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Global Warming Report, Part 1
Getting warmer?
Global warming is one hot topic--and, in the United States at least, the cause of heated political debate. So, last year, Congress asked the National Research Council--whose members come straight from the National Academy of Sciences, National Academy of Engineering, and Institute of Medicine--to review all the science and answer two questions: How has temperature varied over the last 2,000 years? How certain is the answer to this question?
Last week, the National Research Council's panel of experts published their 142-page response, titled Surface Temperature Reconstructions for the Last 2,000 Years. Here are the highlights. We'll have even more in our next issue.
No Fake Bake
"The Earth warmed by roughly 0.6 degrees Centigrade (1 degree Fahrenheit) during the 20th century, and is projected to warm by an additional ~2-6 degrees C during the 21st century."
"Widespread, reliable instrumental records are available only for the last 150 years or so. To study how climatic conditions varied prior to the time of the Industrial Revolution, paleoclimatologists rely on proxy evidence such as tree rings, corals, ocean and lake sediments, cave deposits, fossils, ice cores, borehole temperatures, glacier length records, and documentary evidence."
"Proxy records are meaningful recorders of environmental variables. . . . The connections between proxy records and environmental variables are well justified in terms of physical, chemical, and biological processes."
"The warming is also reflected in a host of other indicators: glaciers are retreating, permafrost is melting, snowcover is decreasing, Arctic sea ice is thinning, rivers and lakes are melting earlier and freezing later, bird migration and nesting dates are changing, flowers are blooming earlier, and the ranges of many insect and plant species are spreading to higher latitudes and higher elevations."
Consistent Climatic Conclusions
"Large-scale surface temperature reconstructions yield a generally consistent picture of temperature trends during the preceding millennium, including relatively warm conditions centered around A.D. 1000 (identified by some as the 'Medieval Warm Period') and a relatively cold period (or 'Little Ice Age') centered around 1700."
"It can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries. This statement is justified by the consistency of the evidence from a wide variety of geographically diverse proxies."
"Less confidence can be placed in large-scale surface temperature reconstructions for the period from A.D. 900 to 1600. Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900."
"Very little confidence can be assigned to statements concerning the hemispheric mean or global mean surface temperature prior to about A.D. 900 because of sparse data coverage and because the uncertainties associated with proxy data . . . are larger than during more recent time periods."
The Proxies Point to Heat
Tree rings - "Measurements of tree ring parameters from regions where temperature limits tree growth can be used to reconstruct surface temperature. These show that the 20th century warming is unusual since at least 1500."
Marine, lake, and cave proxies - "Annual coral records indicate a warming and/or freshening of surface seawater over the last century at most tropical locations, as well as shifts toward warmer and/or fresher waters during the mid-1800s and between 1920 and 1940."
Ice isotopes - "Analyses of stable isotopes in glacial ice provide records of climate changes at high resolution over long time periods. . . . Isotope records from Tibet and the Andes show that the climate of the 20th century was unusual with respect to the preceding 2,000 years."
Glacier length - "Records of glacier length can be used to infer temperature history. These records show global warming of approximately 0.6 degrees C from 1850 to 1990 and cooler conditions for the prior few centuries. The majority of glaciers in high mountain ranges outside the polar regions have retreated during the last 150 years, primarily as a consequence of warming."
Boreholes - "When the temperature at the ground surface changes, the temperature of the underlying substrate (soil, rock, or ice) will also change in response. . . . Measurements of temperature beneath the ground surface show widespread warming during the most recent century and cooler conditions for the four prior centuries."
Michael Himick and Steve Sampson
June 26, 2006
Getting warmer?
Global warming is one hot topic--and, in the United States at least, the cause of heated political debate. So, last year, Congress asked the National Research Council--whose members come straight from the National Academy of Sciences, National Academy of Engineering, and Institute of Medicine--to review all the science and answer two questions: How has temperature varied over the last 2,000 years? How certain is the answer to this question?
Last week, the National Research Council's panel of experts published their 142-page response, titled Surface Temperature Reconstructions for the Last 2,000 Years. Here are the highlights. We'll have even more in our next issue.
No Fake Bake
"The Earth warmed by roughly 0.6 degrees Centigrade (1 degree Fahrenheit) during the 20th century, and is projected to warm by an additional ~2-6 degrees C during the 21st century."
"Widespread, reliable instrumental records are available only for the last 150 years or so. To study how climatic conditions varied prior to the time of the Industrial Revolution, paleoclimatologists rely on proxy evidence such as tree rings, corals, ocean and lake sediments, cave deposits, fossils, ice cores, borehole temperatures, glacier length records, and documentary evidence."
"Proxy records are meaningful recorders of environmental variables. . . . The connections between proxy records and environmental variables are well justified in terms of physical, chemical, and biological processes."
"The warming is also reflected in a host of other indicators: glaciers are retreating, permafrost is melting, snowcover is decreasing, Arctic sea ice is thinning, rivers and lakes are melting earlier and freezing later, bird migration and nesting dates are changing, flowers are blooming earlier, and the ranges of many insect and plant species are spreading to higher latitudes and higher elevations."
Consistent Climatic Conclusions
"Large-scale surface temperature reconstructions yield a generally consistent picture of temperature trends during the preceding millennium, including relatively warm conditions centered around A.D. 1000 (identified by some as the 'Medieval Warm Period') and a relatively cold period (or 'Little Ice Age') centered around 1700."
"It can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries. This statement is justified by the consistency of the evidence from a wide variety of geographically diverse proxies."
"Less confidence can be placed in large-scale surface temperature reconstructions for the period from A.D. 900 to 1600. Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900."
"Very little confidence can be assigned to statements concerning the hemispheric mean or global mean surface temperature prior to about A.D. 900 because of sparse data coverage and because the uncertainties associated with proxy data . . . are larger than during more recent time periods."
The Proxies Point to Heat
Tree rings - "Measurements of tree ring parameters from regions where temperature limits tree growth can be used to reconstruct surface temperature. These show that the 20th century warming is unusual since at least 1500."
Marine, lake, and cave proxies - "Annual coral records indicate a warming and/or freshening of surface seawater over the last century at most tropical locations, as well as shifts toward warmer and/or fresher waters during the mid-1800s and between 1920 and 1940."
Ice isotopes - "Analyses of stable isotopes in glacial ice provide records of climate changes at high resolution over long time periods. . . . Isotope records from Tibet and the Andes show that the climate of the 20th century was unusual with respect to the preceding 2,000 years."
Glacier length - "Records of glacier length can be used to infer temperature history. These records show global warming of approximately 0.6 degrees C from 1850 to 1990 and cooler conditions for the prior few centuries. The majority of glaciers in high mountain ranges outside the polar regions have retreated during the last 150 years, primarily as a consequence of warming."
Boreholes - "When the temperature at the ground surface changes, the temperature of the underlying substrate (soil, rock, or ice) will also change in response. . . . Measurements of temperature beneath the ground surface show widespread warming during the most recent century and cooler conditions for the four prior centuries."
Michael Himick and Steve Sampson
June 26, 2006