Multi-year Drought Research Articles

The role of heavy rainfall in drought in Australia

Droughts are initiated and significantly maintained by precipitation deficits. This study demonstrates that meteorological droughts in Australia are strongly related to changes in the occurrence of heavy rainfall events. We use the standardised precipitation index to identify periods of drought onset, drought development, and recovery from drought. This method successfully identifies the major multi-year drought periods in Australia, which have major impacts on a range of sectors. We show that the occurrence of meteorological (i.e. precipitation-deficit) drought in Australia is regulated by changes in days of significant rainfall. Days with rainfall above 20 mm are responsible for a median of approximately 60% of the reduction in rainfall during drought development periods, and approximately 70% of the rainfall surplus during drought recovery periods. These changes in days of higher rainfall accumulation, which are the greatest contributors to the total reduction in rainfall during drought, equate to very few days per year of rainfall: less than 10 days/year equivalent for the majority of the continent. It is a reduction in these heavy, but infrequent rainfall days that results in a location being classified as in meteorological drought. Similarly, an above-average occurrence of such days will indicate a period of at least partial recovery from drought. Both the frequency of heavy rainfall days and their contribution to accumulated rainfall are demonstrated to be important. These results indicate that an investigation of the synoptic weather systems responsible for heavy rainfall days, as well as changes in the frequency of occurrence of these systems and their relative contributions to total rainfall, is necessary to advance the understanding of drought in Australia.

Forest restoration mitigates drought vulnerability of coast Douglas-fir in a Mediterranean climate

Multi-year drought and climate change can impact tree growth, especially in California's Mediterranean climate where growing season rainfall is limited or absent. Active forest restoration has the potential to mitigate climate impacts by reducing stand density and conversion towards more resilient species' composition. We used dendrochronology methods to examine climate–growth relationships for coast Douglas-fir ( Pseudotsuga menziesii var. menziesii) trees in mixed multiaged stands near the species’ natural southern range margin. We found positive correlations of ring width with spring–early summer and prior October precipitation and an evapotranspiration index. Additionally, cooler spring temperature was negatively correlated with growth. We also studied tree resistance, resilience, and recovery from two multi-year drought events. Restoration treatments enhanced resistance and resilience to drought relative to trees growing in untreated plots. We did not detect differences in drought resistance and resilience between two common restoration methods, giving managers options for restoration to lessen drought impacts on tree growth.

Patterns and Distribution of Botryosphaeriaceae Fungi Related to Dieback in Big Berry Manzanita.

Dieback and mortality in wildland plant species due to climate change have been on the rise in recent decades, and latent fungal pathogens might play a significant role in these events. During a severe multiyear drought, canopy dieback associated with latent pathogens in the Botryosphaeriaceae (Bot) family was observed in stands of a dominant shrub species, big berry manzanita (Arctostaphylos glauca), across chaparral landscapes in California. These fungi are significant pathogens of woody agricultural species, especially in hosts experiencing stress, and have become a threat to economically important crops worldwide. However, little is known regarding their occurrence, distribution, and impact in wildland systems. We conducted a field survey of 300 A. glauca shrubs across an elevational gradient to identify Bot species infection as it relates to (i) A. glauca dieback severity and (ii) landscape variables associated with plant drought stress. Our results show that Bots are widely infecting A. glauca across the landscape, and there is a significant correlation between elevation and dieback severity. Dieback severity was significantly higher at lower elevations, suggesting that infected shrubs at lower elevations are at greater risk than those at higher elevations. Furthermore, two Bot species, Neofusicoccum australe and Botryosphaeria dothidea, were most frequently isolated, with N. australe being the most common and, based on haplotype analysis, likely the most recently introduced of the two. Our results confirm the wide distribution of latent Bot fungi in a wild shrubland system and provide valuable insight into areas of greatest risk for future shrub dieback and mortality. These findings could be particularly useful for informing future wildlands management strategies with regard to introduced latent pathogens.

Shift in precipitation-streamflow relationship induced by multi-year drought across global catchments

Increases in the intensity and frequency of droughts affected by climate changes induce greater uncertainty in precipitation (P) and streamflow (Q) relationship (P–Q relationship). Here, alteration in P–Q relations were assessed resulted from multi-year drought (≥7 years), lag and amplification effects were analyzed between meteorological and hydrological droughts, and then hydrological legacy induced by droughts were presented using the leaf area index (LAI) and the Horton index (ratio of watershed ET to catchment wetness) in different arid regions of 1210 selected catchments across global catchments. Results show that reduced P causes lower Q in arid regions, while tends to induce a higher Q in humid regions than expected. Generally, the severity and intensity were amplified in hydrological drought compared with its triggering meteorological drought. Interestingly, Q reduction was more likely to be induced by meteorological drought in more arid climates, while more likely to be recovered from Q deficit than meteorological drought in the humid regions. Unexpected, vegetation, stimulated by prolonged meteorological and hydrological droughts, tended to maintain higher LAI and subsequently resulted in a lower Horton index, especially in the humid regions. Combined with a traditional bucket model, a conceptual model was developed to elucidate threshold switching characteristics during the drought propagation, and deduced that vegetation played a vital role in partitioning of P and regulating how the catchment coped with climate changes. These new understanding of the hydrological legacy of meteorological drought provides important insights into hydrological mechanisms and the ability of ecology to regulate hydrological processes.

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.

Prolonged drought duration, not intensity, reduces growth recovery and prevents compensatory growth of oak trees

Global forests suffer from increasing drought pressure, as drought are more intense, and last for longer, and occur more frequently, but how drought intensity and duration affect post-drought growth recovery and compensatory growth is still poorly known. Understanding the immediate and subsequent impacts of drought on tree growth is key to predict future forest dynamics. Here, we used a network of tree-ring data of 735 Quercus mongolica trees from 27 sites of North China representing a large portion of its distribution to study the effects of drought intensity and duration on radial growth. We analyzed growth resistance, recovery, and resilience of Q. mongolica by linking it to drought intensity and duration, and further quantified the post-drought growth trajectories. Our results confirmed that current summer drought is a major influence on radial growth of Q. mongolica across its distribution. While drought occurrence was related to decreased growth even for incipient droughts, we found no differences in growth decline with drought severity. Although drought intensity did not alter growth decline in dry years, drought intensity was positively related to recovery and resilience indices. The damage of recovery is due to the extension of drought duration rather than drought intensity. However, post-drought growth was even more strongly related to drought duration and not intensity. Q. mongolica showed compensatory growth in the year following single-year droughts before returning to pre-drought growth-levels, while multi-year drought led to little to no compensatory growth and prolonged growth reductions. Hence, multi-year drought is the key to predict post-drought recovery of Q. mongolica including compensatory growth in an increasingly arid climate.

Floodplain trophic subsidies in a modified river network: managed foodscapes of the future?

ContextCross-boundary subsidies create important growth opportunities for a range of taxa. In modified river systems, remnant patches of floodplain and flood bypasses become ephemeral hotspots of zooplankton production, however, the extent to which these prey items are (or could be) transported downstream is unclear.ObjectivesWe investigated the diet of juvenile salmon under varying hydroclimatic conditions to assess the importance of floodplain-produced prey subsidies in an otherwise food-scarce region.MethodsJuvenile salmon (n = 3033) and zooplankton were sampled across the California Central Valley Sacramento-San Joaquin River Delta in 2014–2018, incorporating a range of climatic conditions including drought and flood. Salmon stomach fullness and diet composition, and ambient zooplankton densities were used to assess spatiotemporal patterns in prey production and consumption.ResultsFloodplain-produced cladocerans provided ephemeral food pulses to juvenile salmon in downstream riverine habitats. Salmon had the fullest stomachs in wetter years (2016–2017) and the emptiest stomachs in the final year of a multi-year drought (2015). Cladoceran abundances in the water column and salmon diets were highest during wet periods and below floodplains, and decreased with increasing distance downstream, consistent with flow-mediated trophic subsidies.ConclusionsThese data emphasize the importance of maintaining diverse, interconnected habitats to support resilient fish populations and the potential for managing floodplains to boost prey production and delivery. Here, the inundation of a flood bypass (or lack of) played a pivotal role shaping the juvenile salmon foodscape. As freshwater ecosystems are increasingly transformed by large-scale engineering, it is important to coordinate infrastructure, habitat and flow modifications to maximize climate resilience and trophic benefits to target species.

Multi-year drought storylines for Europe and North America from an iteratively perturbed global climate model

Throughout history, Europe and North America have experienced intense and long-lasting droughts with large impacts on society and ecosystem such as the recent drought 2018/2019 and historical drought 1540 in Europe, or the US Dust Bowl of the 1930s. To increase resilience and develop adaptation strategies to such extreme droughts, it is important to understand how dry a worst-case drought would be and how long it would take to recover from it. This study introduces and evaluates a methodological framework to generate coherent climate model-based drought storylines of different severities and for different locations. The so-called iterative ensemble resampling method repeatedly runs large ensembles and only keeps those ensemble members, which minimize local precipitation. The drought storylines are developed with the fully coupled global climate model CESM1. The first part of the analysis demonstrates the feasibility of the framework by generating some of the most extreme droughts possible. Using stringent precipitation criteria, accumulated precipitation is reduced by 80% relative to the long-term average in western Europe and by 77% in central North America, respectively, over multiple years. The number of dry days in the Western European storyline corresponds to estimates in the reconstructed drought 1540 in Central Europe. The low precipitation induces soil moisture deficit storylines that are physically consistent but beyond high return levels estimated based on purely statistically fitted generalized extreme value (GEV) distributions. In the second part, the drought storylines are used as a setup to assess the recovery time of such extreme soil moisture deficits. Over the driest regions in central and western Europe as well as central and eastern North America, the soil moisture recovers over a period of a few months up to more than five years, depending on the mean atmospheric circulation rather than on the strength of the soil moisture deficit. The framework of iterative ensemble resampling can generate up to very rare physically consistent storylines to conduct idealized experiments. When lowering the selection criteria for precipitation, the framework can be used to generate less extreme drought storylines that are more likely to occur in the real world. This approach can help to stress test the socio-economic system and adaptation strategies for potential long-lasting drought periods.

The dietary impacts of drought in a traditional pastoralist economy.

Arid pastoralism is often understood as an adaptive strategy to marginal environments. As pastoralists become increasingly market integrated, novel dietary preferences and access to low quality market foods can erode traditional diets. These market-based dietary shifts are particularly problematic during sustained drought, where reductions in traditional foods make pastoralists increasingly reliant on a cash economy. Among the Himba of the Kunene region in Namibia, colonial policies prohibiting access to livestock markets inhibit access to a cash-based economy, leaving them vulnerable to food insecurity when nontraditional foods are needed to supplement traditional lifeways during drought. To understand the impacts of long-term drought on diet and food insecurity, we collected longitudinal survey data on diet breadth and food insecurity across 4 years during a multi-year drought. Participants completed a five-item food insecurity survey and recalled diet breadth survey over the course of 4 years (N= 191-234). Additionally, women completed a short survey of recent stressors, including health and resource stressors (N= 127). We used a set of multilevel models to estimate changes in food insecurity items and diet breadth changes over the course of the study period. Multilevel models predicted score outcomes, as well as individual item responses, by year of data collection. Results indicate a 43% increase in average food insecurity and a 15% decline in average diet breadth over the study period. Dietary recall indicates that drought caused a reduction in sour milk intake, and an increase in nontraditional foods, but no change in meat or maize consumption. Sustained drought in the Kunene region is having long-term impacts on food insecurity, which could result in dietary shifts that outlast the current period of drought. We consider the implications of this change, especially as it relates to increasing market integration and reliance on a cash-based over a subsistence-based economy.

Exploring farmers' perceptions and lessons learned from the 2015–2018 drought in the Western Cape, South Africa

The agricultural sector is one of the most vulnerable sectors to the impacts of climate change. Between 2015 and 2018, the Western Cape Province of South Africa experienced a multi-year severe drought. Projections show that the Western Cape is likely to experience hotter and drier conditions, with more frequent droughts. Without appropriate adaptation actions, climate change is likely to increasingly constrain agricultural activities in the province. Commercial farmers represent a considerable population of decision-makers, which are fundamental to climate change adaptation. Understanding farmers' perceptions is important to develop effective policy, support structures, and communications. This study aimed to understand wheat farmers' and apple producers' perceptions of climate change and adaptation in the Western Cape, South Africa, and establish whether the recent drought offered lessons for adaptation. Study methods included the use of an online questionnaire as well as several in-depth interviews with farmers and producers. Results showed that most farmers and producers agree that climate change is real and is caused by human activities. Most farmers and producers in the region are already actively (or intend to start) preparing for climate change (69%). In response to climate change, apple producers view on-farm water management (such as irrigation management and water recycling) as the most important strategy. Wheat farmers strategies are focused on crop management (including cultivar selection and conservation agriculture). Many farmers and producers further agreed that they had learnt from the past 2015–2018 drought. Notably, results showed that farmers and producers who rely a great deal on weather forecasts were more likely to feel that their farm's response was effective. Furthermore, it was found that farmers and producers who felt they learned from the drought were also more likely to be actively preparing for climate risks. It is recommended that investments into climate change adaptation focus on research and development, particularly with regard to cultivar development, irrigation management, tailored weather forecasting, and localised risk assessments. Policy should prioritise the more vulnerable farmers and producers while focusing on integrated risk reduction measures which account for multiple stressors.

The tale of three floods: From extreme events and cascades of highs to anthropogenic floods

Right after a devastating multi-year drought, a number of flood events with unprecedented spatial extent hit different parts of Iran over the 2-week period of March 17th to April 1st, 2019, causing a human disaster and substantial loss of assets and infrastructure across urban and rural areas. Here, we investigate natural (e.g., rainfall, snow accumulation/melt, soil moisture) and anthropogenic drivers (e.g., deforestation, urbanization, and management practices) of these events using a range of ground-based data and satellite observations. These drivers can range from exceptionally extreme rainfall intensities, to cascades of several extreme and moderate events, and various anthropogenic interventions that exacerbated flooding. Our results reveal strong compounding impacts of natural drivers and anthropogenic triggers in escalating flood risks to unprecedented levels. We argue that a new form of floods, i.e. anthropogenic floods, is becoming more common and should be recognized during the “Anthropocene”. This specific form of floods refers to high to extreme streamflow/runoff events that are primarily caused, or largely exacerbated, by anthropogenic drivers. We demonstrate how the growing risk of anthropogenic floods can be assessed using a wide range of climatic and non-climatic satellite and in-situ data.

Long-term drought aggravates instability of alpine grassland productivity to extreme climatic event.

The frequency and severity of extreme weather events are increasing and expected to increase more in the future, together with global change. However, how extreme events and global change factors interactively influence community structures and ecosystem processes is largely unknown. Here, we investigated the responses of the temporal stability and resilience of aboveground net primary productivity (ANPP) of an alpine meadow to an extreme flooding event under different treatments of experimental drought and clipping. We found that ecosystems that were exposed to drought treatments for 3 years significantly decreased the temporal stability of community productivity but increased resilience to flooding, whereas their resistance to or recovery from flooding did not change. Neither clipping nor its interaction with drought altered the responses of these community stability metrics to flooding. Drought treatments significantly decreased plant species richness and asynchrony and dominant species stability, leading to a decrease in temporal stability and an increase in resilience in response to the extreme flooding event. The study also revealed that the change in species asynchrony was the dominant impact pathway determining the responses of resilience and temporal stability to flooding. Our results highlight that alpine grassland that experiences a multiyear drought may aggravate the instability of community productivity to extreme climatic events by reducing species asynchrony.

Large increases of multi-year droughts in north-western Europe in a warmer climate

Three consecutive dry summers in western Europe (2018–2019–2020) had widespread negative impacts on society and ecosystems, and started societal debate on (changing) drought vulnerability and adaptation measures. We investigate the occurrence of multi-year droughts in the Rhine basin, with a focus on event probability in the present and in future warmer climates. Additionally, we investigate the temporally compounding physical drivers of multi-year drought events. A combination of multiple reanalysis datasets and multi-model large ensemble climate model simulations was used to provide a robust analysis of the statistics and physical processes of these rare events. We identify two types of multi-year drought events (consecutive meteorological summer droughts and long-duration hydrological droughts), and show that these occur on average about twice in a 30 year period in the present climate, though natural variability is large (zero to five events can occur in a single 30 year period). Projected decreases in summer precipitation and increases in atmospheric evaporative demand, lead to a doubling of event probability at 1 ^circC additional global warming relative to present-day and an increase in the average length of events. Consecutive meteorological summer droughts are forced by two, seemingly independent, summers of lower than normal precipitation and higher than normal evaporative demand. The soil moisture response to this temporally compound meteorological forcing has a clear multi-year imprint, resulting in a relatively larger reduction of soil moisture content in the second year of drought, and potentially more severe drought impacts. Long-duration hydrological droughts start with a severe summer drought followed by lingering meteorologically dry conditions. This limits and slows down the hydrological recovery of soil moisture content, leading to long-lasting drought conditions. This initial exploration provides avenues for further investigation of multi-year drought hazard and vulnerability in the region, which is advised given the projected trends and vulnerability of society and ecosystems.

Changes to California Alfalfa Production and Perceptions during the 2011–2017 Drought

California experienced a severe multiyear drought stretching from 2011 through 2017, significantly reducing surface water supply for ecosystems, agriculture, and humans and prompting coordinated conservation efforts. Given that agriculture is the largest consumptive use of water in the state, one anticipated response to a severe drought would be to decrease production of low-value, high-water-use crops such as alfalfa. In this article we use a multimethod approach to examine both spatial distribution and public perceptions of alfalfa production in California over the course of the 2011 through 2017 drought. We find that although California alfalfa production did decline at the state level, it persisted and even increased in specific areas of the state. We also find that alfalfa persisted even though discourses and understandings that were critical of alfalfa production emerged in public forums during this time. We situate these findings within a broader context of California’s water management system, which meant that in practice, infrastructure and water rights allocation practices left many growers with little incentive to change growing practices, even in the face of serious meteorological drought.

Adaptive Reservoir Management by Reforming the Zone-based Hedging Rules against Multi-year Droughts

In this study, the zone-based hedging rule, which is the main operating policy adopted from multipurpose reservoirs in Korea is adjusted to reflect the multi-year droughts caused by climate change. Annual synthetic inflow series with different magnitudes of long memory were generated using the autoregressive fractional integrated moving average (ARFIMA) model. The generated inflow series were then disaggregated into 10-day series and utilized as input variables to derive the alternative hedging rules. The alternative hedging rules from this study were used in adaptive reservoir management by newly updated information. Finally, the performance of the suggested policy is measured in terms of frequency and magnitude under the historical inflow series. As a result, adaptive reservoir management demonstrated improvements in the following terms of the frequency of critical failures (water deficit ratio greater than 30%): 6.14% of the simulation period in the status quo (SQ) policy, and 2.99% in the adaptive management. However, the overall reliability of the reservoir during the simulation horizon was better when operated with the SQ policy (41.19%) than the results from adaptive management (26.42%). Because this result is in a good agreement with the original objective of the hedging rules, the adaptive policy suggested in this study holds promise and may be utilized in further reservoir management with an increase of potential drought risk from climate change.

Large-Scale Sea Surface Temperature Forcing Contributed to the 2013–17 Record-Breaking Meteorological Drought in the Korean Peninsula

Abstract This study examined the contribution of the Pacific decadal oscillation (PDO) to the record-breaking 2013–17 drought in the Korean Peninsula. The meteorological drought signal, measured by the Standardized Precipitation Index (SPI), in 2013 and 2016 co-occurred with a heat wave. The positive phase of the PDO during the mid-2010s was responsible for the precipitation deficit, particularly in 2014, 2015, and 2017, resulting in 5 years of meteorological drought. The enhanced atmospheric heating anomalies over the subtropical central Pacific, induced by the in situ PDO-related sea surface temperature (SST) warming, led to a low-atmospheric cyclonic flow centered over the midlatitude Pacific. The northerly wind anomalies at the western edge of this low-level cyclonic flow were responsible for the horizontal negative advection of moist energy, which contributed to the decreased precipitation and the resultant negative SPI over the Korean Peninsula in 2014, 2015, and 2017. The large-ensemble simulation supported the observational findings that the composited SST anomalies during the 5 years of persistent drought exhibited prominent and persistent SST warming over the subtropical central Pacific, along with large-scale cyclonic flow over the North Pacific. The findings of this study imply that the SST anomalies over the North Pacific and subtropical central Pacific can be a predictable source to potentially increase the ability to forecast multiyear droughts over the Korean Peninsula.

Forest treatment effects on wood production in ponderosa pine

Forest treatments reduce wildfire risk and can promote the vigor and production of remaining trees, but they are also a disturbance. Understanding the type, timing, and longevity of tree response to treatment, as well as the potential for interactive effects of treatments and drought, could help managers plan and evaluate forest management practices. Environmental drivers, biological modifiers, and tree capacity to respond to prior disturbances were concurrently tested to predict ponderosa pine basal area increment (BAI) in a lowland and upland dry pine forest in south central Oregon, USA. Environmental drivers included current year and lags or running averages of a drought index, SPEI, and the sum or count of growing degree days >0°C or 10°C. Biological modifiers of environmental drivers considered pre-treatment response to disturbance, tree vigor, and tree-to-tree competition. A model was developed to predict BAI in both topographic positions for applicability to the landscape level, and then was used to test for specific differences in BAI between paired forest treatments differing by one treatment. Forest treatments tested included no management (NM), undercut and even spacing harvest (HE), prescribed fire (Rx), and their combinations. HE significantly increased BAI shortly after treatment. Post-harvest, one or two Rx did not provide additional BAI benefits, nor in the absence of HE, did 2Rx vs. 1Rx treatment. The 1Rx treatment was imposed between multi-year droughts; BAI significantly increased after the treatment and was resistant to droughts. Upland trees were affected by a single year of drought; lowland trees responded only after sequential drought years. A single treatment, HE or 1Rx appeared to be as effective as multiple or mixed treatments in improving BAI in dry pine forest stands. HE appeared to generate the largest effect. Timing of forest treatments relative to site water balance may affect short term (decadal) wood production.

Large loss and rapid recovery of vegetation cover and aboveground biomass over forest areas in Australia during 2019–2020

Australia experienced multi-year drought and record high temperatures, and massive forest fires occurred across the southeast in 2019 and early 2020. In the fire-affected forest areas, understory and often tree canopies were burned, and in-situ observations in late 2020 reported rapid vegetation recovery, including grasses, shrubs, and tree canopies from burned-but-not-dead eucalyptus trees. Considering the strong fire resilience and resistance of eucalyptus trees and above-average rainfall in 2020, we assessed how much and how quickly vegetation structure and biomass changed from loss to post-fire and drought recovery in 2020 for all forest areas in Australia. Here, we analyzed space-borne optical, thermal, and microwave images to assess changes in the structure and function of vegetation using four vegetation indices (VIs), leaf area index (LAI), solar-induced chlorophyll fluorescence (SIF), gross primary production (GPP), and aboveground biomass (AGB). We found that all eight variables show large losses in 2019, driven by fires and climate (drought and high temperature), but large gains in 2020, resulting from the high resilience of most trees to fire and rapid growth of understory vegetation under wet condition in 2020. In 2019, the forest area has an AGB loss of 0.20 Pg C, which is ~15% of the pre-fire AGB. Attribution analyses showed that both fire and climate (prior and co-occurring severe drought and record high temperatures) are responsible for the AGB loss in 2019, approximately 0.09 Pg C (fire) and 0.11 Pg C (climate), respectively. In 2020, the forest area has a total AGB gain of 0.26 Pg C, composed of 0.22 Pg C from fire-affected forest area and 0.04 Pg C from fire-unaffected forest area. Fire-adapted Eucalyptus forests and above-average annual precipitation in 2020 brought by a moderate La Niña drove the recovery of vegetation cover, productivity, and AGB. The results from this study shows the potential of multiple sensors for monitoring and assessing the impacts of fire and climate on the forest areas in Australia and their post-fire recovery.

Impacts of climate change on global meteorological multi-year droughts using the last millennium simulation as a baseline

Multi-year droughts have been widely studied all over the world for their extraordinary long duration and irreparable impacts. However, they are difficult to be estimated due to the short period of observed time series, especially in the context of climate change and on a global scale. The advanced development of climate model simulations with a long period makes it possible to investigate the multi-year drought change on a global scale. Therefore, this study quantifies the impacts of climate change on global meteorological multi-year droughts for the historical period (1850–2005) and the future period (2006–2100) using the pre-industrial last millennium period (850–1849) as a baseline. Meteorological droughts longer than 5-year are identified using a three-dimensional (latitude-longitude-time) method based on a 12-month scale Standard Precipitation Index (SPI12). Return periods are estimated using tri-variate copulas based on drought duration, area, and severity. The results show that multi-year droughts occur more in North America, South America, Asia, and Africa, accounting for more than 85% during 850–2100. The return period of 61% and 70% multi-year droughts for the historical and future periods becomes shorter than that for the last millennium, indicating more severe drought conditions for these two periods. The main change for the future period is the higher incidence of multi-year droughts with a longer duration and greater severity, especially for North America, Asia, and Africa. Besides, the shift of multi-year drought centroids in Central America, East Asia, Southern Africa, and the Mediterranean implies more severe drought conditions for the future period. Overall, this study indicates that climate change has the potential effects to exacerbate global meteorological multi-year droughts for the future period compared to the last millennium.

Storylines of UK drought based on the 2010–2012 event

Abstract. Spatially extensive multi-year hydrological droughts cause significant environmental stress. The UK is expected to remain vulnerable to future multi-year droughts under climate change. Existing approaches to quantify hydrological impacts of climate change often rely solely on global climate model (GCM) projections following different emission scenarios. This may miss out low-probability events with significant impacts. As a means of exploring such events, physical climate storyline approaches aim to quantify physically coherent articulations of how observed events could hypothetically have unfolded in alternative ways. This study uses the 2010–2012 drought, the most recent period of severe hydrological drought in the UK, as a basis and analyses storylines based on changes to (1) precondition severity, (2) temporal drought sequence, and (3) climate change. Evidence from multiple storylines shows that the maximum intensity, mean deficit, and duration of the 2010–2012 drought were highly influenced by its meteorological preconditions prior to drought inception, particularly for northern catchments at shorter timescales. The influence of progressively drier preconditions reflects both the spatial variation in drought preconditions and the role of physical catchment characteristics, particularly hydrogeology in the propagation of multi-year droughts. There are two plausible storylines of an additional dry year with dry winter conditions repeated either before the observed drought or replacing the observed dramatic drought termination confirm the vulnerability of UK catchments to a “third dry winter” storyline. Applying the UKCP18 climate projections, we find that drought conditions worsen with global warming with a mitigation of drought conditions by wetter winters in northern catchments at high warming levels. Comparison of the storylines with a benchmark drought (1975–1976) and a protracted multi-year drought (1989–1993) shows that, for each storyline (including the climate change storylines), drought conditions could have matched and exceeded those experienced during the past droughts at catchments across the UK, particularly for southern catchments. The construction of storylines based on observed events can complement existing methods to stress test UK catchments against plausible unrealised droughts.