David J. Sauchyn

A paleoclimatic context for the drought of 1999–2001 in the northern Great Plains of North America

The least annual precipitation in the western interior of North America occurs in the northern Great Plains, including an area that encompasses parts of south-eastern Alberta, south-western Saskatchewan and eastern Montana. During 1999–2001, most climate stations in this region had record low precipitation. This paper examines this three-year drought in the context of historical climate records from Medicine Hat, Alberta and Havre, Montana and summer (June–July) and annual (August–July) precipitation reconstructed from standardized tree-ring widths (residual chronologies) from Pinus contorta (lodgepole pine) sampled in the Cypress Hills of Alberta and Saskatchewan and the Bears Paw Mountains of north-central Montana. Drought is operationally defined as precipitation in the lower 10th and 20th percentiles. Plots of reconstructed precipitation and cumulative departure from median values indicate a shift in climate variability prior to the twentieth century, when EuroCanadians settled in this region. The eighteenth and nineteenth centuries are characterized by sustained periods of progressively wetter and drier conditions, including prolonged drought. Various archival sources document the significant impacts of these prolonged droughts. While drought was frequent in the twentieth century, it tended to be of short duration and the impacts also were ameliorated by intervening periods of relatively high precipitation. Increasing aridity in response to global warming could expose a larger area of the northern Great Plains to the impacts of drought.

A continuous record of Holocene pollen from Harris Lake, southwestern Saskatchewan, Canada

Abstract Holocene vegetation change in the West Block of the Cypress Hills, on the northern Great Plains, is interpreted from a continuous 9120-year pollen record. This is one of the few continuous pollen records available from the northern Great Plains which extends to the early Holocene. The West Block rises to a maximum elevation of 1465 m, and is a mosaic of grassland, Populus forest, coniferous forests, and wetland vegetation. Because of the ecotones present climatic changes produced vegetation changes discernible from the pollen record. The record indicates that a Populus forest-grassland complex was established by 9000 yr BP; very little coniferous forest was present at that time. The predominance of grassland and saline-tolerant vegetation between 7700 and 5000 yr B.P. defines the altithermal for this region. Climatic deterioration followed, as indicated by increases in relative amounts of conifer and aquatic taxa pollen. The Pinus contorta and Picea glauca forests characteristics of the Cypress Hills today only have been prevalent since about 4600 yr B.P.; both species were present in very low numbers throughout the altithermal. By 3230 yr B.P. the modern vegetation was established, with little change since then.

The tree-ring record of drought on the Canadian Prairies

Ring-width data from 138 sites in the Canadian Prairie Provinces and adjacent regions are used to estimate summer drought severity during the past several hundred years. The network was divided into five regional groups based on geography, tree species, and length of record: the eastern Rockies, northern Saskatchewan, central Manitoba, southern Manitoba, and northwestern Ontario. Regional tree-ring records are primarily related to summer moisture and drought conditions, and are less responsive to droughts caused by deficits in winter precipitation. These summer-sensitive data are not linearly related to major modes of climate variability, including ENSO and the Pacific decadal oscillation (PDO), which primarily affect the climate of western Canada during winter. Extended drought records inferred from tree rings indicate that drought on the Canadian Prairies has exhibited considerable spatial heterogeneity over the last several centuries. For northern Saskatchewan and northwestern Ontario, intervals of persistently low tree growth during the twentieth century were just as long as or longer than low-growth intervals in the eighteenth or nineteenth centuries. Longer records from southern Alberta suggest that the most intense dry spell in that area during the last 500 yr occurred during the 1720s. At the eastern side of the prairies, the longest dry event is centered around 1700 and may coincide with low lake stands in Manitoba, Minnesota, and North Dakota. Although the Canadian Prairies were dry at times during the 1500s, there is no regional analog to the sixteenth-century ‘‘megadroughts’’ that affected much of the western United States and northern Mexico.

Toward understanding nonstationarity in climate and hydrology through tree ring proxy records

Natural proxy records of hydroclimatic behavior, such as tree ring chronologies, are a rich source of information of past climate-driven nonstationarities in hydrologic variables. In this study, we investigate tree ring chronologies that demonstrate significant correlations with streamflows, with the objective of identifying the spatiotemporal patterns and extents of nonstationarities in climate and hydrology, which are essentially representations of past “climate changes.” First and second-order nonstationarities are of particular interest in this study. As a prerequisite, we develop a methodology to assess the consistency and credibility of a regional network of tree ring chronologies as proxies for hydrologic regime. This methodology involves a cluster analysis of available tree ring data to understand and evaluate their dependence structure, and a regional temporal-consistency plot to assess the consistency of different chronologies over time. The major headwater tributaries of the Saskatchewan River basin (SaskRB), the main source of surface water in the Canadian Prairie Provinces, are used as the case study. Results indicate that stationarity might never have existed in the hydrology of the region, as the statistical properties of annual paleo-hydrologic proxy records across the basin, i.e., the mean and autocorrelation structure, have consistently undergone significant changes (nonstationarities) at different points in the history of the region. The spatial pattern of the changes in the mean statistic has been variable with time, indicating a time-varying cross-correlation structure across the tributaries of the SaskRB. Conversely, the changes in the autocorrelation structure across the basin have been in harmony over time. The results demonstrate that the 89 year period of observational record in this region is a poor representation of the long-term properties of the hydrologic regime, and shorter periods, e.g., 30 year periods, are by no means representative. This paper highlights the need to broaden the understanding of hydrologic characteristics in any basin beyond the limited observational records, as an improved understanding is essential for more reliable assessment and management of available water resources.

A Proxy Record of Drought Severity for the Southwestern Canadian Plains

This paper examines the statistical relationships between Palmer Drought Severity Index (PDSI) data from the southwestern Canadian plains and tree-ring index chronologies from nearby sites. Standardized tree-ring widths from white spruce (Picea glauca) from the Cypress Hills (Alberta and Saskatchewan) and from lodgepole pine (Pinus contorta) from the Bears Paw Mountains (Montana) account for 47% and 39% of the variance in regional July PDSI. The corresponding regressions are the basis for the first reconstructions of PDSI for the Canadian plains. This proxy PDSI record extends from 1597 and demonstrates that, although 1937 was the worst single drought year, the 20th century lacked the prolonged droughts of the 18th and 19th centuries, when decades of July PDSI were consistently below zero. Clusters of drought years in the 1690s, 1720s, 1750s–60s, 1790s–1800s, 1820s, 1850s–60s and 1890s, support the notion of a 20- to 25-year drought cycle for the northern Great Plains. These prolonged droughts lower the resistance of ecosystems and soil landscapes to disturbance from hydroclimatic events, such that thresholds of landscape change are exceeded and the recovery of natural systems can take decades or centuries. These droughts also seriously affect the soil and water resources that support dryland agriculture.

A global multiproxy database for temperature reconstructions of the Common Era

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850–2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.

A Holocene paleosalinity diatom record from southwestern Saskatchewan, Canada: Harris Lake revisited

Fossil diatoms were analysed from a 10.3 m core from Harris Lake, Cypress Hills, Saskatchewan, and a diatom-salinity transfer function was used to construct a history of Holocene salinity changes for the lake. The diatom paleosalinity record indicates that Harris Lake remained fresh <0.5 g l-1 throughout the Holocene, with only slight increases in salinity between approximately 6500 and 5200 years BP. This interval corresponds to the only period in the lakes history when planktonic diatoms were abundant; benthic Fragilaria taxa, mainly F. pinnata, F. construens and F. brevistriata were dominant throughout most of the Holocene. The shift from a benthic to a planktonic diatom flora between 6500 and 5200 years BP may be an indirect response to a warmer climate that reduced forest cover in the watershed and allowed greater rates of inorganic sedimentation. The small salinity increase that accompanies the floristic change is probably not the result of lower lake levels; in fact the lake was probably deeper at this point than in the later Holocene. This paleosalinity record indicates that Harris Lake did not experience episodes of hypersalinity during the mid-Holocene, as suggested by a previous study, and that the lake may have been fresh during the early Holocene as well.

Climate Warming and 21st‐Century Drought in Southwestern North America

Since 2000, southwestern North America has experienced widespread drought. Lakes Powell and Mead are now at less than 50% of their reservoir capacity, and drought or fire-related states of emergency were declared this past summer by governors in six western states. As with other prolonged droughts, such as the Dust Bowl during the 1930s, aridity has at times extended from northern Mexico to the southern Canadian prairies. A synthesis of climatological and paleoclimatological studies suggests that a transition to a more arid climate may be occurring due to global warming, with the prospect of sustained droughts being exacerbated by the potential reaction of the Pacific Ocean to warming.

Modes and Forcing of Hydroclimatic Variability in the Upper North Saskatchewan River Basin Since 1063

In this paper the mean water year (October through September) flow of the North Saskatchewan River (NSR) at Edmonton, Alberta is reconstructed back to 1063 A.D. using a new network of moisture-sensitive tree-ring chronologies from limber pine and Douglas fir at seven sites in the headwater sub-basins of the North Saskatchewan River Basin (NSRB). Over the full extent of the proxy hydrometric record (10632007), we examined 1) the duration and severity of low flow, 2) the dominant frequencies of periodic variability and 3) the correlation between these significant periodicities in proxy streamflow and climate indices, specifically sea surface temperature oscillations, which are known drivers of regional hydroclimatic variability. This new record of the paleohydrology of the NSRB is compared to previous tree-ring reconstructions of the annul flow of the North and South Saskatchewan Rivers. Extending the reference hydrology for the basin from decades to centuries changes perceptions of the reliability of the water supply and understanding of the hydroclimatic variability. The gauge record not does represent the full extent of interannual to multidecadal variability in the tree-ring data; there are periods of low flow in the pre-instrumental record that are longer and more severe than those recorded by the gauge.

Long-term reliability of the Athabasca River (Alberta, Canada) as the water source for oil sands mining

Significance We show that current and projected surface water allocations from the Athabasca River, Alberta, Canada, for the exploitation of the Alberta oil sands are based upon an untenable assumption of the representativeness of the short instrumental gauge record. Our trend analysis of the instrumental data shows declining regional flows. Our tree-ring reconstruction shows periods of severe and prolonged low flows not captured by the instrumental record. Exploitation of the Alberta oil sands, the world’s third-largest crude oil reserve, requires fresh water from the Athabasca River, an allocation of 4.4% of the mean annual flow. This allocation takes into account seasonal fluctuations but not long-term climatic variability and change. This paper examines the decadal-scale variability in river discharge in the Athabasca River Basin (ARB) with (i) a generalized least-squares (GLS) regression analysis of the trend and variability in gauged flow and (ii) a 900-y tree-ring reconstruction of the water-year flow of the Athabasca River at Athabasca, Alberta. The GLS analysis removes confounding transient trends related to the Pacific Decadal Oscillation (PDO) and Pacific North American mode (PNA). It shows long-term declining flows throughout the ARB. The tree-ring record reveals a larger range of flows and severity of hydrologic deficits than those captured by the instrumental records that are the basis for surface water allocation. It includes periods of sustained low flow of multiple decades in duration, suggesting the influence of the PDO and PNA teleconnections. These results together demonstrate that low-frequency variability must be considered in ARB water allocation, which has not been the case. We show that the current and projected surface water allocations from the Athabasca River for the exploitation of the Alberta oil sands are based on an untenable assumption of the representativeness of the short instrumental record.