Chironomid-inferred Temperature Research Articles

Effects of recent climate and environmental changes on the ecology of a boreal forest lake in Manitoba, Canada

Application of paleoenvironmental approaches provides insight into the magnitude and timing of responses to climate warming in aquatic-ecosystems of northern Canada. We examined subfossil biological (Diptera:Chironomidae) and geochemical indicators (organic carbon and elemental nitrogen and stable isotope composition) in a sediment core from Buckland Lake, northern Manitoba, to assess the influence of recent warming (1981–2011) in the central subarctic region of Canada. The earlier part of the paleolimnological record (1830–1980) was characterized by relatively low chironomid diversity (N2 ~ 6), consisting primarily of profundal taxa (Orthocladius consobrinus and Chironomus), low organic matter content of sediments ( 3‰), indicative of a cold-water environment with low nitrogen demand. Between 1910 and 1980, there was a decline in profundal taxa, with small gradual increases in littoral taxa, such as Cladotanytarsus mancus-group. Post-1980 sediment core intervals had distinct geochemistry, with declines in δ13Corg from − 27.5‰ to almost − 29‰, consistent with warming, increased terrestrial influence, and increased snowfall and runoff. We found substantial inferred warming (+ 1–2 °C), with several chironomid-inferred temperatures nearly 3 °C warmer than typical pre-1980 inferences. Concurrently, several warm-water-adapted littoral chironomid taxa (Cladopelma, Cryptochironomus, Polypedilum) recorded increases. The post-1990 records reflected continued increases in warm-water taxa, changes in the benthic:pelagic trophic structure, and reversal of previous trends in the δ13Corg, %Organic Carbon, and δ15N profiles, suggestive of increased aquatic productivity. The meteorological station at Gillam, Manitoba, also recorded warming (1.5 °C) and a reduction in snowfall during that time period. A reduction in spring recharge may have increased littoral habitat associated with lake-level drawdown. The climate-driven shift observed ~ 1980 is in generally good agreement with other regional analyses, which show regime shifts occurred ~ 1995. The earlier lake response (~ 1980), however, demonstrates the role of increased catchment-mediated influences on northern boreal lake productivity that may only appear in records with sufficiently high resolution and multiple paleolimnological indicators.

Complexity and asynchrony of climatic drivers and environmental responses during the Last Glacial-Interglacial Transition (LGIT) in north-west Europe

The Last Glacial-Interglacial Transition (ca 16–8 ka BP) in north-west Europe is an important period of climatic change where millennial-scale climatic evolution led to environmental reorganisation. Imprinted upon these long-term changes are a series of short-lived, centennial-scale events that appear to be spatially and temporally complex across Europe. The complexity of environmental change in response to these climatic events is poorly understood because of a paucity of paired investigations that provide evidence of both driver and response variables. We present a high-resolution palynological, charcoal and stable isotopic record alongside chironomid-inferred temperature data from Tirinie, south-east Grampian Highlands, Scotland. The record is stratigraphically and chronologically constrained using tephra and radiocarbon dating. The isotopic and chironomid data reveal centennial-scale climatic deteriorations at ca 14.0; 13.2 and 11.4 cal ka BP. In response to these cooling events, vegetation became more open, fire frequency increased and landscape erosion was common. The reconstruction of both climate and environment reveals asynchrony in the phasing of annual and summer temperature variability, vegetation change and fire for each climatic event. Whilst responses appear strongest following the convergence of annual and summer temperature variability across all events, the ca 13.2 ka BP event reveals a two-stage environmental and fire response to climatic change, and the ca 11.4 ka BP event exhibits environmental change in the absence of summer temperature variability. The data further suggests that fire is an integral component of abrupt climatic change in this part of north-west Europe.

Late Holocene ecological shifts and chironomid-inferred summer temperature changes reconstructed from Lake Uddelermeer, the Netherlands

This paper presents a late Holocene chironomid-inferred record of July air temperature from a core obtained from Lake Uddelermeer in the Netherlands. The core interval, which dates from 2500 to 400 cal. yr. BP, was analysed at multi-decadal resolution for organic content, pollen, spores and NPPs (Non Pollen Palynomorphs), and chironomid head capsules. These proxies indicate that, from 2500 to 1140 cal. yr. BP, the lake was mesotrophic and sustained a Littorellion community, while the chironomid assemblage was dominated by littoral species associated with macrophytes. At 1140 cal. yr. BP, a shift in the lake ecology occurred from low-nutrient to high-nutrient conditions dominated by algae. This shift might be linked to a concurrent increase in human impact and is reflected in the chironomid assemblage by increases in eurytopic taxa, which are resistant to disturbances. Shifts in the chironomid record between 2500 and 1140 cal. yr. BP do not coincide with changes in lake ecology and are presumably driven by climate change. Using a Norwegian-Swiss calibration dataset as a modern analogue, we produced a chironomid-inferred temperature (C-IT) reconstruction. This reconstruction compares well to other regional temperature reconstructions in timing and duration showing a Roman Warm Period between 2240 and 1760 cal. yr. BP, a Dark Age Cold Period starting at 1760 cal. yr. BP and the Medieval Climate Anomaly beginning at 1280 cal. yr. BP. The C-IT record indicates a temperature drop of 1.5 °C from the Roman Warm Period to the Dark Age Cold Period. Findings improve knowledge of the first millennium AD in NW Europe, which was characterised by changes in landscape, vegetation, society and climate.

The Little Ice Age signature in a 700-year high-resolution chironomid record of summer temperatures in the Central Eastern Alps

Despite the fact that the Little Ice Age (LIA) is well documented for the European Alps, substantial uncertainties concerning the regional spatio-temporal patterns of temperature changes associated with the LIA still exist, especially for their eastern sector. Here we present a high-resolution (4–10 years) 700-year long mean July air temperature reconstruction based on subfossil chironomid assemblages from a remote lake in the Austrian Eastern Alps to gain further insights into the LIA climatic deterioration in the region. The record provides evidence for a prolonged period of predominantly cooler conditions during AD 1530–1920, broadly equivalent to the climatically defined LIA in Europe. The main LIA phase appears to have consisted of two cold time intervals divided by slightly warmer episodes in the second half of the 1600s. The most severe cooling occurred during the eighteenth century. The LIA temperature minimum about 1.5 °C below the long-term mean recorded in the mid-1780 s coincides with the strongest volcanic signal found in the Greenland ice cores over the past 700 years and may be, at least in part, a manifestation of cooling that followed the long-lasting AD 1783–1784 Laki eruption. A continuous warming trend is evident since ca AD 1890 (1.1 °C in 120 years). The chironomid-inferred temperatures show a clear correlation with the instrumental data and reveal a close agreement with paleotemperature evidence from regional high-elevation tree-ring chronologies. A considerable amount of the variability in the temperature record may be linked to changes in the North Atlantic Oscillation.

A mid to late Holocene chironomid-inferred temperature record from northwest Ireland

This study provides the first mid to late Holocene chironomid-inferred temperature (C-IT) model for northwest Ireland, creating a valuable climatic context for the development of Irish society during the Neolithic and Bronze Age. Using a lake sediment core from an isolated catchment, Lough Meenachrinna in northwest Ireland, a multi-proxy approach of chironomid (Diptera: Chironomidae) subfossils, sediment geochemistry (δ13C, δ15N, C:N) and pollen analysis was used to assess any potential limnological impact from prehistoric human activity in the region and provide a quantitative summer temperature estimate for the late Mesolithic to the Iron Age (7050–2050 cal yr BP). The pollen record provides a local signal of human activity, showing low levels of pastoral indicators in the early Neolithic, with increased evidence for pastoral and arable farming during the Bronze Age and in particular the Iron Age. Human activity does not appear to be a driving force in lake system change at Lough Meenachrinna, as peaks in farming indicators (e.g. pastoral pollen indicators and increased δ15N values) were not concurrent with major fluctuations in the chironomid assemblages. C-ITs provide evidence of multiple fluctuations in temperature during the mid to late Holocene with a cold phase during the late Mesolithic (6800–5890 cal yr BP), followed by a warming period during the early Neolithic (5890–5570 cal yr BP). C-ITs reflect a relatively warm climate during the middle Neolithic, with a substantial warming from the late Neolithic into the early Bronze Age (4630–3810 cal yr BP). C-ITs show a general cooling trend from the Bronze Age into the Iron Age, with a cold event occurring at 3340 cal yr BP during the middle Bronze Age and second cold event at 2430 cal yr BP during the Iron Age. The multi-proxy approach has proven an effective strategy for untangling human and climate influences on the chironomid record, and is recommended as a methodological approach in future Holocene climatic reconstructions where prehistoric human influence is a factor.

A chironomid-based record of temperature variability during the past 4000 years in northern China and its possible societal implications

Abstract. Long-term, high-resolution temperature records which combine an unambiguous proxy and precise dating are rare in China. In addition, the societal implications of past temperature change on a regional scale have not been sufficiently assessed. Here, based on the modern relationship between chironomids and temperature, we use fossil chironomid assemblages in a precisely dated sediment core from Gonghai Lake to explore temperature variability during the past 4000 years in northern China. Subsequently, we address the possible regional societal implications of temperature change through a statistical analysis of the occurrence of wars. Our results show the following. (1) The mean annual temperature (TANN) was relatively high during 4000–2700 cal yr BP, decreased gradually during 2700–1270 cal yr BP and then fluctuated during the last 1270 years. (2) A cold event in the Period of Disunity, the Sui-Tang Warm Period (STWP), the Medieval Warm Period (MWP) and the Little Ice Age (LIA) can all be recognized in the paleotemperature record, as well as in many other temperature reconstructions in China. This suggests that our chironomid-inferred temperature record for the Gonghai Lake region is representative. (3) Local wars in Shanxi Province, documented in the historical literature during the past 2700 years, are statistically significantly correlated with changes in temperature, and the relationship is a good example of the potential societal implications of temperature change on a regional scale.

Influence of industrial activity and pollution on the paleoclimate reconstruction from a eutrophic lake in lowland England, UK

Reliable estimates of Holocene temperatures are important for understanding past climate dynamics, the response of biota to climate change, and validating climate models. Chironomids in lake sediment cores are used widely to quantify past summer temperatures, for which high-latitude and/or high-altitude lakes, remote from human influence, are usually considered appropriate. Temperature inferences from lowland lakes are likely influenced by other variables, specifically eutrophication and industrial pollution, but their reliability has never been tested. We used a Norwegian chironomid-based transfer function (r 2 = 0.91; RMSEP = 1.01 °C) to infer mean July air temperature over the last 200 years, using chironomid assemblages in a core collected from a polluted, nutrient-enriched lake at Speke Hall, Liverpool, England. The chironomid-inferred temperatures correlate significantly with the local instrumental temperature record and follow long-term national temperature trends. These results show that chironomids can be used to produce reliable estimates of past mean July air temperature, even when other variables have also influenced the composition of the chironomid community. These findings underline the value of chironomids as sensitive and reliable quantitative proxies for summer temperature.

A temperature reversal within the rapid Younger Dryas-Holocene warming in the North Atlantic?

Abstract The onset of the Holocene has been generally considered rapid and uninterrupted in the circum-Atlantic region. Loss-on-ignition (LOI – an index of organic carbon) profiles from 18 lateglacial-aged lakes in Nova Scotia, Canada, together with chironomid-inferred temperature reconstructions at 5 sites, demonstrate that the rapid warming from the Younger Dryas (GS-1) to the Holocene was interrupted by a cooling of 1.6–6.4 °C in summer surface lake water temperature. The resulting inflection or reversal on the rising temperature curve has also been identified at 35 sites outside Nova Scotia from terrestrial and marine settings, indicating that this cool step is a robust feature throughout the North Atlantic and is likely the result of major oceanic and atmospheric reorganization of the Holocene climate system.

Response of Lacustrine Biota to Late Holocene Climate and Environmental Conditions in Northernmost Ungava (Canada)

Sediment cores from three lakes located in the northernmost region of Ungava, Quebec (Canada) were examined to define aquatic community and ecosystem variability during the Late Holocene period. A chironomid-based transfer function was used to reconstruct August air temperature trends, and lacustrine primary production was inferred from sedimentary biogenic silica content and siliceous microfossil abundances. Trends in primary production, sediment organic matter content (estimated through loss on ignition), and chironomid-inferred temperature were compared to explore potential effects of environmental change on biotic assemblage composition at centennial to millennial time scales. Although no direct correlation between chironomid-inferred August air temperature and primary production was observed, we found indications that both chironomid and diatom communities were responding to the same overarching regional climatic and environmental processes. Over the last decade, northern Quebec has been undergoing notable, rapid warming that contrasts with the relative inertia of the past few millennia. This study provides a baseline against which recent and future environmental changes in this region can be compared.

A model-data comparison of Holocene timberline changes in the Swiss Alps reveals past and future drivers of mountain forest dynamics.

Mountain vegetation is strongly affected by temperature and is expected to shift upwards with climate change. Dynamic vegetation models are often used to assess the impact of climate on vegetation and model output can be compared with paleobotanical data as a reality check. Recent paleoecological studies have revealed regional variation in the upward shift of timberlines in the Northern and Central European Alps in response to rapid warming at the Younger Dryas/Preboreal transition ca. 11 700 years ago, probably caused by a climatic gradient across the Alps. This contrasts with previous studies that successfully simulated the early Holocene afforestation in the (warmer) Central Alps with a chironomid-inferred temperature reconstruction from the (colder) Northern Alps. We use LandClim, a dynamic landscape vegetation model to simulate mountain forests under different temperature, soil and precipitation scenarios around Iffigsee (2065 m a.s.l.) a lake in the Northwestern Swiss Alps, and compare the model output with the paleobotanical records. The model clearly overestimates the upward shift of timberline in a climate scenario that applies chironomid-inferred July-temperature anomalies to all months. However, forest establishment at 9800 cal. BP at Iffigsee is successfully simulated with lower moisture availability and monthly temperatures corrected for stronger seasonality during the early Holocene. The model-data comparison reveals a contraction in the realized niche of Abies alba due to the prominent role of anthropogenic disturbance after ca. 5000 cal. BP, which has important implications for species distribution models (SDMs) that rely on equilibrium with climate and niche stability. Under future climate projections, LandClim indicates a rapid upward shift of mountain vegetation belts by ca. 500 m and treeline positions of ca. 2500 m a.s.l. by the end of this century. Resulting biodiversity losses in the alpine vegetation belt might be mitigated with low-impact pastoralism to preserve species-rich alpine meadows.

Middle to late Holocene chironomid-inferred July temperatures for the central Northwest Territories, Canada

We analyzed subfossil chironomids, sed- iment organic matter and sediment particle size data from a 1.11-m-long freeze core collected from Carl- eton Lake (unofficial name), located approximately 120 km north of the modern treeline. This well-dated core spans the last ca. 6,500 years. Two chironomid transfer functions were applied to infer mean July air temperatures. Our results indicated that the chirono- mid-inferred temperatures from this lake sediment record did not pass a significance test, suggesting that other factors in addition to temperature may have been important in structuring the chironomid community through time. Although not statistically significant, the chironomid-inferred temperatures from this site do follow a familiar pattern, with highest inferred tem- peratures occurring during the Holocene Thermal Maximum (*6-4 cal kyr BP), followed by a long- term cooling trend, which is reversed during the last 600 years. The largest change in the chironomid assemblage, which occurred between ca. 4,600 and 3,900 cal yr BP is possibly related to the well- documented northward advance and subsequent retreat of treeline in this region.

Summer temperature gradients in northwest Europe during the Lateglacial to early Holocene transition (15–8 ka BP) inferred from chironomid assemblages

We present a series of summer air temperature isotherm maps based on chironomid-inferred temperatures from northwest Europe, covering the Lateglacial and early Holocene (15–8 ka BP). These maps are the first of their kind, and use data derived from 22 Lateglacial sites and 34 early Holocene sites. The isotherms are generated by weighted spatial interpolation (kriging). The major patterns of chironomid-inferred summer temperatures are spatially well-resolved in both the Lateglacial and early Holocene. The isotherm maps indicate that there was a strong west to east gradient during the Lateglacial Interstadial (GI-1) due to the influence of thermohaline circulation in the regions bordering the north Atlantic, which diminishes eastwards. A strong north to south temperature gradient is also apparent, particularly in eastern regions, influenced by the extent of the Scandinavian ice-cap. Peak temperatures are achieved early in the Interstadial in the south of the region but occur towards the end of the Interstadial in the north. Holocene warming varies spatially and temporally and is earliest in the south and east, but later in the north and west. During the period covered in our study maximum warmth is reached ca. 10 ka BP. The chironomid-based Lateglacial isotherm maps are compared with previously published isotherm maps from the same region based on beetle-inferred temperatures. While the trends shown in the two datasets are similar, beetle-inferred temperatures are often warmer than chironomid-inferred temperatures. This is especially marked in GI-1e and may be due to microclimatic effects causing the chironomids to underestimate air temperatures and/or the beetles to over-estimate air temperatures. The spatial coherence between sites in both the Lateglacial and early Holocene suggest that the chironomid-based temperature estimates are largely reliable, although data testing suggests that estimates from southern Scandinavia may be less reliable perhaps due to high topographical relief influencing local climate. More data points are required, particularly from northwest Scotland, southwest England and Wales, northeast France, Denmark, Finland and the Baltic States, to confirm trends and provide even coverage and a denser network of sites.

New evidence of warm early-Holocene summers in subarctic Finland based on an enhanced regional chironomid-based temperature calibration model

Paleoclimate reconstructions based on biological proxies present methodological challenges, especially during non-analog conditions, such as the early Holocene. Here, two chironomid-based training sets from Finland were amalgamated to create a more accurate transfer function of summer air temperature. The aim was to reconstruct Holocene paleoclimate in northernmost Lapland, in an area that has been either too warm or too cold for reliable reconstructions using the original calibration models. The results showed that the combined calibration model had improved performance statistics. The temperature trends inferred from the downcore chironomid record using the original and combined models were very similar. However, there were major changes in their absolute values with the combined model showing greatly improved accuracy. The chironomid-based temperature reconstruction showed significant correlation with the previous pollen-based reconstructions from northwestern Finnish Lapland. However, differences were observed in the temperature trends of the early Holocene, when the chironomid-inferred temperatures rapidly increased, but the pollen-based reconstructions lagged behind suggesting that a cool climate continued for much longer. However, similar to the chironomid record, new plant macrofossil evidence from northwestern Finland also showed warmer-than-present early Holocene temperatures. Therefore, we conclude that the early Holocene was probably warm in northern Lapland.

Comparing and cross-validating lake and bog palaeoclimatic records: a review and a new 5,000 year chironomid-inferred temperature record from northern England

This paper reviews the past history of comparisons between lacustrine records of chironomid-inferred summer temperatures (CI-T) and peat bog-derived records of surface wetness (BSW), and develops a new summer temperature record for the mid-late Holocene for Northern Britain from Bigland Tarn, Cumbria. The CI-T reconstruction is well dated, and corresponds well with the only two other CI-T records for Britain. Comparisons are made with BSW records from northern Britain, which show consistent similarities over centennial to millennial timescales, demonstrating that at this resolution cooler summers equated with a wetter climate, at least during the mid- to late-Holocene.

Chironomid-inferred Pleistocene and Holocene temperature records from Tasmania, Australia

Chironomid-inferred Pleistocene and Holocene temperature records from Tasmania, Australia

Impacts of changing climate and land use on vegetation dynamics in a Mediterranean ecosystem: insights from paleoecology and dynamic modeling

Forests near the Mediterranean coast have been shaped by millennia of human disturbance. Consequently, ecological studies relying on modern observations or historical records may have difficulty assessing natural vegetation dynamics under current and future climate. We combined a sedimentary pollen record from Lago di Massacciucoli, Tuscany, Italy with simulations from the LandClim dynamic vegetation model to determine what vegetation preceded intense human disturbance, how past changes in vegetation relate to fire and browsing, and the potential of an extinct vegetation type under present climate. We simulated vegetation dynamics near Lago di Massaciucoli for the last 7,000 years using a local chironomid-inferred temperature reconstruction with combinations of three fire regimes (small infrequent, large infrequent, small frequent) and three browsing intensities (no browsing, light browsing, and moderate browsing), and compared model output to pollen data. Simulations with low disturbance support pollen-inferred evidence for a mixed forest dominated by Quercus ilex (a Mediterranean species) and Abies alba (a montane species). Whereas pollen data record the collapse of A. alba after 6000 cal yr bp, simulated populations expanded with declining summer temperatures during the late Holocene. Simulations with increased fire and browsing are consistent with evidence for expansion by deciduous species after A. alba collapsed. According to our combined paleo-environmental and modeling evidence, mixed Q. ilex and A. alba forests remain possible with current climate and limited disturbance, and provide a viable management objective for ecosystems near the Mediterranean coast and in regions that are expected to experience a mediterranean-type climate in the future.

Using a newly developed chironomid transfer function for reconstructing mean annual air temperature at Lake Potrok Aike, Patagonia, Argentina

In the Southern Hemisphere, the lack of quantitative temperature records hampers the understanding of climate change since the Last Glaciation and refrains the comparison with the Northern Hemisphere records. To provide quantitative data, a 63-lake chironomid transfer functions was developed in Patagonia. Mean annual air temperature (MAT) was one of the most important factors explaining the distribution of chironomids while precipitation did not have any significant relationship with chironomid assemblages. The MAT model had a r2 of 0.64, a RMSE of 0.83 and a maximum bias of 1.81°C, comparable to other transfer functions of this size. This model was applied to the Lake Potrok Aike (PTA) chironomid records which consisted of only four taxa (Phaenopsectra, Cricotopus, Smittia and Polypedilum). The chironomid-inferred air temperatures were colder-than-the-average (10.8°C) during the Lateglacial with the coldest temperatures (9°C in average) during the Antarctic Cold Reversal (ACR). Between ca. 8000 and 3500cal. years BP, the chironomid-inferred air temperatures were warmer-than-the-average with a decreasing trend. From ca. 3500cal. years BP to the present, the chironomid-inferred temperatures oscillated around the average. The difference between the chironomid-inferred air temperature in the surface sample and the climate normal (1961–1990) was 0.6°C, suggesting that chironomids are sensitive enough to quantitatively reconstruct MAT at PTA. The general pattern of temperature changes reconstructed by the PTA chironomid record corresponded well to other quantitative records in the Southern Hemisphere. The results presented here show that investing in the development of chironomid transfer functions for quantitative climate research in the Southern Hemisphere is valuable.

Chironomids can be reliable proxies for Holocene temperatures. A comment on Velle et al. (2010)

Velle et al. (2010) discussed discrepancies between Scandinavian Holocene chironomid-inferred temperature estimates, which they attribute to the response of chironomids to environmental variables other than temperature and to taxonomic shortcomings. They suggest ways in which the reliability of chironomid-based paleotemperature reconstructions could be improved by taking into account ecological complexity. While we agree with many of their recommendations, based on the results of other work, we think their paper is unnecessarily pessimistic regarding the ability of existing chironomid-based temperature inference models to provide reliable estimates of past temperature. We offer a critique of the main points discussed by Velle et al. (2010) and provide evidence that chironomid-based temperature inference models can reliably reconstruct mean July air temperature in the Lateglacial and Holocene over millennial and centennial timescales.

High resolution Lateglacial and early-Holocene summer air temperature records from Scotland inferred from chironomid assemblages

Lateglacial and early-Holocene mean July air temperatures have been reconstructed, using a chironomid-based inference model, from lake-sediment sequences from Abernethy Forest, in the eastern Highlands of Scotland, and Loch Ashik, on the Isle of Skye in north-west Scotland. Chronology for Abernethy Forest was derived from radiocarbon dates of terrestrial plant macrofossils deposited in the lake sediments. Chronology for Loch Ashik was derived from tephra layers of known ages, the first age-depth model of this kind. Chironomid-inferred temperatures peak early in the Lateglacial Interstadial and then gradually decline by about 1 °C to the beginning of the Younger Dryas (YD). At Abernethy Forest, the Lateglacial Interstadial is punctuated by three centennial-scale cold oscillations which appear to be synchronous with the Greenland Interstadial events GI-1d, when temperatures at Abernethy fell by 5.9 °C, GI-1c, when temperatures fell by 2.3 °C, and GI-1b, when temperatures fell by 2.8 °C. At Loch Ashik only the oscillation correlated with GI-1d is clearly defined, when temperatures fell by 3.8 °C. The start of the YD is clearly marked at both sites when temperatures fell by 5.5 °C at Abernethy Forest and 2.8 °C at Loch Ashik. A warming trend is apparent during the late-YD at Abernethy Forest but at Loch Ashik late-YD temperatures became very cold, possibly influenced by its close proximity to the Skye ice-field. The rapidly rising temperatures at the YD – Holocene transition occur about 300 years earlier at both sites than changes in sediment lithology and loss-on-ignition. The temperature trends at both sites are broadly similar, although between-site differences may result from the influence of local factors. Similar climate trends are found at other sites in the northern British Isles. However, the British summer temperature records differ in detail from trends in the oxygen-isotope records from the Greenland ice-cores and from other chironomid-inferred temperature records available from Scandinavia, north-west Europe and central Europe, which suggest important differences in the influence of climatic forcing at regional scales.

A last millennium temperature reconstruction using chironomids preserved in sediments of anoxic Seebergsee (Switzerland): consensus at local, regional and Central European scales

Determining if temperatures of the last century exceed natural variability necessitates the use of high-resolution paleo-reconstructions extending beyond the instrumental data (i.e. >150 years). Although syntheses using tree-ring, stalagmite, and borehole based reconstructions are available, biological proxies preserved in lake sediments have been neglected as a source of high-resolution information on climate change. Here, we present a decadal-scale mean July air temperature reconstruction covering the past millennium from varved Seebergsee, Switzerland. This reconstruction is compared to instrumental data at local, regional and European scales, to another high-resolution chironomid-inferred temperature reconstruction from Lake Silvaplana (Switzerland), to a composite of paleo-climate reconstructions from the Greater Alpine Region and to various millennial scale climate reconstructions from the northern hemisphere. When compared to local and regional instrumental records since ca 1760 AD significant (p < 0.01) relationships (rPearson > 0.5) are obtained, suggesting that chironomids accurately register the changes in temperature for the past ca 250 years. At European scale, the Seebergsee reconstruction correlates (rPearson = 0.40; p < 0.01) with instrumental and early instrumental data back to 1500 AD. On the millennial time scale, the chironomid reconstruction of Seebergsee provides a pattern of temperature changes mirrored by the chironomid reconstruction from Silvaplana (rPearson = 0.44; p < 0.01) and the Greater Alpine Region composite of reconstructions (rPearson = 0.40; p < 0.01). This includes warmer-than-the-last-century mean July air temperatures (+1.2 °C on average) during the end of the “Medieval Climate Anomaly” (MCA) and colder-than-the-last-century temperatures (−0.5 °C on average) during the “Little Ice Age”. Both chironomid reconstructions inferred a warming during the last decades, but this chironomid-inferred warming does not exceed the MCA temperatures. This result is not singular as many millennial temperature reconstructions in the northern Hemisphere do not show unprecedented warming of the last century at local/regional scale. However, the chironomid assemblages found in Seebergsee and Silvaplana since ca 1950 AD seem to be unique (i.e. they show unprecedented assemblage compositions) for the past 1000 years.