David F. Porinchu

Recent warming reverses long-term arctic cooling.

Climate Reversal The climate and environment of the Arctic have changed drastically over the short course of modern observation. Kaufman et al. (p. 1236) synthesized 2000 years of proxy data from lakes above 60° N latitude with complementary ice core and tree ring records, to create a paleoclimate reconstruction for the Arctic with a 10-year resolution. A gradual cooling trend at the start of the record had reversed by the beginning of the 20th century, when temperatures began to increase rapidly. The long-term cooling of the Arctic is consistent with a reduction in summer solar insolation caused by changes in Earths orbit, while the rapid and large warming of the past century is consistent with the human-caused warming. A 2000-year-long Arctic cooling trend seen in a surface air temperature reconstruction was reversed during the last century. The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60°N covering the past 2000 years, which indicates that a pervasive cooling in progress 2000 years ago continued through the Middle Ages and into the Little Ice Age. A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.

The use and application of freshwater midges (Chironomidae: Insecta: Diptera) in geographical research:

The potential of applying the analysis of freshwater midges (Chironomidae) for current questions in geographical research is examined. Chironomids are cosmopolitan in distribution and frequently the most abundant insects found in freshwater ecosystems. The capacity of the family to tolerate large gradients of pH, salinity, depth, oxygen concentration, temperature and productivity enables members of the Chironomidae to occupy virtually every available niche present in freshwater environments. In addition to wide distribution and abundance, Chironomidae are well suited for paleolimnological studies because the larvae possess chitinous head capsules which are well-preserved in lake sediment and relatively easily recovered and identified. As a result, chironomids are increasingly being used to track a number of natural and anthropogenically induced limnological changes resulting from atmospheric contamination, eutrophication and increased lake water salinity. Other areas in which subfossil chironomid analysis has provided valuable insight include climate change, phylogentics and biogeography and aquatic ecosystem dynamics and development. Details describing the biology and ecology of the Chironomidae that are directly relevant to their use in paleoenvironmental and biogeographical studies are presented. The methodology describing the recovery and identification of subfossil chironomid remains is reviewed. A generalized overview of the statistical methods that are commonly employed in relating the modern distribution of chironomids to specific aspects of the environment, i.e., the calibration dataset approach, is briefly discussed. Case studies that highlight the various uses and applications of chironomid analysis in areas of paleoenvironmental and biogeographical research relevant to geographers are described. Lastly, the current status of chironomid research in academic geography is discussed and suggestions of potential future research directions are made.

Subfossil Chironomids As Indicators Of Recent Climate Change In Sierra Nevada, California, Lakes

ABSTRACT High-resolution chironomid (Insecta: Diptera) stratigraphies were developed for three subalpine lakes in the Sierra Nevada of California to assess whether these lakes have been impacted by recent climate change evident in regional instrumental records for the 19th and 20th centuries. Detrended correspondence analysis (DCA) of the chironomid fauna indicates that the lakes have experienced similar unidirectional change in community composition over the 20th century, with two of the lakes showing particularly sharp gradients of change since the 1970s. Application of a chironomid-based surface water temperature inference model (r2jack = 0.73, RMSEPjack = 1.1°C, and a maximum bias of 1.24°C) to the subfossil chironomid assemblages preserved in the lake sediment provided quantitative estimates of surface water temperature changes and revealed the existence of similar water temperature trends between the late 19th century and the present. Above average water temperatures characterized the late 20th century and below average surface water temperatures occurred between a.d. 1910 and a.d. 1980. Fluctuations in the surface water temperature of these lakes closely track changes in mean July air temperature as measured in Fresno, California, over the period a.d. 1895–2001. It appears that 20th century climate change has had an overriding influence on the composition of the chironomid communities within these three lakes. This study demonstrates that subfossil chironomid analysis can provide detailed records of community response to local and regional climatic changes at subdecadal time scales. It also suggests that chironomid communities in subalpine lakes in the Sierra Nevada are already recording the impact of recent climate warming.

Development of a Midge-Based Summer Surface Water Temperature Inference Model for the Great Basin of the Western United States

ABSTRACT Surface sediment recovered from 51 lakes in the Uinta Mountains of northeast Utah was analyzed for subfossil chironomid remains, and incorporated in a midge-based inference model for summer surface water temperature (SSWT). The lakes in the calibration set spanned elevation, depth, and summer surface water temperature ranges of 900 m, 12.7 m, and 11.3°C, respectively. Redundancy analysis (RDA) identified four variables, SSWT, depth, specific conductivity, and Al concentration, that could account for a statistically significant amount of variance in the chironomid distribution, with SSWT accounting for the largest amount of variance. The Uinta Mountain calibration set was merged with a previously developed calibration set from the Sierra Nevada, California, in order to develop a midge-based inference model for SSWT applicable to subfossil chironomid stratigraphies from the Great Basin. A variety of statistical approaches, such as weighted averaging (WA), weighted averaging-partial least squares (WA-PLS), and partial least squares (PLS) were used to assess model performance. The best inference model for SSWT, based on a 3-component WA-PLS approach, had robust performance statistics (r2jack = 0.66, RMSEP = 1.4°C). The newly expanded inference model will enable more accurate estimates of late Pleistocene and Holocene thermal regimes and help address many outstanding questions relating to long-term and recent climate change in this region.

Chironomid-environment relations in northern North America

Chironomid assemblages from the uppermost sediments of 435 lakes spanning northern North America were compared to environmental parameters using direct gradient analysis. This large calibration set was merged from several previously developed regional datasets, and increases the number of modern analogues that are available for use for paleoenvironmental interpretations in this region. Air temperature explained the largest amount of variation in the chironomid assemblages with several other environmental factors accounting for statistical significant amounts of the remaining variance. A robust inference model for deriving past mean July air temperatures from subfossil chironomid assemblages was developed and applied to previously published paleoclimate reconstructions from the High-Arctic, Middle-Arctic, Boreal treeline, and Alpine regions of northern North America. The patterns of the temperature reconstructions from the combined dataset were generally similar to the original reconstructions, but with colder inferred temperatures reflecting the incorporation of a larger number of modern sites from colder climates in the combined dataset. This analysis demonstrated that the larger temperature gradient available in the new training set, when compared to the temperature gradients in the original training sets, provides a better estimation of chironomid-environment relationships. In particular, the optima and tolerances estimated using the larger, combined dataset should be more accurate, and therefore, improve midge-based paleoclimate reconstructions for northern North America. Despite the much larger spatial scale and greater associated environmental heterogeneity now incorporated in the combined dataset, this study suggests that in most cases the overarching constraint governing chironomid distributions in northern North America is temperature.

Prolonged California aridity linked to climate warming and Pacific sea surface temperature

California has experienced a dry 21st century capped by severe drought from 2012 through 2015 prompting questions about hydroclimatic sensitivity to anthropogenic climate change and implications for the future. We address these questions using a Holocene lake sediment record of hydrologic change from the Sierra Nevada Mountains coupled with marine sediment records from the Pacific. These data provide evidence of a persistent relationship between past climate warming, Pacific sea surface temperature (SST) shifts and centennial to millennial episodes of California aridity. The link is most evident during the thermal-maximum of the mid-Holocene (~8 to 3 ka; ka = 1,000 calendar years before present) and during the Medieval Climate Anomaly (MCA) (~1 ka to 0.7 ka). In both cases, climate warming corresponded with cooling of the eastern tropical Pacific despite differences in the factors producing increased radiative forcing. The magnitude of prolonged eastern Pacific cooling was modest, similar to observed La Niña excursions of 1o to 2 °C. Given differences with current radiative forcing it remains uncertain if the Pacific will react in a similar manner in the 21st century, but should it follow apparent past behavior more intense and prolonged aridity in California would result.

Physical and chemical limnology of a 61-lake transect across mainland Nunavut and southeastern Victoria Island, Central Canadian Arctic.

We describe the physical and chemical properties of sixty-one tundra lakes, sampled in a latitudinal transect (65―71°N, 105―108°W) across mainland and island regions of Nunavut, central Canadian Arctic, and examine the influence of geology, geography, climate, and vegetation on lake water chemistry. This dataset complements earlier limnological surveys of the Canadian Arctic and provides valuable information for evaluating the vulnerability of tundra lakes to predicted climate change. Principal components analysis revealed a geographical clustering of lakes; pH, DIC, specific conductivity, and trace metal concentrations reflected major lithological differences between the mainland and Victoria Island. Clustering of mainland lakes by ecoregion was also detected. Lakes of the Queen Maud Gulf Lowland and Garry Lake Lowland ecoregions (north) differed from lakes of the Takijuq Lake Upland ecoregion (south) in depth, pH, and specihc conductivity as well as nutrient, DOC, and chlorophyll-a concentrations. Ionic composition of the northern mainland lakes also indicated that the influence of marine aerosols and/or leaching of residual marine salts from post-glacial marine deposits exposed by isostatic rebound. The northern mainland lakes were the most nutrient-rich and biologically productive of the three lake clusters and were characterized by median concentrations of total dissolved nitrogen (518 μg l ―1 ) and chlorophyll-a (1.6 μg l ―1 ) that were higher than previously reported for tundra lakes in the Canadian Arctic. These lakes were chemically similar to lakes of the Tuktoyaktuk Peninsula, in the western Canadian Arctic. Lakes of the southern mainland were dilute, acidic, and nutrient-poor, in accord with earlier limnological surveys in this ecoregion. Concentrations of nutrients, DOC, and chlorophyll-a in Victoria Island lakes fell in the middle of the ranges reported from other islands in the Canadian Arctic. Lithologic and edaphic factors strongly influenced the limnological properties of the tundra lakes surveyed and must be controlled for in order to fully evaluate the influence of future climate and vegetation change.

Historical trends of mercury and spheroidal carbonaceous particle deposition in sub-alpine lakes in the Great Basin, United States

AbstractThe geochemistry of lake sediments was used to identify anthropogenic factors influencing aquatic ecosystems of sub-alpine lakes in the western United States during the past century. Sediment cores were recovered from six high-elevation lakes in the central Great Basin of the United States. The proxies utilized to examine the degree of recent anthropogenic environmental change include spheroidal carbonaceous particle (SCP), mercury (Hg), and sediment organic content estimated using loss-on-ignition. Chronologies for the sediment cores, developed using 210Pb, indicate the cores span the twentieth century. Mercury flux varied between lakes but all exhibited increasing fluxes during the mid-twentieth century. The mean ratio of modern (post-A.D. 1985) to preindustrial (pre-A.D. 1880) Hg flux was 5.2, which is comparable to the results from previous studies conducted in western North America. Peak SCP flux for all lakes occurred between approximately A.D. 1940 and A.D. 1970, after which time the SCP flux was greatly reduced. The reduction in SCP input is likely due to better controls on combustion sources. Measured Hg concentrations and calculated sedimentation rates suggest atmospheric Hg flux increased in the early 1900s, from A.D. 1920 to A.D. 1990, and at present. Atmospheric deposition is the primary source of the anthropogenic inputs of Hg and SCPs to these high elevation lakes. The input of SCPs, which is largely driven by regional sources, has declined with the implementation of national pollution control regulations. Mercury deposition in the Great Basin has most likely been influenced more by regional inputs.

The modern distribution of chironomid sub-fossils (Insecta: Diptera) in Costa Rica and the development of a regional chironomid-based temperature inference model

Chironomids have been shown to provide robust reconstructions of past temperature change and variability. This is the first study to assess the contemporaneous relationship between the distribution of sub-fossil chironomids and limnological and climatic parameters in Central America. Here, we describe the distribution of chironomids in a suite of 51 lakes in Costa Rica. We identify environmental variables that account for a statistically significant amount of variance in midge distribution, and develop a quantitative chironomid-based inference model for mean annual air temperature (MAT). Psectrocladius, which is documented for the first time in Costa Rica, dominate high-elevation lakes characterized by low MAT and relatively dilute water. Canonical correspondence analysis (CCA) revealed that MAT and conductivity account for large, statistically significant amounts of variance in the distribution of chironomids. A chironomid-based inference model for MAT, developed using a partial least squares approach, provided robust performance statistics with a high coefficient of determination and a relatively low root-mean square error. Application of the chironomid-based inference model for MAT to chironomid stratigraphies spanning the Holocene, together with the ecological information provided by this study, will enable us to address many outstanding questions relating to long-term climate and environmental change in the region.

Regional Climate Change Evidenced by Recent Shifts in Chironomid Community Composition in Subalpine and Alpine Lakes in the Great Basin of the United States

Abstract Chironomids (nonbiting midges) are used to develop centennial length temperature reconstructions for six subalpine and alpine lakes in the central Great Basin of the United States. Faunal turnover, assessed by detrended correspondence analysis (DCA), indicate that substantial compositional change in the midge communities has occurred during the past 100 years. Although the changes in composition are site-specific, increases in Dicrotendipes and decreases in Procladius characterize the late 20th century at a majority of the sites. Notable faunal turnover in midge community composition is observed at five of the six sites beginning at approximately A.D. 1970. Application of a chironomid-based mean July air temperature inference model (r 2 jack = 0.55, RMSEP = 0.9 °C) to the subfossil chironomid assemblages provides site-specific quantitative reconstructions of past temperature variability for the 20th and 21st centuries. Midge-inferred temperature estimates indicate that four of the six lakes were characterized by above average air temperatures during the post—A.D. 1980 interval and below average temperatures during the early 20th century. The rate of temperature change between A.D. 1920 and A.D. 2010 for these four lakes are: Smith Lake = 0.6 °C 100 yr-1; Birdeye Lake = 0.7 °C 100 yr-1; Cold Lake = 1.2 °C 100 yr-1; Stella Lake = 0.4 °C 100 yr-1. Correspondence between fluctuations in the midge-inferred temperature and instrumental measures of mean July air temperature for Nevada Climate Division #2 is also documented. This study adds to the growing body of evidence that subalpine and alpine lakes in the Intermountain West of the United States have been and are increasingly being affected by anthropogenic climate change in the early 21st century.