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Aspects of the Cryosphere

Aspects of the Cryosphere. Meto 401 Warren.J.Wiscombe@nasa.gov. Why study polar regions?. Sea level change from melting Repository of paleoclimatic information Best view of abrupt climate change Unequivocal & integrative signals of climate change Polar amplification

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Aspects of the Cryosphere

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  1. Aspects of the Cryosphere Meto 401 Warren.J.Wiscombe@nasa.gov

  2. Why study polar regions? • Sea level change from melting • Repository of paleoclimatic information • Best view of abrupt climate change • Unequivocal & integrative signals of climate change • Polar amplification • “Canary in the coal mine” Meto 401

  3. International Polar Year: 50th Anniversary of International Geophysical Year (IGY) Meto 401

  4. Meto 401

  5. SHEBA • the first extensive sea ice experiment in 30 yr • since AIDJEX, in which I was also involved • an icebreaker for a year, frozen in the Arctic ice • best modern instruments • helicopter, aircraft, under-ice robotic surveys • I had a tethered balloon there! • confirmed what U.S. nuclear submarines were seeing: dramatic thinning of pack ice • Arctic Oscillation? Meto 401

  6. SHEBA: Approach Meto 401

  7. Ice Station SHEBA • Year long icebreaker drift • 2 Oct 1997: 75 N, 142 W • 11 Oct 1998: 80 N, 166 W • Total drift ~ 2800 km • As crow flies ~ 800 km Meto 401

  8. Seasonal Evolution of Pack Ice Meto 401

  9. Net mass loss during SHEBA year! Meto 401

  10. Ice–Albedo Feedback July: Variegated darker surface April: Uniform white surface Meto 401

  11. Evolution of Average Surface Albedo Gradual seasonal change + Abrupt synoptic change Meto 401

  12. SHEBA: Summary • Albedo evolution: smooth seasonal decrease + discrete synoptic changes • Decrease in average albedo from about 0.9 to 0.4 • Spatial variability of albedo increased during melt • Clouds • In winter: significant (11 C) warming associated with appearance of clouds. • In summer: clouds still had a net warming effect. • The blanket beat the umbrella. • In Fall 1997 the ice cover was thinner than expected; much to our surprise it was even thinner in the fall of 1998 Meto 401

  13. Movie: Thawing Arctic SeaIce Will open the fabled Northwest Passage (summer only) Meto 401

  14. The Two Basic Climate States: Nonglacial and Glacial (Polar Ice Caps) • Continental drift, sealevel change, and CO2 all contributed to planetary temperature shifts on the million-yr timescale • Glacial states are marked by multiple expansions of continental ice sheets into midlatitudes. • Fluctuations on time scales of tens of thousands of years • Fluctuations of ice sheets in last 3M yr correlate with orbitally-induced changes in solar radiation distribution with latitude • terrestrial feedbacks (ice sheet dynamics, ocean temperatures, CO2) are responsible for a significant modulations of the orbital signal Meto 401

  15. We keep bouncing back from ice onslaughts Snowball Earth? Meto 401

  16. Descent into our Ice Epoch Snowball Earth? Meto 401

  17. Our Current Ice Age -- the last 3M yr • Some say it began when the isthmus of Panama closed, cutting off communication between the world oceans and forcing the Gulf Stream to carry vast amounts of warm water northward where it evaporated and helped create the ice sheets • Previously the northern ocean had been too cold to cause enough snowfall to grow ice sheets • Antarctica starting glaciating ~10-30M yr ago, and its chilling effect added to the decline • Main cause of Antarctic glaciation may have been falling CO2 (formerly thought to be opening of Drake Passage by continental drift) Meto 401

  18. Within the current 3M-yr ice age... • there have been ~ 8 glacial periods of roughly 90K yr duration, separated by short interglacials of roughly 10K yr duration • the glacial-interglacial temperature curve is like a sawtooth — slow fall into a glacial, rapid rise out of it (with occasional backslidings) • before the latest 8 glacials, were ~16 others of different character Meto 401

  19. Our understanding of ice ages has increased dramatically since 2004 • … because of the Dome C ice core, which goes back almost 800K yr, or almost 8 glacial cycles • until then, we only had the Vostok ice core, which went back almost 400K yr • based on the Vostok core, we should have been sliding back into a glacial period by now (our 10K-yr interglacial should have been over) • BUT… the Dome C record shows the interglacial most analogous to ours (in terms of Earth’s orbit) last 28K yr, not 10K yr Meto 401

  20. Current Ice Age, 21K to present Movie: Ice Age Map • Asia, Africa, S. America remarkably unglaciated • N. America got the worst of it, by far • No way to migrate into N. America using Bering Strait land bridge until ~13K yr ago • some anthropologists say humans were in N. America 20K yr ago! Movie: Ice Age Bering Strait Meto 401

  21. Milankovitch Theory (1920s) states that cyclical variations in three elements of Earth-Sun geometry combine to produce variations in the amount of solar energy that reaches Earth; these variations are enough to start ice sheets growing or shrinking Earth’s Orbital Mechanics 1. eccentricity —elongation of orbit; 100K-yr period, roughly Meto 401

  22. Earth’s Orbital Mechanics — 2 2. Obliquity — the angle that Earth's axis makes with the plane of Earth's orbit. 40K yr period, roughly. 3. Precession — the Earth's axis of rotation behaves like the spin axis of a top; it traces a circle on the celestial sphere over a period of time. Meto 401

  23. Movie: Precession of Earth axis • 21000-yr period, roughly • Moon has kept Earth from tilting too much (there is a theory that Mars, lacking a big moon, tilted as much as 90 deg in the past) Meto 401

  24. Orbital Elements Have Periodic Behavior lending hope that ice ages might be predictable! Meto 401

  25. Earth’s Orbital Mechanics — 3 • Using these three orbital elements, Milankovitch formulated a mathematical model that calculated latitudinal differences in sunlight and the corresponding surface temperature for 600K yr prior to 1800. He then correlated these changes with the growth and retreat of the Earth’s Ice Ages. • His first monograph on the subject in 1920 was alternately ignored and refuted for about 50 years. Finally, in 1976, a study by Imbrie et al. published in Science examined deep-sea sediment cores over 450,000 yr to seemingly confirm Milankovitch’s theory. • The theory is still controversial, however, because no climate model has been able to create ice ages using the Milankovitch forcing (the changes of sunlight distribution caused by orbital variations) Meto 401

  26. The Smoking Gun for Milankovitch (from Imbrie paper) Fourier spectrum of past 0.5M yr had peaks at exactly the three periods of Earth orbital variation! Meto 401

  27. Ocean Drilling ...has been our main source of information on Ice Age ...confirmed Milankovitch theory of Ice Ages...maybe Sediments don’t go back as far as you’d think: ocean floor is constantly being recycled and is never older a few hundred million yr. Shackleton just won big AGU award for showing in 1970s that global ice volume is related to oxygen isotope fraction in dead plankton (the ones with calcareous (CaCO3) shells) that fall to bottom of ocean. Meto 401

  28. Ocean Cores to 1978 (over 15000) Meto 401

  29. Ocean Core Example Meto 401

  30. Temperature — Last 18K years • it was only ~5C colder at peak of last glaciation • sudden climate shifts are possible • it wasn’t all warming! • the Sun and CO2 may have controlled this, partly Note: Meto 401

  31. Backsliding: Heinrich Event 11K yr ago: parade of icebergs from melting N. American ice sheets chills N. Atlantic Meto 401

  32. Heinrich Events in History (red squares) plot of Sea Surface Temperature (SST) vs time Meto 401

  33. Prof. Ruddiman (U Va) says... • Humans have kept the planet out of the next ice age by doing two things: • deforesting, starting 8K yr ago (releases CO2) • growing rice, starting 5K yr ago (releases CH4) • BUT… this was before the Dome C ice core! • He also sees the signature of the Black Plague in the temperature record of the Little Ice Age, and ascribes it to the regrowth of forests after farms were massively abandoned -- i.e., human dieback lowered atmospheric CO2 and cooled the planet Meto 401

  34. Ice Coring Meto 401

  35. Vostok Ice Core: Temperature, CO2, CH4(Antarctic) Meto 401

  36. Greenland and Antarctic ice cores agree Meto 401

  37. How did we get CO2 and temperature so far back? CO2 and CH4 trapped in air bubbles; stays same over eons once encased (which may take 100s of yr) Temperature inferred from oxygen isotopes or deuterium A heroic endeavor — big technology leap to drill that deep and recover core Cores stored in giant Denver freezer, mainly Meto 401

  38. Ice core drilled at Dome C, Antarctica Meto 401

  39. Ice core drilled at Dome C, Antarctica, goes back 720K yr Meto 401

  40. Deuterium values from Dome C ice core show change in nature of interglacials 400K yr ago Meto 401

  41. Comparison of 400K-yr ago interglacial (blue) with current one (red) Meto 401

  42. Temperature — Last 100M years • the last 2-3M yr has been the “Ice Age” • there have been perhaps 8 glaciations in the Ice Age • 50M yr ago it was warm even at the poles! Meto 401

  43. Why did temperature slide downhill? • probably CO2 (and maybe CH4) decreasing; they may control temperature on both short (10K yr) and long (10M yr) time scales • continental drift played some role • recycling C • changing ocean dynamics (Panama closed 4Myr ago) • creating Antarctica • it happened before: • 250M yr ago • 600M yr ago (Snowball Earth?) • 2.4B yr ago (Sun was 15% less luminous) Meto 401

  44. Current Climate Meto 401

  45. Ice-Covered Areas Deceptively Small Meto 401

  46. Where’s the Ice? Greenland and Antarctica contain about 75% of the world’s fresh water, enough to raise sea level by over 73 m. Measurements of ice elevations have been made by satellite radar altimeters for parts of the polar ice sheets. Now, the IceSAT laser altimeter will provide more accurate measurements and not miss the poles like earlier satellites did. Meto 401

  47. Polar View Complete opposites: Arctic is landlocked sea-ice ocean, Antarctic is sea-locked land ice sheet Meto 401

  48. Ice and Snow Strongly Affect Climate • During Northern Hemisphere winter, they • blanket up to 15% of the Earth’s surface and • (b) reflect up to 80% of the Sun’s radiant energy back to space. • During the Southern Hemisphere winter, they cover about half this area. • But their influence is far larger than their areal coverage would indicate. Meto 401

  49. Global Warming and the Cryosphere Global warming should decrease Earth’s snow and ice cover, which in turn would increase the absorption of solar radiation, amplifying the warming (a positive feedback). The meltback of sea ice would also increase the ocean heat flux to the atmosphere, again amplifying the warming. It would also increase the vapor flux to the atmosphere, which might enhance clouds. What would that do? We don’t really know. Global Climate Models (not all...) have been predicting amplified warming in the Arctic since Manabe in late 1960’s. But...the observational data were too sparse to tell. Meto 401

  50. News on the Polar Warming Front a 40% thinning of Arctic sea ice,1950s to 1990s a 4%/decade reduction in the surface area of sea ice a 0.6C/decade temperature increase since the 1960s in the high northern latitudes marked warming trend on Antarctic Peninsula, faster than the planet as a whole. But Antarctica as a whole is not warming… Some monster Antarctic icebergs have calved recently...but also we are monitoring them better. Greenland is warming, but has only reached the warmth of the 1930s now. Meto 401

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