The simulated impact of land cover change on climate extremes in eastern Australia

In this paper, we investigate the impact of historical land cover change on climate extremes in eastern Australia by analysing data from an ensemble of model simulations using CSIRO AGCM. The model simulations were performed for two sets of prescribed land surface parameters representative of pre- European and modern-day land cover conditions. To evaluate the impact of historical land cover change on Australian regional climate, the CSIRO AGCM was used to complete two sets of model simulations (ensemble of 10 each) for the period 1951-2003. In this study, we used the CSIRO climate model consisting of atmospheric and land surface components forced by observed sea surface temperature and sea ice data for the period 1951-2003 (Rayner et al., 1996). This experimental set-up followed the design of the Climate of the 20th Century project (Folland et al., 2002) and allows for direct comparison between observed and model simulated ENSO events which are known to strongly influence Australian climate. The only difference between the experiments was the land surface characteristics for Australian continent used by the CSIRO model. The first set of model simulations used the modern-day and the second used the pre-European land cover characteristics. Outside Australia, the land cover characteristics were set at modern day conditions for both experiments. The modern-day land surface conditions were derived using data from the AVHRR satellite imagery for the period 1981 to 2001 at an 8km spatial footprint (Lawrence, 2004). The monthly long-term average values of vegetation cover class, leaf area index, vegetation fraction and surface albedo were used as an input to the Simple Biosphere Model (SiB) derivation methods described in Sellers et al.(1986) to compute land surface characteristics used by the CSIRO climate model. Pre-clearing land surface parameters of vegetation fraction, leaf area index, surface albedo and stomatal resistance were generated by extrapolating the modern-day monthly values of remnant native vegetation to the pre-European coverage (see Lawrence, 2004). The extrapolation was performed for the Australian continent at an ~8×8km resolution and aggregated to ~200×200km resolution used by CSIRO AGCM using the approach of Shuttleworth, (1991), thereby ensuring the seasonal dynamics captured by satellite imagery were represented in pre-European parameters. The impact of land cover change on mean climate in Australia was described in McAlpine et al.(2007). The results showed a statistically significant increase in mean annual surface temperature and decrease in mean annual rainfall in southeast Australia. On a seasonal basis, the impact of land cover change was strongest during the summer season and was especially pronounced during strong El Nino events such as the 2002/03 event. In this paper, we focus on the impact of land cover change on the climate extremes by analysing the daily statistics of rainfall and temperature change over the period 1951-2003. To quantify the changes in annual distribution of daily rainfall and temperature, we computed the probability distribution functions (pdfs) of daily maximum surface temperature (tmax) and daily rainfall for selected locations in eastern Australia. In addition, the daily rainfall and temperature data was used to derive climate extreme indices of dry days (number of days with rainfall <1 mm), daily rainfall intensity (total annual rainfall / number of rain days), rain days (number of days with rainfall ≥1 mm) and hot days (number of days with tmax ≥35ºC) (Frich et al., 2002). The analysis results showed statistically significant changes in the annual pdfs of rainfall and temperature in southeast Australia, which corresponds well with areas with largest fragmentation of pre-European vegetation cover. The fragmentation of vegetation resulted in an increase in the number of hot days, a decrease in daily rainfall intensity and a decrease in cumulative rainfall on rainy days in southeast Australia. These changes were especially pronounced during strong El Nino events

The influence of air-sea interaction on the Madden-Julian Oscillation: The role of the seasonal mean state

The CSIRO Mark 3 general circulation model at T63 resolution is used to explore the potential effect of air-sea interaction in enhancing the eastward propagation of the Madden-Julian Oscillation (MJO). Principal component analysis is used to define a seasonal lower-tropospheric wind signal. When the model is coupled with an interactive ocean, the monsoon wind anomalies in December-February (DJF) propagate from the Indian Ocean to the Pacific Ocean. In versions with a thinner mixed layer, the propagation speed approaches that seen in the observational ERA40 data set. However, in the non-interactive model with specified sea surface temperatures (SSTs) there is no propagation. Similar contrasts are seen in other seasons. The upper tropospheric long-wave signal determined through spectral analysis is also more realistic in the coupled model, although power around the 80 day period remains too large. Positive SST anomalies form to the east of low-level convergence, in part due to evaporative flux that is modified by the mean monsoon westerly belt in DJF. Interannual variations in this belt appear to have an effect on the propagation of the wind anomalies in the coupled model, while only the amplitude varies in the non-interactive model. This contrast is also seen in partitions of years by the state of ENSO. Propagation of the MJO signal is faster and extends farther into the Pacific in El Niño years in observations and the coupled model, although model biases, in particular a short westerly belt, appear to limit the effect. It is concluded that air-sea interaction is potentially very important to the MJO and its interannual variability, and that the westerly belt has an influence on its evolution

Modelling impacts of vegetation cover change on regional climate

Extensive areas of native vegetation in Queensland and other states have been cleared for agriculture, improved pastures and urban development. However, the potential impact of land clearing on Australia’s climate has been largely ignored in current climate change projections and policies. In this study, we addressed the question - is Australia’s regional climate sensitive to land cover change? We conducted simulation experiments using the CSIRO MARK 3 climate model to compare the effects on regional climate based on differences between pre-European and 1990 vegetation cover. The two experiments aimed to reproduce the Australian climate for the period 1951-2003, with the only difference being the conversion of land cover from native vegetation to pastures and crops. Consistent with actual climate trends since the 1950s, simulated annual and seasonal surface temperatures showed statistically significant warming for eastern Australia (0.4-2°C) and southwest Western Australia (0.4-0.8°C), being most pronounced in summer. Mean summer rainfall showed a decrease of 4-12% in eastern Australia and 4-8% in southwest Western Australia which coincided with regions where the most extensive land clearing has occurred. Further, the study found an increase in temperatures on average by 2°C, especially in southern Queensland and New South Wales, for the recent 2002/2003 drought. The findings suggest that the large scale clearance of native vegetation is amplifying the adverse impacts associated with El Niño drought periods, which together with rainfall deficiency, is having a strong impact on Australia’s already stressed natural resources and agriculture. Implications for Policy: We suggest that policy needs to recognise that climate change is a two-way process, and that broad scale clearing of native vegetation cover has a strong influence on climate in addition to greenhouse gases. Protecting and restoring Australia's native vegetation therefore needs to be a critical policy and management consideration in mitigating the effects of climate change

Anthropogenic effects on the subtropical jet in the Southern Hemisphere: aerosols versus long-lived greenhouse gases

We use single-forcing historical simulations with a coupled atmosphere-ocean global climate model to compare the effects of anthropogenic aerosols (AAs) and increasing long-lived greenhouse gases (LLGHGs) on simulated winter circulation in the Southern Hemisphere (SH). Our primary focus is on the subtropical jet, which is an important source of baroclinic instability, especially in the Australasian region, where the speed of the jet is largest. For the period 1950 to 2005, our simulations suggest that AAs weaken the jet, whereas increasing LLGHGs strengthen the jet. The different responses are explained in terms of thermal wind balance: increasing LLGHGs preferentially warm the tropical mid-troposphere and upper troposphere, whereas AAs have a similar effect of opposite sign. In the mid-troposphere, the warming (cooling) effect of LLGHGs (AAs) is maximal between 20S and 30S; this coincides with the descending branch of the Hadley circulation, which may advect temperature changes from the tropical upper troposphere to the subtropics of the SH. It follows that LLGHGs (AAs) increase (decrease) the mid-tropospheric temperature gradient between low latitudes and the SH mid-latitudes. The strongest effects are seen at longitudes where the southward branches of the Hadley cell in the upper troposphere are strongest, notably at those that correspond to Asia and the western Pacific warm pool

Aerosol- and greenhouse gas-induced changes in summer rainfall and circulation in the Australasian region: a study using single-forcing climate simulations

We use a coupled atmosphere-ocean global climate model (CSIRO-Mk3.6) to investigate the drivers of trends in summer rainfall and circulation in the vicinity of northern Australia. As part of the Coupled Model Intercomparison Project Phase 5 (CMIP5), we perform a 10-member 21st century ensemble driven by Representative Concentration Pathway 4.5 (RCP4.5). To investigate the roles of different forcing agents, we also perform multiple 10-member ensembles of historical climate change, which are analysed for the period 1951-2010. The historical runs include ensembles driven by "all forcings" (HIST), all forcings except anthropogenic aerosols (NO-AA) and forcing only from long-lived greenhouse gases (GHGAS). Anthropogenic aerosol-induced effects in a warming climate are calculated from the difference of HIST minus NO-AA. CSIRO-Mk3.6 simulates a strong summer rainfall decrease over north-western Australia (NWA) in RCP4.5, whereas simulated trends in HIST are weakly positive (but insignificant) during 1951-2010. The weak rainfall trends in HIST are due to compensating effects of different forcing agents: there is a significant decrease in GHGAS, offset by an aerosol-induced increase. Observations show a significant increase of summer rainfall over NWA during the last few decades. The large magnitude of the observed NWA rainfall trend is not captured by 440 unforced 60-yr trends calculated from a 500-yr pre-industrial control run, even though the model's decadal variability appears to be realistic. This suggests that the observed trend includes a forced component, despite the fact that the model does not simulate the magnitude of the observed rainfall increase in response to "all forcings" (HIST). We investigate the mechanism of simulated and observed NWA rainfall changes by exploring changes in circulation over the Indo-Pacific region. The key circulation feature associated with the rainfall increase in reanalyses is a lower-tropospheric cyclonic circulation trend off the coast of NWA, which enhances the monsoonal flow. The model shows an aerosol-induced cyclonic circulation trend off the coast of NWA in HIST minus NO-AA, whereas GHGAS shows an anticyclonic circulation trend. This explains why the aerosol-induced effect is an increase of rainfall over NWA, and the greenhouse gas-induced effect is of opposite sign. Possible explanations for the cyclonic (anticyclonic) circulation trend in HIST minus NO-AA (GHGAS) involve changes in the Walker circulation or the local Hadley circulation. In either case, a plausible atmospheric mechanism is that the circulation anomaly is a Rossby wave response to convective heating anomalies south of the Equator. We also discuss the possible role of air-sea interactions, e.g. an increase (decrease) of sea-surface temperatures off the coast of NWA in HIST minus NO-AA (GHGAS). Further research is needed to better understand the mechanisms and the extent to which these are model-dependent. In summary, our results suggest that anthropogenic aerosols may have "masked" greenhouse gas-induced changes in rainfall over NWA and in circulation over the wider Indo-Pacific region. Due to the opposing effects of greenhouse gases and anthropogenic aerosols, future trends may be very different from trends observed over the last few decades

Converting tropical forests to agriculture increases fire risk by fourfold

Deforestation exacerbates climate change through greenhouse gas emissions, but other climatic alterations linked to the local biophysical changes from deforestation remain poorly understood. Here, we assess the impact of tropical deforestation on fire weather risk—defined as the climate conditions conducive to wildfires—using high-resolution convection-permitting climate simulations. We consider two land cover scenarios for the island of Borneo: land cover in 1980 (forest scenario) and land cover in 2050 (deforestation scenario) to force a convection-permitting climate model, using boundary conditions from ERA-Interim reanalysis for the 2002–2016 period. Our findings revealed significant alterations in post-deforestation fire precursors such as increased temperature, wind speed and potential evapotranspiration and decreased humidity, cloud cover and precipitation. As a result, fire weather events that would occur once a year in the forested scenario, are likely to occur four times a year following deforestation. Likewise, for extreme conditions, such as those occurring on longer time-horizons than 20 years, the magnitude of extreme fire weather is likely to double following deforestation. These increases in extreme fire weather conditions demonstrate the key role of tropical forests in regulating regional climate processes, including reduced fire weather risk

Impact of historical land cover change on daily indices of climate extremes including droughts in eastern Australia

There is growing scientific evidence that anthropogenic land cover change (LCC) can produce a significant impact on regional climate. However, few studies have quantified this impact on climate extremes and droughts. In this study, we analysed daily data from a pair of ensemble simulations using the CSIRO AGCM for the period 1951–2003 to quantify the impact of LCC on selected daily indices of climate extremes in eastern Australia. The results showed: an increase in the number of dry and hot days, a decrease in daily rainfall intensity and wet day rainfall, and an increase in the decile‐based drought duration index for modified land cover conditions. These changes were statistically significant for all years, and especially pronounced during strong El Niño events. Therefore it appears that LCC has exacerbated climate extremes in eastern Australia, thus resulting in longer‐lasting and more severe droughts

The CSIRO-Mk3.6.0 Atmosphere-Ocean GCM: Participation in CMIP5 and data publication

The participation of the CSIRO-Mk3.6.0 Atmosphere Ocean Global Climate Model (AOGCM) in the Coupled Model Intercomparison Project Phase 5 (CMIP5) is a joint initiative between the Queensland Climate Change Centre of Excellence and the Commonwealth Scientific and Industrial Research Organisation (CSIRO). It now has approximately 10 research and support scientists working on this project which first began in 2009. This on-going project consists of the following four main components:• A model design and testing period to ensure that the model had acceptable configuration for participation in CMIP5, in particular, exhibiting a realistic present-day climate and a stable preindustrial climate;• A model integration phase where CMIP5 experiments were performed. These were to include the so-called "core" experiments plus a number of "tier1" and "tier2" experiments, which will constitute a significant submission to CMIP5 and to address local climate modelling needs and applications;• Post-processing of the raw CSIRO-Mk3.6.0 model output into internationally recognised and standardized CMIP5 form; and • Quality control and publication phase of the CSIRO-Mk3.6.0 data to ensure entry into the Earth System Grid (ESG) Federation, allowing it to be disseminated to the CMIP5 international community. In this paper the four phases of this climate modelling project will be discussed in detail. The main emphasis is to make potentially interested researchers aware of the CSIRO-Mk3.6.0 climate model submission and to elucidate the range and features of the datasets that are now available. The CMIP5 datasets are being hosted on the ESG which consists of international data nodes and gateways, including Australia's own node hosted by the National Computing Infrastructure (NCI) National Facility in Canberra. A key outcome of our efforts is the generation of over 150, mostly high priority, uniquely defined parameters from the list of requested model output to understand climate processes and also produce new climate change projection data for impact assessment. Some preliminary results of the CSIRO-Mk3.6.0 model are presented to illustrate the usefulness of this dataset in this research area

Modeling the impact of historical land cover change on Australia's regional climate

The Australian landscape has been transformed extensively since European settlement. However, the potential impact of historical land cover change (LCC) on regional climate has been a secondary consideration in the climate change projections. In this study, we analyzed data from a pair of ensembles (10 members each) for the period 1951-2003 to quantify changes in regional climate by comparing results from pre-European and modern-day land cover characteristics. The results of the sensitivity simulations showed the following: a statistically significant warming of the surface temperature, especially for summer in eastern Australia (0.4-2°C) and southwest Western Australia (0.4-0.8°C); a statistically significant decrease in summer rainfall in southeast Australia; and increased surface temperature in eastern regions during the 2002/2003 El Niño drought event. The simulated magnitude and pattern of change indicates that LCC has potentially been an important contributing factor to the observed changes in regional climate of Australia