The Antarctic Ice Sheet and Global Warming

Transcription

1 6th Annual S.T. Lee Lecture in Antarctic Studies INTRODUCTION and WELCOME Professor Pat Walsh Vice-Chancellor Victoria University of Wellington S. T. LEE LECTURE Through a Crevasse Darkly: An Update on the Future of the Antarctic Ice Sheet Professor Richard Alley (via Live Video-Link) Evan Pugh Professor of Geosciences, The Pennsylvania State University, United States of America Dr Lee Seng Tee FORUM Dr Nancy Bertler Antarctic Research Centre Professor Jonathan Boston Institute of Policy Studies Dr Andrew Mackintosh School of Geography, Environment and Earth Sciences Professor Martin Manning Climate Change Research Institute Professor Peter Barrett Antarctic Research Centre and Climate Change Research Institute QUESTIONS

2 Through a Crevasse Darkly An update on the future of the Antarctic Ice Sheet Richard B. Alley, Penn State Please note: I work for Pennsylvania State University, And help UN IPCC, NRC, etc., But I am not representing them, Just me. G. Comer Foundation

3 High scientific confidence Our fossil-fuel burning and other activities are changing the atmosphere, especially by raising CO 2 We know how much we are burning, and we see that CO 2 in the air and ocean We measure other sources and they are MUCH smaller (volcanoes 1-2% of human) We see the fall in oxygen used for burning, and the changes in carbon isotopes that confirm fossil-fuel source.

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5 High scientific confidence Earth s climate is warming in response Data: warming unequivocal --shown by thermometers, including those far from cities, in ground, in ocean, on balloons, looking down from satellites) Physics: CO 2 warming effect unavoidable Forcings: sun isn t getting brighter, cosmic rays not changing, no other plausible cause pushing toward warming Fingerprint: Warming pattern in space and time matches known human forcings, but incompatible with sun, etc.

6 Blue=Nature Only Pink=Humans+Nature Black=What Happened Warming is occurring because of us

7 Antarctic sea ice high; trend not significant Fig. 4.9 Arctic summer sea ice Fig. 4.2 Northern snow cover Fig Greenland ice sheet Fig. 4.15b, Glaciers Fig Antarctic ice sheet

8 7 Muir Glacier, Alaska, August 13, 1941, photo by W.O. Field Photo pairs by Bruce Molnia, USGS, archived at the National Snow and Ice Data Center,

9 Muir Glacier, Alaska, August 31, 2004, photo by B.F. Molnia

10 ??? 9??? Rise to come Rise so far Year IPCC, 2001

11 6.4 o C 4.0 o C 2.4 o C Warming so far Warming to come (world continues past 2100 ) Future warming could be large

12 CO2 in atmosphere has followed IPCC-linked projections closely. Temperature rise has been a bit larger than central projections, but well within error bars. Sea-level rise has been well above central projections, and barely within error bars. Rahmstorf et al., Science, 2007

13 Warmer Has global warming stopped? Here are temperatures from 1998 to 2004, from GISTEMP. No warming there, right??? Year

14 13 Actually, global warming is clearly continuing. Be careful of cherrypicking, and weather. Previous slide Climate usually a 30-year average, for good reasons! Source: Gavin Schmidt, NASA GISS

15 Previous slide High emissions scenarios might in just over a century give the warming shown in this cartoon

16 IPCC on Ice Sheets 2001: Noted large uncertainties, but suggested central estimate of slight net growth over next century, with snowfall increase in cold parts exceeding melting increase in warmer parts, and with little ice-flow change 2007: Ice sheets now shrinking, in part because of ice-flow change in response to warming Models used to date do not include the full effects of changes in ice sheet flow, because a basis in published literature is lacking understanding of these effects is too limited to provide a best estimate or an upper bound for sea level rise. (IPCC WG1 AR4 SPM 2007)

17 Model-based projections of global average sea level rise at the end of the 21st century ( ) are shown in Table SPM-3. For each scenario, the midpoint of the range in Table SPM-3 is within 10% of the TAR model average for The ranges are narrower than in the TAR mainly because of improved information about some uncertainties in the projected contributions 15. {10.6} 15 TAR projections were made for 2100, whereas projections in this Report are for The TAR would have had similar ranges to those in Table SPM-2 if it had treated the uncertainties in the same way. [p. 15]

18 Model-based projections of global average sea level rise at the end of the 21st century ( ) are shown in Table SPM-3. For each scenario, the midpoint of the range in Table SPM-3 is within 10% of the TAR model average for The ranges are narrower than in the TAR mainly because of improved information about some uncertainties in the projected contributions 15. {10.6} 15 TAR projections were made for 2100, whereas projections in this Report are for The TAR would have had similar ranges to those in Table SPM-2 if it had treated the uncertainties in the same way. [p. 15]

19 Darker orange is more-or-less what would be under water if just West Antarctica (~5 m), or just Greenland (~7 m), melted. We lack a generally accepted worst-case scenario, but 20 m over many centuries might be defensible. New Zealand clearly has vulnerabilities, but is not the biggest worry on Earth

20 I know of no credible estimate of meters of sea-level rise in decades, but we might within decades warm enough to commit us to meters or more over centuries.

21 Accelerating mass loss from Greenland. S Alley et al., 2007, Ann. Glac., +Shepherd&Wingham, 2007, Science IPCC: warming-induced icesheet growth next century

22 S Accelerating mass loss from Antarctica? R R R Alley et al., 2007, Ann. Glac., Shepherd&Wingham, 2007, Science; Rignot et al., Nature, 2008 IPCC: warming-induced icesheet growth next century

23 All piles tend to spread under own weight: Strong things resist spreading (a block of wood), but weak things spread easily (pancake batter); Lubrication speeds spreading (pancake batter spreads faster on a greased griddle than on a waffle iron); Supports oppose spreading (a flying buttress keeps a cathedral from spreading and falling apart).

24 What controls size of Antarctic ice sheet? Scott (1905)--ice shrank when cooling reduced snowfall. Wrong. Ice bigger in colder, less-snowfall past; Penck (1928)--ice shrank when melting of Canadian ice floated Antarctic edges, reducing friction and speeding flow. Widely accepted, looking inadequate; Now, looks like warming melts ice. Not great news in a warming world

25 Warming increases mass-loss from self-lubricating ice sheets In places, ice sits on a waterand-mud-lubricated pancake griddle, in other places on a bumpy bedrock waffle iron ; can be mapped through 3 km of ice, but job far from done; In places, ice is selflubricating --surface meltwater plunges to bottom to make it more slippery, so warming may bring faster flow, but depends on griddle vs. waffle iron character; This matters, but not as much as worst hype, and not much in Antarctic (yet) Zwally et al., 2002, Science

26 Ice sheets have flying buttresses, too Floating extensions called ice shelves --ice flows over water for a while before breaking off to make bergs; Ice shelves may run aground on islands or scrape past rocky sides of bays; Friction from this slows ice-sheet spreading; Warming air or water can attack ice shelves quickly, speeding ice-sheet spreading and sea-level rise.

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29 Antarctic Peninsula (gothic cathedral) Larsen B Ice Shelf (flying buttress) Melt ponds Island Ocean Island Icebergs 12 mi 20 km January 31, 2002

30 January 31, mi 20 km

31 March 7, Then tributary sped up 8x. 12 mi 20 km

32 Shepherd et al., 2004, GRL 1 o C warming gives 10 m/year more sub-ice-shelf melt. So ice shelves expected to be very sensitive to even small climate changes.

33 Shepherd et al., 2004, GRL Strong thinning of ice shelves receiving more ice (faster flow); increased basal melting from warmer waters implicated. Leading (fuzzy) hypothesis (e.g., WALSE, 2007)-- increased Ekman divergence from positive-phase AAO/SAM increased warm Circumpolar Deep Water delivery. Thinning 5 m/yr Balance 100 km

34 Removing little ice shelves lets ice go faster. Would removing big ice shelves let lots more ice go faster? Yes Anandakrishnan et al., 2003, GRL This is motion of ice stream D, flowing into Ross Ice Shelf. When tide falls (~1 m), the push back on the ice stream is reduced a little, and the ice stream velocity doubles. Over on ice stream B, the rising tide stops the ice, with all motion when tide falling. VERY sensitive.

35 A good reason for ANDRILL Penck thought rising sea level would float the ice-sheet edge and speed flow; But the rising tide slows the ice. Why? The ice dumps sediment under the ice shelves, and friction with sediment slows the ice; Ice shelf starts where the sediment ends (the ice flows off a minor sediment cliff ); (We need to know about ice-sediment interactions, and similar things, one of many reasons to study the history of that sediment) (ANDRILL is NZ+US+UK+Italy+Germany program coring in Ross Embayment to study that sediment)

36 Another good reason for ANDRILL ANDRILL, earlier work (Scherer et al., 2008; Naish et al., 2008, etc.) shows Antarctic ice has responded to Antarctic temperature Millions of years ago, before much ice in north, Antarctic ice still changed a lot; ~1 million years ago (MIS31), the Ross Embayment was warm with reduced ice during peak southern sunshine, when north was cold with reduced sunshine; Together, point to southern control, not to sea-level control from north.

37 Temperature control of Antarctic ice sheet--model Here, tripling CO 2 above pre-human level triggers East Antarctic melting. Other models need more CO 2 for that. For range of models and uncertainties, may or may not be enough fossil fuels to affect East Antarctic. D. Pollard and R.M. DeConto, 2005, Global and Planetary Change 45, 9-21.

38 Antarctic temperature control Antarctic Peninsula changes at least partly from meltwater in crevasses breaking shelves--may get much meltwater on big shelves this century (Oppenheimer & Alley, 2004, Clim. Change); West Antarctic sub-ice-shelf melting increase from ocean circulation change water involved has been in deep ocean for a while (not heated by humans yet); Possible human influence in wind changes bringing up warmer waters; Suspect human heat will show up in future

39 Linking Southern Ocean to ice sheet central Models do a lot of things really well, but not yet Southern Ocean (see next slide on sea ice ); Global models terminate before reaching ice shelves, so can t tell us how much heat will be carried under the shelves to melt them; A lot of work to get ocean/sub-iceshelf/ice-shelf/ice-stream/ice-sheet coupling right.

40 March sea ice Red is real. Gray to green is model range. September sea ice Red is average sea-ice extent (15% concentration). Of the 14 IPCC models considered, gray is the model with the smallest sea-ice extent, and light green is the model with the largest sea-ice extent. This is not reassuring about model skill in the Southern Ocean. (Figure 8.10, IPCC WG1 AR )

41 Future of Antarctic ice sheet? It is big, old and cold; But it has changed much, growing with cooling and shrinking with warming; Whole-ice-sheet full-physics model projections not yet available; Simple scalings suggest at least centuries to lose a lot of ice; But might trigger loss within decades.

42 Some thoughts on futures Economic analyses of greenhouse-gas emissions tend to indicate that damages will cost more than solutions, and that beginning investment now is economically optimal (Nordhaus; IPCC); These analyses tend to omit events such as ice-sheet collapses because of poor scientific understanding; Previous projections for Antarctic ice change (slight growth) are more optimistic than most possible outcomes; And are looking less and less likely.

43 Synopsis LOTS of work to do! Antarctic ice sheet probably losing mass now, because of flow changes; Comprehensive models didn t project this; Warming above & below ice shelves probably most important issue, but poorly understood; Probably centuries or more to lose an ice sheet, but might trigger loss within decades; And might give sea-level rise notably faster than in IPCC table. Recall the IPCC s words to the wise: understanding too limited to provide a best estimate or an upper bound for sea level rise (IPCC WG1 AR4 SPM). 42

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