Current Climate Warming Research Articles

Environmental change and challenge in the Himalaya. A historical perspective

This overview, or retrospective, has two objectives. The first is to demonstrate how the principles of ‘mountain geoecology’ were applied in an attempt to counteract the political and socio-economic impacts of a major and misguided environmental orthodoxy-the Theory of Himalayan Environmental Degradation (henceforth to be referred to as the ‘Theory’). The second is to explore the difficulties of transferring the results of on-going scholarly mountain research into the public and political decision-making process. In this sense the paper should be regarded as a case study of the potentially serious effects of exaggerated and emotionally based responses to orthodoxies founded on assumptions and latter-day myths. A third objective, reserved for the companion paper in this issue, outlines the origins of mountain geoecology and explores how academic research influenced the inclusion of high level concern for mountain problems within AGENDA 21, one of the principal results of the 1992 UN Conference on Environment and Development (popularly known as the Rio Earth Summit) and declaration of 2002 as the International Year of Mountains. The original environmental orthodoxy (the Theory) has been eclipsed since the turn of the Millennium by a new populist alarm proposing that the current climate warming will cause all the Himalayan glaciers to disappear in the near future. From this it would follow that, as the glacier melt progresses, numerous large glacial lakes, forming as a consequence, would burst and the ensuing floods would annihilate many millions of people. Eventually, as the glaciers disappeared vital rivers, such as the Ganges and Brahmaputra, would wither to seasonal streams heralding further massive loss of life due to desertification and starvation. This current environmental alarm could be regarded as a present day parallel to the original Theory and will be examined in the final section of the paper. Between 1970 and about 1985 it was almost universal wisdom amongst scholars and development specialists, as well as conservationists, that the Himalaya were on the brink of environmental, and hence socio-economic and political collapse. This theme of gloom and doom was taken up avidly by journalists, politicians, and diplomats; it influenced the expenditure of large sums of aid and development money, and augmented periodic international confrontations. In concise terms, in the early 1970s an assumed approaching environmental disaster was perceived to be driven by relentless growth in the population of subsistence hill communities and their dependence on mountain forests for fuel, fodder, building materials, and conversion to agricultural land. The assumption of rapid and catastrophic deforestation of steep hillslopes under a monsoon climate (the World Bank predicted that there would be no accessible forest remaining in Nepal by the year 2000) led inexorably to a series of dependent assumptions: increasing soil erosion and worsening landslide incidence; accelerated flooding and siltation on the plains of Gangetic India and Bangladesh; social and political unrest, if not serious armed conflict – the notion of a world super-crisis, considering that the region in question contained about ten percent of the world’s entire human population and about thirty percent of its poorest. As will be emphasized later, none of this all-embracing construct was based on reliable evidence, but it was accepted world-wide as a given. It represents a prime example of the dangers associated with convenient adoption of environmental myths, or environmental orthodoxies, especially where the myth is a Western ‘scientific’ construct. I characterized it as The Theory of Himalayan Environmental Degradation (Ives, 1985).

A comparison of the physics of the northern and southern shelves of the eastern Bering Sea and some implications for the ecosystem

Sufficient oceanographic measurements have been made in recent years to describe the latitudinal variation in the physics of the eastern Bering Sea shelf and the potential impact of climate change on the species assemblages in the two ecosystems (north and south). Many of the predicted ecosystem changes will result from alterations in the timing and extent of sea ice. It is predicted that the sea ice in the northern Bering Sea will be less common in May, but will continue to be extensive through April. In contrast, the southern shelf will have, on average, much less sea ice than currently observed, but with large interannual and multiyear variability until at least 2050. Thus, even under current climate warming scenarios, bottom temperatures on the northern shelf will remain cold. Based on biophysical measurements, the southern and northern ecosystems were divided by a North–South Transition at ∼60°N. The northern middle shelf was characterized by a freshwater lens at the surface, cold bottom temperatures, and a thicker pycnocline than found on the southern shelf. Subsurface phytoplankton blooms were common. In contrast, the southern shelf stratification was largely determined by temperature alone; the pycnocline was thin (often<3m) and subsurface blooms were uncommon. Biological responses to climate warming could include greater north–south differences in zooplankton community structure, the transport of large Outer Shelf Domain crustacean zooplankton to the middle shelf, and the disappearance of two principal prey taxa (Calanus spp. and Thysanoessa spp.) of planktivorous fish, seabirds and whales. The response of commercially and ecologically important fish species is predicted to vary. Some species of fish (e.g., juvenile sockeye salmon, Oncorhynchus nerka) may expand their summer range into the northern Bering Sea; some (e.g., pink salmon, O. gorbuscha) may increase in abundance while still other species (e.g., walleye pollock and arrowtooth flounder; Theragra chalcogramma and Atheresthes stomias, respectively) are unlikely to become common in the north. The projected warming of the southern shelf will limit the distribution of arctic species (e.g., snow crab, Chionoecetes opilio) to the northern shelf and will likely permit expansion of subarctic species into the southern Bering Sea. The distribution and abundance of baleen whales will respond to shifts in prey availability; for instance, if prey are advected northward from the southeastern Bering Sea, an extension of range and an increase in seasonally migratory baleen whale numbers is anticipated. Thus, alteration of this ecosystem in response to climate change is expected to result in something other than a simple northward shift in the distribution of all species.

Extreme summer temperatures in Western Europe

Abstract. We discuss the evolution of summer temperature extremes over Western Europe during 1961–2004 in the context of current climate warming. Using a parametric approach, we investigate the role of properties and changes in probability density functions of daily temperatures in modifying the frequency of severe, isolated events. In this perspective, the recent intensification of extremely warm events over Europe turns out to be well consistent with a pure, nonuniform shift of mean values, with no room for conjectures about increasing temperature variability.

Annual plants colonizing the Arctic? Phylogeography and genetic variation in theEuphrasia minimacomplex (Orobanchaceae)

AbstractThe high‐arctic environment supports very few annual plants, but more annuals can be expected to move northwards following climate warming. Here we address the history of the amphi‐Atlantic, hemiparasiticEuphrasia minimacomplex, which is taxonomically intricate and consists of annuals typically growing in alpine, north boreal, and low‐arctic habitats. Recently it has also been discovered at three distantly separated sites in the high‐arctic archipelago of Svalbard. We analyzed Amplified Fragment Length Polymorphism (AFLP) and chloroplast DNA (cpDNA) variation in populations from most of the range of the complex. The main split observed in both datasets, dated to nearly 6 Ma ago, was between one central/southern European alpine lineage (E. minimas.str.) and one northern amphi‐Atlantic lineage (E. wettsteinii). TheE. wettsteiniilineage consisted of two cpDNA sublineages, both amphi‐Atlantic and partly sympatric, estimated to have diverged 1.7 Ma ago. Two main northern subgroups were also revealed in the AFLP dataset, partly consistent with the cpDNA sublineages: one E Atlantic subgroup (N Russia, extending into Scotland and Colesdalen in Svalbard), and one mainly W Atlantic subgroup (NE Canada, Greenland, and Iceland, extending into N Norway). A third subgroup was restricted to two of the three Svalbard populations, Bockfjorden and Ossian Sarsfjellet. Assignment tests of the Svalbard populations suggest that Colesdalen was derived from N Russian source populations and Bockfjorden from N Norwegian ones; the assignment of the Ossian Sarsfjellet population was uncertain. However, as all Svalbard populations were distinctly divergent from each other as well as from all other populations in AFLP marker frequencies, it is unlikely that their establishment in Svalbard is caused by the current climate warming or recent human activities. Their divergence is probably due to founder effects and genetic drift following independent earlier immigrations, possibly during the postglacial warm period 9500 to 4000 years ago.

Photoperiodic response in the subtropical and warm‐temperate zone populations of the southern green stink bug Nezara viridula: why does it not fit the common latitudinal trend?

The photoperiodic response of diapause induction is studied in females of five subtropical and warm‐temperate zone populations of Nezara viridula (L.) (Heteroptera: Pentatomidae) in Japan (26.4–34.7°N; 127.4–135.7°E). Laboratory tests at 25 °C demonstrate that both warm‐temperate and subtropical populations have pronounced photoperiodic responses of adult diapause induction. Under short‐day conditions (LD 10 : 14 h), 73–100% of females enter diapause, whereas, under long‐day conditions (LD 15 : 9 h), 87–100% of females are nondiapause and reproduce. When the critical photoperiod for diapause induction is plotted against the latitude of origin of each population, the data points do not show the expected tendency of increasing critical photoperiod northwards but, instead, vary between 12 h 15 min and 13 h 30 min. It is suggested that adults from different populations of such a highly migratory species move often among subtropical islands (by themselves or being assisted by typhoons), thus constantly destabilizing the photoperiodic responses. Although important for general synchronization of seasonal development in the tropics and subtropics, winter diapause might not be so critical for survival in mild winters in these zones. Such circumstances might make the selective pressure over the photoperiodic response less severe in the tropics and subtropics than in the temperate zone. It is suggested that the current climate warming might contribute to this situation as well by promoting polewards migrations and lessening the selective pressure of overwintering conditions.

Spatiotemporal scaling of surface air temperature fields in China

The characteristics of spatiotemporal variability of surface air temperature anomaly fields in China are computed from the data of instrumental observations carried out from 1907 to 2000. The fields are represented by their empirical orthogonal function expansion. The temporal variations of the coefficients of this expansion are considered at the scale range from one month to five years. It turned out that both the orthogonal function structure and the type of the temporal variability of expansion coefficients in China differ from those in Northern Eurasia. Investigating the current climate warming in China, it is enough to consider the first two empirical orthogonal functions of temperature fields only. The characteristic time of the existence of temperature anomalies described by these functions is almost the same as that of the temperature anomalies in Northern Eurasia described by the first empirical orthogonal function of the respective fields.

Distribution of total mercury and methylmercury in lake sediments in Arctic Ny-Ålesund

The toxicities and bioavailabilities of total mercury (THg) and methylmercury (MeHg) in aquatic systems have made them the subjects of recent research. In this study, we collected a lake sediment core from Ny-Ålesund in Svalbard and analyzed the distributions of THg and MeHg in the sediments. The increased trend of THg was caused by anthropogenic contamination since the 14th century through long-range transportation, especially after the industrial era. However, the peak values of Hg in surface sediment samples could be explained by the increased algal scavenging process in recent decades. All the biogeochemical proxies (e.g., pigments and diatom biomass) revealed recent sharp increases in aquatic primary production due to the current climate warming. Rock–Eval analyses indicated that algal-derived organic matter took up a large portion, and quantitative calculation showed that 89.6–95.8% of the Hg in post-1950 could be explained by scavenging. The distribution of MeHg has a close relationship with total Hg and organic matter. The oxidation–reduction condition is one of the possible factors affecting the methylation rates in H2 lake sediments. Higher algal productivity and organic matter actually led to the increased trend of methylation in the uppermost sediment. This study supports some new key hypotheses on climate-driven factors affecting Hg and MeHg cycling in High Arctic lake sediments.

Properties and changes in empirical orthogonal components of temperature fields in Northern Eurasia in the 20th century

Using data from 55 meteorological stations in the Northern Hemisphere for the 20th century as a whole and separately for 1901–1950 and 1951–2000, the norms and empirical orthogonal components (EOCs) of the fields of the annual mean surface air temperature were calculated. These fields were found to have two components: smooth and nonsmooth. The field of differences between the temperature norms of the first and second halves of the 20th century was found to be projected almost completely on the very first component of fixed sign; i.e., the current climate trend of the Northern Hemisphere is spatially homogeneous. No substantial changes were found in the transition from the first half to the second half of the 20th century, either in the form of components or in the spectrum of eigenvalues, with the only exception being a small growth (in magnitude) in part of the eigenvalues corresponding to the nonsmooth component. This can be explained by the progressing urbanization of northern Eurasia. All coefficients of the temperature-field expansion by natural components are distributed normally, and their temporal correlation functions for smooth components of fields have a form characteristic of processes with long-term memory. The latter manifests itself particularly in the motions of the two main waves of the smooth component and reflects two stages of the current climate warming.

10.1007/s11983-008-1003-4

A teleconnection is analyzed which exists between long-term variations of temperature and precipitation in St. Petersburg in March and April and North Atlantic and European atmospheric zonal circulation during the 20th century. It is shown that current climate warming in the region is almost entirely explained by a significant enhancement of warm advection of Atlantic air onto the continent.

Which are the phenologically flexible species? A case study with common passerine birds

One of the most consensual ecological effects of the current climate warming is the alteration of the environmental timing of ecosystems. Phenological shifts, at different levels of food webs, are predicted to have major effects on species assemblages. Indeed it is unlikely that all species should be able to respond to the phenological shifts of their environment evenly. Yet questions remain about the specific traits that predict the ability of a species to track the temporal fluctuations of its environment. In this study, we use data from the French Constant Effort Site ringing program over a 20 years period (1989–2008) to estimate the ability of 20 common passerine species to adjust their breeding phenology to spring temperature variations. We show that the sensitivity of species breeding phenology to climate relates to species mean migration distance, species’ thermal and habitat niche breadth and brain mass. Species with the broadest ecological and thermal niches, the shortest mean migration distances and the largest brains were most able to adjust their breeding phenology to temperature variations. Our results thus identify long distance migrants and ecological specialists as species that could most suffer from the future expected climate change and suggests phenological adjustment as one possible mechanism underlying the replacement of specialist species by more generalist ones, the so called functional biotic homogenization.

Influence of the Atlantic Ocean on the magnitude of air temperature trends under current warming

An analysis is made of the magnitudes of the linear trends of daily mean temperature in the city of Minsk for the period from 1966 to 2008. It is found that the largest magnitudes of trends in the annual variation correspond to mid-January as well as to the end of July–August, whereas in May and in November they are poorly pronounced (positive or even negative). The maximum magnitudes of the air temperature trends are in good agreement with maximum warm advection by currents of the Gulf Stream system, while modest warm advection by currents corresponds to weak positive and even negative magnitudes of the temperature trends. Consequently, the magnitude of the temperature trends in the annual variation is modulated by warm advection by currents of the Gulf Stream system which contains a half-yearly component and is determined by natural factors. Therefore, the driving forces of current climate warming cannot be attributed solely to the increase of greenhouse gases in the atmosphere.

Effect of climate change on human health and some adaptive strategies – a review

The impact of human-induced climate change and ozone depletion are now observed to compromise the sustainability of human development as it threatens the ecological support system on which life depends. Evidences abound to show that there is climate change and ozone layer depletion in the last 2-4 decades. The Earth is warming, the warming is accelerating, and human actions are responsible. If the current climate change and warming trends remain uncontrolled, humanity will face more injury, disease and death related to natural disasters and heat waves; higher rates of food-borne, water-borne and vector-borne illnesses; and death that is more premature and disease related to air pollution. Moreover, in many parts of the world, large populations will be displaced by rising sea level, and many others will be affected by drought and famine. As glaciers melt, the hydrological cycle shifts and the productivity of arable land will change. That is the bad news. Nevertheless, the good news is that through increased interdisciplinary and intersectoral collaboration, and rapid deployment of mitigation strategies to stabilize climate change as well as development of proactive adaptation programmes to minimize health impact of climate change, the global community will be better prepared to cope with the challenges of protecting health from the impact of climate change. Key Words: Climate Change, Human Health, Adaptive Strategies

Summer freezing resistance decreased in high-elevation plants exposed to experimental warming in the central Chilean Andes

Alpine habitats have been proposed as particularly sensitive to climate change. Shorter snow cover could expose high-elevation plants to very low temperatures, increasing their risk of suffering damage by freezing, hence decreasing their population viability. In addition, a longer and warmer growing season could affect the hardening process on these species. Thus, understanding the ability of these species to withstand freezing events under warmer conditions is essential for predicting how alpine species may respond to future climate changes. Here we assessed the freezing resistance of 11 species from the central Chilean Andes by determining their low temperature damage (LT(50)) and freezing point (FP) after experimental warming in the field. Plants were exposed during two growing seasons to a passive increase in the air temperature using open top chambers (OTCs). OTCs increased by ca. 3 K the mean air and soil daytime temperatures, but had smaller effects on freezing temperatures. Leaf temperature of the different species was on average 5.5 K warmer inside OTCs at midday. While LT(50) of control plants ranged from -9.9 to -22.4, that of warmed plants ranged from -7.4 to -17.3 degrees C. Overall, high-Andean species growing inside OTCs increased their LT(50) ca. 4 K, indicating that warming decreased their ability to survive severe freezing events. Moreover, plants inside OTCs increased the FP ca. 2 K in some studied species, indicating that warming altered processes of ice crystal formation. Resistance of very low temperatures is a key feature of high-elevation species; our results suggest that current climate warming trends will seriously threaten the survival of high-elevation plants by decreasing their ability to withstand severe freezing events.

An increase in the upper tree-limit of silver fir ( Abiesalba Mill.) in the Alps since the mid-20th century: A land-use change phenomenon

Global environmental changes observed during recent decades are likely to have had an impact on the distribution of species. Currently, silver fir ( Abies alba) is becoming established in the subalpine forests of the west central Alps at elevations higher than 2000 m a.s.l.; prior to the 1970s its upper altitudinal limit was 2000 m. Several hypotheses could explain this recent expansion of the upper tree-limit. Silver fir regeneration could be linked (1) to land-use changes or (2) to current climatic warming. Using dendrochronology, the age structure of 31 forest plots containing at least one silver fir was examined in order to elucidate the population dynamics of subalpine communities. This allowed us to decipher the timing of fir regeneration in relation to the mean age of the stands examined and of the other tree-canopy species present. The majority of the firs germinated sporadically since 1950, before the regional temperature increase. The pattern of fir recruitment did not appear to relate to altitude, but followed a pattern characteristic of secondary succession. The age structures identified showed an exponential increase in tree-density during the 20th century; the stands were first dominated by Larix decidua during the 18th and 19th centuries, and then by Pinus cembra during the 20th century. In most stands, fir regeneration occurred after Larix and before P. cembra dominated, following a similar pattern to Picea abies regeneration. The number of local inhabitants and temperature both exhibited a negative relationship with fir tree recruitment, thus supporting the land-use change hypothesis. There has been a significant upward shift of the altitudinal range of fir, amounting to an increase of about 300 m since 1950. This followed the abandonment of low-productivity land. This trend is likely to continue during the 21st century, because of new agricultural and forestry practices which involve limited intervention in low-productivity areas and may be because of the effects of global warming.

Forecasting temperate alpine glacier survival from accumulation zone observations

Abstract. Temperate alpine glacier survival is dependent on the consistent presence of an accumulation zone. Frequent low accumulation area ratio values, below 30%, indicate the lack of a consistent accumulation zone, which leads to substantial thinning of the glacier in the accumulation zone. This thinning is often evident from substantial marginal recession, emergence of new rock outcrops and surface elevation decline in the accumulation zone. In the North Cascades 9 of the 12 examined glaciers exhibit characteristics of substantial accumulation zone thinning; marginal recession or emergent bedrock areas in the accumulation zone. The longitudinal profile thinning factor, f, which is a measure of the ratio of thinning in the accumulation zone to that at the terminus, is above 0.6 for all glaciers exhibiting accumulation zone thinning characteristics. The ratio of accumulation zone thinning to cumulative mass balance is above 0.5 for glacier experiencing substantial accumulation zone thinning. Without a consistent accumulation zone these glaciers are forecast not to survive the current climate or future additional warming. The results vary considerably with adjacent glaciers having a different survival forecast. This emphasizes the danger of extrapolating survival from one glacier to the next.

A weak upward elevational shift in the distributions of breeding birds in the Italian Alps

AbstractAim To test whether bird assemblages are shifting upwards in their elevational distribution in synchrony with current climate warming and/or habitat changes.Location A gradient of elevation in the Italian Alps (Alta Valsessera, Piedmont).Methods We used data from two recent atlas surveys performed on a 1 × 1 km grid at an 11‐year interval (1992–94 and 2003–05). We modelled the elevational gradient of avifaunal composition, using a sample‐based approach, in an effort to detect evidence for an upward elevational shift of bird zonation. Changes in species richness were controlled for. The results from this analysis were compared with those obtained using a species‐based approach. Changes in climate and landscape between the two surveys were assessed using local meteorological data and Corine Land Cover maps, respectively.Results We detected small avifaunal changes between the two surveys: (1) mean elevations increased for the majority of species, but the average change was not significantly different from zero; (2) the species richness increased, but this was mainly due to an increase in sampling effort; and (3) a change in species composition was detected, which was at the limit of significance and corresponded on average to a 29‐m upward elevational shift in the distribution of the avifauna. The shift was the same for open land and forest bird communities. During the same period, the mean temperature increased by c. 1 °C in the area, and a slight trend towards vegetation closure by woody plants was detected.Main conclusions The use of fine‐scale breeding bird atlases in mountainous regions, together with ordination methods, provides a sensitive tool to test and measure elevational shifts in species ranges, but the results have to be interpreted carefully. In our case, the observed elevational shift in the distributions of the avifauna cannot unambiguously be attributed to climate warming. This shift is smaller than expected from the regional increase in temperature, which raises the question of how closely bird distributions match climate change.

Changes in the cenoflora of dark-coniferous forests of the Northern Baikal region under current climate warming

Calculations of the activity indices and species richness of belt-zonal elements of the flora of dark-coniferous forests have been done in order to reveal the presence of climatogenic changes. It was found that there is an increase in activity of the species of the dark-coniferous belt-zonal group peculiar to these forests. It is suggested that this phenomenon is associated with current climate warming. It is concluded that with increasing climate warming and with the persisting tendency for its humidification, in the lower belt of the mountains there will be taking place an enhancement in the landscape role of dark-coniferous forests through succession from light-coniferous forests to them.

Growth–climate responses of high-elevation conifers in the central Hengduan Mountains, southwestern China

Improved understanding of tree-growth responses to climate is needed to model and predict forest ecosystem responses to current and future climatic variability. We applied dendroclimatological techniques to assess the effects of inter-annual climate variations on radial growth of high-elevation conifers in the central Hengduan Mountains, southwestern China. Eight tree-ring width chronologies of the major tree genera Abies and Picea that are aligned along an elevation gradient from 3200 to 4200 m a.s.l. were developed. Correlation and principal component analyses for the eight chronologies identified three groups of sites, representing different patterns of growth–climate relationships. Correlation and redundancy analyses with regional climate data revealed that radial growth of fir growing at high-elevation sites is enhanced by normal or warm summer temperatures (June and July) during the current growing season. In addition, radial growth of trees growing from high to middle elevations is sensitive to low temperatures during winter season. At low-elevation sites, trees display low sensitivity to temperature variation. However spring moisture availability becomes crucial for radial growth regardless of tree species. High- to middle-elevation conifers in the central Hengduan Mountains may benefit from the current climate warming, especially from rising winter temperatures.

Winter climate change in alpine tundra: plant responses to changes in snow depth and snowmelt timing

Snow is an important environmental factor in alpine ecosystems, which influences plant phenology, growth and species composition in various ways. With current climate warming, the snow-to-rain ratio is decreasing, and the timing of snowmelt advancing. In a 2-year field experiment above treeline in the Swiss Alps, we investigated how a substantial decrease in snow depth and an earlier snowmelt affect plant phenology, growth, and reproduction of the four most abundant dwarf-shrub species in an alpine tundra community. By advancing the timing when plants started their growing season and thus lost their winter frost hardiness, earlier snowmelt also changed the number of low-temperature events they experienced while frost sensitive. This seemed to outweigh the positive effects of a longer growing season and hence, aboveground growth was reduced after advanced snowmelt in three of the four species studied. Only Loiseleuria procumbens, a specialist of wind exposed sites with little snow, benefited from an advanced snowmelt. We conclude that changes in the snow cover can have a wide range of species-specific effects on alpine tundra plants. Thus, changes in winter climate and snow cover characteristics should be taken into account when predicting climate change effects on alpine ecosystems.

Composition and carbon dynamics of forests in northeastern North America in a future, warmer worldThis article is one of a selection of papers from NE Forests 2100: A Synthesis of Climate Change Impacts on Forests of the Northeastern US and Eastern Canada.

Increasing temperatures, precipitation extremes, and other anthropogenic influences (pollutant deposition, increasing carbon dioxide) will influence future forest composition and productivity in the northeastern United States and eastern Canada. This synthesis of empirical and modeling studies includes tree DNA evidence suggesting tree migrations since the last glaciation were much slower, at least under postglacial conditions, than is needed to keep up with current and future climate warming. Exceedances of US and Canadian ozone air quality standards are apparent and offset CO2-induced gains in biomass and predispose trees to other stresses. The deposition of nitrogen and sulfate in the northeastern United States changes forest nutrient availability and retention, reduces reproductive success and frost hardiness, causes physical damage to leaf surfaces, and alters performance of forest pests and diseases. These interacting stresses may increase future tree declines and ecosystem disturbances during transition to a warmer climate. Recent modeling work predicts warmer climates will increase suitable habitat (not necessarily actual distribution) for most tree species in the northeastern United States. Species whose habitat is declining in the northeastern United States currently occur in Canadian forests and may expand northward with warming. Paleoecological studies suggest local factors may interact with, even overwhelm, climatic effects, causing lags and thresholds leading to sudden large shifts in vegetation.