmed that the graph is essentially accurate, though its shape is less certain for earlier dates.
IN CONTEXT: TEMPERATURE RECORDS
Recent World Meteorological Organization (WMO) reports provide an extensive portrait of the increasing severity of weather activity around the globe. The report noted not only higher average temperatures worldwide, but wide variations in rainfall, with some regions experiencing flooding and others plagued with drought. The WMO report from 2005 echoed similar themes, with higher-than-average temperatures resulting in what was then the second warmest year on record. The year 2005 also brought a record number of strong Atlantic hurricanes, which some scientists claim are due to higher ocean temperatures brought on by global warming and climate change.
at the very end of the graph—recent, observed, global warming. Since the overall shape of this curve resembles the outline of a hockey stick on its side with its blade pointing up, such a curve is often called a hockey-stick graph.
Impacts and Issues
Throughout the 1990s, skeptics of global warming noted that surface instrumental temperature records did not agree well with satellite records measuring temperatures higher up in the atmosphere. How, they asked, if the satellite and surface records did not agree, could climatologists say with any confidence that the world was warming? As the U.S. Climate Change Science Program stated in 2005, “previously reported discrepancies between the amount of warming near the surface and higher in the atmosphere have been used to challenge the reliability of climate models and the reality of human-induced global warming. Specifically, surface data showed substantial global-average warming, while early versions of satellite and radiosonde data showed little or no warming above the surface.” However, the scientists of the Climate Change Science Program concluded that the discrepancy “no longer exists because errors in the satellite and radiosonde data have been identified and corrected. New data sets have also been developed that do not show such discrepancies.”
Another climate controversy erupted in 1998, when Michael E. Mann, Raymond S. Bradley, and Malcolm K. Hughes published an article in the journal Nature. The authors reconstructed average air temperatures in the Northern Hemisphere from 1500 to the present from various proxies and presented a hockey-stick graph of average temperatures in the Northern Hemisphere from 1500 to the present day. A version of the graph appeared in the Intergovernmental Panel on Climate Change’s third Assessment Report in 2001 and in former U.S. Vice President Al Gore’s Academy Award-winning docmentary An Inconvenient Truth (2006). Scientific critics of the graph, including W. Soon, S. Baliunas, S. McIntyre, R. McKitrick, and Hans von Storch, argued that it was deeply flawed and that the Medieval Warm Period was probably just as warm as our present period. However, the hockey-stick shape is typical of all published reconstructions of average surface temperature over the last 1,300 to 2,000 years.
Moreover, in early 2007, the Intergovernmental Panel on Climate Change reviewed the hockey-stick controversy in detail. The criticisms of Soon, Baliunas, and others were dismissed as having been adequately answered. The panel published a graph overlaying a dozen different reconstructions of average Northern Hemisphere air surface temperature for the last 1,300 years. All the reconstructions, created using various proxy temperature records, agreed that it was at least 66% probable that the twentieth century was the warmest century in the last 1,300 years, and at least 90% probable that the second half of the twentieth century was the warmest 50-year period in the last 500 years. A 2006 study by the National Academy of Sciences also affirmed the basic validity of the hockey-stick temperature record.
North, Gerald R., et al. Surface Temperature Reconstructions for the Last 2,000 Years. Washington, DC: National Academy of Sciences, 2006.
Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.
Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.
Brumfiel, Geoff. “Academy Affirms Hockey-Stick Graph.” Nature 441 (2006): 1032–1033.
Kerr, Richard A. “Politicians Attack, But Evidence for Global Warming Doesn’t Wilt.” Science 313 (2006): 421.
Kerr, Richard A. “Yes, It’s Been Getting Warmer in Here Since the CO2 Began to Rise.” Science 312 (2006): 1854.
Mann, Michael E., et al. “Global-Scale Temperature Patterns and Climate Forcing Over the Past Six Centuries.” Nature 392 (1998): 779-105.
McIntyre, S., and R. McKitrick. “Corrections to the Mann et al. (1998) Proxy Database and Northern Hemispheric Average Temperature Series.” Annual Review of Energy and the Environment 14 (2003): 751-771.
National Oceanographic and Atmospheric Administration. “Global Surface Temperatures
IN CONTEXT: TEMPERATURE RECORDS AND INCREASES IN GREENHOUSE GASES
Globally, all of the warmest years since instrumental measurements of surface temperature began to be recorded (around 1880) have occurred since the late 1980s. Typically, these warm years have averaged about 1.5-2.0°F (0.8-1.0°C) warmer than those during the decade of the 1880s. As of 2007, the average temperature of Earth’s atmosphere near the surface had risen about 1.33°F (0.74°C) since 1906. This warming closely matches increases in greenhouse gases emitted by human activities.
over the Past Two Millennia.” http://www.ncdc.noaa.gov/paleo/pubs/mann2003b/mann2003b.html (accessed May 12, 2008)
U.S. Climate Change Science Program. “Temperature Trends in the Lower Atmosphere—Understanding and Reconciling Differences [Executive Summary].” http://www.climatescience.gov/Library/sap/sap1-1/finalreport/sap1-1-final-execsum.pdf (accessed May 12, 2008).