Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
ABSTRACT: The relation between changes in modern glaciers, not including the ice sheets of Greenland and Antarctica, and their climatic environment is investigated to shed light on paleoglacier evidence of past climate change and for projecting the effects of future climate warming on cold regions of the world. Loss of glacier volume has been more or less continuous since the 19th century, but it is not a simple adjustment to the end of an "anomalous" Little Ice Age. We address the 1961-1997 period, which provides the most observational data on volume changes. These data show trends that are highly variable with time as well as within and between regions; trends in the Arctic are consistent with global averages but are quantitatively smaller. The averaged annual volume loss is 147 mm yr-1 in water equivalent, totaling 3.7 x 103 km3 over 37 yr. The time series shows a shift during the mid-1970s, followed by more rapid loss of ice volume and further acceleration in the last decade; this is consistent with climatologic data. Perhaps most significant is an increase in annual accumulation along with an increase in melting; these produce a marked increase in the annual turnover or amplitude. The rise in air temperature suggested by the temperature sensitivities of glaciers in cold regions is somewhat greater than the global average temperature rise derived largely from low altitude gauges, and the warming is accelerating.
ABSTRACT: Winter and summer mass balance measurements from four French glaciers have been used to assess the sensitivity of mass balance to climatic fluctuations. The sensitivity of summer ablation to temperature is maximum in low-elevation zones (1.4 m water equivalent (w.e.) °C-1 at 1800 m above sea level (asl)) and decreases with altitude (0.5 m w.e. °C-1 at 2900 m asl). As a consequence, the sensitivity of equilibrium line altitude to temperature is 60–70 m °C-1 . This is half the value previously reported in the literature, implying that alpine glacier retreat scenarios for the 21st Century have been largely overestimated. Winter accumulation can be as high as 3 times the amount of precipitation recorded downvalley. These relationships between mass balance and meteorological data were then used to reconstruct the mass balances of these four glaciers back to 1907 using old maps and photogrammetric measurements. Model sensitivity analysis shows that a 25–30% increase in precipitation would compensate a 1°C temperature rise for the mass balances of glaciers. From these results the 20th Century may be divided into four periods: two steady state periods, 1907–1941 and 1954–1981, during which the mass of glaciers remained almost constant, and two deficit periods, 1942–1953 and 1982–1999, marked by a sharp reduction in glacier mass. Regarding mean ablation at 2800 m asl, a 22 W m-2 increase in energy balance is required to explain the ablation difference between the two most recent periods, 1954–1981 and 1982–1999. According to the energy balance analysis the increase in air temperature explains more than 60% of this ablation rise.
ABSTRACT: For more than a century it has been postulated that the Holocene vegetation of western Europe has changed in significant ways. A half-century ago a lively debate revolved on whether there were one or two dry intervals causing bogs to dry out and become forested, or whether instead the climate warmed to a maximum and then cooled. Today none of these climatic schemes is accepted without reservation, because two nonclimatic factors are recognized as significant: the differential immigration rates of dominant tree types (e.g., spruce in the north and beech in the south) brought unexpected changes in forest composition, and Neolithic man cleared the forest for agriculture and thereby disrupted the natural plant associations.
In North America some of the same problems exist. In the hardwood forests of the Northeast, which are richer than but otherwise not unlike those of western Europe, the successive spread of white pine, hemlock, beech, hickory, and chestnut into oak dominated forests provides a pollen sequence that may yield no climatic message. On the other hand, on the ecotone between these hardwood forests and the conifer forests of the Great Lakes-St. Lawrence area, the southward expansion of spruce, fir, and tamarack in the late Holocene implies a climatic cooling of regional importance, although the progressive conversion of lakes to wetlands favored the expansion of wetland forms of these genera.
In the southeastern states the late-Holocene expansion of southern pines has uncertain climatic significance. About all that can be said about the distribution and ecology of the 10 or so species is that some of them favor sandy soils and are adapted to frequent fires. In coastal areas the expansion of pines was accompanied by development of great swamps like Okefenokee and the Everglades—perhaps related to the stabilization of the water table after the early Holocene rise of sea level. The vegetation replaced by the pines in Florida consisted of oak scrub with prairie-like openings, indicating dry early Holocene conditions, which in fact had also prevailed during the time of Wisconsin glaciation.
In the Midwest the vegetation history provides a clearer record of Holocene climatic change, at least along the prairie border in Minnesota. With the withdrawal of the boreal spruce forest soon after ice retreat, pine forest and hardwood forest succeeded rapidly, as in the eastern states. But prairie was not far behind. By 7000 years ago the prairie had advanced into east-central Minnesota, 75 miles east of its present limit. It then withdrew to the west, as hardwoods expanded again, followed by conifers from the north. The sequence easily fits the paleoclimatic concept of gradual warming and drying to a maximum, followed by cooling to the present day. It is supported by independent fossil evidence from lake sediments, showing that lakes were shallow or even intermittently dry during mid-Holocene time.
Here we have a paleoclimatic pattern that is consistent with the record from glaciers in the western mountains—a record that involves a late-Holocene Neoglaciation after a mid-Holocene interval of distant glacial recession. Just as the Neoglaciation is time-transgressive, according to the review of its evidence by Porter and Denton, so also is the mid-Holocene episode of maximum warmth, and they are thus both geologic climate units. The warm episode is commonly termed the Hypsithermal, which, however, was defined by Deevey and Flint as a time-stratigraphic unit that is supposed to have time-parallel rather than time-transgressive boundaries. It was defined on the basis of pollen-zone boundaries in western Europe and the northeastern United States that have a sound biogeographic but questionable paleoclimatic basis. Perhaps it should be redefined as Porter and Denton suggest, as a geologic-climate unit with recognizable time-transgressive boundaries that match the gradual geographic shifts in the general circulation of the atmosphere and the resulting location of storm tracks and weather patterns. Holocene glacial and vegetational progressions provide a good record of climatic change, if one can work out the lag effects related to the glacial economy and the geographic factors controlling tree migration. The terminology for the Holocene, where so much time control is available, should indicate the dynamic character not only of the climate but also of the geologic and biogeographic processes controlled by climate.
ABSTRACT: It is hypothesised that the Medieval Warm Period was preceded and followed by periods of moraine deposition associated with glacier expansion. Improvements in the methodology of radiocarbon calibration make it possible to convert radiocarbon ages to calendar dates with greater precision than was previously possible. Dating of organic material closely associated with moraines in many montane regions has reached the point where it is possible to survey available information concerning the timing of the medieval warm period. The results suggest that it was a global event occurring between about 900 and 1250 A.D., possibly interrupted by a minor readvance of ice between about 1050 and 1150 A.D.
G. D. Thackray (2001). Extensive early and middle Wisconsin glaciation on the western Olympic Peninsula, Washington, and the variability of Pacific moisture delivery to the Northwestern United States. Quaternary Research 55 (3): 257-270
ABSTRACT: Large glaciers descended western valleys of the Olympic Mountains six times during the last (Wisconsin) glaciation, terminating in the Pacific coastal lowlands. The glaciers constructed extensive landforms and thick stratigraphic sequences, which commonly contain wood and other organic detritus. The organic material, coupled with stratigraphic data, provides a detailed radiocarbon chronology of late Pleistocene ice-margin fluctuations. The early Wisconsin Lyman Rapids advance, which terminated prior to ca. 54,00014 C yr B.P., represented the most extensive ice cover. Subsequent glacier expansions included the Hoh Oxbow 1 advance, which commenced between ca. 42,000 and 35,00014 C yr B.P.; the Hoh Oxbow 2 advance, ca. 30,800 to 26,30014 C yr B.P.; the Hoh Oxbow 3 advance, ca. 22,000–19,30014 C yr B.P.; the Twin Creeks 1 advance, 19,100–18,30014 C yr B.P.; and the subsequent, undated Twin Creeks 2 advance. The Hoh Oxbow 2 advance represents the greatest ice extent of the last 50,000 yr, with the glacier extending 22 km further downvalley than during the Twin Creeks 1 advance, which is correlative with the global last glacial maximum. Local pollen data indicate intensified summer cooling during successive stadial events. Because ice extent was diminished during colder stadial events, precipitation—not summer temperature—influenced the magnitude of glaciation most strongly. Regional aridity, independently documented by extensive pollen evidence, limited ice extent during the last glacial maximum. The timing of glacier advances suggests causal links with North Atlantic Bond cycles and Heinrich events.
ABSTRACT: Mapping and radiocarbon dates of cirque moraines in the Sierra Nevada demonstrate that the last significant pre-Little Ice Age glacier advance in the range, the Recess Peak, resulted from snowline lowering roughly twice that of the Matthes (Little Ice Age) advance, and that the Recess Peak advance is late Pleistocene in age. We mapped Recess Peak and Matthes deposits in 64 cirques along a profile of the main Sierran crest that spans the north-south limits of `Neoglacial' deposits in the range. Equilibrium-line altitudes for the reconstructed Recess Peak glaciers vary greatly but coherently with those of the Matthes advance. The variability of both sets of deposits reflects strong topographic influences on snow accumulation and ablation patterns in their deep cirques.
Tephrochronology and radiocarbon dates from lake-sediment cores provide limits on the timing of the two advances. Previous work documenting the absence of a young, regionally extensive tephra on Matthes moraines in the central Sierra demonstrates that they formed after ~70014 C years BP (~650 cal. years BP). The age of the Recess Peak advance has been less certain; we therefore collected and dated sediment cores from lakes dammed behind terminal moraines correlating to the Recess Peak advance in four widely separated drainages along the Sierran crest (north to south): South Fork American River, Lee Vining Creek, Middle Fork San Joaquin River, and Bishop Creek. Twenty-three high-precision AMS radiocarbon dates on gyttja, peat, and macrofossils from the cores are internally consistent and demonstrate that the Recess Peak advance, previously thought to be of late Holocene age (~2500 years BP), ended before 11,190±7014 C years BP (~13,100±85 cal. years BP). Recess Peak is therefore late Pleistocene in age and probably predates the North Atlantic Younger Dryas climatic reversal. The absence of any glacial deposits on the bedrock between the Recess Peak and Matthes deposits indicates that: (1) any advance related to the Younger Dryas event in central California was smaller than the Matthes advance; (2) the Matthes advance was the most extensive, and possibly the only, Neoglacial event in the range; and (3) climate in the Sierra between ~13,000 cal. years BP and 650 cal. years BP was apparently too warm and/or dry to support glaciers larger than those of the Little Ice Age. Other mapping indicates that the Recess Peak is the first significant glacier advance after retreat of Tioga (local late-Wisconsin maximum) glaciers. These results suggest a regionally variable climate in western North America during the Younger Dryas event, because glaciers appear to have expanded in the Canadian Rockies at that time.
The new Recess Peak age limits, combined with other dated lake cores, indicate that the Sierra was essentially deglaciated by 14,000–15,000 cal. years BP (~12,000–13,00014 C years BP), substantially earlier than previously estimated. This finding indicates that current production rates of some in situ cosmogenic nuclides, calibrated on an assumed deglaciation of the range at 11,000 cal. years BP (~10,00014 C years BP), may be systematically too high by as much as 20%.
ABSTRACT: New cosmogenic surface-exposure ages of moraine-crest boulders from southwestern Colorado are compared with published surface-exposure ages of boulders from moraine complexes in north-central Colorado and in west-central (Fremont Lake basin) Wyoming.10 Be data sets from the three areas were scaled to a single10 Be production rate of 5.4 at/g/yr at sea level and high latitude (SLHL), which represents the average10 Be production rate for two high-altitude, mid-latitude sites in the western United States (US) and Austria. Multiple nuclide ages on single boulders indicate that this10 Be production rate yields ages comparable to those calculated with a commonly used36 Cl production scheme. The average age and age range of moraine-crest boulders on terminal moraines at the southwestern Colorado and Wyoming sites are similar, indicating a retreat from their positions 16.836 Cl ka (Cosmogenic ages in this paper are labeled10 Be or36 Cl ka or just ka when both10 Be or36 Cl ages are being discussed; radiocarbon ages are labeled14 C ka, calibrated radiocarbon are labeled cal ka, and calendar ages are labeled calendar ka. Errors (±1s) associated with ages are shown in tables. Radiocarbon ages were calibrated using the data of Hughen et al. (Science 303 (2004) 202). This suggests a near-synchronous retreat of Pinedale glaciers across a 470-km latitudinal range in the Middle and Southern Rocky Mountains. Hypothetical corrections for snow shielding and rock-surface erosion shifts the time of retreat to between 17.2 and 17.510 Be ka at Pinedale, Wyoming, and between 16.3 and 17.336 Cl ka at Hogback Mountain, Colorado.
ABSTRACT: Glacial geology and14 C dating in the central Wasatch Mountains indicate: an early canyon-mouth glaciation (Dry Creek till), probably during isotope stage 6; on that till, a paleosol (Majestic Canyon soil) dated at about 26,000 yr B.P.; overriding that soil, a later canyon-mouth glaciation (Bells Canyon till) probably beginning prior to about 19,000 yr B.P.; a midcanyon deglacial pause (Hogum Fork till) prior to 12,300 yr B.P.; an upper-canyon deglacial pause (Devils Castle till) prior to 7500 yr B.P.; and late Holocene periglaciation. Pollen ratios from bog profiles in the mid to upper reaches of the canyon suggest that temperatures cooler than the Holocene average occurred until after about 8000 yr B.P. Warmer and dryer than average conditions were initiated about 8000 to 7500 yr B.P. During the later portion of this Altithermal period conditions became relatively warm and wet. Two subsequent episodes of cooler than average temperatures correspond chronologically to the initial stades of Neoglaciation elsewhere in the Rocky Mountains. However, there is no geomorphic evidence of corresponding glacial activity in the canyon area. Relative moisture during these two periods differs significantly, suggesting that Neoglacial conditions were controlled primarily by changes in summer temperature.
ABSTRACT: Unlike the North Atlantic, the North Pacific Ocean probably remained free of sea ice during the last glacial maximum (LGM), 22,000 to 17,000 BP. Following a eustatic low in sea level of ca. -120 m at 19,000 BP, a marine transgression had flooded the Bering and Chukchi shelves by 10,000 BP. Post-glacial sea-level history varied widely in other parts of the North Pacific coastline according to the magnitude and timing of local tectonism and glacio-isostatic rebound. Glaciers covered much of the continental shelf between the Alaska Peninsula and British Columbia during the LGM. Maximum glacier extent during the LGM was out of phase between southern Alaska and southern British Columbia with northern glaciers reaching their outer limits earlier, between 23,000 and 16,000 BP, compared to 15,000–14,000 BP in the south. Glacier retreat was also time-transgressive, with glaciers retreating from the continental shelf of southern Alaska before 16,000 BP but not until 14,000–13,000 BP in southwestern British Columbia. Major climatic transitions occurred in the North Pacific at 24,000–22,000, 15,000–13,000 and 11,000–9000 BP. Rapid climate changes occurred within these intervals, including a possible Younger Dryas episode. An interval of climate warmer and drier than today occurred in the early Holocene. Cooler and wetter conditions accompanied widespread Neoglaciation, beginning in some mountain ranges as early as the middle Holocene, but reaching full development after 3000 BP.
ABSTRACT: Forty individually named ranges, plateaus, and massifs draining wholly or partly into the Great Basin of the western United States show definite evidence of Pleistocene glaciation. The most obvious deposits are a family of moraines designated, among other names, "Tioga", "Angel Lake", and "Pinedale". Such moraines generally can be traced from range to range away from described type moraines. These deposits have been numerically assigned to Late Wisconsinan glaciation in the Wasatch Range, White Mountains, Boulder Mountain, and Sierra Nevada on the basis of radiocarbon and surface-exposure ages, and have been assigned to Late Wisconsinan time in several other ranges on the basis of relative-age studies. The type Angel Lake moraine, and most other equivalent moraines across the Great Basin, are thick, hummocky, lobate piles of till rather than looping ridges. The thicknesses of the moraines (often 60+m) can be explained by heavy debris loads, and/or glacial advance, retreat, and readvance to the same positions a number of times, which is consistent with recent evidence that multiple Late Wisconsinan advances, possibly related to Heinrich and Dansgaard-Oeschger events, occurred in the Sierra Nevada. Pre-Angel Lake deposits occur in many Great Basin ranges, but it is currently difficult or perhaps impossible to determine if these deposits are equivalent to each other and what their relationship is to pre-Tioga deposits in the Sierra Nevada. Numerical ages are rare and relative-age studies suggest that pre-Angel Lake deposits may be products of more than one glaciation. Mapped pre-Angel Lake glaciers were longer than their Angel Lake counterparts, but the length differences do not translate into large differences in ELA depression. There is evidence of two minor latest Pleistocene or early Holocene advances in some ranges, judging from the presence of overlying Mazama tephra and/or weathering comparisons to local Angel Lake moraines. In the latter part of the Holocene, ELA's were sufficiently high that only the highest, wettest ranges developed Neoglacial glaciers. There does not appear to be a consistent pattern of latest Pleistocene/Holocene glacial fluctuations along an east–west transect through the Cordillera, or even through the Great Basin.
ABSTRACT: This paper integrates concepts derived from the literature to focus upon interactions between riparian vegetation and river channel dynamics in alpine glacier basins. Discussion of the nature and variability of discharge and sediment regimes of alpine glacier-fed rivers; downstream variations in the physical character of the river channel and corridor; consequent downstream variations in lateral processes; and regional variations in alpine glacier dynamics, lead to the proposal of a conceptual model of proglacial river channel-riparian vegetation interactions under changing climatic conditions.
ABSTRACT: An integrated program of ecosystem modeling and field studies in the mountains of the Pacific Northwest (U.S.A.) has quantified many of the ecological processes affected by climatic variability. Paleoecological and contemporary ecological data in forest ecosystems provided model parameterization and validation at broad spatial and temporal scales for tree growth, tree regeneration and treeline movement. For subalpine tree species, winter precipitation has a strong negative correlation with growth; this relationship is stronger at higher elevations and west-side sites (which have more precipitation). Temperature affects tree growth at some locations with respect to length of growing season (spring) and severity of drought at drier sites (summer). Furthermore, variable but predictable climate-growth relationships across elevation gradients suggest that tree species respond differently to climate at different locations, making a uniform response of these species to future climatic change unlikely. Multi-decadal variability in climate also affects ecosystem processes. Mountain hemlock growth at high-elevation sites is negatively correlated with winter snow depth and positively correlated with the winter Pacific Decadal Oscillation (PDO) index. At low elevations, the reverse is true. Glacier mass balance and fire severity are also linked to PDO. Rapid establishment of trees in subalpine ecosystems during this century is increasing forest cover and reducing meadow cover at many subalpine locations in the western U.S.A. and precipitation (snow depth) is a critical variable regulating conifer expansion. Lastly, modeling potential future ecosystem conditions suggests that increased climatic variability will result in increasing forest fire size and frequency, and reduced net primary productivity in drier, east-side forest ecosystems. As additional empirical data and modeling output become available, we will improve our ability to predict the effects of climatic change across a broad range of climates and mountain ecosystems in the northwestern U.S.A.
ABSTRACT: Climatic warming during the last 100-150 years has resulted in a significant glacier ice loss from mountainous areas of the world. Certain natural processes which pose hazards to people and development in these areas have accelerated as a result of this recent deglaciation. These include glacier avalanches, landslides and slope instability caused by glacier debuttressing, and outburst floods from moraine- and glacier-dammed lakes. In addition, changes in sediment and water supply induced by climatic warming and glacier retreat have altered channel and floodplain patterns of rivers draining high mountain ranges.
The perturbation of natural processes operating in mountain environments, caused by recent climatic warming, ranges from tens of decades for moraine-dam failures to hundreds of years or more for landslides. The recognition that climatic change as modest as that of the last century can perturb natural alpine processes has important implications for hazard assessment and future development in mountains. Even so, these effects are probably at least an order of magnitude smaller than those associated with late Pleistocene deglaciation ca. 15,000 to 10,000 years ago.