High-frequency Climate Variability Research Articles

Detrending climate data prior to climate–growth analyses in dendroecology: A common best practice?

Tree growth varies closely with high–frequency climate variability. Since the 1930s detrending climate data prior to comparing them with tree growth data has been shown to better capture tree growth sensitivity to climate. However, in a context of increasingly pronounced trends in climate, this practice remains surprisingly rare in dendroecology. In a review of Dendrochronologia over the 2018–2021 period, we found that less than 20 % of dendroecological studies detrended climate data prior to climate-growth analyses. With an illustrative study, we want to remind the dendroecology community that such a procedure is still, if not more than ever, rational and relevant. We investigated the effects of detrending climate data on climate–growth relationships across North America over the 1951–2000 period. We used a network of 2536 tree individual ring-width series from the Canadian and Western US forest inventories. We compared correlations between tree growth and seasonal climate data (Tmin, Tmax, Prec) both raw and detrended. Detrending approaches included a linear regression, 30-yr and 100-yr cubic smoothing splines. Our results indicate that on average the detrending of climate data increased climate–growth correlations. In addition, we observed that strong trends in climate data translated to higher variability in inferred correlations based on raw vs. detrended climate data. We provide further evidence that our results hold true for the entire spectrum of dendroecological studies using either mean site chronologies and correlations coefficients, or individual tree time series within a mixed-effects model framework where regression coefficients are used more commonly. We show that even without a change in correlation, regression coefficients can change a lot and we tend to underestimate the true climate impact on growth in case of climate variables containing trends. This study demonstrates that treating climate and tree-ring time series “like-for-like” is a necessary procedure to reduce false negatives and positives in dendroecological studies. Concluding, we recommend using the same detrending for climate and tree growth data when tree-ring time series are detrended with splines or similar frequency-based filters.

Giant clam (Tridacna) distribution in the Gulf of Oman in relation to past and future climate

The Oman upwelling zone (OUZ) creates an unfavorable environment and a major biogeographic barrier for many coral reef species, such as giant clams, thus promoting and maintaining faunal differences among reefs on the east and west side of the Arabian Peninsula. We record the former existence of Tridacna in the Gulf of Oman and review its stratigraphic distribution in the Persian Gulf to provide new insights on the connectivity of coral reef habitats around southern Arabia under changing climate and ocean conditions. Fossil shells were carbon-14 dated and employed as sclerochronological proxy archives. This reveals that the Omani population represents a last glacial colonization event during the Marine Isotope Stage 3 interstadial under colder-than-present temperatures and variable upwelling intensity linked to Dansgaard-Oeschger climate oscillations. It was favored by temperatures just above the lower threshold for the habitat-forming reef coral communities and instability of the upwelling barrier. We conclude that the distribution of Tridacna in the northern Arabian Sea is generally limited by either strong upwelling or cool sea surface temperature under gradually changing climate conditions at the interglacial-glacial scale. Opportunities for dispersal and temporary colonization existed only when there was a simultaneous attenuation of both limiting factors due to high-frequency climate variability. The OUZ will unlikely become a future climate change refuge for giant clams because they will be exposed either to thermal stress by rapid anthropogenic Indian Ocean warming or to unfavorable upwelling conditions.

Reconstructing climate variability during the last four millennia from trans-Himalaya (Ladakh-Karakoram, India) using multiple proxies

In this communication we reconstruct climate variability of Ladakh, Trans-Himalaya region, on sub-century timescale spanning the last four millennia (~4130 to 260 cal yr ‘before present’ (BP)). To achieve this objective, we measured a suite of physical, biotic and inorganic proxies of a lacustrine sequence. In general, between ~4130 to ~2640 cal yr BP proxy records reveal climate variability following a typical pattern of the northern hemispheric climate and appear to be dominated by westerlies. Thereafter, the Indian Summer Monsoon (ISM) forcing appears to dominate regional climate. The former period appears to have experienced human presence as evidenced by an invasive species of diatom –Didymosphenia and charcoal, likely originating from human activities on trading routes with Central Asia, China and Tibet. After ~2640 cal yr BP, when ISM started governing the climate of the Ladakh- Karakoram region, it probably induced high-frequency climate variability leading to disasters related to landslides, debris flows, avalanches etc., eventually reducing the intra-boundary trade. A similar situation appears to be gripping the region again as meteorologically recorded precipitation data from Ladakh reveals arrival of high-frequency monsoonal variability in the region with recurrent flash floods (e.g., 2010, 2013).

Abrupt climate change and its influences on hominin evolution during the early Pleistocene in the Turkana Basin, Kenya

Rapid climate variability has been hypothesized to play an important role in hominin evolution, yet our knowledge of Plio-Pleistocene climate change on short timescales is poor. Here, we developed centennial-scale reconstructions of precipitation from leaf wax biomarker hydrogen isotope ratios (δDwax) using lacustrine sediment from West Turkana, Kenya. We analyzed two time intervals (∼1.72 and ∼1.60 Ma) with different orbital configurations (0.043 and 0.025 eccentricity, respectively) to examine the influence of seasonal insolation forcing on high-frequency climate variability and the rates of climate transitions. Our data indicate that under low summer insolation, which should induce high latitude glaciation and tropical African aridity, millennial-scale climate variability was stronger. This suggests that hominins may have been forced to contend with increased climate variability during already extreme environmental conditions. Additionally, we observe a rapid shift from arid to humid conditions occurring in less than 200 years under high-amplitude precessional-scale insolation change. The rate of this transition is similar to that observed in some proxy records of the onset of the African Humid Period, indicating high sensitivity to gradual insolation forcing in the Turkana Basin. Such abrupt climate changes could induce evolutionary selection for generalist behavioral traits in hominins.

High-frequency climate variability in the Holocene from a coastal-dome ice core in east-central Greenland

Abstract. An ice core drilled on the Renland ice cap in east-central Greenland contains a continuous climate record dating through the last glacial period. The Renland record is valuable because the coastal environment is more likely to reflect regional sea surface conditions compared to inland Greenland ice cores that capture synoptic variability. Here we present the δ18O water isotope record for the Holocene, in which decadal-scale climate information is retained for the last 8 kyr, while the annual water isotope signal is preserved throughout the last 2.6 kyr. To investigate regional climate information preserved in the water isotope record, we apply spectral analysis techniques to a 300-year moving window to determine the mean strength of varying frequency bands through time. We find that the strength of 15–20-year δ18O variability exhibits a millennial-scale signal in line with the well-known Bond events. Comparison to other North Atlantic proxy records suggests that the 15–20-year variability may reflect fluctuating sea surface conditions throughout the Holocene, driven by changes in the strength of the Atlantic Meridional Overturning Circulation. Additional analysis of the seasonal signal over the last 2.6 kyr reveals that the winter δ18O signal has experienced a decreasing trend, while the summer signal has predominantly remained stable. The winter trend may correspond to an increase in Arctic sea ice cover, which is driven by a decrease in total annual insolation, and is also likely influenced by regional climate variables such as atmospheric and oceanic circulation. In the context of anthropogenic climate change, the winter trend may have important implications for feedback processes as sea ice retreats in the Arctic.

Identifying Precipitation and Reference Evapotranspiration Trends in West Africa to Support Drought Insurance

West Africa represents a wide gradient of climates, extending from tropical conditions along the Guinea Coast to the dry deserts of the south Sahara, and it has some of the lowest income, most vulnerable populations on the planet, which increases catastrophic impacts of low and high frequency climate variability. This paper investigates low and high frequency climate variability in West African monthly and seasonal precipitation and reference evapotranspiration from the early 1980s to 2016. We examine the impact of those trends and how they interact with payouts from index insurance products. Understanding low and high frequency variability in precipitation and reference evapotranspiration at these scales can provide insight into trends during periods critical to agricultural performance across the region. For index insurance, it is important to identify low-frequency variability, which can result in radical departures between designed/planned and actual insurance payouts, especially in the later part of a 30-year period, a common climate analysis period. We find that evaporative demand and precipitation are not perfect substitutes for monitoring crop deficits and that there may be space to use both for index insurance design. We also show that low yields—aligned with the need for insurance payouts—can be predicted using classification trees that include both precipitation and reference evapotranspiration.

Assessing Bjerknes Compensation for Climate Variability and Its Time-Scale Dependence

Abstract The Bjerknes compensation (BJC) refers to the tendency for changes in the atmosphere heat transport (AHT) and ocean heat transport (OHT) to compensate each other. However, the nature of this compensation varies with the time scale of changes. In this study, a new approach was developed to diagnose BJC for climate variability by considering the correlation between AHT and OHT and their relative magnitudes. The correlation is equivalent to the cosine of phase difference between AHT and OHT. For high-frequency climate variability, AHT lags or leads OHT by π/2, the correlation is zero, and BJC does not occur concurrently. For low-frequency climate variability, AHT lags or leads OHT by π, the correlation is −1, and BJC is concurrent. With increasing time scale, the phase difference between AHT and OHT changes from π/2 to π, and the BJC reaches equilibrium. A coupled box model is used to justify the approach and to understand the temporal change of BJC from a theoretical perspective. The correlation and BJC rate derived from theory and from the box model exhibit similar transient behaviors, approaching equilibrium monotonically with increasing time scale. The equilibrium BJC is established at decadal time scale. Since the BJC is closely related to climate feedback, a proper identification of BJC processes in climate variability can reveal the nature of dominant climate feedback processes at different time scales.

Stable carbon and oxygen isotopes in tree rings show physiological responses of Pericopsis elata to precipitation in the Congo Basin

Abstract:In equatorial regions, where tree rings are less distinct or even absent, the response of forests to high-frequency climate variability is poorly understood. We measured stable carbon and oxygen isotopes in anatomically distinct, annual growth rings of four Pericopsis elata trees from a plantation in the Congo Basin, to assess their sensitivity to recorded changes in precipitation over the last 50 y. Our results suggest that oxygen isotopes have high common signal strength (EPS = 0.74), and respond to multi-annual precipitation variability at the regional scale, with low δ18O values (28–29‰) during wetter conditions (1960–1970). Conversely, δ13C are mostly related to growth variation, which in a light-demanding species are driven by competition for light. Differences in δ13C values between fast- and slow-growing trees (c. 2‰), result in low common signal strength (EPS = 0.37) and are driven by micro-site conditions rather than by climate. This study highlights the potential for understanding the causes of growth variation in P. elata as well as past hydroclimatic changes, in a climatically complex region characterized by a bimodal distribution in precipitation.

Late Holocene vegetation changes in relation with climate fluctuations and human activity in Languedoc (southern France)

Abstract. Holocene climate fluctuations and human activity since the Neolithic have shaped present-day Mediterranean environments. Separating anthropogenic effects from climatic impacts to better understand Mediterranean paleoenvironmental changes over the last millennia remains a challenging issue. High-resolution pollen analyses were undertaken on two cores from the Palavasian lagoon system (Hérault, southern France). These records allow reconstruction of vegetation dynamics over the last 4500 years. Results are compared with climatic, historical and archeological archives. A long-term aridification trend is highlighted during the late Holocene, and three superimposed arid events are recorded at 4600–4300, 2800–2400 and 1300–1100 cal BP. These periods of high-frequency climate variability coincide in time with the rapid climatic events observed in the Atlantic Ocean (Bond et al., 2001). From the Bronze Age (4000 cal BP) to the end of the Iron Age (around 2000 cal BP), the spread of sclerophyllous taxa and loss of forest cover result from anthropogenic impact. Classical Antiquity is characterized by a major reforestation event related to the concentration of rural activity and populations in coastal plains leading to forest recovery in the mountains. A major regional deforestation occurred at the beginning of the High Middle Ages. Around 1000 cal BP, forest cover is minimal while the cover of olive, chestnut and walnut expands in relation to increasing human influence. The present-day vegetation dominated by Mediterranean shrubland and pines has been in existence since the beginning of the 20th century.

Vegetation and environmental changes during the last interglacial in eastern Anatolia (Turkey): a new high-resolution pollen record from Lake Van

A high-resolution multi-proxy record from Lake Van, eastern Anatolia, derived from a lacustrine sequence cored at the 357 m deep Ahlat Ridge (AR), allows a comprehensive view of paleoclimate and environmental history in the continental Near East during the last interglacial (LI). We combined paleovegetation (pollen), stable oxygen isotope (delta O-18(bulk)) and XRF data from the same sedimentary sequence, showing distinct variations during the period from 135 to 110 ka ago leading into and out of full interglacial conditions. The last interglacial plateau, as defined by the presence of thermophilous steppe-forest communities, lasted ca. 13.5 ka, from similar to 129.1-115.6 ka BP. The detailed palynological sequence at Lake Van documents a vegetation succession with several climatic phases: (I) the Pistacia zone (ca. 131.2-129.1 ka BP) indicates summer dryness and mild winter conditions during the initial warming, (II) the Quercus-Ulmus zone (ca. 129.1-1272 ka BP) occurred during warm and humid climate conditions with enhanced evaporation, (III) the Carpinus zone (ca. 1272-124.1 ka BP) suggest increasingly cooler and wetter conditions, and (IV) the expansion of Pinus at similar to 124.1 ka BP marks the onset of a colder/drier environment that extended into the interval of global ice growth. Pollen data suggest migration of thermophilous trees from refugial areas at the beginning of the last interglacial. Analogous to the current interglacial, the migration documents a time lag between the onset of climatic amelioration and the establishment of an oak steppe-forest, spanning 2.1 ka. Hence, the major difference between the last interglacial compared to the current interglacial (Holocene) is the abundance of Pinus as well as the decrease of deciduous broad-leaved trees, indicating higher continentality during the last interglacial. Finally, our results demonstrate intra-interglacial variability in the low mid-latitudes and suggest a close connection with the high-frequency climate variability recorded in Greenland ice cores. (C) 2015 Elsevier B.V. All rights reserved.

Orbital forcing as principal driver for fine-grained deep-marine siliciclastic sedimentation, Middle-Eocene Ainsa Basin, Spanish Pyrenees

The relative importance of climate, tectonics and/or autocyclicity in driving deposition of laminated deep-marine fine-grained siliciclastic sediments in tectonically active basins remains controversial. Consequently, the recognition of Milankovitch periodicities in such successions remains poorly documented. This study evaluates the cyclostratigraphy and its likely drivers in the Middle Eocene Ainsa Basin, Spanish Pyrenees, a basin that is known to have been associated with major syndepositional tectonics. This basin was infilled by a thick succession of siliciclastic sediments, characterised by an alternation of coarse-grained sandbodies interpreted as submarine fans and finer grained packages of interfan, fan lateral-margin, and channel levée–overbank sediments. This study presents time-series analysis of outcrop spectral gamma-ray and cumulative sandstone turbidite abundance data from eight fine-grained, essentially interfan sections located between coarse-grained sandbodies (submarine fans). The ~1.4km of cumulative stratigraphy studied show that Milankovitch cyclicity can be recognised throughout the entire stratigraphic evolution of the Ainsa Basin, even though the basin was tectonically active and suggest that the accumulation of the interfan deposits was orbitally controlled. Orbital parameters most likely paced the cyclic delivery of the fine-grained sediments mainly by river- and delta-supplied turbidity currents from the non-marine and shallow-marine Tremp Basin immediately to the east. Sediment accumulation rates determined from spectral analysis show a gradual decrease throughout the deep-marine stratigraphy from ~57 to ~24cm/kyr. This study shows that orbitally-induced climate change acted as a principal driver on deep-marine terrigenous sediment accumulation within a tectonically active basin. High-frequency climate variability, therefore, should be considered when studying controls on siliciclastic sedimentation in tectonically active regions.

Solar influence on climate variability and human development during the Neolithic: evidence from a high-resolution multi-proxy record from Templevanny Lough, County Sligo, Ireland

The relationship between climatic variations, vegetation dynamics and early human activity between c. 4150–2860 BC was reconstructed from a high-resolution pollen and geochemical record obtained from a small lake located in County Sligo, Ireland. The proxy record suggests the existence of a woodland with a largely closed canopy at the start of the fourth millennium BC. Only minor human disturbance is recorded. Following an episode of increased rainfall at c. 3990 BC, a decrease in the elm population occurred between c. 3970 and 3820 BC. This coincided with a period of warming and drying climatic conditions and an initial increase in anthropogenic activities. A second episode of high precipitation between c. 3830–3800 BC was followed by a steep increase in human impact on the landscape, which became most pronounced between c. 3740 and 3630 BC. At this time, the lake level of Templevanny Lough was at its lowest during the Neolithic.The onset of wetter and cooler conditions after c. 3670 BC, representing the transition from the Early to the Middle Neolithic, coincided with a period of woodland recovery. The Middle Neolithic was characterised by pronounced climatic oscillations including periods of substantial rainfall between c. 3600 and 3500 BC and between c. 3500 and 3460 BC. A nearly century-long climatic amelioration between c. 3460–3370 BC facilitated a revival of human activity on a small scale around the lake. Abandonment of the area and full woodland recovery occurred after a period of particularly wet and cool conditions ranging from c. 3360–3290 BC. The pollen and geochemistry data suggest that the Late Neolithic was marked by a period of ameliorated conditions between c. 3110–3050 BC that was followed by two episodes of high rainfall at c. 3060–3030 BC and c. 2940–2900 BC.The timing of the climatic shifts inferred from the Templevanny Lough record is in agreement with those of moisture/precipitation and temperature reconstructions from northern and western Europe and the Alps, suggesting that the studied period was characterised by a high-frequency climate variability. The results of the present study imply that human development during the Irish Neolithic was influenced by climatic variations. These climatic shifts correspond to variations in solar activity, suggesting a solar forcing on climate.

Trends in Daily Solar Radiation and Precipitation Coefficients of Variation since 1984

Abstract This study investigates the possibility of changes in daily scale solar radiation and precipitation variability. Coefficients of variation (CVs) were computed for the daily downward surface solar radiation product from the International Satellite Cloud Climatology Project and the daily precipitation product from the Global Precipitation Climatology Project. Regression analysis was used to identify trends in CVs. Statistically significant changes in solar radiation variability were found for 35% of the globe, and particularly large increases were found for tropical Africa and the Maritime Continent. These increases in solar radiation variability were correlated with increases in precipitation variability and increases in deep convective cloud amount. The changes in high-frequency climate variability identified here have consequences for any process depending nonlinearly on climate, including solar energy production and terrestrial ecosystem photosynthesis. To assess these consequences, additional work is needed to understand how high-frequency climate variability will change in the coming decades.

High frequency climate variability of the Norwegian Atlantic Current during the early Holocene period and a possible connection to the Gleissberg cycle

A high-resolution sediment core (MD95-2011) from the Vøring Plateau has been studied to document the variability of the surface water conditions during the Younger Dryas—Preboreal (13—9 kyr BP) in the eastern Norwegian Sea, in order to assess the climate variability in a period considered to be highly dynamic. Quantitative summer sea surface temperatures (SSSTs) with a time resolution of 20—40 years are reconstructed using three different diatom transfer function methods. The Younger Dryas—Preboreal transition at site MD95-2011 is documented with a temperature increase of 3.5°C/3°C within 100 years. Following, the record resolves for the first time PBO as two prominent cooling events centred at 11.2 and 11.3 kyr BP. In addition, a broad cooling event documented from 10.3—9.9 kyr BP coincides with previously documented climatic events as the 10.3 event and Erdalen Event 1 (10.1—9.9 kyr BP) and 2 (~9.7 kyr BP), respectively. Constrained by the observed SSST changes and diatom assemblages, short-term SSST changes with a periodicity of 80—120 years are observed, and the length of this period might indicate a possible connection to the solar Gleissberg cycle. The North Atlantic heat transport is highly sensitive to freshwater inputs, and the interplay of freshwater forcing with insolation forcing is considered as the controlling mechanism of climate changes at site MD05-2011 during the early Holocene. However, the century-scale variability of 80—120 years cannot explain the large-scale variability during the early Holocene, but is of importance for understanding the underlying small-scale oscillations.

Isotope measurements of single ostracod valves and gastropod shells for climate reconstruction: evaluation of within-sample variability and determination of optimum sample size

Sediment cores from Lakes Punta Laguna, Chichancanab, and Pet,n Itza on the Yucatan Peninsula were used to (1) investigate "within-horizon" stable isotope variability (delta O-18 and delta C-13) measured on multiple, single ostracod valves and gastropod shells, (2) determine the optimum number of individuals required to infer low-frequency climate changes, and (3) evaluate the potential for using intra-sample delta O-18 variability in ostracod and gastropod shells as a proxy measure for high-frequency climate variability. Calculated optimum sample numbers ("n") for delta O-18 and delta C-13 in the ostracod Cytheridella ilosvayi and the gastropod Pyrgophorus coronatus vary appreciably throughout the cores in all three lakes. Variability and optimum "n" values were, in most cases, larger for C. ilosvayi than for P. coronatus for delta O-18 measurements, whereas there was no significant difference for delta C-13 measurements. This finding may be explained by differences in the ecology and life history of the two taxa as well as contrasting modes of calcification. Individual delta O-18 measurements on C. ilosvayi in sediments from Lake Punta Laguna show that samples from core depths that have high mean delta O-18 values, indicative of low effective moisture, display lower variability, whereas samples with low mean delta O-18 values, reflecting times of higher effective moisture, display higher variability. Relatively dry periods were thus consistently dry, whereas relatively wet periods had both wet and dry years. This interpretation of data from the cores applies to two important periods of the late Holocene, the Maya Terminal Classic period and the Little Ice Age. delta O-18 variability during the ancient Maya Terminal Classic Period (ca. 910-990 AD) indicates not only the driest mean conditions in the last 3,000 years, but consistently dry climate. Variability of delta C-13 measurements in single stratigraphic layers displayed no relationship with climate conditions inferred from delta O-18 measurements.

Low- and high-frequency climate variability in eastern Beringia during the past 25 000 yearsThis article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.

We present new temperature and precipitation reconstructions for the past 25 000 cal. years BP from across eastern Beringia based on a network of pollen diagrams, an updated modern pollen calibration database, and an improved methodology using the modern analogue technique (MAT). Time series show July temperatures were around 4 °C lower during full glacial and January temperatures were about 2 °C lower than present. Annual temperatures rose beginning around 16 000 cal. years BP, reaching a maximum around 12 000 cal. year BP. The warming was more rapid in southern Beringia. Annual precipitation varied by 250 mm during the past 25 000 cal. years BP. Maps of reconstructed precipitation patterns show increasingly drier conditions since 12 000 cal. years BP. that vary regionally, suggesting Holocene atmospheric circulation changes at multiple time and space scales. Orbitally forced seasonality changes during the late glacial and early Holocene resulted in reversed seasonal temperature reconstructions due to methodological constraints using the MAT and (or) non-analogue conditions. The magnitude of millennial-scale climate variability in this region was greater during the last glacial and late glacial periods than during the past 8000 years.

Late Pliocene climate variability on Milankovitch to millennial time scales: A high-resolution study of MIS100 from the Mediterranean

Astronomically tuned high-resolution climatic proxy records across marine oxygen isotope stage 100 (MIS100) from the Italian Monte San Nicola section and ODP Leg 160 Hole 967A are presented. These records reveal a complex pattern of climate fluctuations on both Milankovitch and sub-Milankovitch timescales that oppose or reinforce one another. Planktonic and benthic foraminiferal δ 18O records of San Nicola depict distinct stadial and interstadial phases superimposed on the saw-tooth pattern of this glacial stage. The duration of the stadial–interstadial alterations closely resembles that of the Late Pleistocene Bond cycles. In addition, both isotopic and foraminiferal records of San Nicola reflect rapid changes on timescales comparable to that of the Dansgard–Oeschger (D–O) cycles of the Late Pleistocene. During stadial intervals winter surface cooling and deep convection in the Mediterranean appeared to be more intense, probably as a consequence of very cold winds entering the Mediterranean from the Atlantic or the European continent. The high-frequency climate variability is less clear at Site 967, indicating that the eastern Mediterranean was probably less sensitive to surface water cooling and the influence of the Atlantic climate system. Concomitant changes in the colour reflectance of ODP Site 967 and the calcium carbonate record of San Nicola probably indicate that part of the high-frequency climate variability (3–5 kyr) in the eastern Mediterranean is related to changes in Saharan dust supply. Evidently, enhanced dust deposition in the Mediterranean correlates with the cold intervals of the millennial-scale D–O oscillations suggesting that the Atlantic pressure system may have played a critical role in varying the wind strength and/or aridification of northern Africa.

Population effects of increased climate variation

Global circulation models predict and numerous observations confirm that anthropogenic climate change has altered high-frequency climate variability. However, it is not yet well understood how changing patterns of environmental variation will affect wildlife population dynamics and other ecological processes. Theory predicts that a population's long-run growth rate is diminished and the chance of population extinction is increased as environmental variation increases. This results from the fact that population growth is a multiplicative process and that long-run population growth rate is the geometric mean of growth rates over time, which is always less than the arithmetic mean. However, when population growth rates for unstructured populations are related nonlinearly to environmental drivers, increasing environmental variation can increase a population's long-run growth rate. This suggests that patterns of environmental variation associated with different aspects of climate change may affect population dynamics in different ways. Specifically, increasing variation in rainfall might result in diminished long-run growth rates for many animal species while increasing variation in temperature might result in increased long-run growth rates. While the effect of rainfall is theoretically well understood and supported by data, the hypothesized effect of temperature is not. Here, I analyse two datasets to study the effect of fluctuating temperatures on growth rates of zooplankton. Results are consistent with the prediction that fluctuating temperatures should increase long-run growth rates and the frequency of extreme demographic events.

From proxy data to paleoclimate interpretation: the mid-Holocene paradox of the Atacama Desert, northern Chile

The question whether the mid-Holocene climate (between ca. 9 and 4 cal kyr B.P.) in the Atacama Desert and the Central Andes in general was humid or dry has wide implications with regard to the understanding of long-term climate variability in South America. Paleosols, regional groundwater tables, abiotic proxy data and pollen of aquatic plants in lake sediments show a marked and rapid shift from very humid late-glacial/early Holocene climatic conditions (between ca. 14 and 9.5 kyr B.P.) to extremely dry mid-Holocene conditions (more arid than today between ca. 9 and 4 kyr B.P.). An exception during this hyperarid period is a century-scale more humid interval around ca. 5.5–6 kyr B.P. that appears systematically in lake sediment archives. In contrast, pollen for most terrestrial plants preserved in lake sediments do not show major changes during the Holocene, whereas more humid mid-Holocene conditions (compared with late Holocene conditions) were inferred from plant macrofossils in rodent middens. Is the reason for this disagreement to be attributed to misinterpretation of the paleoenvironments or of the proxy records themselves, or to incomplete paleoclimatic interpretation of the paleoenvironments? We argue that these different paleoclimate archives record different aspects and facets of ‘climate’. While paleosols and groundwater in the Atacama Desert record low-frequency climate variability at century to millennium scales, lake sediments on the Altiplano record decade- to century-scale variability. Terrestrial vegetation responds to shorter high-frequency climate variability at seasonal to inter-annual scales and preferably to humid years. Vegetation remains in ‘hibernation’ or does not germinate during arid years. Thus information from these three types of archives is not a priori comparable and requires careful site-specific, archive-specific and time-scale-specific evaluation. What is natural in modern climatology is also true for paleoclimatology: a comprehensive assessment must account for the complex daily and seasonal cycles, for the range of climate variability and trends at different scales in space and time, for impacts of short-term extreme events, and for specific, often non-linear responses of individual bio-geo-physical archives to any of the numerous aspects of ‘climate’.

Modeling the effect of freshwater pulses on the early Holocene climate: The influence of high‐frequency climate variability

The effect of freshwater pulses on the early Holocene climate is investigated with a global coupled atmosphere‐sea ice‐ocean model. In the model an early Holocene equilibrium climate state is perturbed by releasing a fixed amount of freshwater (4.67 × 1014 m3) into the Labrador Sea at three different constant rates: 1.5 Sv (1 Sv = 1 × 106 m3 s−1) in 10 years, 0.75 Sv in 20 years, and 0.3 Sv in 50 years. For each rate, five ensemble experiments have been performed, varying in initial conditions. The freshwater pulses produce a weakening of the thermohaline circulation. The perturbed state is in agreement with proxy evidence for the 8.2 ka event. Two types of recovery of the thermohaline circulation occurred, differing in time‐scale: (1) ≤200 years and (2) >200 years. In the experiments with 10 year and 20 year pulses, both types of recovery were observed. This suggests that the model response is unpredictable in the range of parameters studied here. It is hypothesized that the unpredictability is associated with annual‐to‐decadal climate variability. Our results demonstrate that several types of recovery may exist with the same kind of perturbation. The interpretation of events observed in proxy data may be thus more complex than realized until now since the magnitude and duration of climatic events caused by freshwater pulses is likely to depend strongly on nonlinear dynamics inside the coupled atmosphere–sea ice–ocean system.