Climate Change and...
Effects of Climate Change
- Climate Variability
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Effects of Climate Change
Miller, J., D. Germanoski, K. Waltman, R. Tausch, J. Chambers (2001). Influence of late Holocene hillslope processes and landforms on modern channel dynamics in upland watersheds of central Nevada. Geomorphology 38 (3-4): 373-391
ABSTRACT: Stratigraphic, geomorphic, and paleoecological data were collected from upland watersheds in the Great Basin of central Nevada to assess the relationships between late Holocene climate change, hillslope processes and landforms, and modern channel dynamics. These data indicate that a shift to drier, warmer climatic conditions from approximately 2500 to 1300 YPB led to a complex set of geomorphic responses. The initial response was massive hillslope erosion and the simultaneous aggradation of both side-valley alluvial fans and the axial valley system. The final response was fan stabilization and axial channel incision as fine-grained sediments were winnowed from the hillslope sediment reservoirs, and sediment yield and runoff processes were altered. The primary geomorphic response to disturbance for approximately the past 1900 years has been channel entrenchment, suggesting that the evolutionary history of hillslopes has produced watersheds that are prone to incision. The magnitude of the most recent phase of channel entrenchment varies along the valley floor as a function of geomorphic position relative to side-valley alluvial fans. Radial fan profiles suggest that during fan building, fan deposits temporarily blocked the flow of sediment down the main stem of the valley, commonly creating a stepped longitudinal valley profile. Stream reaches located immediately upvalley of these fans are characterized by low gradients and alternating episodes of erosion and deposition. In contrast, reaches coincident with or immediately downstream of the fans exhibit higher gradients and limited valley floor deposition. Thus, modern channel dynamics and associated riparian ecosystems are strongly influenced by landforms created by depositional events that occurred approximately 2000 years ago.
Pederson, J., Smith, G., Pazzaglia, F. (2001). Comparing the modern, Quaternary, and Neogene records of climate-controlled hillslope sedimentation in southeast Nevada. Geological Society of America Bulletin 113 (3): 305-319
ABSTRACT: The vast majority of all sediment is derived from hillslopes. Attempts to understand what controls variability in the sedimentary record should therefore consider the primary variability of hillslope sediment yield to depositional basins. But our understanding of controls on sedimentation, particularly climatic controls, is limited by poor understanding of the links between hillslopes and depositional systems. This is partly because the applications of geomorphic research to sedimentology are not fully realized. The long-term hillslope stratigraphic records of this study provide a crucial physical link between hillslope sediment sources and depositional basins, and between geomorphology and sedimentology. We compare rare Neogene colluvium and buried hillslopes preserved in superproximal basin-fill exposures to their Quaternary and modern equivalents in two tectonically quiescent basins in southeastern Nevada. Field and laboratory geomorphic and sedimentologic methods are employed to document the provenance of proximal basin sediment, the character and relative amount of sediment produced on local hillslopes at different times, the hillslope weathering and transport processes occurring through time, and the role that rock-type differences have played. Physical weathering processes have dominated the production of angular, pebbly colluvium on both ancient and modern slopes, and overland flow has been the main process transporting detritus off slopes. Although hillslope processes and products in the study area remained the same in upper Miocene, Pliocene, Pleistocene, and modern records, process rates have varied greatly, indicating that orbital-scale climatic cyclicity can be, but is not always, well expressed in the stratigraphy of continental basins. The vast majority of basin sediment in the study area is derived from hillslopes underlain by volcanic rather than carbonate bedrock, and rock type is the dominant control on sediment yield and landscape development in this tectonically inactive, dry setting.
Forman, S.L., Oglesby, R., Webb, R.S. (2001). Temporal and spatial patterns of Holocene dune activity on the Great Plains of North America: megadroughts and climate links. Global and Planetary Change 29: 1-29
ABSTRACT: The Holocene record of eolian sand and loess deposition is reviewed for numerous presently stabilized dune fields on the Great Plains of North America. Dune field activity reflects decade-to-century-scale dominance of drought that exceeded historic conditions, with a growing season deficit of precipitation >25%. The largest dune fields, the Nebraska Sand Hills and ergs in eastern Colorado, Kansas and the Southern High Plains showed peak activity sometime between ca. 7 and 5 cal. ka. Loess deposition between ca. 10 and 4 cal. ka also signifies widespread aridity. Most dune fields exhibit evidence for one or more reactivation events sometime in the past 2 cal. ka; a number of localities register two events post 1 cal. ka, the latest potentially after 1400 AD. However, there is not a clear association of the latest dune remobilization events with up to 13 droughts in the past 2 cal. ka identified in dendroclimatic and lacustrine records. Periods of persistent drought are associated with a La Niña-dominated climate state, with cooling of sea surface temperatures in the tropical Pacific Ocean and later of the tropical Atlantic Ocean and the Gulf of Mexico that significantly weakens cyclogenesis over central North America. As drought proceeds, reduced soil moisture and vegetation cover would lessen evaporative cooling and increase surface temperatures. These surface changes strengthen the eastward expansion of a high-pressure ridge aloft and shift the jet stream northward, further enhancing continent-wide drought. Uncertainty persists if dune fields will reactivate in the future at a scale similar to the Holocene because of widespread irrigation, the lack of migratory bison herds, and the suppression of prairie fires, all of which enhance stabilization of dune fields in the Great Plains.
Meyer, G.A., J. L. Pierce (2003). Climatic controls on fire-induced sediment pulses in Yellowstone National Park and central Idaho: a long-term perspective. Forest Ecology and Management 178 (1-2): 89-104
ABSTRACT: Fire management addressing postfire erosion and aquatic ecosystems tends to focus on short-term effects persisting up to about a decade after fire. A longer perspective is important in understanding natural variability in postfire erosion and sedimentation, the role of these processes in structuring habitat, and future expectations in light of a warming climate and environmental change. In cool high-elevation forests of northern Yellowstone National Park, stand ages indicate infrequent large stand-replacing fires. In warmer low-elevation forests of the Payette River region of Idaho, fire-scarred tree-rings record frequent low-severity fires before 1900; stand-replacing fires and resulting debris flows in recent decades are usually attributed to 20th-century fire suppression, grazing, and other land uses. In both areas, however, tree-ring records extend back only about 500 years. We use14 C-dated geologic records to examine spatial and temporal patterns of fire-induced sedimentation and its relation to climate over the last 10 000 years. We review sedimentation processes in modern postfire events, which vary in magnitude and impact on stream systems depending on burn severity, basin geomorphology, and the timing and characteristics of postfire storms. Modern deposits also provide analogs for identification of fire-related deposits in alluvial fans. In Yellowstone, episodes of fire-induced sedimentation occurred at intervals of about 300–450 years during the last 3500 years, indicating a regime of infrequent high-severity fires. Millennial-scale variations in the fire-sedimentation record appear to relate to hemispheric-scale climatic change. Fire-related sedimentation is rare in Yellowstone during cooler episodes (e.g., the Little Ice Age ~1200–1900), probably because effectively wetter conditions prevented most fires from spreading. During some of the same cool periods, the Payette region experienced light surface fires and frequent, small pulses of fire-induced sediment. Between 900 and 1200, however, large fire-related debris flows occurred in both study areas, coincident with the Medieval Warm Period. During that time, drought may have limited grass growth in xeric Payette-region forests, restricting surface fire spread and allowing understory shrubs and trees to create ladder fuels. Although fire suppression and land-use effects are clearly involved in recent catastrophic fires in the Payette region, a warming climate and severe drought are probable contributors to major stand-replacing fires and postfire sedimentation, both past and present. Restoration and maintenance of conditions prior to European settlement may be unrealistic because of the potent influence of climate, and the incidence of severe fires will likely increase in both areas with future warming.
McFadden, L.D., J.R. McAuliffe (1997). Lithologically influenced geomorphic responses to Holocene climate changes in the southern Colorado Plateau, Arizona: a soil-geomorphic and ecological perspective. Geomorphology 19 (3-4): 303-332
ABSTRACT: The semiarid landscape occupied by the Navajo and Hopi peoples on the Colorado Plateau in northeastern Arizona in the southwestern United States is characterized by an extensive network of deeply incised arroyos. Since the early 20th century, many researchers have proposed that the recent formation of arroyos in this region and also many attributes of modern vegetation communities were caused directly by overgrazing of vegetation by domestic livestock of the Navajo. Other researchers, however, have proposed other causes for such features, such as climatic change. We believe that the landforms, soils and vegetation of a small area located on Antelope Mesa in this region, underlain by the highly erodible materials of the Miocene Bidahochi Formation, may have been more sensitive to minor climatic changes of the Holocene than landscapes of massive Mesozoic sandstones that dominate the Colorado Plateau. In the Antelope Mesa area, the presence of actively filling channels rather than arroyos in the upper parts of many drainage basins and associated soils and ecologic patterns indicate that the aggradation (1) was initiated in downstream reaches and within the past two centuries, (2) may be linked to recently accelerated slope erosion, and (3) is unrelated to past or ongoing grazing. This suggests the ongoing aggradation may be related to recent minor climatic changes. Geochronologic and soil-geomorphic evidence indicate that the most recent cycle of arroyo incision and filling may be a small-scale analogue for larger-magnitude, older cycles that produced regionally recognizable, paired terraces that are attributable to previous Holocene climate changes.
We propose that climatic change, and more specifically, increases in precipitation, caused an acceleration in the erosion of the steep, typically minimally vegetated slopes of the Bidahochi Formation. The beginning of the `Neoglacial Period' (ca. 2-3 ka), effects of which are documented by other proxy records in this region, may be the climatic change that triggered widespread deposition of a large, mid-Holocene alluvial unit in this region. Alternating episodes of eolian activity and soil formation in the study area and in this general region provide independent evidence of Holocene climatic changes. Our interpretations of the origin of the cut-and-fill cycles that emphasize the role of drainage basin lithology and differ significantly from previous interpretations emphasize the linkage of climatic changes, groundwater levels and arroyo incision. This study reveals the importance of integrating soil and ecologic studies with geomorphologic research; such an approach may be critical in helping understand how anthropogenically induced climatic changes of the next century could impact geomorphic processes and the ecology of arid and semiarid regions.
Ritter, J. B., J.R. Miller, J. Husek-Wulforst (2000). Environmental controls on alluvial fan evolution in Buena Vista valley, north central Nevada, during the late Quaternary time. Geomorphology 36: 63-87
ABSTRACT: Alluvial fans in Buena Vista Valley, north-central Nevada, formed during at least four time periods during the late Quaternary as evidenced by four temporally and spatially constrained fan deposits: Qf1, Qf2, Qf3, and Qf4, from oldest to youngest. The stratigraphic relation between Qf3 and remnant beach ridge deposits of pluvial Lake Lahontan, the latter of which forms a paleoclimatic proxy for this region, indicate that climate and climatic change are primary controls on the evolution of fans. Qf3 deposition commenced during or immediately following the highstand of Lake Lahontan, between 13.5 and 12.5 ka, and continued as lake levels plummeted to levels below the sill elevation of the Buena Vista subbasin. We interpret Qf3 to be the result of a dynamic interplay between hillslope vegetation, soils, and hydrology as they are conditioned by climate and climatic change. Accelerated pedogenesis, because of aerosolic silts and clays from exposed lake sediments, coupled with decreased density of vegetation reduced infiltration capacity of the hillslope soils in interscrub areas. In contrast to fans in more arid climates, the increased runoff was not associated with increased coarse-grained sediment yield and major fan aggradation. Discharge from sediment-deficient basins entrenched older, proximal fan deposits and transported the sediment downfan to form secondary fans. The process–response of Qf3 to climatic change in north-central Nevada is especially illustrative of the impact of climate on landscapes in that: (1) it results from changes in basin sediment and/or water yield that occur in less than 500 years, and (2) it differs from responses in arid climates to the south and more semiarid climates to the north. The magnitude of vegetation change, including type and density of vegetation and the relative proportion of interscrub area, is particularly critical to water and sediment yield. Unit Qf2, an early late Pleistocene deposit, may also have been deposited during a transition from a wetter to drier climate. The synchronism of aggradation, stability, and entrenchment on all fans along the perimeter of Buena Vista Valley in the late Quaternary is further support for the dominance of climatic control on fan process; on the other hand, the impact of other extrinsic variables on fan process, including tectonism, base level, and basin lithology, all of which vary along the perimeter, is negligible.
ABSTRACT: Recent investigations have shown that the extent of the channel network in some drainage basins is controlled by a threshold for overland flow erosion. The sensitivity of such basins to climate change is analyzed using a physically based model of drainage basin evolution. The GOLEM model simulates basin evolution under the action of weathering processes, hillslope transport, and fluvial bedrock erosion and sediment transport. Results from perturbation analyses reveal that the nature and timescale of basin response depends on the direction of change. An increase in runoff intensity (or a decrease in vegetation cover) will lead to a rapid expansion of the channel network, with the resulting increase in sediment supply initially generating aggradation along the main network, followed by downcutting as the sediment supply tapers off. By contrast, a decrease in runoff intensity (or an increase in the erosion threshold) will lead to a retraction of the active channel network and a much more gradual geomorphic response. Cyclic changes in runoff intensity are shown to produce aggradational-degradational cycles that resemble those observed in the field. Cyclic variations in runoff also lead to highly punctuated denudation rates, with denudation concentrated during periods of increasing runoff intensity and/or decreasing vegetation cover. The sediment yield from threshold dominated basins may therefore exhibit significant variability in response to relatively subtle environmental changes, a finding which underscores the need for caution in interpreting modern sediment-yield data.
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: Successive surveys of the cross section of ephemeral channels in New Mexico over a period of 15 years 1960–1975, show that arroyos that were actively eroding early in the century have reversed the trend and are alluviating. This appears to be associated with the worldwide cooling trend that began about 1940. If the inference proves to be correct, it is significant hydrologically, for it provides some specific knowledge of the climatic conditions associated with the alternate periods of valley erosion and valley alluviation that created the widespread terraces of the semiarid West.
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.
Wells, S. G., L.D. McFadden, J.C. Dohrenwend (1987). Influence of late Quaternary climatic changes on geomorphic and pedogenic processes on a desert piedmont, eastern Mojave Desert, California. Quaternary Research 27 (2): 130-146
ABSTRACT: Radiocarbon dating of late Quaternary deposits and shorelines of Lake Mojave and cation-ratio numerical age dating of stone pavements (Dorn, 1984) on the adjacent Soda Mountains piedmont provide age constraints for alluvial and eolian deposits. These deposits are associated with climatically controlled stands of Lake Mojave during the past 15,000 yr. Six alluvial fan units and three eolian stratigraphic units were assigned ages based on field relations with dated shorelines and piedmont surfaces, as well as on soil-geomorphic data. All but one of these stratigraphic units were deposited in response to time-transgressive climatic changes beginning approximately 10,000 yr ago. Increased eolian flux rates occurred in response to the lowering of Lake Mojave and a consequent increase in fine-sediment availability. Increased rates of deposition of eolian fines and associated salts influenced pedogenesis, stone-pavement development, and runoff-infiltration relations by (1) enhancing mechanical weathering of fan surfaces and hillslopes and (2) forming clay- and silt-rich surface horizons which decrease infiltration. Changes in alluvial-fan source areas from hillslopes to piedmonts during the Holocene reflect runoff reduction on hillslopes caused by colluvial mantle development and runoff enhancement on piedmonts caused by the development of less-permeable soils. Inferred increased in early to middle Holocene monsoonal activity resulted in high-magnitude paleo-sheetflood events on older fan pavements; this runoff triggered piedmont dissection which, in turn, caused increased sediment availability along channel walls. Thus, runoff-infiltration changes during the late Quaternary have occurred in response to eolian deposition of fines, pedogenesis, increased sheetflood activity in the Holocene, and vegetational changes which are related to many complicated linkages among climatic change, lake fluctuations, and eolian, hillslope, and alluvial-fan processes.
ABSTRACT: Short-term episodic cycles of wet and dry patterns of climate are common in southern California. Wet intervals, like the one in 1978-83, are often characterized by more than double the average annual precipitation. The impact of these episodic climatic fluctuations on landforms and surficial processes has not been well documented for areas inland of the coast. The response to these cycles may be significant in the evolution of hillslopes and fluvial landforms, and may have significant implications for geologic hazards in this rapidly developing region.
Using aerial photographs and field investigations we found little response to the 1978-83 wet interval on upland hillslopes, but documented significant response on alluvial fans and in channels in desert piedmont areas. These observations may lend support to the Langbein-Schumm (1958) model relating sediment yield to precipitation. A variety of techniques, including dendrogeomorphology, studies of the weathering of clasts, soil stratigraphy, and aerial photo mapping were used to discern at least six units on alluvial fans ranging from Late Pleistocene to present. Terraces along active fan channels and the San Felipe River record a geomorphic record of the most recent wet intervals (ca. 1940 and 1980) as a significant depositional event. Geomorphic responses to the wet interval along the San Felipe River were complex, varying locally according to controls on sediment storage and downstream transfer through a recently integrated drainage system. Additional complex responses to the wet period were experienced in selected sites where antecedence and response times may be measured in months or even years.