Examining current bias and future projection consistency of globally downscaled climate projections commonly used in climate impact studies

The associated uncertainties of future climate projections are one of the biggest obstacles to overcome in studies exploring the potential regional impacts of future climate shifts. In remote and climatically complex regions, the limited number of available downscaled projections may not provide an accurate representation of the underlying uncertainty in future climate or the possible range of potential scenarios. Consequently, global downscaled projections are now some of the most widely used climate datasets in the world. However, they are rarely examined for representativeness of local climate or the plausibility of their projected changes. Here we explore the utility of two such global datasets (CHELSA and WorldClim2) in providing plausible future climate scenarios for regional climate change impact studies. Our analysis was based on three steps: (1) standardizing a baseline period to compare available global downscaled projections with regional observation-based datasets and regional downscaled datasets; (2) bias correcting projections using a single observation-based baseline; and (3) having controlled differences in baselines between datasets, exploring the patterns and magnitude of projected climate shifts from these datasets to determine their plausibility as future climate scenarios, using Hawaiʻi as an example region. Focusing on mean annual temperature and precipitation, we show projected climate shifts from these commonly used global datasets not only may vary significantly from one another but may also fall well outside the range of future scenarios derived from regional downscaling efforts. As species distribution models are commonly created from these datasets, we further illustrate how a substantial portion of variability in future species distribution shifts can arise from the choice of global dataset used. Hence, projected shifts between baseline and future scenarios from these global downscaled projections warrant careful evaluation before use in climate impact studies, something rarely done in the existing literature. © 2023, This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply

Distinguishing Variability Regimes of Hawaiian Summer Rainfall: Quasi-Biennial and Interdecadal Oscillations

Summer precipitation in Hawai\u27i accounts for 40% of the annual total and provides important water sources. However, our knowledge about its variability remains limited. Here we show that statewide Hawai\u27i summer rainfall (HSR) variability exhibits two distinct regimes: quasi-biennial (QB, ~2 years) and interdecadal (~30–40 years). The QB variation is linked to alternating occurrences of the Western North Pacific (WNP) cyclone and anticyclone in successive years, which is modulated by the intrinsic El Niño–Southern Oscillation biennial variability and involves a positive feedback between atmospheric Rossby waves and underlying sea surface temperature (SST) anomalies. The interdecadal variation of HSR is largely modulated by the Pacific Decadal Oscillation through affecting upstream low-level humidity that affects topographic rainfall. HSR shows weak long-term drying trend during 1920–2019. This first description of the major physical drivers of summer rainfall variability provides key information for seasonal rainfall prediction in Hawai\u27i

Climatology of Haleakalā

The steep mountain slopes of Haleakalā Volcano (Maui, HI) support some of the most spatially diverse environments on the planet. Microclimates found across vertical gradients on the mountain slopes can change over relatively short differences in slope exposure and elevation and are strongly influenced by a persistent temperature inversion and northeast trade winds that are characteristic of this region. Eleven climate stations, which comprise the HaleNet climate network, have been monitoring climatic conditions along a 2030-m leeward (960 to 2990 m) and a 810-m windward (1650 to 2460 m) elevational transect, beginning as early as June of 1988. Hourly measurements of solar radiation, net radiation, relative humidity, wind speed, temperature, precipitation and soil moisture, and derived variables including potential evapotranspiration, vapor pressure deficit, soil heat flux, and daytime cloud attenuation of sunlight are analyzed in this study. This report documents the annual, diurnal and elevational characteri tics of these climatic variables as well as their behavior over the period-of-record (~1988 to 2013) in both the 6-month dry (May – October) and wet (November to April) seasons. Results show that the climate gradients along both leeward and windward elevation transects are highly influenced by the trade wind inversion in both dry and wet seasons. Period-of-record trends in the dry-season, show increases in energy and decrease in moisture at high elevations (>2000 m). Significant dry season changes include: decreases in precipitation (5 to 8% decade-1), relative humidity (3 to 5% decade-1) and cloud attenuation of sunlight (-2 to -5% decade-1) and increase in solar radiation (2 to 4% decade-1), vapor pressure deficit (9 to 10 % decade-1), zero precipitation days (4 to 5% decade-1) and potential evapotranspiration (3 to 7% decade -1). For the wet season, an opposite signal of change was observed at high elevation although trends were not as robust as the dry season trends. Reported dry season trends are potenti lly explained by a 4% significant increase in TWI frequency identified over a similar observation period (1991-2013). In addition to a climate variable analysis, this report also highlights other past and ongoing research projects that have taken place on the mountain and provides a summary of the history of the HaleNet climate network, the people and organizations that have contributed to its operation, and a list of publications that have made use of HaleNet climate data. It is the authors’ hope that this information will aid resource managers in protecting the ecosystems and other natural resources, and provide insight into ongoing and future climate changes on Haleakalā.The data analysis presented here and the preparation of this report were supported by the acific Island Climate Science Center (PICSC) and the Pacific Island Climate Change Cooperative (PICCC) and the Pacific Island Ecosystem Research Center (PIERC). We also thank Paul Krushelnycky, Shelley Crausbay, Abby Frazier, Henry Diaz, Erica von Allmen, Thomas Schroeder and Ross Sutherland for their contributions to this report. In conducting fieldwork on Maui, the authors were given support, encouragement, and assistance by numerous ndividuals. We extend our gratitude especially to Jotoku and Doris Asato, Dennis Nullet, Bill Minyard, Ryan Mudd, Dave Penn, Ron Nagata, Ted Rodrigues, Timmy Bailey, Matt Brown, Pamela Waiolena, Chuck Chimera, Kathy Wakely, Philip Thomas, and Sabine Jessel. We thank Haleakalā National Park and PIERC, and the USGS, for their long support of the HaleNet system. We owe a special debt of gratitude to Gordon Tribble of PIERC for his unwavering commitment to sustaining HaleNet. We would also like to thank Jeff Burgett of PICCC, Deborah Solis of the U.S. Army Corps of Engineers and Neil Fujii and Jeremy Kimura of the Commission on Water Resource Management. Over the years, HaleNet research has also been supported with funding from the University of Hawai‘i Research Council, the Water Resources Institute Program of the U.S. Geological Survey, the Cooperative National Parks Resources Study Unit, NSF EPSCoR (under award 0903833), and PICCC

A Century of Drought in Hawaiʻi: Geospatial Analysis and Synthesis across Hydrological, Ecological, and Socioeconomic Scales

Drought is a prominent feature of Hawaiʻi’s climate. However, it has been over 30 years since the last comprehensive meteorological drought analysis, and recent drying trends have emphasized the need to better understand drought dynamics and multi-sector effects in Hawaiʻi. Here, we provide a comprehensive synthesis of past drought effects in Hawaiʻi that we integrate with geospatial analysis of drought characteristics using a newly developed 100-year (1920–2019) gridded Standardized Precipitation Index (SPI) dataset. The synthesis examines past droughts classified into five categories: Meteorological, agricultural, hydrological, ecological, and socioeconomic drought. Results show that drought duration and magnitude have increased significantly, consistent with trends found in other Pacific Islands. We found that most droughts were associated with El Niño events, and the two worst droughts of the past century were multi-year events occurring in 1998–2002 and 2007–2014. The former event was most severe on the islands of O’ahu and Kaua’i while the latter event was most severe on Hawaiʻi Island. Within islands, we found different spatial patterns depending on leeward versus windward contrasts. Droughts have resulted in over $80 million in agricultural relief since 1996 and have increased wildfire risk, especially during El Niño years. In addition to providing the historical context needed to better understand future drought projections and to develop effective policies and management strategies to protect natural, cultural, hydrological, and agricultural resources, this work provides a framework for conducting drought analyses in other tropical island systems, especially those with a complex topography and strong climatic gradients

The hydrology of the humid tropics

Hydrological processes in the humid tropics differ from other regions in having greater energy inputs and faster rates of change, including human-induced change. Human influences on population growth, land use and climate change will profoundly influence tropical hydrology, yet understanding of key hydrological interactions is limited. We propose that efforts to collect tropical data should explicitly emphasize characterizing moisture and energy fluxes from below the ground surface into the atmosphere. Research needs to chiefly involve field-based characterizations and modelling of moisture cycling and catchment processes, as well as long-term data acquisition and organization

Mycorrhizal feedbacks influence global forest structure and diversity

One mechanism proposed to explain high species diversity in tropical systems is strong negative conspecific density dependence (CDD), which reduces recruitment of juveniles in proximity to conspecific adult plants. Although evidence shows that plant-specific soil pathogens can drive negative CDD, trees also form key mutualisms with mycorrhizal fungi, which may counteract these effects. Across 43 large-scale forest plots worldwide, we tested whether ectomycorrhizal tree species exhibit weaker negative CDD than arbuscular mycorrhizal tree species. We further tested for conmycorrhizal density dependence (CMDD) to test for benefit from shared mutualists. We found that the strength of CDD varies systematically with mycorrhizal type, with ectomycorrhizal tree species exhibiting higher sapling densities with increasing adult densities than arbuscular mycorrhizal tree species. Moreover, we found evidence of positive CMDD for tree species of both mycorrhizal types. Collectively, these findings indicate that mycorrhizal interactions likely play a foundational role in global forest diversity patterns and structure

Climate sensitive size-dependent survival in tropical trees

Survival rates of large trees determine forest biomass dynamics. Survival rates of small trees have been linked to mechanisms that maintain biodiversity across tropical forests. How species survival rates change with size offers insight into the links between biodiversity and ecosystem function across tropical forests. We tested patterns of size-dependent tree survival across the tropics using data from 1,781 species and over 2 million individuals to assess whether tropical forests can be characterized by size-dependent life-history survival strategies. We found that species were classifiable into four ‘survival modes’ that explain life-history variation that shapes carbon cycling and the relative abundance within forests. Frequently collected functional traits, such as wood density, leaf mass per area and seed mass, were not generally predictive of the survival modes of species. Mean annual temperature and cumulative water deficit predicted the proportion of biomass of survival modes, indicating important links between evolutionary strategies, climate and carbon cycling. The application of survival modes in demographic simulations predicted biomass change across forest sites. Our results reveal globally identifiable size-dependent survival strategies that differ across diverse systems in a consistent way. The abundance of survival modes and interaction with climate ultimately determine forest structure, carbon storage in biomass and future forest trajectories

Mycorrhizal feedbacks influence global forest structure and diversity

One mechanism proposed to explain high species diversity in tropical systems is strong negative conspecific density dependence (CDD), which reduces recruitment of juveniles in proximity to conspecific adult plants. Although evidence shows that plant-specific soil pathogens can drive negative CDD, trees also form key mutualisms with mycorrhizal fungi, which may counteract these effects. Across 43 large-scale forest plots worldwide, we tested whether ectomycorrhizal tree species exhibit weaker negative CDD than arbuscular mycorrhizal tree species. We further tested for conmycorrhizal density dependence (CMDD) to test for benefit from shared mutualists. We found that the strength of CDD varies systematically with mycorrhizal type, with ectomycorrhizal tree species exhibiting higher sapling densities with increasing adult densities than arbuscular mycorrhizal tree species. Moreover, we found evidence of positive CMDD for tree species of both mycorrhizal types. Collectively, these findings indicate that mycorrhizal interactions likely play a foundational role in global forest diversity patterns and structure

Hydrologic change and accelerated erosion in Northern Thailand

This journal has been published at different time periods under the following titles: Explorations: A Graduate Student Journal of Southeast Asian Studies, Explorations in Southeast Asian Studies, and The Journal of the Southeast Asian Studies Association.The Student Activities Program Fee Boar

Changes in atmospheric circulation patterns associated with high and low rainfall regimes in the Hawaiian Islands region on multiple time scales

We examine a suite of climate variables in the North Pacific associated with different temporal scales of dry and wet episodes in the Hawaiian Islands. The goal is to ascertain whether varying the length in the occurrence of such events from seasonal to multi-decadal results from different ocean–atmosphere circulation patterns over the North Pacific region.We find that as the dryness period lengthens, the spatial scale of the anomalies widens, but the characteristics of the anomalous circulation fields exhibit elements evident at each time scale. At the longest (multi-decadal) time scales during the wet season (November–April), the major anomalous circulation features associatedwith above (below) average rainfall in Hawai\u27i are a strengthening (weakening) of the North Pacific mid-latitude westerlies associated with a strengthened (weakened) Aleutian Low system, a slight weakening (strengthening) of the northeast trade winds, with anomalous northerly (southerly) component to the south of the Islands, together with diminished (enhanced) precipitation across most of the tropical North Pacific. During the summer dry season (May–October) a characteristic pattern that is evident at different time scales during drier than normal periods is a strengthening of the trade winds to the south of the Islands, with sinking motion extending from the latitude of Hawai\u27i eastward to Central America. Conversely, wet summers are associated with generally weaker trades to the south with a tendency for anomalous southwesterly component.We also consider possible changes in the relationship of Hawaiian rainfall to changes in the Pacific-Decadal Oscillation (PDO) but conclude that the record is too short to establish the significance of any changes in an overall negative correlation