We evaluated the influence of climate change on fire weather in Australia using the McArthur Forest Fire Danger Index (FFDI) and climate simulations from an ensemble of dynamically downscaled CMIP6 projections. Extreme FFDI events were assessed under a range of Global Warming Levels (GWLs) using the Generalised Extreme Value (GEV) distribution, with a focus on southeast Australia’s wildfire prone eucalyptus forests. The magnitude and frequency of extreme FFDI events are projected to increase substantially, particularly in southern Australia. For the eucalyptus forests of southeast Australia, 20-year and 50-year return interval 7-day FFDI events (i.e. weekly average FFDI extremes) are projected to become approximately 2.1 and 2.5 times more likely under 3 °C of global warming. The increases are most notable for the eucalyptus forests of Tasmania, with projected 20-year and 50-year return interval 7-day FFDI events becoming approximately 3.2 and 4.1 times more likely at 3 °C GWL.

Severe convective events are impactful and costly for Australia’s economy and environment. The impact of climate change on these events is unclear due to competing changes in atmospheric instability, vertical wind shear and convective inhibition. Here, we use an ensemble of high-resolution (10 km) climate simulations, including a set of atmosphere-only and ocean-coupled simulations, dynamically downscaled from CMIP6 global climate models (GCMs), to examine the impact of climate change on severe convective environments (SEV), and environments conducive to hail (SEV-hail) in Australia. We found days with SEV are projected to increase across most of the continent and in most capital cities. The largest increases to SEV and SEV-hail were found in summer (DJF) and spring (SON), the seasons where SEV and SEV-hail occur most often in the present. We use global warming levels (GWLs) to look at the impact of climate change and found high model agreement on the sign of change, with the signal of SEV increases emerging from the noise at 3°C of global warming in DJF across most of Australia. The largest increases in SEV days in capital cities were found in Sydney and in the Australian Capital Territory (ACT), with both areas having approximately 6 additional SEV days in DJF with 3°C of global warming. Trends in agricultural areas were similar to trends in the cities, with New South Wales (NSW) agricultural areas having approximately an additional 7 SEV days in DJF, and Queensland having an additional 3 days. SEV-hail decreased or did not change in most areas, but increased in south-western Australia, and in Perth and Adelaide by about 1–2 days at 3°C of global warming in DJF. Our results show generally increasing risk to cities and agricultural areas from a warmer future and highlight the need to enhance resilience to mitigate and adapt to climate change.

Climate change is projected to lead to changes in rainfall patterns, which, when coupled with increasing evapotranspiration, have the potential to exacerbate future droughts. This study investigates the impacts of climate change on meteorological droughts in Australia using downscaled high-resolution CMIP6 climate models under three Shared Socioeconomic Pathway (SSP) scenarios. The Standardised Precipitation Index (SPI) and the Standardised Precipitation Evapotranspiration Index (SPEI) were used to assess changes to the frequency, duration, percent time, and spatial extent of droughts. There were consistent increases in droughts projected for southwest Western Australia, southern Victoria, southern South Australia, and western Tasmania using SPI and SPEI. There were significantly larger increases for SPEI-derived droughts, with consistent increases projected for most of the country. Increases in drought appear to have mostly come at the expense of “normal” climatic conditions, with similar or increased time spent under extreme wet conditions, indicating an overall shift towards more extreme climatic conditions. The largest increases occurred at the end of the century and under the high-emissions scenario (SSP370), demonstrating the influence of emissions on extreme droughts. For instance, if emissions reached high levels by the end of the century, the area subject to extreme drought in drought-prone Southern Australia would be 2.8 times greater than if they were kept to low levels using SPI and 4 times greater if assessed using SPEI. The insights generated from these results and supplementary tailored datasets for Australian local government areas and river basins are essential to better inform decision-making and future adaptation strategies at national, regional, and local scales.

Droughts and heatwaves are amongst the most frequent hazards in Australia. They cause severe impacts to society and the environment, and the magnitude of impacts increase when they are compounded- occurring simultaneously or sequentially. Using CMIP6 projections dynamically downscaled with the Conformal Cubic Atmospheric Model to a 10 km resolution over Australia, we examine the impact of climate change on compound drought heatwave events (CDHWs). We define droughts using the Standardized Precipitation Index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI), and for heatwaves use the Excess Heat Factor. In the historical period, most models capture the temperature-precipitation correlation well, however, most fail to capture the observed strengthening of this relationship over time. The models also over-estimate the number of CDHWs. In the historical period, the choice of SPEI or SPI has little impact on CDHWs, however for climate change impacts, using SPEI results in more severe impacts with less uncertainty. Under SSP370, by the end of the century, for SPEI CDHWs, there are around 3 more events/year, versus little change for SPI CDHWs. However, the duration of both events increase by around 3 days. A global warming level (GWL) analysis shows spikes in impacts when moving from 2 to 3°C, with a doubling in the number of SPEI CDHWs for Northern Australia. Moving from 3 to 4°C of warming, the length of SPEI CDHWs increases from 8 to 13 days in Northern Australia. This highlights the urgent need to reduce emissions to avoid more than 2°C of warming. A storyline analysis reveals that even in a wetter future, CDHWs will increase due to increasing temperatures and heatwaves, showing that as the climate warms, CDHWs are likely to increase despite uncertainty in precipitation projections. This research applies a novel approach to examine CDHWs using downscaled climate simulations, considering two drought metrics, GWLs and storylines, offering new insights to disentangle uncertainty in future compound extreme events.

Incorporating climate change into intensity duration frequency (IDF) curves is broadly conducted using either a climate model simulation–based approach or a covariate–based approach. However, as of now, there has been no research comparing these two approaches in the context of rainfall IDF derivation under climate change. To this end, this study evaluates the 1–h annual rainfall maxima from an ensemble of 60 CORDEX–CMIP6 simulations using a high–resolution regional climate model, the Conformal Cubic Atmospheric Model, for Australia. We quantify rainfall changes for the near (2041–2070) and far (2071–2100) future compared to a reference period (1961–1990) across various durations and Annual Exceedance Probabilities (AEPs) under three emissions scenarios for 39 locations across Australia. We then compare these projections with covariate–based frequency model projections. The 1–h extreme (1 in 100 AEP) hindcast event shows a negative bias relative to observations, with a wide degree of variability across the ensemble. Projected changes for a high emissions scenario with a 3°C of global temperature increase show a median increase of 33.9% for 1–h and 18.9% for 1–day extreme events by the end of the century. Additionally, the reference 1 in 100 AEP event is projected to be 2.3 and 1.6 times more frequent for the 1–h and 1–day durations, respectively. Projections also indicated extreme rainfall increases at the rate of 8.7%°C−1, which exceeds Clausius–Clapeyron (CC) scaling for 1–h duration events, and nearly equal CC scaling for longer 1–day durations. Covariate–based projections indicated larger quantile increases for 1–h events with no change for 1–day events. Regional downscaling provides robust evidence for extreme rainfall changes despite uncertainties.

Quantifying the impact of climate change on actual and potential evapotranspiration (AET and PET) is essential for water security, agricultural production and environmental management. Using dynamically downscaled CMIP6 models at 10 km resolution, we assess AET and PET at a daily time step using the Morton method and projected future changes to both PET and AET under three emission scenarios (SSP126, 245, 370) for Australia. The performance of observation- and downscaled climate model-based AET is assessed against measured AET from 26 OzFlux sites in Australia. We show that high resolution downscaled climate models can provide reasonably accurate estimations of AET, with an ensemble mean error of 17 % for historical period 1981–2010. This compared favourably to observation- and reanalysis-based products, which reported mean errors ranging from 15.7 %–44 %. Annual average end-of-century AET projections for low and intermediate emission scenarios (SSP126 and SSP245) show a decrease of −4.5 % and −3.5 % respectively in Australia, while under high emissions (SSP370) AET was projected to increase by 1.8 %. In contrast, PET was projected to increase by 5.0 % for SSP126, 8.4 % for SSP245 and 11.5 % for SSP370. Using a random forest model, we show that the primary controlling factors for changes in AET are precipitation and solar radiation, and solar radiation and maximum temperature for PET. Our results offer new insights into future AET and PET changes estimated using downscaled CMIP6 simulations with implications for agriculture, water supply and natural resource management.

We investigated projected changes to daily mean, common extreme (99th and 99.7th percentile), and rare extreme (annual exceedance probability (AEP) 1 in 10, 50, and 100) precipitation events across Australia and its greater capital cities using a large ensemble of downscaled CORDEX-CMIP6 simulations. The largest increases in precipitation extremes were seen over northern Australia, with the 1 in 100 AEP event in Darwin projected to increase by approximately 11.9% K−1. Other capital cities had lower increases but still substantial (7.6% K−1 for Brisbane, 7.3% K−1 for Sydney, 3.4% K−1 for Melbourne, and 4.4% K−1 for Perth). Large spatial differences were noted among the downscaled ensembles, highlighting the need for large ensembles to ensure uncertainties in host models and downscaling methods can be accounted for. The findings can inform decision making around flood management, urban planning, urban water supply and agriculture around Australia, in addition to revealing globally relevant scientific insights.

Regional climate projections are critical for climate risk assessments and planning. Previous studies have mostly focused on evaluating climate models for temperature and precipitation biases without identifying the origins of these biases, which remain poorly understood. To address this gap, we conducted a process-based evaluation to examine the atmospheric processes and physical mechanisms influencing regional climate simulations. We used downscaled data from the Conformal Cubic Atmospheric Model (CCAM) driven by European Centre for Medium-Range Weather Forecasts Reanalysis v5 (ERA5) over Australia at 10 km spatial resolution. Using ERA5 driven downscaling is useful for model evaluation, as it avoids the inherent biases from initial and boundary conditions present in free running CMIP6 coupled global climate models. We found that CCAM well represented the atmospheric circulation at various vertical levels, performing comparably to or better than previous studies. CCAM also successfully captured key large-scale circulation features, such as the monsoonal westerly, subtropical jet, and Australian precipitation teleconnections with climate modes of variability (e.g., El Ni & ntilde;o-Southern Oscillation and Indian Ocean Dipole). CCAM underestimated mean sea level pressure in northern Australia, resulting in stronger wind circulation (monsoonal westerly) over ocean north of Australia. This led to dry biases in humidity over northern Australia and contributed to dry biases over the north and northeast coasts. CCAM simulated stronger north-south wind and moisture flux in central Australia, which may be associated with wet biases over western Queensland. This study highlights CCAM's ability to simulate atmospheric circulation and sheds light on the source of biases.

High-resolution climate change projections are required to evaluate local and regional climate change impacts. We used CCAM (Conformal Cubic Atmospheric Model) to dynamically downscale CMIP6 GCMs (Global Climate Models) over Australia under three emissions scenarios, producing a set of 60 simulations at a 10 km resolution. Previous work has evaluated the performance of the downscaled models in the historical period. Here, we evaluate the impact of end-of-century climate change in the downscaled CMIP6-CCAM models for mean and extreme climate under three Shared Socioeconomic Pathways (SSP126, 245 and 370). We find the changes in mean climate are in general similar in the host CMIP6 and downscaled models. For extreme temperature, we find that extreme maximum temperatures (TXx) increase by 3.4 degrees C, while extreme minimum temperatures (TNn) warm by 3.0 degrees C. Extreme precipitation generally increases in summer and decreases in winter; however, there is a large amount of inter-model variation in the location and magnitude of change. Consecutive dry days also decrease in most areas in Austral summer and increase in Austral winter. Heatwaves become more frequent and hotter by the end of the century. These results suggest a hotter, wetter Austral summer, with longer, more frequent and more intense heatwaves, and a hotter and drier Austral winter in most areas. This dataset provides useful new high-resolution information on how climate change is likely to impact Australia, which will be a valuable resource to underpin local adaptation responses to future impacts.

Abstract. Climate change is projected to lead to changes in rainfall patterns, which, when coupled with increasing evapotranspiration, has the potential to exacerbate future droughts. This study investigates the impacts of climate change on meteorological droughts in Australia using downscaled high-resolution CMIP6 climate models under three Shared Socioeconomic Pathway (SSP) scenarios. The Standardised Precipitation Index (SPI) and the Standardised Precipitation Evapotranspiration Index (SPEI) were used to assess changes to the frequency, duration, percent time, and spatial extent of droughts. There were consistent increases in droughts projected for south-west Western Australia, southern Victoria, southern South Australia, and western Tasmania using SPI and SPEI. There were significantly larger increases for SPEI derived droughts, with consistent increases projected for most of the country. The largest increases occurred at the end of the century and under the high emissions scenario (SSP370), demonstrating the influence of emissions on extreme droughts. For instance, if emissions reached high levels by the end of the century, the area subject to extreme drought in drought prone Southern Australia would be 2.8 greater than if they were kept to low levels using SPI, and 4 times greater if assessed using SPEI. The insights generated from these results and supplementary tailored datasets for Australian Local Government Areas and River Basins are essential to better inform decision making and future adaptation strategies at national, regional, and local scales.

Climate change is predicted to significantly alter hydrological cycles across the world, affecting runoff, streamflow, and pollutant loads from diffuse sources. The objectives of this study were to examine the impacts of climate change on streamflow, total nitrogen (TN), total phosphorus (TP), and total suspended sediment (TSS) loads in the subtropical Logan-Albert catchment, Queensland, Australia. We calibrated the Soil Water Assessment Tool (SWAT) against event monitoring data in the Logan and Albert rivers, respectively. Hydrological and water quality effects of an ensemble of 11 dynamically downscaled high-resolution climate models were assessed with SWAT under high (Representative Concentration Pathway 8.5 - RCP8.5) and intermediate (RCP4.5) emission scenarios. Streamflow decreased most in winter and spring and decreased least in summer. This followed the predicted seasonal changes for precipitation, although decreases tended to be amplified due to increasing evaporative loss. TSS, TN, and TP loads showed a similar pattern to streamflow, with the largest decreases predicted for the dry season under RCP8.5 by the 2080s. Annual TSS load decreased by 34.3 and 54.2%, TN load decreased by 29.8 and 30.5%, and TP load by 24.9 and 4.4% for the Logan and Albert sites, respectively. The results of this study indicate that for subtropical river-estuary systems, climate warming may lead to lower streamflow and contaminant loads, reduced flushing, and greater relative importance of point source loads in urbanising catchments.

•Effects of climate change and sea level rise on flows and floods were assessed.•High and mean flows decreased substantially in winter and spring.•Large variability in the projections of major flood events.•Larger flood events tended towards an increase and smaller events did not change.•Sea level rise significantly increased floodplain inundation by the 2050s and 2080s.

The Logan-Albert estuary in southeast Queensland, Australia, has high biodiversity and supports multiple economic and recreational services. Elevated nutrient and sediment loads have been a longstanding management issue for the estuary. We investigated the spatial and seasonal patterns of nutrients and turbidity along the Logan-Albert estuary and assessed the effects of a recently constructed upstream dam. Nutrient concentrations and turbidity levels were analysed using 15 years of monitoring data from 19 water quality sites throughout the estuary. We hypothesised that the construction of Wyaralong Dam would act as a nutrient and sediment sink which may have positive effects on downstream water quality. Long-term trends of water quality constituents were evaluated using a non-parametric seasonal Mann-Kendall test and the effect of upstream impoundment was assessed with a Before-After Control-Impact (BACI) test. Nutrient concentrations and turbidity levels declined significantly with time in the upper Logan estuary and, to a lesser extent, in the lower Albert estuary. The general improvement of water quality in the upper Logan estuary was attributed to construction of the Wyaralong Dam. Significant decreases in concentrations of total phosphorus (TP) and oxidised nitrogen (NOx-N) along the lower Albert were principally attributed to wetter conditions over the 15-year dataset, which diluted point-source loads from a nearby wastewater treatment plant (WWTP). Our results show that estuarine water quality changes can be highly dynamic with interactions amongst climate and management practices that necessitate long-term monitoring programs with good spatial coverage.

Tropical and subtropical regions can be particularly severely affected by flooding. Climate change is expected to lead to more intense precipitation in many regions of the world, increasing the frequency and magnitude of flood events. This paper presents a review of studies assessing the impacts of climate change on riverine flooding in the world's tropical and subtropical regions. A systematic quantitative approach was used to evaluate the literature. The majority of studies reported increases in flooding under climate change, with the most consistent increases predicted for South Asia, South East Asia, and the western Amazon. Results were more varied for Latin America and Africa where there was a notable paucity of studies. Our review points to the need for further studies in these regions as well as in Australia, in small to mid-sized catchments, and in rapidly urbanising catchments in the developing world. Adoption of non-stationary flood analysis techniques and improved site-specific socio-economic and environmental model scenarios were identified as important future directions for research. Data accessibility and mitigation of model uncertainty were recognised as the principal issues faced by researchers investigating the impacts of climate change on tropical and subtropical rivers.