U22A-01 INVITED
Milestones and Lacunae in Quaternary Paleoclimatology
It has been just over 40 years since Nick Shackleton submitted his PhD thesis on, 'The Measurement of Palaeotemperatures in the Quaternary Era'. Only a few years earlier, Libby was awarded the Nobel Prize for his work on radiocarbon dating. Looking back, we recognize that these were seminal events which provided essential insight and tools for generations of future researchers, opening the window to our interpretation of the earth's recent history. Research in paleoclimatology and paleoceanography has made enormous advances since these early steps were taken, and our understanding of how climates have changed, and why, has exploded. Hardly a week goes by without a new and interesting record or model simulation being published. Yet gaps remain, and new questions continue to emerge. New analytical techniques provide higher and higher resolution data sets, yet chronology remains a challenge in many records. This is especially important in deciphering times of abrupt change in earth history, when the synchronism of geographically dispersed events (or lack thereof) is of critical importance. The role of abrupt climate change in driving societal change is also controversial. Certainly there is evidence from many regions for abrupt, unprecedented and persistent climate anomalies for which we commonly have no explanation, and such episodes appear to have had significant effects of societies in the past. Deciphering the causes of such episodes, and how they affected societies has important implications for our understanding of the past and the future. Understanding the role of forcing and feedbacks is also essential. For example, many questions remain about the role of solar forcing. If small changes in solar irradiance have driven climate changes (as many have argued) large feedbacks must be involved. Modelling may help in resolving such questions. Many new proxies have been developed, though often our understanding of how these relate to climate is rudimentary at best. In fact, this is true even for some of our most cherished proxies. Improvements in the calibration of these proxies, through both mechanistic (process-based) studies and modeling will pay dividends and help avoid misinterpretations and the pursuit of archives that may not yield useful results. Paleoclimatologists and paleoceanographers have made spectacular discoveries over the past 40 years. Although anthropogenic effects will increasingly dominate the climate system in coming decades, establishing a firm understanding of pre-anthropogenic climate variability is still an essential challenge: whatever anthropogenic climate changes occur in the future, they will be superimposed on, and interact with, underlying natural variability. Therefore, to anticipate future changes, we must continue our efforts to understand how and why climates varied in the past.
U22A-02 INVITED
An Imminent Revolution in Modeling Interactions of Ice Sheets With Climate
Modeling continental ice sheets was inaugurated by meteorologists William Budd and Uwe Radok, with mathematician Richard Jenssen, in 1971. Their model calculated the thermal and mechanical regime using measured surface accumulation rates, temperatures, and elevations, and bed topography. This top-down approach delivered a basal thermal regime of temperatures or melting rates for an assumed basal geothermal heat flux. When Philippe Huybrechts and others incorporated time, largely unknownpast surface conditions had a major effect on present basal thermal conditions. This approach produced ice-sheet models with only a slow response to external forcing, whereas the glacial geological record and climate records from ice and ocean cores show that ice sheets can have rapid changes in size and shape independent of external forcing. These top-down models were wholly inadequate for reconstructing former ice sheets at the LGM for CLIMAP in 1981. Ice-sheet areas,elevations, and volumes provided the albedo, surface topography, and sea-surface area as input to climate models. A bottom-up model based on dated glacial geology was developed to provide the areal extent and basal thermal regime of ice sheets at the LGM. Basal thermal conditions determined ice-bed coupling and therefore the elevation of ice sheets. High convex ice surfaces for slow sheet flow lower about 20 percent when a frozen bed becomes thawed. As further basal melting drowns bedrock bumps that "pin" basal ice, the ice surface becomes concave in fast stream flow that ends as low floating ice shelves at marine ice margins. A revolution in modeling interactions between glaciation, climate, and sea level is driven by new Greenland and Antarctic data from Earth-orbiting satellites, airborne and surface traverses, and deep drilling. We anticipate continuous data acquisition of surface albedo, accumulation/ablation rates, elevations, velocities, and temperatures over a whole ice sheet, mapping basal thermal conditions by radar, seismic, and magnetic profiling, and direct measurement of basal conditions by deep drilling and coring into the ice and the bed. These data allow calculating the geothermal heat flux and mapping flow of basal meltwater from geothermal sources to sinks at the termini of ice streams, which discharge up to 90 percent of the ice. James Fastook has a preliminary solution of the full momentum equation needed to model ice streams. Douglas MacAyeal has pioneered modeling catastrophic ice-shelf disintegration that releases "armadas" of icebergs into the world ocean, to extract heat from ocean surface water and thereby reduce the critical ocean-to-atmosphere heat exchange that drives global climate. Ice sheets are the only component of Earth's climate machine that can destroy itself-- swiftly--and thereby radically and rapidly alter global climate and sea level.
U22A-03 INVITED
Orbital Forcing of Monsoons and Ice Sheets
Two great planetary-scale systems dominate orbital-scale climate change: low-latitude monsoons and high- latitude ice sheets. Oxygen-isotopic signals from accurately dated cave calcite deposits show that the Asian monsoon system varied mainly at the 22,000-year precession cycle and with a phase of August 1. This evidence confirms the orbital monsoon hypothesis (now theory) of John Kutzbach: low-latitude insolation forces monsoon strength, with subsequent amplification from climate-system feedbacks. In contrast. the current understanding of ice-sheet variations is incomplete. Milankovitch's theory of summer insolation forcing has been confirmed in part, but discrepancies remain. High-latitude summer insolation has strong power at the 22,000-year precession period, but early glaciations (2.75-0.9 Myr ago) varied mainly at the 41,000-year obliquity (tilt) period, and subsequent glaciations occurred within a broad band of power near the ~100.000-year eccentricity band. Recent competing explanations for these discrepancies between insolation forcing and ice-responses include interhemispheric cancellation of opposite-phased 23,000-year ice volume responses, prevalence of 41,000-year insolation forcing in indices that account for changes in length of day, and strong CO2 feedback at the 41,000-year period. A major uncertainty is whether independent CO2 variations arise within the climate system and force ice sheets or whether ice sheets drive their own positive CO2 feedback.
U22A-04 INVITED
Modeling of Past Climates: Some Perspectives
Important new ideas related to modeling of past climates go hand in hand with new observations, with advances in our understanding and ability to represent physical and biogeochemical processes, and with advances in computer capacity and speed. Important first steps in quantitative climate modeling using energy balance models were underway in the early 20th century. Dynamical climate models began to be used to study past climates in the 1970s and 1980s, with a focus first on the atmosphere, and then on coupled models of atmosphere and upper ocean. In the past decades, coupled dynamical models include atmosphere, global ocean, vegetation, cryosphere and carbon cycle components. This astonishingly rapid development in modeling potential has been greatly facilitated by the rapid increase in computational power. Equally important is the rapid development of more diverse, accurate and worldwide observations of present and past environments from land, lakes, oceans and ice. The topics of early, more recent, and current research on modeling of past climates come from a diverse range of ideas about the mechanisms that might force fundamental changes in climate – for example: changes in greenhouse gases, changes in insolation caused by orbital changes, changes in land-sea distribution, changes in orography, and changes in ocean gateways. Past and current research on these topics, using climate models, illustrates the process and the progress. Certain fundamental principles of modeling and analysis have been important in the past, are important now, and most likely will continue to be important. These principles will be enumerated. Looking toward the future, new observations, improved models and even faster computers are to be expected. But there will also be new challenges: intermodel comparisons and analysis and correction of model bias, understanding feedback processes, understanding non-linear responses, understanding the response to combinations of forcing, and studying variability and abrupt change. Another huge challenge will be to represent, with the climate models, the processes that create the environmental records of past climate: if that goal is reached, then the 'output' of the model will be simulated environmental records that can, in turn, be compared directly to observed environmental records. In most cases, that kind of comparison of simulations and observations isn't possible now. What's past is prologue.
U22A-05 INVITED
Shifting Rainfall; A Paleo Perspective
Twenty years ago my concern regarding the impacts of the ongoing CO2 buildup were centered on the ocean's conveyor circulation. Would the predicted increase in rainfall and runoff lead to a sudden shutdown? In the meantime, model simulations have made clear that this is highly unlikely. Of late my thoughts have shifted to another aspect - the hydrologic cycle; i.e., the prediction of Held and Soden that precipitation will be increasingly focused on the tropics with a consequence of that the earth's drylands will become ever more parched. While no adequate warm analogue exists allowing a direct paleo check on this prediction, the record from glacial time is certainly entirely consistent with this scenario. In a colder world, the tropics were less wet and the drylands much less dry.
U22A-06 INVITED
The third climate state: Proterozoic pan-glacial events
In Climate Through the Ages (1926), C.E.P. Brooks distinguished two discrete climate states in the
Phanerozoic, non-glacial (no continental ice sheets) and glacial-interglacial (1-4 continental ice
sheets). It now appears that pan-glacial climate states (ice sheets on all continents) existed near the
beginning and end of the Proterozoic eon, around 0.64, 0.71 and 2.22 Ga. Their existence is inferred from
(1) integrated paleomagnetic and sedimentologic evidence for tidewater glaciers at paleolatitudes
<20°, (2) submarine ice grounding-line wedges within carbonate-dominated successions, (3) synglacial
banded iron-formations, and (4) distinctive syndeglacial cap carbonates. Pan-glacial models envision
tropical oceans either ice-covered (snowball) or ice-free (slushball). The latter are biologically more
benign but do not explain the iron-formations or cap carbonates. Glacioeustatic changes of ~1.5 km
provide fresh support for the 0.64-Ga pan-glacial event and new evidence for high pCO2 from
boron, calcium, carbon and sulfate triple-oxygen isotopes in cap carbonates favors the snowball option.
Microfossils and biomarkers suggest that the advent of multicellular animal life was nearly coincident with the
same pan-glacial event. Some view this as favoring the slushball option, while others infer that the
environmental stress of a snowball drove the biological revolution. Future research directions include
causation in light of pronounced negative δ13C excursions that precede the two younger pan-
glacial events, geochemical cycling in variably ice-covered low-pH oceans, apparent hyperfast geomagnetic
field reversals during the last pan-glacial event, atmosphere-ocean dynamics attending deglaciation,
depauperate planktonic microbiota between the younger pan-glacial events, and the 1.5-billion-year non-
glacial interval following the 2.22-Ga event. The author thanks geologist-paleomagnetist Edward (Ted) Irving
for bringing to his attention the book by Brooks.
http://www.snowballearth.org