Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
Paleoecology and Paleohydrology
Leavitt, S. W., Follett, R. F., Kimble, J. M., Pruessner, E. G. (2007). Radiocarbon andd13 C depth profiles of soil organic carbon in the U.S. Great Plains: A possible spatial record of paleoenvironment and paleovegetation. Quaternary International 162 (1): 21-34
ABSTRACT: Soil profiles from undisturbed grassland sites around the Great Plains of the USA were sampled for analysis of soil organic carbon (SOC) content (%), radiocarbon age and stable-carbon isotope composition (d13 C). With the exception of a few pronounced dating anomalies, SOC radiocarbon age generally increases steadily with depth back to 10–15,000 cal yr BP, the deepest soil intervals in 9 of the 12 sites. The radiocarbon ages were used to establish the chronology of changes in past plant distribution over time and space, based on SOCd13 C as an indicator of C3 and C4 plant abundance. Changes were referenced to an SOCd13 C value of−20‰, which is the approximate mid-point between C3 and C4 carbon isotope composition, i.e., an equal mixture of C3 and C4 carbon. Prior to 10,000 cal yr BP, the region was dominated by C3 plants with the exception of the southernmost Texas sites. From 10,000 to 2000 cal yr BP, C4 plants expanded their range, initially as a peninsula of C4-predominant grasses extending northeastward and ultimately dominating all but the northernmost border of the region. Finally, the C3-predominant region re-expanded after 1000 cal yr BP, perhaps as a response to the Little Ice Age cooling. Despite uncertainties associated with using radiocarbon-dated SOC-depth profiles as a proxy, the past C3 and C4 plant distribution inferred from SOCd13 C conforms well to results from other paleoclimate proxies, and differences may be helpful in targeting future research.
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: The existing warm (Larrea) deserts of the Southwest are Holocene expansions replacing late—Pleistocene, evergreen woodlands of low—statured junipers, pinyon pines, and live oaks; these woodlands have been isolated by complementary contraction to the slopes of higher mountains that rise like islands from the modern desert sea. Because pinyon—juniper woodland is now so widespread on the similar fault—block mountains of the Great Basin, even as far north as southern Idaho, it would seem reasonable to suppose that the modern "cold" (Artemisia ,Atriplex ) deserts were similarly wooded during the last glacial. However, conclusive Neotoma macrofossil evidence (45 14°C—dated assemblages are reported here) documents major latitudinal displacement of vegetation that precludes pinyon—juniper woodland in the northern and central Great Basin at that time. On the other hand, the entire Mohave Desert sector (south of 37°N) served as an extensive Pleistocene refugium for pinyon—juniper woodland, as documented by an additional 48 dated Neotoma deposits. During the Wisconsinan glacial in the southeastern corner of Oregon, a 42°27'N, there was a subarctic landscape of hyperboreal, prostrate shrublet—junipers (Juniperus horizontalis andJ. communis ) and widespread patterned ground, even at the near—basal elevation of 1460 m. The pleniglacial vegetation of the central Great Basin at 39°N in eastern Nevada and western Utah, was dominated by a regional subalpine forest of bristlecone pine (Pinus longaeva ), associated with minor but consistent boreal juniper (J. communis ) down to 1660 m, close to the base level imposed by pluvial Lake Bonneville. Spruce has not been recorded below 1900 m during the last glacial. At a lower range of elevation (1350—1525 m), available south of the southeastern rim of the Bonneville basin at 37°30'N. Pinus longaeva was replaced by limber pine (P. flexilis ), Douglas—fir (Pseudotsuga ), and montane red cedar (J. scopulorum ); existing woodland juniper (J. osteosperma ) was lacking, but the subalpineJ. communis was present at this local base level. Theory of island biogeography, as applied to ecological islands atop the high mountains of the Great Basin, is reexamined in the light of the drastic vegetational displacements documented in the detailed Quaternary macrofossil record. Species/area plots of montane—subalpine conifers presently distributed on 54 Great Basin mountaintops show an overall insular pattern that is especially well developed on the subset of 38 ecological islands east of 116°W; the slope of z = 0.26 is close to the theoretical value for islands in equilibrium. All 11 taxa of montane—subalpine conifers that penetrate the Great Basin deeply have their main distributions in the Rocky Mountains; only three wide—ranging species occur also in the Sierra Nevada. A long sundering trough in the western Great Basin parallels and isolates the Cascade—Sierran uplift with low—elevation barriers that impede migration, but in the eastern Great Basin there are high connecting divides to the western Rockies, especially via an axial route southeast of the Bonneville basin. There is an east—west pattern of declining species richness of11 montane conifers in the Great Basin that correlates with distance from the rocky Mountain pool of 12 coniferous species. The pleniglacial subalpine forests in the lowlands of the central Great Basin had only one to three species of conifers (e.g.,Pinus longaeva ,Picea engelmannii ,Juniperus communis ). During the great late—glacial/Holocene (12 000—8000 yr BP) warming of climate, these shifted upward in elevation and were augmented in the east (but not in the west) by as many as five additional species of montane conifers. Macrofossil evidence indicted that the later Holocene arrivals dispersed across barriers of woodland and desert that by then isolated the shrunken montane islands. Moderately long—range transport of seeds by birds is deduced as follows: a northward latitudinal shift of 500—640 km during the Holocene is documented for several species of relatively thermophilous conifers, including the heavy—seeded, late—maturing pinyon pine. A 640—km migration in 8000 yr (80 m/yr) is indicated for pinyon, but the most generous estimate of its dispersal rate via the wind/gravity mode is a plodding 0.4 m/yr (3.2 km/8000 yr), orders of magnitude too slow. Seed dispersal by Clark's Nutcrackers and Pinyon Jays, however, is both the prevalent mode and amply swift enough to fit the known migrational history. Hence, the islands—in—equilibrium pattern indicated by the typically insular slope (z = 0.26) for montane conifers on the eastern set of mountaintops in the Great Basin is a reflection of Holocene immigration via sweepstakes dispersal being offset by extinction on the smaller islands. Both extinction and immigration of conifers are documented in the late—glacial/early—Holocene Neotoma record from the small Confusion Range in the east—central Great Basin of western Utah.
ABSTRACT: The climate system consists of the atmosphere, the oceans, the cryosphere (land ice, snow, sea ice), the lithosphere, and the biomass. The behavior of the individual components of the system is governed by processes occurring over a broad range of time and space scales. The components are coupled by physical, biological, and chemical processes, and the coupled system seems capable of undergoing fluctuations on all time scales. In addition to these "internal" climatic processes, external processes (such as variability in the solar irradiance or human activities) must also be considered. Space and time scales of climatic variability are reviewed, with emphasis on the Holocene. Regional patterns of climatic variability may be associated with changes in the amplitude and longitudinal position of the long waves in the westerlies of midlatitudes, and with changes in the intensity and latitude of meridional circulation features such as the Hadley cell. Possible examples of this are mentioned. The variance spectrum of climatic time series is described and certain implications for climate modeling are suggested.
ABSTRACT: Pollen and plant macrofossils from eight sedimentary basins on the west slope of the Colorado Rocky Mountains document fluctuations in upper and lower timberline since the latest Pleistocene. By tracking climatically sensitive forest boundaries, the moisture-controlled lower timberline and the temperature-controlled upper timberline, paleoclimatic estimates can be derived from modern temperature and precipitation lapse rates. Pollen data suggest that prior to 11 000 yr B.P., a subalpine forest dominated byPicea (spruce) andPinus (pine) grew 300–700 m below its modern limit. The inferred climate was 2–5 °C cooler and had 7–16 cm greater precipitation than today.Abies (fir) increased in abundance in the subalpine forest around 11 000 yr B.P., probably in response to cooler conditions with increased winter snow. Pollen and plant macrofossil data demonstrate that from 9000 to 4000 yr B.P. the subalpine forest occupied a greater elevational range than it does today. Upper timberline was 270 m above its modern limit, suggesting that mean annual and mean July temperatures were 1–2 °C warmer than today. Intensification of the summer monsoon, coupled with increased summer radiation between 9000 and 6000 yr B.P., raised mean annual precipitation by 8–11 cm and allowed the lower limit of the subalpine and montane forests to descend to lower elevations. The lower forest border began to retreat upslope between 6000 and 4000 yr B.P. in response to drier conditions, and the upper timberline descended after 4000 yr B.P., when temperatures cooled to about 1 °C warmer than today. The modern climatic regime was established about 2000 yr B.P., when the summer precipitation maxima of the early and middle Holocene were balanced by increased winter precipitation.
ABSTRACT: A large assemblage of salmon bones excavated 50 yr ago from an ~10,000-yr-old archaeological site near The Dalles, Oregon, USA, has been the primary evidence that early native people along the Columbia River subsisted on salmon. Recent debate about the human role in creating the deposit prompted excavation of additional deposits and analysis of archaeologic, geologic, and hydrologic conditions at the site. Results indicate an anthropogenic source for most of the salmonid remains, which have associated radiocarbon dates indicating that the site was occupied as long ago as 9300 cal yr B.P. The abundance of salmon bone indicates that salmon was a major food item and suggests that migratory salmonids had well-established spawning populations in some parts of the Columbia Basin by 9300–8200 yr ago.
ABSTRACT: Significant climate anomalies have characterized the last 1000 yr in the Sierra Nevada, California, USA. Two warm, dry periods of 150- and 200-yr duration occurred during AD 900–1350, which were followed by anomalously cold climates, known as the Little Ice Age, that lasted from AD 1400 to 1900. Climate in the last century has been significantly warmer. Regional biotic and physical response to these climatic periods occurred. Climate variability presents challenges when interpreting historical variability, including the need to accommodate climate effects when comparing current ecosystems to historical conditions, especially if comparisons are done to evaluate causes (e.g., human impacts) of differences, or to develop models for restoration of current ecosystems. Many historical studies focus on “presettlement” periods, which usually fall within the Little Ice Age. Thus, it should be assumed that ecosystems inferred for these historical periods responded to different climates than those at present, and management implications should be adjusted accordingly. The warmer centuries before the Little Ice Age may be a more appropriate analogue to the present, although no historic period is likely to be better as a model than an understanding of what conditions would be at present without intervention. Understanding the climate context of historical reconstruction studies, and adjusting implications to the present, should strengthen the value of historical variability research to management.
ABSTRACT: Aquatic resources management has become increasingly challenging as human demands on water supplies compete with the needs of natural ecosystems, particularly in arid lands. A wide range of factors, both natural and human, influence aquatic environments, but an important underlying component is climate variability. Instrumental records of hydroclimatic variability from precipitation, streamflow, and snowpack are limited to 100 years or less in most areas of the western U.S., and are too short to provide more than a subset of the full range of natural climate variability. Paleoclimatic proxy data from a variety of sources can be used to extend instrumental records of climate back centuries to tens of thousands of years and longer. In this review, four drought events over the past three millennia, each documented with a number of proxy records, illustrate natural hydroclimatic variability characteristics over the western U.S. Although a small sample of paleoclimate data, these four events exemplify the wide range of natural hydroclimatic variability over space and time. Climate is now, and will continue to be, impacted by human activities, but natural climatic variability will likely be an important underlying factor in future climate variability and change.
ABSTRACT: Attempts to increase European biodiversity by restoring rivers and floodplains are based on inadequate data on natural systems. This is particularly the case for NW European rivers because all catchments have been impacted by agriculture and river engineering. If river restoration is to have an ecological, as opposed to `cosmetic' design basis then baseline models are required. However, this poses three questions; (a) what is the natural river-floodplain state, (b) how can it be defined and modelled and (c) can this state be recreated today? The first two questions can only be addressed by using palaeohydrological and palaeoecological data. A second and equally vital consideration is the stability/instability of any restored system to change in external forcing factors (e.g. climate) and in this context it may not be realistic to expect baseline models to provide equilibrium solutions but instead to define process-form domains. Over the last two decades evidence has accumulated that the natural state of lowland rivers in much of NW Europe was multi rather than single thread-braided, anastomosing or anabranching. Until recently our knowledge of floodplain palaeoecology was generally derived from pollen diagrams, which have source-area of problems and lack of taxonomic specificity. The precision and breadth of palaeoecological reconstruction (including richness and structure) has been greatly increased by the use of multiple palaeo-indicators including macrofossils, diatoms and beetles. The dynamics of small to medium sized, low-energy, predeforestation floodplains were dominated by disturbance (windthrow, beavers, etc.) and large woody debris. In order to compare the hydrogeomorphological basis of floodplain ecology, both temporally and spatially, a simple index of fluvial complexity is presented. Palaeoecological and geomorphological investigations have the potential to provide in-depth models of the natural range of channel conditions and sensitivity to external change that can be used to provide a scientific basis for floodplain restoration. There is also the possibility that floodplain-channel restoration may be a valuable tool in the mitigation of future geomorphological change forced by climatic instability.
ABSTRACT: Applied historical ecology is the use of historical knowledge in the management of ecosystems. Historical perspectives increase our understanding of the dynamic nature of landscapes and provide a frame of reference for assessing modern patterns and processes. Historical records, however, are often too brief or fragmentary to be useful, or they are not obtainable for the process or structure of interest. Even where long historical time series can be assembled, selection of appropriate reference conditions may be complicated by the past influence of humans and the many potential reference conditions encompassed by nonequilibrium dynamics. These complications, however, do not lessen the value of history; rather they underscore the need for multiple, comparative histories from many locations for evaluating both cultural and natural causes of variability, as well as for characterizing the overall dynamical properties of ecosystems. Historical knowledge may not simplify the task of setting management goals and making decisions, but 20th century trends, such as increasingly severe wildfires, suggest that disregarding history can be perilous.
We describe examples from our research in the southwestern United States to illustrate some of the values and limitations of applied historical ecology. Paleoecological data from packrat middens and other natural archives have been useful for defining baseline conditions of vegetation communities, determining histories and rates of species range expansions and contractions, and discriminating between natural and cultural causes of environmental change. We describe a montane grassland restoration project in northern New Mexico that was justified and guided by an historical sequence of aerial photographs showing progressive tree invasion during the 20th century. Likewise, fire scar chronologies have been widely used to justify and guide fuel reduction and natural fire reintroduction in forests. A southwestern network of fire histories illustrates the power of aggregating historical time series across spatial scales. Regional fire patterns evident in these aggregations point to the key role of interannual lags in responses of fuels and fire regimes to the El Niño–Southern Oscillation (wet/dry cycles), with important implications for long-range fire hazard forecasting. These examples of applied historical ecology emphasize that detection and explanation of historical trends and variability are essential to informed management.
ABSTRACT: Hydroclimatic variability over a wide range of spatial and temporal scales is directly linked to attendant variations in the magnitude and frequency of severe regional flooding events in the western U.S. Understanding this linkage is critical for improving flood-frequency forecasting and water resources management in this region. The spatial and temporal distributions of large floods in the western U.S. are largely controlled by persistent, anomalous patterns in hemispheric to global-scale atmospheric and oceanic circulation that directly influence flood-generating storm systems. Variability in flood frequency over short and long time scales thus provides an insight into variability in related larger-scale climatic phenomena over the same time scales.
This study employed paleoflood analyses to compare the influence of decadal- to millennialscale climatic variability on the magnitude and frequency of large floods on rivers in three distinct hydroclimatic regions of the western U.S: the Southwest, Northwest, and western Great Basin. This is the first study to construct regional paleoflood chronologies for rivers in the Northwest and Great Basin, and this aspect of the project alone will greatly increase the accuracy of flood-frequency forecasting in these areas. In addition, our comparison of paleoflood chronologies from three areas of the western U.S. will allow us to go one step further and determine whether there are consistent, predictable, long-term similarities or differences in the occurrence of large floods among these distinct hydroclimatic regions and investigate whether the regional differences in the timing and controls on floods in the short-term records hold true for the response of large floods in the three regions to longer-term climatic variations.
Whitlock, C. (1992). Vegetational and climatic history of the Pacific Northwest during the last 20,000 Years: implications for understanding present-day biodiversity. Northwest Environmental Journal 8 (1): 5-28
ABSTRACT: During the last 20,000 years, the world climate system has moved from a glacial state into the present interglaciation, known as the Holocene. In the course of the transition, the vast continental ice sheets disappeared, sea level rose worldwide, land and ocean surfaces warmed, and moisture became redistributed (Ruddiman and Wright 1987). These global events also set in motion a series of adjustments to regional climate that caused changes in vegetational composition, the formation of new plant communities, and shifts in the biogeographic range of particular species. The legacy of these events is the present diversity—species richness or number of taxa—of plants and their distribution on the landscape. Thus, a knowledge of the environmental history of a region is important in understanding present and future landscape change.
In the Pacific Northwest, the retreat of glacial ice created a landscape of stagnant ice and glacial meltwater debris in northern Washington, Idaho, and western Montana. This region was colonized by the biota that survived in the unglaciated region to the south, along the exposed coastal shelf, and in the highlands. What was the nature of vegetation in the unglaciated region? How did glacial-age communities respond as climates changed, deglaciated terrain became available, and new species entered the region? What environmental controls shaped the subsequent development of modern forests within both the glaciated and unglaciated regions? In what ways have present-day vegetation and plant communities in the Pacific Northwest been influenced by long-term changes in climate, substrate, biological interactions, and natural disturbance?
Present patterns of biodiversity in the Pacific Northwest represent the culmination of ecological, climatological, and geological processes spanning several time scales. Ecological research provides information on recent responses of vegetation to disturbance and rapid environmental change; the fossil record, however, offers complementary information by disclosing ways in which vegetation has responded to large-scale environmental perturbations in the past. The insights gained from paleoecology take on particular importance with the prospect of a 4-5° C warming in the next century as a result of increasing greenhouse gases (Houghton et al. 1990). To appreciate the changes in vegetation and plant communities that will ensue from a warming of this magnitude necessitates an examination of the paleoecologic record, and the limitations and assumptions associated with it.
The objective of this paper is to describe the vegetational and climatic history of the Pacific Northwest during the late-Quaternary period from 20 ka (kiloannum = 1,000 years before present) to the present day. My remarks are confined to Washington State and southern British Columbia, where the fossil record is richest and our understanding of the landscape history is most refined (Fig. 1). A broader discussion of the late-Quaternary vegetational history of the western United States (U.S.) can be found in review papers by Baker (1983), Heusser (1983), Mehringer (1985), Barnosky et al. (1987), and Thompson et al. (in press).
P.D. Jones, K.R. Briffa, T.J. Osborn, J.M. Lough, T.D. van Ommen, B.M. Vinther, J. Luterbacher, E.R. Wahl, F.W. Zwiers, M.E. Mann, G.A. Schmidt, C.M. Ammann, B.M. Buckley, K.M. Cobb, J. Esper, H. Goosse, N. Graham, E. Jansen, T. Kiefer, C. Kull, M. Küttel, E. Mosley-Thompson, J.T. Overpeck, N. Riedwyl, M. Schulz, A.W. Tudhope, R. Villalba, H. Wanner, E. Wolff, E. Xoplaki (2009). High-resolution palaeoclimatology of the last millennium: a review of current status and future prospects. The Holocene 19 (1): 3-49
ABSTRACT: This review of late-Holocene palaeoclimatology represents the results from a PAGES/CLIVAR Intersection Panel meeting that took place in June 2006. The review is in three parts: the principal high-resolution proxy disciplines (trees, corals, ice cores and documentary evidence), emphasizing current issues in their use for climate reconstruction; the various approaches that have been adopted to combine multiple climate proxy records to provide estimates of past annual-to-decadal timescale Northern Hemisphere surface temperatures and other climate variables, such as large-scale circulation indices; and the forcing histories used in climate model simulations of the past millennium. We discuss the need to develop a framework through which current and new approaches to interpreting these proxy data may be rigorously assessed using pseudo-proxies derived from climate model runs, where the `answer' is known. The article concludes with a list of recommendations. First, more raw proxy data are required from the diverse disciplines and from more locations, as well as replication, for all proxy sources, of the basic raw measurements to improve absolute dating, and to better distinguish the proxy climate signal from noise. Second, more effort is required to improve the understanding of what individual proxies respond to, supported by more site measurements and process studies. These activities should also be mindful of the correlation structure of instrumental data, indicating which adjacent proxy records ought to be in agreement and which not. Third, large-scale climate reconstructions should be attempted using a wide variety of techniques, emphasizing those for which quantified errors can be estimated at specified timescales. Fourth, a greater use of climate model simulations is needed to guide the choice of reconstruction techniques (the pseudo-proxy concept) and possibly help determine where, given limited resources, future sampling should be concentrated.