Michael-Shawn Fletcher

Zonally symmetric changes in the strength and position of the Southern Westerlies drove atmospheric CO2 variations over the past 14 k.y.

Terrestrial records from 41 to 52°S across the Southern Hemisphere reveal nearly synchronous multimillennial trends in moisture derived from the Southern Westerly Winds (SWW) since 14 ka, pointing to a marked zonal symmetry in SWW changes across a broad swath of the southern middle latitudes. The data suggest a southward shift of the SWW that coincided with a rapid atmospheric CO 2 rise starting ca. 12.5 ka, a widespread decline in SWW strength between ca. 10 and 7 ka contemporaneous with an ∼8 ppm reversal in the deglacial atmospheric CO 2 rise, followed by stronger SWW and a steady multimillennial increase in CO 2 until the preindustrial maximum. We conclude that zonally symmetric changes in the intensity of the SWW at multimillennial time scales have covaried with atmospheric CO 2 variations since 14 ka, and suggest that changes in the SWW–Southern Ocean coupled system have influenced the atmospheric CO 2 concentration through wind-driven upwelling of CO 2 -rich deep waters in the high southern latitudes.

A fire-driven shift from forest to non-forest: evidence for alternative stable states?

We test the validity of applying the alternative stable state paradigm to account for the landscape-scale forest/non-forest mosaic that prevails in temperate Tasmania, Australia. This test is based on fine-scale pollen, spore, and charcoal analyses of sediments located within a small patch of non-forest vegetation surrounded by temperate forest. Following nearly 500 years of forest dominance at the site, a catastrophic fire drove an irreversible shift from a forested Cyperaceae–Sphagnum wetland to a non-forested Restionaceae wetland at ca. 7000 calibrated (cal) yr BP. Persistence of the non-forest/Restionaceae vegetation state over 7000 years, despite long fire-free intervals, implies that fire was not essential for the maintenance of the non-forest state. We propose that reduced interception and transpiration of the non-forest state resulted in local waterlogging, presenting an eco-hydrological barrier to forest reestablishment over the succeeding 7000 years. We further contend that the rhizomatous natur...

A Holocene record of sea level, vegetation, people and fire from western Tasmania, Australia

The analysis of a 10 000 calendar year (cal. ka) pollen record on the west coast of Tasmania has revealed a suite of changes that can be related to sea level, fire and people. Fire-promoted moorland has occupied the site for the entire period and challenges the long-held assumption that rainforest dominated the landscape of western Tasmania through the early to mid Holocene. Changes in wetland taxa and the occurrence of benthic marine diatoms indicate a Holocene sea-level high-stand between 6.3 and 5.8 cal. ka. A significant and sustained rise in charcoal concentration occurs after 6 cal. ka, reflecting the combined effects of anthropogenic burning and hydrological changes that were probably modulated by regional climatic forcing. Finally, European colonisation resulted in a significant decrease in charcoal, rapid peat accumulation and a suite of vegetation changes.

Natural hazards in Australia: extreme bushfire

Bushfires are one of the most frequent natural hazards experienced in Australia. Fires play an important role in shaping the landscape and its ecological dynamics, but may also have devastating effects that cause human injuries and fatalities, as well as broad-scale environmental damage. While there has been considerable effort to quantify changes in the occurrence of bushfire in Australia, a comprehensive assessment of the most extreme bushfire cases, which exact the greatest economic and environmental impacts, is lacking. In this paper we reflect upon recently developed understanding of bushfire dynamics to consider (i) historical changes in the occurrence of extreme bushfires, and (ii) the potential for increasing frequency in the future under climate change projections. The science of extreme bushfires is still a developing area, thus our conclusions about emerging patterns in their occurrence should be considered tentative. Nonetheless, historical information on noteworthy bushfire events suggests an increased occurrence in recent decades. Based on our best current understanding of how extreme bushfires develop, there is strong potential for them to increase in frequency in the future. As such there is a pressing need for a greater understanding of these powerful and often destructive phenomena.

ENSO controls interannual fire activity in southeast Australia

El Nino Southern Oscillation (ENSO) is the main mode controlling the variability in the ocean-atmosphere system in the South Pacific. While the ENSO influence on rainfall regimes in the South Pacific is well documented, its role in driving spatiotemporal trends in fire activity in this region has not been rigorously investigated. This is particularly the case for the highly flammable and densely populated southeast Australian sector, where ENSO is a major control over climatic variability. Here we conduct the first region-wide analysis of how ENSO controls fire activity in southeast Australia. We identify a significant relationship between ENSO and both fire frequency and area burnt. Critically, wavelet analyses reveal that, despite substantial temporal variability in the ENSO system, ENSO exerts a persistent and significant influence on southeast Australian fire activity. Our analysis has direct application for developing robust predictive capacity for the increasingly important efforts at fire management.

Variance and Rate‐of‐Change as Early Warning Signals for a Critical Transition in an Aquatic Ecosystem State: A Test Case From Tasmania, Australia

Critical transitions in ecosystem states are often sudden and unpredictable. Consequently, there is a concerted effort to identify measurable early warning signals (EWS) for these important events. Aquatic ecosystems provide an opportunity to observe critical transitions due to their high sensitivity and rapid response times. Using palaeoecological techniques, we can measure properties of time series data to determine if critical transitions are preceded by any measurable ecosystem metrics, that is, identify EWS. Using a suite of palaeoenvironmental data spanning the last 2,400 years (diatoms, pollen, geochemistry, and charcoal influx), we assess whether a critical transition in diatom community structure was preceded by measurable EWS. Lake Vera, in the temperate rain forest of western Tasmania, Australia, has a diatom community dominated by Discostella stelligera and undergoes an abrupt compositional shift at ca. 820 cal yr BP that is concomitant with increased fire disturbance of the local vegetation. This shift is manifest as a transition from less oligotrophic acidic diatom flora (Achnanthidium minutissimum, Brachysira styriaca, and Fragilaria capucina) to more oligotrophic acidic taxa (Frustulia elongatissima, Eunotia diodon, and Gomphonema multiforme). We observe a marked increase in compositional variance and rate-of-change prior to this critical transition, revealing these metrics are useful EWS in this system. Interestingly, vegetation remains complacent to fire disturbance until after the shift in the diatom community. Disturbance taxa invade and the vegetation system experiences an increase in both compositional variance and rate-of-change. These trends imply an approaching critical transition in the vegetation and the probable collapse of the local rain forest system.

Long-term environmental change in eastern Tasmania: vegetation, climate and fire at Stoney Lagoon

Tasmania’s dry, inland east is ideally positioned to inform models of late Quaternary environmental change in southern Australasia. Despite this, it remains poorly represented in the palaeoecological record. Here, we seek to address this with a >13,000-year vegetation and fire history from Stoney Lagoon, a site at the eastern margin of Tasmania’s inland Midlands plains. Pollen and charcoal analysis indicates that here, a relatively moist early deglacial was followed by a dry later deglacial (ca. 14,000–12,000 cal. BP), when sclerophyll forests became well established and burning increased. This suggests that the Midlands’ vegetation responded to the climatic signals characterising Australia’s south-eastern coast rather than those governing developments in western Tasmania. Dry sclerophyll forest persisted throughout the Holocene; with a pronounced transition from more to less grassy understoreys between ca. 9000 and 7000 cal. BP. From the mid-Holocene, the sclerophyll community remains relatively stable. However, increased fire activity and trends in moisture-sensitive taxa suggest generally drier conditions coupled with greater hydroclimatic variability under the strengthening influence of the El Niño–Southern Oscillation (ENSO). Overall, these results highlight the role of macro-scale climatic shifts in shaping vegetation development in Tasmania’s inland east, while hinting at the concurrent importance of local ecological drivers. This highlights the need for spatially diverse studies to understand interactions between drivers of long-term environmental change in sub-humid southern Australia. This research also supports conservation by strengthening understandings of pre-colonial baselines in this highly modified landscape.

Centennial-scale trends in the Southern Annular Mode revealed by hemisphere-wide fire and hydroclimatic trends over the past 2400 years

Millennial-scale latitudinal shifts in the southern westerly winds (SWW) drive changes in Southern Ocean upwelling, leading to changes in atmospheric CO2 levels, thereby affecting the global climate and carbon cycle. Our aim here is to understand whether century-scale shifts in the SWW also drive changes in atmospheric CO2 content. We report new multiproxy lake sediment data from southwest Tasmania, Australia, that show centennial-scale changes in vegetation and fire activity over the past 2400 yr. We compare our results with existing data from southern South America and reveal synchronous and in-phase centennial-scale trends in vegetation and fire activity between southwest Tasmania and southern South America over the past 2400 yr. Interannual to centennial-scale rainfall anomalies and fire activity in both these regions are significantly correlated with shifts in the SWW associated with the Southern Annular Mode (SAM; atmospheric variability of the Southern Hemisphere). Thus, we interpret the centennial-scale trends we have identified as reflecting century-scale SAM-like shifts in the SWW over the past 2400 yr. We identify covariance between our inferred century-scale shifts in the SWW and Antarctic ice core CO2 values, demonstrating that the SWW-CO2 relationship operating at a millennial scale also operates at a centennial scale through the past 2400 yr. Our results indicate a possible westerly-driven modulation of recent increases in global atmospheric CO2 content that could potentially exacerbate current greenhouse gas–related warming.

Biological responses to the press and pulse of climate trends and extreme events

The interaction of gradual climate trends and extreme weather events since the turn of the century has triggered complex and, in some cases, catastrophic ecological responses around the world. We illustrate this using Australian examples within a press–pulse framework. Despite the Australian biota being adapted to high natural climate variability, recent combinations of climatic presses and pulses have led to population collapses, loss of relictual communities and shifts into novel ecosystems. These changes have been sudden and unpredictable, and may represent permanent transitions to new ecosystem states without adaptive management interventions. The press–pulse framework helps illuminate biological responses to climate change, grounds debate about suitable management interventions and highlights possible consequences of (non-) intervention.A press–pulse framework is used to understand the interactive ecological effects of gradual climate trends and extreme weather events. Australian case studies include population collapses, loss of relictual communities and novel ecosystems.

Climate Change Amplifications of Climate‐Fire Teleconnections in the Southern Hemisphere

Recent changes in trend and variability of the main Southern Hemisphere climate modes are driven by a variety of factors, including increasing atmospheric greenhouse gases, changes in tropical sea-surface temperature and stratospheric ozone depletion and recovery. One of the most important implications for climatic change is its effect via climate teleconnections on natural ecosystems, water security and fire variability in proximity to populated areas, thus threatening human lives and properties. Only sparse and fragmentary knowledge of relationships between teleconnections, lightning strikes, and fire is available during the observed record within the Southern Hemisphere. This constitutes a major knowledge gap for undertaking suitable management and conservation plans. Our analysis of documentary fire records from Mediterranean and temperate regions across the Southern Hemisphere reveals a critical increased strength of climate-fire teleconnections during the onset of the 21st century including a tight coupling between lightning-ignited fire occurrences, the upward trend in the Southern Annular Mode and rising temperatures across the Southern Hemisphere.