Century Drought Research Articles

A multi-proxy stalagmite record indicates a shift in forcing of twentieth century drought events in Normandy

Drought events are increasingly impacting Europe. The study of past droughts helps disentangle the different factors that trigger hydrological drought, helping to forecast future drought severity. Here we identify the historical drought events of the twentieth and twenty-first centuries in geochemical records of a stalagmite from Caumont cave in Northern France and develop a mechanistic understanding of their root causes. Subannually-resolved stable carbon isotope (δ13C) and trace element data are directly compared with historical climatic records. δ13C, Mg, and Sr peaks align well with most of the historical drought events of the twentieth and twenty-first centuries. The comparison reveals a good correspondence between summer effective rainfall and δ13C, Mg, and Sr concentrations. Further comparison of geochemical records with the precipitation-evapotranspiration index (SPEI) reveals a change in the drought forcings for the past two decades. During most of the twentieth century, droughts appear driven by a pluvial regime, whereas since the late 1990s, drought was driven by higher evapotranspiration due to increased temperature.

Projected 21st Century Drought Condition in the South Saskatchewan River Watershed: A Case Study in the Canadian Prairies

In this study, a CMIP6 ensemble of 26 GCMs and SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios from CanDCS-U6 is used to project drought conditions in the South Saskatchewan River Watershed. The near-current period (2015–2030) and two future periods (2041–2060 and 2071–2100) are analyzed based on the Standardized Precipitation Evapotranspiration Index (SPEI) at 1-, 3-, 6-, 12-, and 24-month timescales. Projections indicate a shift in average SPEI values from above zero (no drought) in the base period (1951–1990) to more negative values in the future. Results show an increase in drought severity and frequency under climate change conditions. The percentage of time with no drought conditions is projected to decline from 55–70% in the base period to 25–45% by 2071–2100. Severe and extreme droughts, rare in the base period (below 4%), are projected to increase to up to 19% by 2071–2100. The area experiencing drought is expected to expand from 36–49% (for different SPEI timescales) in the base period to up to 76% by 2071–2100. Drought frequency is projected to be higher under SSP1-2.6 and less frequent under SSP2-4.5. Results showed that longer SPEI timescales are associated with higher drought occurrence rates and severity. The spatial pattern of drought is also projected to significantly change, with higher frequencies expected in the eastern parts of the watershed under climate change.

The Impact of Drought on Terrestrial Carbon in the West African Sahel: Implications for Natural Climate Solutions

AbstractTerrestrial ecosystems store more than twice the carbon of the atmosphere, and are critical to climate change mitigation efforts. This has led to a proliferation of land‐based carbon sequestration efforts, such as re/afforestation associated with the Great Green Wall in the West African Sahel (WAS GGW). However, we currently lack comprehensive assessments of the long‐term viability of these ecosystems' carbon storage in the context of increasingly severe climate extremes. The WAS is particularly prone to recurrent and disruptive extremes, exemplified by the persistent and severe late‐20th century drought. We assessed the response and recovery of WAS GGW carbon stocks and fluxes to this late‐20th century drought, and the subsequent rainfall recovery, by leveraging a suite of terrestrial ecosystem models. While multi‐model mean carbon fluxes (e.g., gross primary production, respiration) partly recovered to pre‐drought levels, modeled total (above and below ground) ecosystem carbon stock falls to as much as two standard deviations below pre‐drought levels and does not recover even ∼20 years after the maximum drought anomaly. Furthermore, to the extent that the modeled regional carbon stock recovers, it is nearly entirely driven by atmospheric CO2 trends rather than the precipitation recovery. Uncertainties in regional ecosystem carbon simulation are high, as the models' carbon responses to drought displayed a nearly 10‐standard deviation spread. Nevertheless, the multi‐model average response highlights the strong and persistent impact of drought on terrestrial carbon storage, and the potential risks of relying on terrestrial ecosystems as a “natural climate solution” for climate change mitigation.

Development of maximum relevant prior feature ensemble (MRPFE) index to characterize future drought using global climate models

Drought is one of the foremost outcomes of global warming and global climate change. It is a serious threat to humans and other living beings. To reduce the adverse impact of drought, mitigation strategies as well as sound projections of extreme events are essential. This research aims to strengthen the robustness of anticipated twenty-first century drought by combining different Global Climate Models (GCMs). In this article, we develop a new drought index, named Maximum Relevant Prior Feature Ensemble index that is based on the newly proposed weighting scheme, called weighted ensemble (WE). In the application, this study considers 32 randomly scattered grid points within the Tibetan Plateau region and 18 GCMs of Coupled Model Intercomparison Project Phase 6 (CMIP6) of precipitation. In this study, the comparative inferences of the WE scheme are made with the traditional simple model averaging (SMA). To investigate the trend and long-term probability of various classes, this research employs Markov chain steady states probability, Mann–Kendall trend test, and Sen’s Slope estimator. The outcomes of this research are twofold. Firstly, the comparative inference shows that the proposed weighting scheme has greater efficiency than SMA to conflate GCMs. Secondly, the research indicates that the Tibetan Plateau is projected to experience “moderate drought (MD)” in the twenty-first century.

Streamflow response to drought in a managed coast redwood catchment

AbstractA 60‐year precipitation and streamflow record from the Caspar Creek Experimental Watersheds in northern California was used to explore the propagation of meteorological drought to hydrological drought. Standardized precipitation and runoff indices were calculated for the two forested catchments using integration periods of 12, 24, and 36 months. The resulting time series were used to define three severe drought events (1976–1977, 2013–2014, and 2020–2022). The earliest drought followed the 1971–1973 harvest of the 417 ha South Fork (SF) watershed, a second followed the 1989–1992 harvest of the 479 ha North Fork watershed, and a third followed the 2017–2019 reentry harvest of the SF. From these time series, we calculated drought metrics and anomalies to model differences in catchment responses in the context of climate and management. The meteorological drought in the 1977 event was more severe and extreme than the streamflow response. Both of the 21st Century droughts were hydrologically more severe than the 1977 drought. Timber harvest initially shortened and reduced streamflow drought (1977 and 2021) but prolonged and intensified the 2014 streamflow drought. Declining fall precipitation has reduced streamflows, thereby impeding salmonid migration and exacerbating impacts on native fish. Our results provide new insights into the role of climate variation, particularly long‐term and seasonal drought dynamics, in managed forests along the North American Pacific coast.

A novel semi data dimension reduction type weighting scheme of the multi-model ensemble for accurate assessment of twenty-first century drought

A novel semi data dimension reduction type weighting scheme of the multi-model ensemble for accurate assessment of twenty-first century drought

A novel self-adjusting weight approximation procedure to minimize non-identical seasonal effects in multimodel ensemble for accurate twenty-first century drought assessment

A novel self-adjusting weight approximation procedure to minimize non-identical seasonal effects in multimodel ensemble for accurate twenty-first century drought assessment

A 19th century hydroclimate chronology for the semi-arid Karoo, South Africa: Droughts and dry periods in perspective

The semi-arid central Karoo of South Africa is an important livestock farming region, oftentimes characterized by water scarcity and limited grazing resources. This paper builds on previous research that has predominantly focused on post 19th century Karoo rainfall characteristics. The paper aims to: 1) establish the long-term frequency and longevity of droughts or dry periods; 2) explore spatial hydroclimatic variability across the Karoo during given dry/drought periods; 3) compare the timing and duration of 19th century Karoo droughts with those of neighboring regions to the east (Lesotho) and north (southern Kalahari); 4) establish the possible role of the El Niño Southern Oscillation (ENSO) in drought occurrence across the Karoo-Kalahari-Lesotho region; 5) determine whether the so called ‘global drought’ of 1877-78 is evident in the central Karoo; and 6) place 19th century droughts in a 20th– 21st century context (or visa versa) for the Karoo. Documentary sources were accessed, photographed and digitized from a variety of repositories (Libraries, Museums, and online sites such as READEX). Multi-station instrumental rainfall records (1861–2022) were made available by the Rubidge family at Wellwood farm. Historical droughts (1861–2022) in the Karoo were spatially highly variable in severity. However, severe droughts in the Karoo have decreased in frequency and amplitude (rainfall deficit) between 1861 and the present.

Drought variations in the southeastern Tibetan Plateau region since 1704 and their link to the Atlantic Multidecadal Oscillation

Understanding the long-term history of drought in the Tibetan Plateau region is important to improve understanding of drought frequency in response to future global warming. Although the Tibetan Plateau has become generally wetter recently, the southeastern part of the Tibetan Plateau has become drier since the 2000s. To place this drying trend in a historical context, we conducted tree-ring studies for Abies spectabilis and Tsuga dumosa at three sites in the southeastern Tibetan Plateau, and developed tree-ring width (TRW), earlywood width (EWW), and latewood width (LWW) chronologies to permit local drought reconstruction. Based on correlation between these tree-ring parameters and instrumental meteorological data, the EWW chronologies were identified as suitable variables for reconstructing the average April–July self-calibrating Palmer Drought Index (scPDSI). The reconstruction accounted for 45.86% of the variance in the instrumental record and allowed us to extend the drought record back to 1704 CE. Based on this new dataset, the driest interval was from 1907 to 1919 CE during which time agricultural production fell by about 70% according to written historical records, leading to severe famine. Our findings suggest that this early twentieth century drought was in phase with the previously known drought over the northern Daxing'an Mountains. However, the southeastern Tibetan Plateau drought occurred one decade earlier than the known widespread drought event in North China during the 1920s–1930s. We show that this spatial drought pattern may have been modulated by the Atlantic Multidecadal Oscillation.

A Common Factor Analysis Based Data Mining Procedure for Effective Assessment of 21st Century Drought under Multiple Global Climate Models

A Common Factor Analysis Based Data Mining Procedure for Effective Assessment of 21st Century Drought under Multiple Global Climate Models

Resilience of riparian vegetation productivity to early 21st century drought in northern California, USA

AbstractDrought and intensive land use can interact as stressors on riparian vegetation, especially along rivers flowing through seasonally dry landscapes. Knowledge of past riparian vegetation response to drought and land use change can provide land managers with a better understanding of changes induced by upstream management actions, climate change, and chronic stressors. To investigate the response of riparian vegetation productivity to drought and land use, we developed a 21‐year time series (2000–2020) of growing season vegetation dynamics using near‐infrared reflectance of vegetation (NIRV) derived from satellite data across 30 watershed subbasins that drain into the San Francisco Bay Delta in central California, USA. We observed a strong response of riparian vegetation to drought, but rapid recovery and very few long‐term declines in productivity. At a local level, vegetation communities' response to drought and post‐drought productivity dynamics were highly variable across biophysical settings and land use gradients. Most of the riparian areas with long‐term declines in NIRV were located in the lower elevation Coast Range on the western side of the study area where there is little to no water engineering or agricultural irrigation runoff to subsidize riparian vegetation. Riparian areas with the greatest long‐term increase were along rivers draining the higher elevation Sierra Nevada range to the east. Our results suggest that river systems with a high proportion of water originating as snowmelt may be more buffered against long‐term drought‐driven declines in productivity than those dependent exclusively on winter rainfall. The long‐term increase in NIRV in the vast majority of riparian areas within our study area may also have been driven in part by increasing atmospheric CO2 concentrations, which have been shown to increase plant water use efficiency.

Changes in the surface water – Groundwater interactions of the Murray-Darling basin (Australia) over the past half a century

The Murray-Darling Basin is Australia’s most stressed water resource and most contentious in the sharing of those water resources. Our hydrological models used for water management have no information on the surface water – groundwater interactions within a reach and so implicitly assume that their relationship is stationary. This study seeks to test this assumption by looking at the direction of surface water – groundwater exchanges and how they have changed over the 49-year period from 1970 to 2019 at the whole of basin scale (492 river reaches). This was achieved by comparing the groundwater level at 59,340 bores to the elevation of the surface water at the closest point on the stream network. Each point evaluated on the stream network was classified as either gaining (water from groundwater) or losing (water to groundwater) at an annual scale. These point scale observations were aggregated to the reach scale (as used in river systems models) for analysis of trends though time. The proportion of bores predicting losing conditions was falling through the 1970’s and 1980’s before reaching a minimum of 48% in 1995/96. From this period the proportion of bores predicting losing conditions increased to a maximum at the end of the analysis period of 78% in 2018/19. This finding at the point scale is replicated at the reach scale. The reaches with greater than 80% of bores predicting losing conditions had a minimum in 1993/94 (at 40%) and increased to 66% at the end of the analysis period. These observations are consistent with the management challenges of the time. In the 1980’s and 1990’s gaining conditions leading to increasing salt loads in the streams caused by groundwater discharge were a major problem for downstream water users. Since the turn of the century drought and water scarcity have been the major challenges with falling groundwater levels leading to increases in losing conditions. This study has shown that the surface water – groundwater interactions are not stationary and have been changing through time with many reaches having changed direction from gaining to losing throughout the analysis period. This is highlighting a deficiency in our hydrological models that cannot replicate this change in flux direction and therefore leading to errors in predicting low flows.

Water balance characterization of the early 21st century drought in the western United States

AbstractMonthly temperature and precipitation data for 923 United States Geological Survey 8‐digit hydrologic units are used as inputs to a monthly water balance model to compute monthly actual evapotranspiration, soil moisture storage, and runoff across the western United States (U.S.) for the period 1900 through 2020. Time series of these water balance variables are examined to characterize and explain the dry conditions across the western U.S. since the year 2000. Results indicate that although precipitation deficits account for most of the changes in actual evapotranspiration and runoff, increases in temperature primarily explain decreases in soil moisture storage. Specifically, temperature has been particularly impactful on the magnitude of negative departures of soil moisture storage during the spring (April through June) and summer (July through September) seasons. These effects on soil moisture may be particularly detrimental to agriculture in regions already stressed by drought such as the western U.S.

Rangewide climatic sensitivities and non-timber values of tall Sequoia sempervirens forests

Heavily exploited for its reddish, decay-resistant heartwood, the tallest conifer, Sequoia sempervirens, is a major component of coastal forests from extreme southwestern Oregon to California’s Santa Lucia Mountains. Primary Sequoia forests are now restricted to < 5 % of their former distribution, and mature secondary forests with trees over 60 m tall are even scarcer due to repeated logging. Leveraging allometric equations recently derived from intensive work in both forest types, we climbed, measured, and core-sampled 235 trees in 45 locations distributed across the species range to examine growth trends and understand how tall Sequoia are responding to recent environmental changes. Paired samples of sapwood and heartwood collected along the height gradient were used to quantify Sequoia investment in decay resistance. During the 20th century, trees in most locations began producing more wood than expected for their size with this growth surge becoming pronounced after 1970 and ending around 2000. Radial increments—ring widths—correlate with climatic variables related to water availability, and these relationships are strengthening as temperatures rise. Sensitivity to drought increased from north to south along a 6° latitudinal gradient of decreasing precipitation and summer fog frequency. Sequoia trees north of 40° were least sensitive to drought, producing similar biomass annually during dry and wet years, whereas trees farther south produced less biomass during individual drought years. Hotter 21st century drought barely affected Sequoia growth efficiency (biomass increment per unit leaf mass) north of 40° until the fourth consecutive year (2015), when growth efficiency dropped precipitously, recovering within two years. South of 40°, Sequoia trees exhibited steadily declining growth efficiency during the multi-year drought followed by recovery, but recovery did not occur south of 37° despite ample precipitation in 2017. Sequoia growth efficiency is currently highest in secondary forests north of 40°, where trees produce relatively small amounts of heartwood with the lowest decay resistance (least fungicide) while receiving the most nocturnal summer fog. Increasing sink limitations, whereby rising temperatures, drier air at night, and extreme tree height collectively lower turgor pressure to inhibit cambial activity, may reduce Sequoia growth efficiency while contributing to more durable biomass production. Heartwood and fungicide increments are higher in primary than secondary forests across the species range. Crown structural complexity promotes development of vascular epiphytes and arboreal soil habitats in Sequoia forests with sufficient moisture availability. These habitats are lacking in secondary forests and rare in primary forests south of 40°. After logging, restoration of tall Sequoia forests can be achieved via silviculture that maximizes height increments during early stand development and then retains some dominant trees in perpetuity, allowing them to gain full stature, produce increasingly decay-resistant heartwood, and support significant arboreal biodiversity.

Featured Collection Introduction: Severe Sustained Drought Revisited: Managing the Colorado River System in Times of Water Shortage 25 Years Later — Part I

Featured Collection Introduction: Severe Sustained Drought Revisited: Managing the Colorado River System in Times of Water Shortage 25 Years Later — Part I

Tree Rings Reveal Unmatched 2nd Century Drought in the Colorado River Basin

AbstractThe ongoing 22‐year drought in the Upper Colorado River Basin (UCRB) has been extremely severe, even in the context of the longest available tree‐ring reconstruction of annual flow at Lees Ferry, Arizona, dating back to 762 CE. While many southwestern drought assessments have been limited to the past 1,200 years, longer paleorecords of moisture variability do exist for the UCRB. Here, gridded drought‐atlas data in the UCRB domain along with naturalized streamflow data from the instrumental period (1906–2021) are used in a K‐nearest neighbor nonparametric algorithm to develop a streamflow reconstruction for the Lees Ferry gage starting in 1 CE. The reconstruction reveals a second‐century drought unmatched in severity by the current drought or by well‐documented medieval period droughts in the UCRB. Although data are sparse, analysis of individual long tree‐ring records and other paleoclimatic data also support the occurrence of an exceptional second‐century drought.

Twenty-first century drought analysis across China under climate change

Twenty-first century drought analysis across China under climate change

Sensitivity and threshold dynamics of Pinus strobus and Quercus spp. in response to experimental and naturally occurring severe droughts.

Increased drought frequency and severity are a pervasive global threat, yet the capacity of mesic temperate forests to maintain resilience in response to drought remains poorly understood. We deployed a throughfall removal experiment to simulate a once in a century drought in New Hampshire, USA, which coupled with the region-wide 2016 drought, intensified moisture stress beyond that experienced in the lifetimes of our study trees. To assess the sensitivity and threshold dynamics of two dominant northeastern tree genera (Quercus and Pinus), we monitored sap flux density (Js), leaf water potential and gas exchange, growth and intrinsic water-use efficiency (iWUE) for one pretreatment year (2015) and two treatment years (2016-17). Results showed that Js in pine (Pinus strobus L.) declined abruptly at a soil moisture threshold of 0.15m3m-3, whereas oak's (Quercus rubra L. and Quercus velutina Lam.) threshold was 0.11 m3m-3-a finding consistent with pine's more isohydric strategy. Nevertheless, once oaks' moisture threshold was surpassed, Js declined abruptly, suggesting that while oaks are well adapted to moderate drought, they are highly susceptible to extreme drought. The radial growth reduction in response to the 2016 drought was more than twice as great for pine as for oaks (50 vs 18%, respectively). Despite relatively high precipitation in 2017, the oaks' growth continued to decline (low recovery), whereas pine showed neutral (treatment) or improved (control) growth. The iWUE increased in 2016 for both treatment and control pines, but only in treatment oaks. Notably, pines exhibited a significant linear relationship between iWUE and precipitation across years, whereas the oaks only showed a response during the driest conditions, further underscoring the different sensitivity thresholds for these species. Our results provide new insights into how interactions between temperate forest tree species' contrasting physiologies and soil moisture thresholds influence their responses and resilience to extreme drought.

Twenty-first century droughts have not increasingly exacerbated fire season severity in the Brazilian Amazon

Biomass burning in the Brazilian Amazon is modulated by climate factors, such as droughts, and by human factors, such as deforestation, and land management activities. The increase in forest fires during drought years has led to the hypothesis that fire activity decoupled from deforestation during the twenty-first century. However, assessment of the hypothesis relied on an incorrect active fire dataset, which led to an underestimation of the decreasing trend in fire activity and to an inflated rank for year 2015 in terms of active fire counts. The recent correction of that database warrants a reassessment of the relationships between deforestation and fire. Contrasting with earlier findings, we show that the exacerbating effect of drought on fire season severity did not increase from 2003 to 2015 and that the record-breaking dry conditions of 2015 had the least impact on fire season of all twenty-first century severe droughts. Overall, our results for the same period used in the study that originated the fire-deforestation decoupling hypothesis (2003–2015) show that decoupling was clearly weaker than initially proposed. Extension of the study period up to 2019, and novel analysis of trends in fire types and fire intensity strengthened this conclusion. Therefore, the role of deforestation as a driver of fire activity in the region should not be underestimated and must be taken into account when implementing measures to protect the Amazon forest.

Upper Colorado River Basin 20th century droughts under 21st century warming: Plausible scenarios for the future

This study builds on a collaboration with a water resource management community of practice in the Upper Colorado River Basin to develop scenarios of future drought and assess impacts on water supply reliability. Water managers are concerned with the impacts of warming on water year streamflow, but uncertainties in projections of climate make the application of these projections to planning a challenge. Instead, water managers considered a plausible scenario for future drought to be historical droughts to which warming is added. We used a simple statistical model of water year streamflow with temperatures increased by 1 °C to 4 °C, and then examined reductions in flow and runoff efficiency (RE) with each degree of warming for the six droughts defined in the observed streamflow record. In order to place these results into a management context, we employed an existing framework for system reliability, and in particular, a vulnerability assessment for water delivery metrics. Using modeled streamflow resulting from 1 °C to 4 °C warming, we found vulnerable condition thresholds for the two water delivery metrics assessed, Upper Basin Shortage and Lees Ferry Deficit, were crossed relatively infrequently at +1 °C, but with a substantially increased frequency under additional warming. Results are more relevant to resource management because the impacts of warming on Upper Colorado River streamflow were assessed in the context of management metrics and vulnerability thresholds, in collaboration with members of the water management community of practice.