The Heat Is Online

The Scientific Case for Human-Induced Global Warming

The 1995 finding by the Intergovernmental Panel on Climate Change of a "discernible human influence" on the global climate was based on several "signature" experiments in which researchers were able to distinguish natural warming from warming due to human burning of fossil fuels. Since that 1995 declaration, subsequent studies have made the case much more robust.


[This list of nine signature studies excludes research on the impacts of climate change as well as findings that simply document the warming. In focusing solely on the scientific case for human-induced warming, the list excludes studies of warming of surface and deep ocean waters, animal, plant and insect migrations, impacts on agriculture and forests, measurements from boreholes, research into glacier retreat, sea level rise, the warming-driven spread of infectious diseases, and measurement of carbon levels which had remained the same for 10,000 years and which are now at levels not seen for the past 420,000 years.]

· In 1995, a team of researchers led by Dr. Benjamin Santer of the Lawrence Livermore Labs examined the pattern of heating in the atmosphere. That pattern of warming -- over land and water and warm and cold areas -- produced a very specific structure of gases in the atmosphere. That structure matches the structure projected by computer models of "greenhouse gas" plus sulfate warming. When the vertical structure of the atmosphere was examined, it was found to be graphically different from the structure produced by natural warming.

Citation: "A search for human influences on the thermal structure of the atmosphere," Nature, Vol. 382, July 4, 1996, B.D. Santer, et al.

· A second "smoking gun" was published in 1995 when a team of scientists at NOAA's National Climatic Data Center verified an increase of extreme weather events in the US. They concluded the growing weather extremes are due, by a probability of 90 percent, to rising levels of greenhouse gases. Those extremes -- which reflect an intensification of the planet's hydrological cycle from atmospheric heating -- are not consistent with natural warming and, instead, resemble the changes that were projected for emissions from fossil fuels. The researchers declared the climate in the US is becoming more "greenhouse-like" -- with more intense rain and snowfalls, more winter precipitation, more droughts, floods and heat waves. It concluded: "[T]he late-century changes recorded in US climate are consistent with the general trends anticipated from a greenhouse-enhanced atmosphere." Citation: Trends in U.S. Climate during the Twentieth Century, Consequences, Spring, 1995, Vol. 1, No. 1, Thomas Karl et al. Also: "The Coming Climate," by Thomas R. Karl, Neville Nicholls and Jonathan Gregory, Scientific American, May, 1997.

·A third contribution to our understanding of the global climate appeared in the spring of 1995 when David J. Thomson, a signals analyst at AT&T Bell Labs, evaluated a century of summer and winter temperature data. While some scientific skeptics had attributed this century's atmospheric warming to solar variations, Thomson discovered the opposite: the accumulation of greenhouse gases had overwhelmed the relatively weak effects of solar cycles on the climate. He also discovered that since the beginning of World War II, when accelerating industrialization led to a skyrocketing of carbon dioxide emissions, the timing of the seasons began to shift. Since 1940, he wrote in the journal, Science, the seasonal patterns "of the previous 300 years began to change and now appear to be changing at an unprecedented rate."

Citation: "The Seasons, Global Temperature and Precession," by David J. Thomson, Science, Vol. 268, April 7, 1995; also, "Dependence of global temperatures on Atmospheric CO2 and solar irradiance," David J. Thomson, 1997

Since the IPCC's 1995 declaration, a succession of studies have strengthened the case for human-induced global warming.

· In 1997, a research team led by David Easterling of NOAA's National Climatic Data Center found the night-time and winter-time low temperatures are rising nearly twice as fast as the daytime and summer-time high temperatures. Easterling called the findings a "fingerprint" study of "greenhouse warming." The research was based on data from 5400 observing stations around the world. "The rise in [minimum-temperatures] is due to higher humidity and more water vapor, especially in the winter in northern latitudes of the Northern Hemisphere. In an increasingly 'greenhouse' world this is the kind of rise you’d expect to see," Easterling said. He added that If the warming were natural, and not driven by fossil fuel emissions, the high and low temperatures would more or less rise and fall in parallel.

Citation: "Temperature Range Narrows between Daytime Highs and Nighttime Lows," Science, July 18, 1997, David Easterling et al.

· In 1998, researchers examining weather records for the previous 600 years declared that 1997 was the hottest year at least since the 1400s. Using written records and information gleaned from tree rings, ice cores and coral reefs, researchers reconstructed the world's climate record for the past 600 years. The record revealed that the warmest years in that span were 1997, 1995, and 1990. Michael Mann and Raymond Bradley, of the University of Massachusetts, and Malcolm Hughes, of the University of Arizona, examined the correlation between temperature changes and other factors such as volcanic activity and variations in the sun's brightness. Those other factors showed a strong relationship with temperatures in earlier centuries, 'but during the 20th century, with its abrupt warming, there is little relationship between any of the natural factors we looked at' and the rising temperatures, Mann said. In the past century, he said, 'we see a remarkable correlation with carbon dioxide emissions, which swamps these natural factors.'

Citation: "Global-scale temperature patterns and climate forcing over the past six centuries, Nature, April 23, 1998, No. 392 pp 779-787, Michael E. Mann, Raymond S. Bradley & Malcolm K. Hughes

· Mann, Bradley and Hughes followed their Nature study with another climate reconstruction from the year 1000 AD to 1998. They found that the decade of the 1990s was the hottest in the last millennium. Strikingly, the team found that the planet had been undergoing a slight but steady cooling trend from 1000 to about 1880. That trend was abruptly reversed as temperatures began to rise rapidly in tandem with large-scale industrialization based on our use of coal and oil.

Citation: "Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations" Geophysical Research Letters, March 15, 1999, Volume 26 Issue #6 Pages 759-762

·In June, 1999, British researchers examined the planet's temperature record between 1900 and 1998. In particular, they assessed the relative roles of four "climate forcing" components -- solar irradiance and stratospheric volcanic aerosols (which occur naturally) and greenhouse gases and sulfate aerosols (which are generated by human fossil fuel combustion). The team, led by Simon Tett of the Hadley Centre in the U.K., found: "The temperature changes over the 20th century cannot be explained by any combination of natural internal variability and the response to natural forcings alone." A commentary in Nature concluded: "The researchers’ findings were unambiguous…All in all, it seems we can lay to rest the idea that recent climate warming is just a freak of nature."

Citation: Nature, Vol. 399, pp 569-572 10 June 1999



· An analysis of the climate of the last 1,000 year published in the July 14, 2000 issue of Science suggests that human activity is the dominant force behind the sharp global warming trend seen in the 20th century. The study, by Dr. Thomas J. Crowley, a geologist at Texas A&M University, found that natural factors, like fluctuations in sunshine or volcanic activity, were powerful influences on temperatures in past centuries. But he found that they account for only 25 percent of the warming since 1900. The lion's share, he said, can be attributed to human influences, particularly to rising levels of carbon dioxide and other heat-trapping "greenhouse gases" that come from the burning of fuels and forests. These twin lines of evidence provide further support for the idea that the greenhouse effect is already here," Dr. Crowley wrote in describing the work in today's issue of the journal Science. Several climate experts said his findings offer the most direct link yet between people and the 1.1 degree rise in average global temperature over the last 100 years.


Citation: "Causes of Climate Change Over the Past 1000 Years," Dr. Thomas J. Crowley, Science, 14 July 2000, v. 289


·In March, 2001, researchers found a significant increase between 1970 and 1997 in the amount of CO2 and Methane in space -- which had migrated out from earth's atmosphere. The study was published in the journal Nature.


Citation:Nature, 15 March, 2001, v. 410, pp. 355-357 "Increases in greenhouse forcing inferred from the outgoing longwave radiation spectra of the Earth in 1970 and 1997"

John E. Harries, Helen E. Brindley, Pretty J. Sagoo & Richard J. Bantges

·In 2005, a team of researchers, lead by NASA's James Hansen, found that the earth had become a "net importer" of heat -- due to the build-up of human-generated carbon dioxide and other greenhouse gases.  Hansen called the study a "smoking Gun" proof that humans are changing the climate.  

 "Earth's Energy Imbalance: Confirmation and Implications," Science, Vol 308, Issue 5727, 1431-1435, 3 June 2005