MAN-INDUCED CLIMATE CHANGE Wallace S

MAN-INDUCED CLIMATE CHANGE wallace S. broecker Wallace Smith Broecker received his PhD in geology from Columbia University in 1958, joined the faculty in 1959, and, since 1977, has been Newberry Professor of Earth and Environmental Sciences at Columbia’s Lamont-Doherty Earth Observatory. His major research interest has been the ocean’s role in climate change. He was among the pioneers in radiocarbon and isotope dating, and the first person to recognize and name the Ocean Conveyor Belt. A prolific researcher, teacher, and author, he has published over 400 scientific articles and is the author or coauthor of several textbooks. His most recent book, The Great Ocean Conveyor: Discovering the Trigger for Abrupt Climate Change, published in spring by Princeton University Press. In 1979, he was elected to membership in the National Academy of Sciences. He is also a member of the American Academy of Arts and Sciences, a Fellow of both the American and European Geophysical Unions, and a Foreign Member of the Royal Society. Among numerous other awards, in 2009 he received the Frontiers of Knowledge Award in Climate Change from the BBVA Foundation. 174-185_PAL.indd 174 19/11/09 19:41:50 INTRODUCTION 1.2 FIGURE 1 A A) Diagram reproduced from my 1975 Sci- Nearly everyone on the planet has by now en- ence paper. The solid curve is the global change (°C) 0.8 temperature record available to me at that countered the words “global warming.” They Manmade co2 plus Camp Century cycles time. The dotted curve is my guess regard- appear almost daily in our newspapers and on ing the temperature rise attributable to man- television. Dozens of books on this subject and Global temperature 0.4 made CO2. The light dashed curve is my countless magazine articles appear each year. Obs. global temperature guess regarding the natural fluctuations in global temperature based on the 80- and I hold the perhaps dubious distinction of hav- 180-year cycles seen in the oxygen isotope ing been the first to use them in print. In 1975, 0.0 record from the Camp Century Greenland Manmade co I published a paper in Science entitled “Climate 2 ice core. The bold dashed curve is the sum of my two guesses. change: Are we on the brink of a pronounced camp century 18o CYCLES -0.4 B) The updated global temperature record. global warming?” In it I offered an explanation Although my prediction turned out to be as to why, despite a continuing rise in the at- yEAR 1800 1850 1900 1950 2000 qualitatively correct, the 80- and 180-year mosphere’s CO content, Earth temperature re- cycles on which it is based have not shown 2 up in any other long climate record. mained very nearly constant from 1940 to 1975. 0.6 B My hypothesis was that an extension of the 80- 0.4 and 180-year periodicities in air temperature re- anomaly (°C) corded in the Camp Century Greenland ice core 0.2 in the centuries before the Industrial Revolution 0.0 suggested that the expected man-induced CO2 warming had, by chance, been compensated by Global land-temperature a natural cooling. Further, if this were the case, -0.2 the Earth was poised for a turn around, for the -0.4 natural cooling was about to turn the corner and yEAR 1880 1900 1920 1940 1960 1980 2000 become a natural warming. If so, nature would join forces with made-made CO2 and the Earth teaus and downturns. Until we rein in the rising would warm. As it turned out, my prediction was greenhouse gases, these natural ups and downs right on. A year after my paper was published, will likely only serve to modulate the steepness the Earth began warming and has continued to of a continuing warming. do so right up to today. But to my chagrin, the 80- and 180-year cycles so prominent in the GREENHOUSE PHySICS record from northern Greenland have not shown up in any subsequent climate record Physics demands that the increases in the con- (including those from ice cores in central and centrations of gases like CO2, which are capa- southern Greenland). ble of capturing quanta of outgoing Earth light, To be sure, the naysayers point to this unex- must warm the planet. The reason is that when plained temperature plateau as support for their these gases reemit the captured energy, only claim that global warming is little more than a one half is sent toward outer space. The other tempest in a teapot. With equal vigor, they point half is sent back toward the Earth’s surface. As to the advances of mountain glaciers that took a result, in order to balance the energy received place during the seventeenth and nineteenth from the Sun, the Earth must compensate by centuries and to the warm condition that allowed emitting more earth light. To do so, the Earth’s the Vikings to colonize Greenland a millennium surface must become warmer. ago as evidence that CO2 is not to blame. Rather, Were CO2 and the other manmade green- they would like to believe that it’s all natural, house gases (i.e., methane, nitrous oxide, perhaps driven by the Sun. CFCs…) the only players, our impact on Earth Those of us who firmly believe that CO2 has temperature would be not nearly so worrisome. warmed and will continue to warm the planet, But a powerful feedback occurs. As the planet look upon these past temperature changes as gets warmer, the vapor pressure of water rises. background fluctuations that will surely con- Because they are so much more abundant than tinue. Further, we believe that the impacts of CO2 molecules or those of any other greenhouse CO2 and other so-called greenhouse gases are gas, water molecules dominate the atmosphere’s not yet large enough to have pushed us beyond ability to capture outgoing Earth light (i.e., in- the reach of these natural fluctuations. But we fra-red rays). For each degree Celsius Earth have likely experienced the last of natural pla- temperature rises, the vapor pressure of water man-induced climate change Wallace S. Broecker 175 174-185_PAL.indd 175 19/11/09 19:41:50 the naysayers increases by 7% and consequently the amount While Lindzen’s reputation as a scientist re- point to this of water vapor in the atmosphere increases by mains intact, his battle against mainstream sci- a similar amount. This mushrooms the prima- ence wears thin. Further, the fact that he will unexplained ry impact of manmade greenhouse gases by a argue with equal forcefulness that there is no temperature factor of about three. So, a doubling of atmo- proof that cigarette smoking is linked to lung plateau as support spheric CO2 content from its 280 parts per mil- cancer makes it tempting to write him off as for their claim lion pre-industrial value to the 560 parts per a contrarian. million value we are likely to reach by this cen- that global tury’s end is projected to warm the planet by AEROSOLS AND CLOUDS warming is little about 3.6°C instead of the 1.2°C rise expected more than a tempest from CO2 alone. Bedeviling attempts to understand what is go- in a teapot. To my knowledge there is only one scientist ing on today and what the future holds are the with unquestioned credentials who denies this impacts of man-made aerosols on the Earth’s ra- water vapor enhancement of the CO2 warming. diation budget. These aerosols both reflect away He is Richard Lindzen, a renowned atmospheric incoming sunlight and absorb outgoing Earth physicist at MIT. While not questioning the ba- light. The sulfur released by burning coal ends sic physics described above, he concludes that, up as light-colored sulfuric acid aerosols, which rather than enhancing, the warming water vapor act primarily as solar reflectors and hence tend will work to reduce it. His argument involves a to cool the Earth. The carbon released during redistribution of the atmosphere’s water vapor; biomass burning ends up in dark-colored soot, he admits that more will be present in the tropi- which, like greenhouse gases, captures Earth cal atmosphere but hypothesizes that less will light and hence tends to warm the Earth. Un- be present in the extra tropical atmosphere (i.e., like greenhouse gases, which have well-defined above its drylands). Upwelling of high moisture- optical properties and are uniformly distributed content air in the tropics creates a dense cloud throughout the atmosphere, aerosols have com- cover that greatly impedes the loss of Earth plex optical properties and are concentrated in light. By contrast, down welling of low moisture the regions adjacent to their sources. Because content air in the extra tropics provides a major of this, a large uncertainty exists regarding their escape hatch for Earth light. So Lindzen would contribution to man-induced climate change. put the extra water vapor where it would be least While opinion has it that cooling by sulfate cur- effective and by drying the air in the extra trop- rently outweighs warming by soot, there is con- ics, he opens the escape hatch even wider. The cern that the ongoing increase in soot emissions problem is that no one, including Lindzen, has will turn the tables and aerosols will soon en- created a fully-fledged computer simulation that hance rather than impede warming. accomplishes this feat. All such simulations In thinking about this one has to keep in lead to a more nearly uniform increase in water mind that as aerosols remain aloft for only days vapor and hence a large enhancement of the to weeks before being purged from the atmo- primary warming.

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