Past, Present, and Future Temperatures: the Hockeystick FAQ

1. How do we calculate Earth's temperature over time?

Since the 1880s, surface air and water temperatures have been measured regularly at a sufficient number of stations around the world to calculate a global average temperature each year. Despite rare daily temperature records going as far back as 1741 in Sweden and 1871 in China, it is difficult to accurately reconstruct global average temperatures prior to 1880 because written records are not available in sufficient quantity. To understand climate trends when Henry VIII ruled England, for example, climate experts must rely on biological or physical archives—known as "proxies"—that preserve past temperature. Tree rings, coral skeletons, and glacial ice cores are proxies for annual temperature records, while boreholes (holes drilled deep into Earth's crust) can show temperature shifts over longer periods of time.

2. How do tree rings, ice cores, and other "proxies" help us determine historic temperatures?

The way scientists measure and interpret various proxies depends on the information each proxy provides. For example, to obtain temperature records from tree rings, scientists drill cores into several trees that are growing in a region. They identify site-specific factors that influence tree growth such as temperature, precipitation, altitude, and tree age, and then compare these factors against the width or density of the tree rings over the lifetime of the tree. The scientists then standardize the regional data and remove or adjust for individual tree growth responses that are not related to climatic factors. These can include forest density (trees in an open location face less competition for moisture and light than trees growing in a densely forested area) and tree age (a tree grows differently at the beginning and end of its life).

Once researchers are confident about how local tree growth correlates to air temperature, they then seek out older trees in the region that are preserved (perhaps in a local swamp or lake). By matching rings from trees that partially overlap life spans, scientists can construct a continuous climate record over thousands of years.

3. What is the "hockey stick" graph?

This graph, created by a group of climate researchers in the late 1990s, reflects average Northern Hemisphere temperature changes over the past several centuries. It was the first comprehensive study combining data from many different archives of temperature including tree rings, ice cores, and coral reefs. It demonstrated that Northern Hemisphere temperatures rose sharply during the late 20th century, in marked contrast to the relatively small temperature fluctuations during the previous six centuries. The graph got its name because its shape resembles a hockey stick, with the blade end representing the sharp temperature rise over recent years.

4. Is there legitimate scientific debate about the accuracy of the hockey stick graph?

Yes, but mainly about the details, not the essential point. Temperature fluctuations that predate written records are preserved in natural archives (e.g., tree rings, ice cores, boreholes) with various time periods (e.g., seasonal, annual, decadal). The scientific discussion has focused on the best statistical method for combining these various records to accurately capture temperature fluctuations for the Northern Hemisphere. As is typical of the scientific process, independent teams of researchers have worked to reproduce the results of the "hockey stick" by using their own approaches and even by using slightly different data. These studies sometimes produce slightly higher temperature fluctuations in the past compared with the initial study. But despite their differences, they still yield the same essential conclusion: the past 10- to 20-year period was likely the warmest of the past millennium.

5. How much does our understanding of global warming depend on the hockey stick graph?

The short answer is "very little." The hockey stick graph constitutes only one among literally thousands of pieces of evidence that have contributed to the present scientific consensus on the human influence on global warming. In 2001, the Intergovernmental Panel on Climate Change (IPCC) concluded in its authoritative third assessment report that "there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities." As one climate expert observed: The IPCC report Climate Change 2001: The Scientific Basis is 881 pages in length. It weighs 5.5 pounds and contains over 200 figures and 80 tables. It would be absurd to think that the weight of its conclusions rests on any one figure or table; rather it paints a convincing picture in the totality of its science, as noted succinctly in its title."¹

We are now observing real changes due to higher temperatures. Here are some examples:

  • The Mt. Kilimanjaro glacier, which has survived the past 11,000 years, is currently at risk of disappearing by 2020 if present rates of melting continue;
  • Enormous tracts of Siberian peatlands, with vast stores of carbon, are beginning to thaw and release carbon dioxide and methane into the atmosphere;
  • The Larsen B ice shelf in Antarctica has lost volume as large chunks (some as large as the state of Rhode Island) have recently broken free and melted;
  • The annual surface area of Arctic sea ice has declined eight percent over the past several decades;
  • Large-scale increases in ocean temperatures have been detected over the past 45 years; and
  • Plants and animals are changing their habitation ranges, sometimes dramatically, such as robins and mosquitoes in the Arctic that were previously unknown there.

Antarctic ice core records vividly illustrate that atmospheric carbon dioxide (CO2) levels today are higher than levels recorded over the past 650,000 years (see figure below).  Atmospheric CO2 levels have risen 30 percent in the last 150 years, with half of that rise occurring only in the last three decades. It is a well-established scientific fact that CO2 (and other gases emitted from industrial and agricultural sources) traps heat in the atmosphere, so it is no surprise that we are now witnessing a dramatic increase in temperature.

Compared with other factors that influence climate (including solar variation, volcanic eruptions, and pollutant emissions such as sulfur dioxide), human activities-primarily burning fossil fuels and deforestation-have been a major contributor to climate change over the last 50 years.

Atmospheric carbon dioxide record data sources: Keeling and Whorf (2004), Petit et al. (1999), IPCC (2001), Ahn et al. (2004). Art credits: astronaut, pyramid: (c); car, mammoth: (c)

6. What evidence demonstrates that the recent increase in global temperature is unprecedented?

The National Climate Data Center (NCDC) has maintained global average monthly and annual records of combined land and ocean surface temperatures since 1880, the earliest year for which reliable instrumental records were available worldwide. Based on NCDC data, nine of the top 10 warmest years globally have occurred since 1995. Adding to the evidence of direct temperature measurements, multiple studies by independent teams of researchers indicate that, across the Northern Hemisphere, the 1990s were likely the warmest decade of the past millennium-and possibly the past 2,000 years.

7. How do past climate trends help us understand future temperature?

Reconstructions of past climates help us build accurate projections of how future climates will be affected by global warming. Much as the Air Force builds computer programs to simulate aircraft flight under different conditions, climate scientists build computer programs to help simulate global climate under different conditions. These computer programs, called General Circulation Models (GCMs), use various assumptions about physical, chemical, and biological processes that occur within Earth's atmosphere and oceans and on its land surfaces. To ensure accuracy, each model is checked to see if it generates results that correctly reproduce the past and current climate. Once accuracy is established, the computer program can then be used to explore the likely future climate if, for example, we double the atmospheric concentration of carbon dioxide.


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Arctic Climate Impact Assessment. 2004. Impacts of a Warming Arctic. Arctic Climate Impact Assessment. Cambridge, U.K.: Cambridge University Press. Available at

Ahn J. et al. 2004. A record of atmospheric CO2 during the last 40,000 years from the Siple Dome, Antarctica ice core, Journal of Geophysical Research, 109, D13305, doi:10.1029/2003JD004415.

Barnett, T.P., D.W. Pierce, and R. Schnur. 2001. Detection of anthropogenic climate change in the world's oceans. Science 292:270-274.

Huybers, P., Comment on "Hockey sticks, principal components, and spurious significance" by McIntyre and McKitrick, Geophysical Research Letters (In Press).

Intergovernmental Panel on Climate Change. 2001. Climate Change 2001: The Scientific Basis. Cambridge, U.K.: Cambridge University Press.

Jones, P.D. and M.E. Mann. 2004. Climate over past millennia, Reviews of Geophysics 42(2):1-42.

Keeling, C.D. and T.P. Whorf. 2004. Atmospheric CO2 records from sites in the SIO air sampling network. In Trends: A Compendium of Data on Global Change. Oak Ridge, TN: Carbon Dioxide Information Analysis Center.

Mann M.E., R.S. Bradley, and M.K. Hughes. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations, Geophysical Research Letters 26(6):759-762.

Mann, M. et al. 2003. On past temperatures and anomalous late-20th century warmth, EOS, Transactions, American Geophysical Union, 84: 8.

Meko, D. et al. 1993. Spatial patterns of tree-growth anomalies in the United States and Southeastern Canada, Journal of Climate 6:1773-1786.

Moberg, A. et al. 2005. Highly variable northern hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433:613-617.

National Climate Data Center. 2005. Climate of 2005: June in Historical Perspective. Available at

Petit J.R. et al. 1999. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399:429-436.

Siegenthaler, U. et al. 2005. Stable carbon cycle-climate relationship during the late Pleistocene. Science 310:1313–1317.

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World Meteorological Organization. 2004. WMO Statement on the Status of the Global Climate in 2004: Global Temperature in 2004 Fourth Warmest. WMO-No. 718. Press Release. December 15.

¹Bradley, R.S. 2005. Letter to Representative Joseph Barton (R-TX). July 13. Dr. Raymond Bradley is a University Distinguished Professor at the University of Massachusetts at Amherst.

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