by Max Schulz



Manhattan Institute/Zogby Survey of Adults Question Frequencies

Manhattan Institute/Zogby Survey of Adults Question X-tabs





Over 70 percent of survey respondents agreed that the Earth’s temperature has risen steadily during the last century, including the last decade. Global temperatures indeed rose over the course of the last hundred years, but the rise was not steady. By most accounts, the Earth’s temperature rose about 0.6 degree Celsius (about 1 degree Fahrenheit) during the twentieth century;[111] and just as the climate has warmed and cooled throughout recorded history, temperatures fluctuated during the 1900s. A Science magazine article reports that two distinct periods of warming—from 1910 to 1945 and again since 1976—were separated by a period of very gradual cooling.[112] Thus, contrary to popular opinion, recent warming did not occur steadily. More recently, satellite data indicate that temperatures have not risen appreciably since 1998 and that temperatures have actually dropped since 2007.[113]

Recent temperature declines are at odds with the warming projected by various computer models. Such models serve as the basis for predictions from the United Nations Intergovernmental Panel on Climate Change (IPCC), whose pronouncements are widely viewed as authoritative. According to the IPCC, “Warming of the climate system is unequivocal, as is now evident from xobservations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level.”[114] Perhaps more widely disseminated than any other projections, the IPCC’s global temperature estimates indicate a major, long-term warming trend. These estimates, in turn, confirm for many that the Earth’s warming is increasing steadily and inform the belief that recent warming is out of the ordinary.

An important question, then, is whether the temperature swings of the twentieth century were atypical. Opinions range from those who feel the twentieth century’s temperature rise is atypically large to those who feel it was just another normal phase in a natural climate cycle. In their 2001 synthesis report, the IPCC stated that “the rate and duration of warming of the 20th century has been much greater than in any of the previous nine centuries.”[115] According to a report from the Australian government’s Department of the Environment and Heritage, “All reliable estimates of Northern Hemisphere temperatures over the past 1000 to 2000 years confirm that the 20th century has been unusually warm.”[116] On the other hand, certain examinations of the geological record indicate that recent temperature changes are well within the range of natural variability.[117] A September 2007 analysis of peer-reviewed literature reports evidence that a natural, moderate 1,500-year climate cycle has produced more than a dozen global warming cycles (similar to the most recent warming cycle) since the last Ice Age.[118] A November 2007 paper examined the temperature records of eighteen locations over a 2,000-year period, concluding that the Medieval Warm Period (roughly the ninth through thirteenth centuries) was 0.3 degree Celsius warmer than the twentieth century.[119]

Just as the climate has warmed and cooled throughout
recorded history, temperatures fluctuated during the 1900s.

To estimate temperatures for the distant past, scientists extrapolate data from proxies, such as tree rings, ice cores, boreholes, pollen remains, glacier lengths, ocean sediments, and changes in the Earth’s orbit.[120] However, according to a recent National Research Council study, “very little confidence can be assigned to estimates of hemisphere average or global average temperature prior to A.D. 900 due to limited data coverage and challenges in analyzing older data.”[121] Such limitations and challenges highlight the difficulties of accurately determining how much our temperature has changed. Yet one thing is certain: climate changes and always has. According to the U.S. Environmental Protection Agency: “From glacial periods (or ‘ice ages’) where ice covered significant portions of the Earth to interglacial periods where ice retreated to the poles or melted entirely—the climate has continuously changed.”[122]

Our limited knowledge and understanding of the myriad intricacies of the Earth’s complex climate system make climate-change discussions necessarily inconclusive.123 “While most scientists agree that anthropogenic [man-made] global warming is a threat, they’re not certain about its scale or its timing or its precise consequences,” writes John Tierney in the New York Times.124 Until our collection of climate data becomes more uniform and reliable, and until our understanding of such data improves, many of our questions about the Earth’s climate will remain unanswered. Clearly, the Earth has warmed since the late nineteenth century, but the key is to judge such warming in historical context, continually refining our interpretation of varying climate data. Moreover, the important task for policymakers is to proceed with caution, in order to avoid implementing dramatic public-policy steps based upon an incomplete understanding of global-climate issues.

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