Joëlle Gergis

A Tripole Index for the Interdecadal Pacific Oscillation

Abstract A new index is developed for the Interdecadal Pacific Oscillation, termed the IPO Tripole Index (TPI). The IPO is associated with a distinct ‘tripole’ pattern of sea surface temperature anomalies (SSTA), with three large centres of action and variations on decadal timescales, evident in the second principal component (PC) of low-pass filtered global SST. The new index is based on the difference between the SSTA averaged over the central equatorial Pacific and the average of the SSTA in the Northwest and Southwest Pacific. The TPI is an easily calculated, non-PC-based index for tracking decadal SST variability associated with the IPO. The TPI time series bears a close resemblance to previously published PC-based indices and has the advantages of being simpler to compute and more consistent with indices used to track the El Niño–Southern Oscillation (ENSO), such as Niño 3.4. The TPI also provides a simple metric in physical units of °C for evaluating decadal and interdecadal variability of SST fields in a straightforward manner, and can be used to evaluate the skill of dynamical decadal prediction systems. Composites of SST and mean sea level pressure anomalies reveal that the IPO has maintained a broadly stable structure across the seven most recent positive and negative epochs that occurred during 1870–2013. The TPI is shown to be a robust and stable representation of the IPO phenomenon in instrumental records, with relatively more variance in decadal than shorter timescales compared to Niño 3.4, due to the explicit inclusion of off-equatorial SST variability associated with the IPO.

Southern Hemisphere high-resolution palaeoclimate records of the last 2000 years

This study presents a comprehensive assessment of high-resolution Southern Hemisphere (SH) paleoarchives covering the last 2000 years. We identified 174 monthly to annually resolved climate proxy (tree ring, coral, ice core, documentary, speleothem and sedimentary) records from the Hemisphere. We assess the interannual and decadal sensitivity of each proxy record to large-scale circulation indices from the Pacific, Indian and Southern Ocean regions over the twentieth century. We then analyse the potential of this newly expanded palaeoclimate network to collectively represent predictands (sea surface temperature, sea level pressure, surface air temperature and precipitation) commonly used in climate reconstructions. The key dynamical centres-of-action of the equatorial Indo-Pacific are well captured by the palaeoclimate network, indicating that there is considerable reconstruction potential in this region, particularly in the post AD 1600 period when a number of long coral records are available. Current spatiotemporal gaps in data coverage and regions where significant potential for future proxy collection exists are discussed. We then highlight the need for new and extended records from key dynamical regions of the Southern Hemisphere. Although large-scale climate field reconstructions for the SH are in their infancy, we report that excellent progress in the development of regional proxies now makes plausible estimates of continental- to hemispheric-scale climate variations possible.

Paleoclimate Data-Model Comparison and the Role of Climate Forcings over the Past 1500 Years*

The past 1500 years provide a valuable opportunity to study the response of the climate system to external forcings. However, the integration of paleoclimate proxies with climate modeling is critical to improving the understanding of climate dynamics. In this paper, a climate system model and proxy records are therefore used to study the role of natural and anthropogenic forcings in driving the global climate. The inverse and forward approaches to paleoclimate data–model comparison are applied, and sources of uncertainty are identified and discussed. In the first of two case studies, the climate model simulations are compared with multiproxy temperature reconstructions. Robust solar and volcanic signals are detected in Southern Hemisphere temperatures, with a possible volcanic signal detected in the Northern Hemisphere. The anthropogenic signal dominates during the industrial period. It is also found that seasonal and geographical biases may cause multiproxy reconstructions to overestimate the magnitude of the long-term preindustrial cooling trend. Inthesecondcasestudy,themodelsimulationsarecomparedwithacorald 18 OrecordfromthecentralPacific Ocean. It is found that greenhouse gases, solar irradiance, and volcanic eruptions all influence the mean state of the central Pacific, but there is no evidence that natural or anthropogenic forcings have any systematic impact on El Ni~ Oscillation. The proxy climate relationship is found to change over time, challenging the assumption of stationarity that underlies the interpretation of paleoclimate proxies. These case studies demonstrate the value of paleoclimate data–model comparison but also highlight the limitations of current techniques and demonstrate the need to develop alternative approaches.

Multi-centennial tree-ring record of ENSO-related activity in New Zealand

It is unclear how global warming will affect the El Nino/Southern Oscillation (ENSO), in part because the instrumental record is too short to understand how ENSO has changed in the past. Now a 700-year-long tree-ring record indicates that ENSO-related climate variability may increase in New Zealand with continued warming.

A global multiproxy database for temperature reconstructions of the Common Era

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850–2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.

A climate reconstruction of Sydney Cove, New South Wales, using weather journal and documentary data, 1788-1791

This study presents the first analysis of the weather conditions experienced at Sydney Cove, New South Wales, during the earliest period of the European settlement of Australia. A climate analysis is presented for January 1788 to December 1791 using daily temperature and barometric pressure observations recorded by William Dawes in Sydney Cove and a temperature record kept by William Bradley on board the HMS Sirius anchored in Port Jackson (Sydney Harbour) in the early months of the First Fleet’s arrival in Australia. Remarkably, the records appear comparable with modern day measurements taken from Sydney Observatory Hill, displaying similar daily variability, a distinct seasonal cycle and considerable inter-annual variability. To assess the reliability of these early weather data, they were cross-verified with other data sources, including anecdotal observations recorded in First Fleet documentary records and independent palaeoclimate reconstructions. Some biases in the temperature record, likely associated with the location of the thermometer, have been identified. Although the 1788–1791 period experienced a marked La Nina to El Nino fluctuation according to palaeoclimatic data, the cool and warm intervals in Sydney over this period cannot be conclusively linked to El Nino– Southern Oscillation (ENSO) conditions. This study demonstrates that there are excellent opportunities to expand our description of pre-20th century climate variability in Australia while contributing culturally significant material to the emerging field of Australian environmental history.

Continental-Scale Temperature Variability during the Past Two Millennia: Supplementary Information

Past global climate changes had strong regional expression. To elucidate their spatio-temporal pattern, we reconstructed past temperatures for seven continental-scale regions during the past one to two millennia. The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century. At multi-decadal to centennial scales, temperature variability shows distinctly different regional patterns, with more similarity within each hemisphere than between them. There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between ad 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century. The transition to these colder conditions occurred earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere regions. Recent warming reversed the long-term cooling; during the period ad 1971–2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years.

Australasian Temperature Reconstructions Spanning the Last Millennium

AbstractMultiproxy warm season (September–February) temperature reconstructions are presented for the combined land–ocean region of Australasia (0°–50°S, 110°E–180°) covering 1000–2001. Using between 2 (R2) and 28 (R28) paleoclimate records, four 1000-member ensemble reconstructions of regional temperature are developed using four statistical methods: principal component regression (PCR), composite plus scale (CPS), Bayesian hierarchical models (LNA), and pairwise comparison (PaiCo). The reconstructions are then compared with a three-member ensemble of GISS-E2-R climate model simulations and independent paleoclimate records. Decadal fluctuations in Australasian temperatures are remarkably similar between the four reconstruction methods. There are, however, differences in the amplitude of temperature variations between the different statistical methods and proxy networks. When the R28 network is used, the warmest 30-yr periods occur after 1950 in 77% of ensemble members over all methods. However, reconstru...

Paleoclimate studies and natural-resource management in the Murray-Darling Basin II: unravelling human impacts and climate variability

The management of the water resources of the Murray-Darling Basin (MDB) has long been contested, and the effects of the recent Millennium drought and subsequent flooding events have generated acute contests over the appropriate allocation of water supplies to agricultural, domestic and environmental uses. This water-availability crisis has driven demand for improved knowledge of climate change trends, cycles of variability, the range of historical climates experienced by natural systems and the ecological health of the system relative to a past benchmark. A considerable volume of research on the past climates of southeastern Australia has been produced over recent decades, but much of this work has focused on longer geological time-scales, and is of low temporal resolution. Less evidence has been generated of recent climate change at the level of resolution that accesses the cycles of change relevant to management. Intra-decadal and near-annual resolution (high-resolution) records do exist and provide evidence of climate change and variability, and of human impact on systems, relevant to natural-resource management. There exist now many research groups using a range of proxy indicators of climate that will rapidly escalate our knowledge of management-relevant, climate change and variability. This review assembles available climate and catchment change research within, and in the vicinity of, the MDB and portrays the research activities that are responding to the knowledge need. It also discusses how paleoclimate scientists may better integrate their pursuits into the resource-management realm to enhance the utility of the science, the effectiveness of the management measures and the outcomes for the end users.

Increasing the understanding and use of natural archives of ecosystem services, resilience and thresholds to improve policy, science and practice

Despite the great potential of palaeo-environmental information to strengthen natural resource policy, science and practical outcomes naturally occurring archives of palaeo-environmental and ecosystem service information have not been fully recognised or utilised to inform the development of environmental policy. In this paper, we describe how Australian palaeo-environmental science is improving environmental understanding through local studies and regional syntheses that inform us about past conditions, extreme conditions and altered ecosystem states. Australian innovations in ecosystem services research and palaeo-environmental science contribute in five important contexts: discussions about environmental understanding and management objectives, improving access to information, improved knowledge about the dynamics of ecosystem services, increasing understanding of environmental processes and resource availability, and engaging interdisciplinary approaches to manage ecosystem services. Knowledge of the past is an important starting point for setting present and future resource management objectives, anticipating consequences of trade-offs, sharing risk and evaluating and monitoring the ongoing availability of ecosystem services. Palaeo-environmental information helps reframe discussions about desirable futures and collaborative efforts between scientists, planners, managers and communities. However, further steps are needed to translate the ecosystem services concept into ecosystem services policy and tangible management objectives and actions that are useful, feasible and encompass the range of benefits to people from ecosystems. We argue that increased incorporation of palaeo-environmental information into policy and decision-making is needed for evidence-based adaptive management to enhance sustainability of ecosystem functions and reduce long-term risks.