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Global Warming

Global Warming. Warren Wiscombe NASA Goddard. Movie GWE-Intro.mov. It’s a hot-button political issue!. Why the big concern?. Because Earth is near its carrying capacity for humans. Slight changes in climate cause massive disruptions in agriculture, commerce, etc.

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Global Warming

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  1. Global Warming Warren Wiscombe NASA Goddard

  2. Movie GWE-Intro.mov Meto 401

  3. It’s a hot-button political issue! Meto 401

  4. Why the big concern? • Because Earth is near its carrying capacity for humans. Slight changes in climate cause massive disruptions in agriculture, commerce, etc. • A unique situation. Before, mankind adapted by • migration • war • population control Meto 401

  5. Aspects of Global Warming • Is it really happening? the “skeptics”… • Carbon cycle (predict future levels of CO2) • Climatic impact (temperature, precip, sea ice, ...) • Range of plausible scenarios (energy use, ...) • Society’s response, if any Meto 401

  6. Is it really happening? - 1 • Atmosphere • global average temperature — up • polar temperature — up even faster • stratospheric temperature — down fast • H2O vapor — up • Ocean • sea surface temperature — up • sea level (and day length) — up Meto 401

  7. Is it really happening? - 2 • Cryosphere • Arctic sea ice — receding and thinning • Mountain glaciers — disappearing • snow line — rising • Growing season — lengthening • The problem is...even though we see these effects, are they: • due to other causes than humans? • due to natural internal variability? Meto 401

  8. The following slides are adapted from a presentation given to Pres. Bush and his cabinet by a NOAA scientist (hence the use of Fahrenheit rather than Celsius).It was so well done that some of the slides were reproduced in the newspapers. Meto 401

  9. Greenhouse Basics – 1 • If an object is bathed in visible light... • it warms up, and... • emits infrared radiation • Water vapor (2%) and CO2 (0.03%) have been part of our atmopshere for millions of years. • Their presence gives us an average surface temperature of ~60° F • Without them, the average would be ~5° F. • This is the natural greenhouse effect. Meto 401

  10. Greenhouse effect under a tree Meto 401

  11. Movie: Greenhouse_Effect.mov Meto 401

  12. Greenhouse Basics – 2 • Greenhouse gases are increasing in the atmosphere, and increasingly trapping more heat • The rise in CO2 is impeccably documented back at least 1000 years (from bubbles in ice cores); it was constant until 1850, since then has risen ~30% • Methane (CH4) has also increased and has 1/3 the effect of the additional CO2 • All of the CO2 increase and most of the CH4 increase are human-caused Meto 401

  13. Greenhouse Basics – 3 • Most indicators point to a warming world • Temperature rise is 0.7-1.4° F over past 100 years • Glacial retreat ubiquitous • Snow-cover decrease • Freeze-free periods lengthened • Sea level rise of 4-8 inches (thermal expansion) Meto 401

  14. Greenhouse Basics – 4 • Most warming over past 30 yr is likely due to greenhouse gas increases • Reasons: comparisons of simulated vs observed temperatures • simulations including both natural and human factors give best match to observations • the correspondences increase with time • probability is loow that a “natural-only” Earth would have such correspondences Meto 401

  15. Greenhouse Basics – 5 • A continued growth in greenhouse gases is projected to lead to very significant increases in global temperatures and sea level • CO2 abundance will likely double before 2100 • cuts in emissions needed just to stabilize there • For a range of future emission scenarios: • global temperature will rise by 2.5–10° F • (if so, this would exceed the natural changes over the past 10,000 years) • corresponding sea level rise: 4–35 inches Meto 401

  16. Greenhouse Basics – 6 • A greenhouse warming could be reversed only very slowly • Reason: the oceans are sluggish • It is a complex planet of which we have imperfect knowledge, so prediction of further details suffers • regional changes cannot yet be predicted reliably • but some projections are likely robust: • land areas warm more than oceans (N. North America 40% above average), and night more than day • increased mid-continental soil drying • more vigorous hydrological cycle Meto 401

  17. Greenhouse Basics – 7 • More extreme events? • likely heavier rains, with more rain in extreme events, and larger variance between regions • hurricanes? don’t know… • Abrupt shifts? • has happened before (but mainly in ice ages) • climate is a non-linear system; such systems are subject to chaos and regime changes Meto 401

  18. Greenhouse Basics – 8 • The vast-majority scientific viewpoint: • the issue is a real one • the first signs of human-caused climate change have likely occurred already • some degree of further change appears inevitable • exactly where, when (rate of change), how much is hard to predict • human-caused climate change would be slow to reverse — 100s of years at the least Meto 401

  19. The Three Sisters: Differing Greenhouses Decrease in temperature with increasing distance from Sun (units of 10^6 km) for non-reflecting planets with no atmospheres. Open circles take account of the actual planetary reflection of sunlight. Solid circles : actual surface temperatures. The length of each dashed line is a measure of the greenhouse effect. Meto 401

  20. Meto 401

  21. History of the CO2 Theoryof Climate Change Warren Wiscombe NASA Goddard

  22. “A great deal has been written about the influence of the absorption of the atmosphere upon the climate.” Was this written by a present-day climate scientist, weary of trying to keep up with the thousands of papers on this subject? No, it is the first sentence of the classic 1896 paper of Nobel-Prize winning chemist Svante Arrhenius. Has the CO2-climate problem, then, riveted the attention of scientists for so long? Far from it. Whole decades passed without the slightest attention being paid to it. It is characteristic of the CO2 theory of climate change that it has repeatedly been incinerated by its critics, only to rise again from its own ashes. Meto 401

  23. Fourier (1768-1830), French mathematician Became interested in dissipation of heat while serving Napoleon in Egypt. Proposed that the Earth’s atmosphere behaves like a “hothouse”. Misunderstood how a hothouse works — assumed that the glass let in the sun's infrared rays and reflected heat back into the structure Meto 401

  24. John Tyndall (1820-1893) Investigated radiant (infrared) heat in 1859. Was first to measure absorptive powers of gases such as H2O, CO2, O3 and hydrocarbons (all greenhouse gases). Demonstrated that H2O, CO2, O3 are some of the best absorbers of infrared radiation. Meto 401

  25. Wrote that without water vapor, Earth’s atmosphere would be “held fast in the iron grip of frost” Meto 401

  26. Tyndall Speculates on Climate “Now if, as the above experiments indicate, the chief influence be exercised by the aqueous vapor, every variation of this constituent must produce a change of climate. Similar remarks would apply to the carbonic acid (CO2) diffused through the air, while an almost inappreciable admixture of any of the hydrocarbon vapors would produce great effects on the terrestrial [infrared] rays and corresponding changes of climate... A slight change in these variable constituents may have produced all the mutations of climate which the researches of geologists reveal.” Meto 401

  27. Svante Arrhenius (1859-1927),Swedish physical chemist Said rising atmospheric CO2 due to human industry. Predicted doubling CO2 would increase average global temperature by ~5°C. Meto 401

  28. Movie: GlobalWarming(1.15).mov Meto 401

  29. Arrhenius’ radiation balance equations — the first climate model Meto 401

  30. Arrhenius anticipated: Role of carbon cycle, including coal-burning (rediscovered 1950s) H2O feedback with fixed relative humidity (rediscovered 1960s) Ice-albedo feedback (rediscovered 1960s) but he lacked: – Heat transport terms in his equations Meto 401

  31. Disbelief Period #1: 1900-1930 CO2 infrared absorption bands are “saturated” H2O absorbs in same IR spectral region (“overlap problem”) Meto 401

  32. Overlapping gas absorption bands are made up of lines which don’t really interfere with one another Gas absorption (normalized) Meto 401

  33. Callendar (1938):Temperature Change vs. CO2 concentration Meto 401

  34. Callendar answers his critics at the Royal Meteorological Society Mr. Dines: “Was Mr. Callendar satisfied the change in the temperature of the air which he had found [in the historical record] was significant, and not merely a casual variation?” Callendar replies: “If any substance is added to the atmosphere which delays the transfer of low temperature radiation, without interfering with the arrival or distribution of the heat supply, some rise of temperature appears to be inevitable in those parts which are furthest from outer space.” Meto 401

  35. Disbelief Period #2: 1940-1970 Rising temperature trend from 1880 reversed around 1940 and a cooling commenced. Nature itself had rejected the CO2 theory. Meto 401

  36. Roger Revelle (1909-1991): CO2 in the Oceans In 1936 Revelle began studying the chemical interaction of CO2 with seawater. He left the subject, returning two decades later (here in 1958) to discover that the absorption of CO2 was taking place at a much lower rate than he or anyone else had thought. His first published statements to this effect were taped on to a manuscript just before publication (revealed from his archives at Scripps). Meto 401

  37. Revelle wrote the now legendary "human beings are now carrying out a large scale geophysical experiment" • 1957: Revelle and Hans Suess, a founder of radiocarbon dating, demonstrated in a famous article published in Tellus that CO2 had increased in the air as a result of the use of fossil fuels. • Revelle chaired National Academy of Sciences Energy and Climate Panel in 1977, which found that ~ 40% of the anthropogenic CO2 has remained in the air, 2/3 of that from fossil fuel, and 1/3 from the clearing of forests. Meto 401

  38. Manabe & Wetherald invented “radiative-convective modeling” (1960’s) (considers whole atmosphere, not just surface) Two positive feedbacks: (1) more H2O vapor in air (~ constant relative humidity) (2) troposphere warms which both radiate more IR to surface. Net result: 4x amplification of bare CO2 effect Meto 401

  39. 1970’s: CO2 a lesser concern; eclipsed by... • – Aerosols: the “Human Volcano” • – Ozone layer in danger: • * spray cans (CFCs) • * supersonic transports • – Nitrogen fertilizers • – Contrail cirrus • – Stratospheric H2O • – Surface albedo changes (desertification) • – Glaciers are coming! (cooling trend continues...) Meto 401

  40. Highlights of the modern period – 1 1975 Ramanathan: CFC greenhouse effect Global cooling stops; warming begins 1978 Clouds begin to eclipse aerosols; Oxford meeting begins 20+ year focus on clouds 1986-9 Hansen says greenhouse warming already here (testimony to U.S. Congress) 1990 IPCC Reports begin IPCC = Intergovernmental Panel on Climate Change Meto 401

  41. Ramanathan’s famous 1976 graph of global surface temperature vs. CFC concentration Meto 401

  42. Fraction of global warming contributed by each trace gas Meto 401

  43. Properties of trace greenhouse gases – 1 Meto 401

  44. Properties of trace greenhouse gases – 2 Meto 401

  45. Highlights of the modern period – 2 1992 Rio Climate Treaty; U.S. signs 1995 IPCC report says anthropogenic greenhouse warming has been detected; attacked 1997 Kyoto agreement; legally binding emission targets for greenhouse gases (developed countries only); U.S. won’t sign 1990’s warmest decade in recorded history SHEBA, submarines find Arctic sea ice disappearing at alarming rate Meto 401

  46. SO2 has no greenhouse effect of its own... but it converts to sulfate particles in the atmosphere in a few days … which, being relatively transparent, can cause a cooling by reflecting sunlight back to space • Pinatubo SO2 emissions did this Charlson et al. estimated sulfate aerosols counteracted 20–30% of greenhouse gas warming Human emissions of SO2 are twice natural ones (volcanic and biological “exhalations”) Meto 401

  47. Aerosol studies died after a burst of study in 1960’s, but revived in the late 1980s Big discovery — most aerosol important for climate is anthropogenic; not realized pre-1985 Climate models can only get agreement with measured temperatures by including sulfate aerosols – supposedly solves 40s cooling Aerosol “indirect effect” (aerosol changes cloud drop size and liquid water content and therefore cloud reflection) also revived Meto 401

  48. 1980’s Earth Radiation Budget Experiment (3 satellites) settled an issue dating back to Arrhenius: do clouds warm or cool the Earth? • Answer was...a large net cooling, 15–20 W/m2. That is, cloud solar reflection beats their infrared heat-trapping effect. • Around 1989, a comparison of 19 GCM’s revealed that most of the range of temperature response (2-5 C) to 2xCO2 was due to differing model treatment of clouds. Meto 401

  49. Evidence for Global Warming Meto 401

  50. The warming trend suffered a reversal in 1992 Pinatubo! Meto 401

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