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1.
The effects of terrestrial ecosystems on the climate system have received most attention in the tropics, where extensive deforestation and burning has altered atmospheric chemistry and land surface climatology. In this paper we examine the biophysical and biogeochemical effects of boreal forest and tundra ecosystems on atmospheric processes. Boreal forests and tundra have an important role in the global budgets of atmospheric CO2 and CH4. However, these biogeochemical interactions are climatically important only at long temporal scales, when terrestrial vegetation undergoes large geographic redistribution in response to climate change. In contrast, by masking the high albedo of snow and through the partitioning of net radiation into sensible and latent heat, boreal forests have a significant impact on the seasonal and annual climatology of much of the Northern Hemisphere. Experiments with the LSX land surface model and the GENESIS climate model show that the boreal forest decreases land surface albedo in the winter, warms surface air temperatures at all times of the year, and increases latent heat flux and atmospheric moisture at all times of the year compared to simulations in which the boreal forest is replaced with bare ground or tundra. These effects are greatest in arctic and sub-arctic regions, but extend to the tropics. This paper shows that land-atmosphere interactions are especially important in arctic and sub-arctic regions, resulting in a coupled system in which the geographic distribution of vegetation affects climate and vice versa. This coupling is most important over long time periods, when changes in the abundance and distribution of boreal forest and tundra ecosystems in response to climatic change influence climate through their carbon storage, albedo, and hydrologic feedbacks.  相似文献   

2.
In the boreal biome, fire is the major disturbance agent affecting ecosystem change, and fire dynamics will likely change in response to climatic warming. We modified a spatially explicit model of Alaskan subarctic treeline dynamics (ALFRESCO) to simulate boreal vegetation dynamics in interior Alaska. The model is used to investigate the role of black spruce ecosystems in the fire regime of interior Alaska boreal forest. Model simulations revealed that vegetation shifts caused substantial changes to the fire regime. The number of fires and the total area burned increased as black spruce forest became an increasingly dominant component of the landscape. The most significant impact of adding black spruce to the model was an increase in the frequency and magnitude of large-scale burning events (i.e., time steps in which total area burned far exceeded the normal distribution of area burned). Early successional deciduous forest vegetation burned more frequently when black spruce was added to the model, considerably decreasing the fire return interval of deciduous vegetation. Ecosystem flammability accounted for the majority of the differences in the distribution of the average area burned. These simulated vegetation effects and fire regime dynamics have important implications for global models of vegetation dynamics and potential biotic feedbacks to regional climate.  相似文献   

3.
《大气与海洋》2013,51(3):305-320
Abstract

Satellite and conventional snow water equivalent (SWE) dataseis reveal a well‐defined zone of high winter season SWE (>100 mm) that extends across the northern boreal forest of Canada. SWE coefficient of variation (CV) patterns derived from a monthly averaged (1978–2002) passive microwave derived time series show a high degree of interannual variability across open prairie, southern boreal, and open tundra regions of North America while SWE across the northern boreal forest was highly invariant. The potential existence of a consistent SWE zone resistant to interannual climatic variability over the past 25 years is intriguing in the context of the sensitivity of snow cover to climate variability and change. A ground sampling campaign conceived specifically to evaluate SWE distribution across the northern boreal forest was conducted in northern Manitoba during the 2003–04 winter season. Data from this survey confirmed the SWE gradient across the boreal forest, although satellite‐derived retrievals for the tundra were consistently low.

A series of Canadian Regional Climate Model (CRCM) simulations were conducted to identify feedbacks between the atmosphere and land surface for a domain focused on the northern boreal forest. A control simulation produced monthly patterns of SWE distribution that closely matched the passive microwave retrievals. Water budget computations showed the SWE accumulation pattern to be a function of the modelled regional precipitation pattern, and not the result of surface processes such as melt or evaporation/sublimation. Mean monthly patterns of 850‐hPa fronto genesis forcing corresponded closely to the patterns of accumulated SWE suggesting that lower tropospheric frontal activity was responsible for the snowfall events that led directly to the deposition of the northern boreal SWE band. CRCM sensitivity experiments were conducted with perturbed land cover and terrain. Only subtle differences in SWE accumulation and frontogenesis patterns relative to the control run were found when complete grassland cover was prescribed, though removing orography greatly enhanced the magnitude and zonal extent of the SWE band.  相似文献   

4.
A deforestation experiment is performed using the Laboratoire de Meteorologie Dynamique Atmospheric General Circulation Model (LMD GCM) to determine the climatic role of the largest vegetation formation in the Northern Hemisphere, localized mostly north of latitude 45°N, which is called the temperate and boreal forest. For this purpose, an iterative albedo scheme based on vegetation type, snow age, snowfall rate and area of snow cover, is developed for snow-covered surfaces. The results show a cooling of Northern Hemisphere soil and an increase in the snow cover when the forest is removed, as found by previous similar experiments.In our study this cooling is related to different causes, depending on the season. It is linked to modifications in the soil radiative properties, like surface albedo, due to the disappearance of forest, and consequently, to a greater exposure of the snow-covered soil underneath. It is also related to alterations in the hydrological cycle, observed mainly in summer and autumn at middle latitudes. The model shows a strong sensitivity to the coupled surface albedo — soil temperature — fractional snow cover response in the spring. A later and longer snowmelt season is also detected.This study adds to our understanding of climatic variation on longer time scales, since it is widely accepted that the formation and disappearance of different vegetation formations is closely related to climatic evolution patterns, in particular on the time scale of the glacial oscillations.  相似文献   

5.
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.  相似文献   

6.
Over the last 100?years, Arctic warming has resulted in a longer growing season in boreal and tundra ecosystems. This has contributed to a slow northward expansion of the boreal forest and a decrease in the surface albedo. Corresponding changes to the surface and atmospheric energy budgets have contributed to a broad region of warming over areas of boreal forest expansion. In addition, mesoscale and synoptic scale patterns have changed as a result of the excess energy at and near the surface. Previous studies have identified a relationship between the positioning of the boreal forest-tundra ecotone and the Arctic frontal zone in summer. This study examines the climate response to hypothetical boreal forest expansion and its influence on the summer Arctic frontal zone. Using the Weather Research and Forecasting model over the Northern Hemisphere, an experiment was performed to evaluate the atmospheric response to expansion of evergreen and deciduous boreal needleleaf forests into open shrubland along the northern boundary of the existing forest. Results show that the lower surface albedo with forest expansion leads to a local increase in net radiation and an average hemispheric warming of 0.6°C at and near the surface during June with some locations warming by 1–2°C. This warming contributes to changes in the meridional temperature gradient that enhances the Arctic frontal zone and strengthens the summertime jet. This experiment suggests that continued Northern Hemisphere high-latitude warming and boreal forest expansion might contribute to additional climate changes during the summer.  相似文献   

7.
We present a study that suggests greening in the circumpolar high-latitude regions amplifies surface warming in the growing season (May–September) under enhanced greenhouse conditions. The investigation used a series of climate simulations with the Community Atmospheric Model version 3—which incorporates a coupled, dynamic global vegetation model—with and without vegetation feedback, under both present and doubled CO2 concentrations. Results indicate that climate warming and associated changes promote circumpolar greening with northward expansion and enhanced greenness of both the Arctic tundra and boreal forest regions. This leads to additional surface warming in the high-latitudes in the growing season, primarily through more absorption of incoming solar radiation. The resulting surface and tropospheric warming in the high-latitude and Arctic regions weakens prevailing tropospheric westerlies over 45–70N, leading to the formation of anticyclonic pressure anomalies in the Arctic regions. These pressure anomalies resemble the anomalous circulation pattern during the negative phase of winter Arctic Oscillation. It is suggested that these circulation anomalies reinforce the high-latitude and Arctic warming in the growing season.  相似文献   

8.
Measurements, made at a high subarctic, maritime, wetland tundra site, are presented for three different growing seasons. These are divided into hot-dry, normal-dry and normal-wet years and the behaviour of their surface energy and water balances is examined within the framework of a combination model. For periods of comparable energy availability, evapotranspiration during hot-dry conditions can be larger than during cooler and wetter periods. This results from small stomatal resistance in the sparse canopy of well-rooted sedges, and from the ability of peat soils to supply water under conditions of large atmospheric demand. This demand is expressed in terms of the vapour pressure deficit and it counteracts the large surface resistances which develop during dry periods. In many respects, the energy balance of a subarctic wetland tundra is comparable to observations and models for temperate agricultural and forest lands, in spite of the fact that the soils are organic, the vegetation canopy is sparse and there is continuous permafrost. A dry year promotes deeper thaw depths in the permafrost soils, during the growing season, than does a wet one. This is due to larger ground heat fluxes and larger soil thermal diffusivities. We concluded that maritime, wetland tundra, growing on peat soils, displays feedback mechanisms, which can offset the effects of moisture stress, caused by summer climate warming of a similar magnitude to that simulated by General Circulation Models for a 2 × CO2 scenario.  相似文献   

9.
A full global atmosphere-ocean-land vegetation model is used to examine the coupled climate/vegetation changes in the extratropics between modern and mid-Holocene (6,000 year BP) times and to assess the feedback of vegetation cover changes on the climate response. The model produces a relatively realistic natural vegetation cover and a climate sensitivity comparable to that realized in previous studies. The simulated mid-Holocene climate led to an expansion of boreal forest cover into polar tundra areas (mainly due to increased summer/fall warmth) and an expansion of middle latitude grass cover (due to a combination of enhanced temperature seasonality with cold winters and interior drying of the continents). The simulated poleward expansion of boreal forest and middle latitude expansion of grass cover are consistent with previous modeling studies. The feedback effect of expanding boreal forest in polar latitudes induced a significant spring warming and reduced snow cover that partially countered the response produced by the orbitally induced changes in radiative forcing. The expansion of grass cover in middle latitudes worked to reinforce the orbital forcing by contributing a spring cooling, enhanced snow cover, and a delayed soil water input by snow melt. Locally, summer rains tended to increase (decrease) in areas with greatest tree cover increases (decreases); however, for the broad-scale polar and middle latitude domains the climate responses produced by the changes in vegetation are relatively much smaller in summer/fall than found in previous studies. This study highlights the need to develop a more comprehensive strategy for investigating vegetation feedbacks.  相似文献   

10.
To assess the influence of global climate change at the regional scale, we examine past and future changes in key climate, hydrological, and biophysical indicators across the US Northeast (NE). We first consider the extent to which simulations of twentieth century climate from nine atmosphere-ocean general circulation models (AOGCMs) are able to reproduce observed changes in these indicators. We then evaluate projected future trends in primary climate characteristics and indicators of change, including seasonal temperatures, rainfall and drought, snow cover, soil moisture, streamflow, and changes in biometeorological indicators that depend on threshold or accumulated temperatures such as growing season, frost days, and Spring Indices (SI). Changes in indicators for which temperature-related signals have already been observed (seasonal warming patterns, advances in high-spring streamflow, decreases in snow depth, extended growing seasons, earlier bloom dates) are generally reproduced by past model simulations and are projected to continue in the future. Other indicators for which trends have not yet been observed also show projected future changes consistent with a warmer climate (shrinking snow cover, more frequent droughts, and extended low-flow periods in summer). The magnitude of temperature-driven trends in the future are generally projected to be higher under the Special Report on Emission Scenarios (SRES) mid-high (A2) and higher (A1FI) emissions scenarios than under the lower (B1) scenario. These results provide confidence regarding the direction of many regional climate trends, and highlight the fundamental role of future emissions in determining the potential magnitude of changes we can expect over the coming century.
Katharine HayhoeEmail:
  相似文献   

11.
We examined the annual exchange of CO2 between the atmosphere and moist tussock and dry heath tundra ecosystems (which together account for over one-third of the low arctic land area) under ambient field conditions and under increased winter snow deposition, increased summer temperatures, or both. Our results indicate that these two arctic tundra ecosystems were net annual sources of CO2 to the atmosphere from September 1994 to September 1996 under ambient weather conditions and under our three climate change scenarios. Carbon was lost from these ecosystems in both winter and summer, although the majority of CO2 evolution took place during the short summer. Our results indicate that (1) warmer summer temperatures will increase annual CO2 efflux from both moist and dry tundra ecosystems by 45–55% compared to current ambient temperatures; (2) deeper winter snow cover will increase winter CO2 efflux in both moist and dry tundra ecosystems, but will decrease net summer CO2 efflux; and (3) deeper winter snow cover coupled with warmer summer temperatures will nearly double the annual amount of CO2 emitted from moist tundra and will result in a 24% increase in the annual CO2 efflux of dry tundra. If, as predicted, climate change alters both winter snow deposition and summer temperatures, then shifts in CO2 exchange between the biosphere and atmosphere will likely not be uniform across the Arctic tundra landscape. Increased snow deposition in dry tundra is likely to have a larger effect on annual CO2 flux than warmer summer temperatures alone or warmer temperatures coupled with increased winter snow depth. The combined effects of increased summer temperatures and winter snow deposition on annual CO2 flux in moist tundra will be much larger than the effects of either climate change scenario alone.  相似文献   

12.
Projections of vegetation distribution that incorporate the transient responses of vegetation to climate change are likely to be more efficacious than those that assume an equilibrium between climate and vegetation. We examine the non-equilibrium dynamics of a temperate forest region under historic and projected future climate change using the dynamic ecosystem model LPJ-GUESS. We parameterized LPJ-GUESS for the New England region of the United Sates utilizing eight forest cover types that comprise the regionally dominant species. We developed a set of climate data at a monthly-step and a 30-arc second spatial resolution to run the model. These datasets consist of past climate observations for the period 1901?C2006 and three general circulation model projections for the period 2007?C2099. Our baseline (1971?C2000) simulation reproduces the distribution of forest types in our study region as compared to the National Land Cover Data 2001 (Kappa statistic?=?0.54). Under historic and nine future climate change scenarios, maple-beech-basswood, oaks and aspen-birch were modeled to move upslope at an estimated rate of 0.2, 0.3 and 0.5?m?yr?1 from 1901 to 2006, and continued this trend at an accelerated rate of around 0.5, 0.9 and 1.7?m?yr?1 from 2007 to 2099. Spruce-fir and white pine-cedar were modeled to contract to mountain ranges and cooler regions of our study region under projected future climate change scenarios. By the end of the 21st century, 60% of New England is projected to be dominated by oaks relative to 21% at the beginning of the 21st century, while northern New England is modeled to be dominated by aspen-birch. In mid and central New England, maple-beech-basswood, yellow birch-elm and hickories co-occur and form novel species associations. In addition to warming-induced northward and upslope shifts, climate change causes more complex changes in our simulations, such as reversed conversions between forest types that currently share similar bioclimatic ranges. These results underline the importance of considering community interactions and transient dynamics in modeling studies of climate change impacts on forest ecosystems.  相似文献   

13.
Changing climate could affect the functioning of grassland ecosystems through variation in climate forcings and by altering the interactions of forcings with ecological processes. Both the short and long-term effects of changing forcings and ecosystem interactions are a critical part of future impacts to ecosystem ecology and hydrology. To explore these interactions and identify possible characteristics of climate change impacts to mesic grasslands, we employ a low-dimensional modeling framework to assess the IPCC A1B scenario projections for the Central Plains of the United States; forcings include increased precipitation variability, increased potential evaporation, and earlier growing season onset. These forcings are also evaluated by simulations of vegetation photosynthetic capacity to explore the seasonal characteristics of the vegetation carbon assimilation response for species at the Konza Prairie in North Central Kansas, USA. The climate change simulations show decreases in mean annual soil moisture and and carbon assimilation and increased variation in water and carbon fluxes during the growing season. Simulations of the vegetation response show increased variation at the species-level instead of at a larger class scale, with important heterogeneity in how individual species respond to climate forcings. Understanding the drivers and relationships behind these ecosystem responses is important for understanding the likely scale of climate change impacts and for exploring the mechanisms shaping growing season dynamics in grassland ecosystems.  相似文献   

14.
Over the last century, the Arctic has warmed at twice the rate of the planet as a whole. Observational evidence indicates that this rapid warming is affecting the tundra and boreal forest biomes by changing their structure and geographic distribution. A global climate model (GCM) was used to explore the atmospheric response to boreal forest expansion by applying a one-grid cell shift of the forest into tundra. This subtle shift is meant to represent the expansion that would occur this century rather than more extreme scenarios predicted by dynamic vegetation models. Results show that this shift causes an average annual warming of 0.3 °C over the region because of a reduction in the surface albedo and an increase in net radiation. A warming of ~1.0 °C occurs in spring when the forest masks the higher albedo snow-covered surface and results in snowmelt and a reduction in cloud cover. Results fail to show a larger-scale dynamical response although some warming of the lower and mid troposphere occurs in July. No changes were found in the position or strength of the Arctic frontal zone as some studies have indicated will occur with a shift in the boreal forest-tundra boundary. These findings suggest that coupled model simulations that predict larger changes in vegetation distribution are likely overemphasizing the amount of Arctic warming that will occur this century. These findings also indicate that a realistic dynamical response to subtle land cover change might not be correctly simulated by GCMs run at coarse spatial resolutions.  相似文献   

15.
Precipitation from the Eastern Sierra Nevada watersheds of Owens Lake and Mono Lake is one of the main water sources for Los Angeles’ over 4 million people, and plays a major role in the ecology of Mono Lake and of these watersheds. We use the Variable Infiltration Capacity (VIC) hydrologic model at daily time scale, forced by climate projections from 16 global climate models under greenhouse gas emissions scenarios B1 and A2, to evaluate likely hydrologic responses in these watersheds for 1950–2099. Comparing climate in the latter half of the 20th Century to projections for 2070–2099, we find that all projections indicate continued temperature increases, by 2–5 °C, but differ on precipitation changes, ranging from ?24 % to +56 %. As a result, the fraction of precipitation falling as rain is projected to increase, from a historical 0.19 to a range of 0.26–0.52 (depending on the GCM and emission scenario), leading to earlier timing of the annual hydrograph’s center, by a range of 9–37 days. Snowpack accumulation depends on temperature and even more strongly on precipitation due to the high elevation of these watersheds (reaching 4,000 m), and projected changes for April 1 snow water equivalent range from ?67 % to +9 %. We characterize the watershed’s hydrologic response using variables integrated in space over the entire simulated area and aggregated in time over 30-year periods. We show that from the complex dynamics acting at fine time scales (seasonal and sub-seasonal) simple dynamics emerge at this multi-year time scale. Of particular interest are the dynamic effects of temperature. Warming anticipates hydrograph timing, by raising the fraction of precipitation falling as rain, reducing the volume of snowmelt, and initiating snowmelt earlier. This timing shift results in the depletion of soil moisture in summer, when potential evapotranspiration is highest. Summer evapotranspiration losses are limited by soil moisture availability, and as a result the watershed’s water balance at the annual and longer scales is insensitive to warming. Mean annual runoff changes at base-of-mountain stations are thus strongly determined by precipitation changes.  相似文献   

16.
We use a state of the art climate model (CAM3–CLM3) to investigate the sensitivity of surface climate and land surface processes to treatments of snow thermal conductivity. In the first set of experiments, the thermal conductivity of snow at each grid cell is set to that of the underlying soil (SC-SOIL), effectively eliminating any insulation effect. This scenario is compared against a control run (CTRL), where snow thermal conductivity is determined as a prognostic function of snow density. In the second set of experiments, high (SC-HI) and low (SC-LO) thermal conductivity values for snow are prescribed, based on upper and lower observed limits. These two scenarios are used to envelop model sensitivity to the range of realistic observed thermal conductivities. In both sets of experiments, the high conductivity/low insulation cases show increased heat exchange, with anomalous heat fluxes from the soil to the atmosphere during the winter and from the atmosphere to the soil during the summer. The increase in surface heat exchange leads to soil cooling of up to 20 K in the winter, anomalies that persist (though damped) into the summer season. The heat exchange also drives an asymmetric seasonal response in near-surface air temperatures, with boreal winter anomalies of +6 K and boreal summer anomalies of −2 K. On an annual basis there is a net loss of heat from the soil and increases in ground ice, leading to reductions in infiltration, evapotranspiration, and photosynthesis. Our results show land surface processes and the surface climate within CAM3–CLM3 are sensitive to the treatment of snow thermal conductivity.  相似文献   

17.
The timing and nature of ice sheet variations on Greenland over the last ～5 million years remain largely uncertain. Here, we use a coupled climate-vegetation-ice sheet model to determine the climatic sensitivity of Greenland to combined sets of external forcings and internal feedbacks operating on glacial-interglacial timescales. In particular, we assess the role of atmospheric pCO2, orbital forcing, and vegetation dynamics in modifying thresholds for the onset of glaciation in late Pliocene and Pleistocene. The response of circum-Arctic vegetation to declining levels of pCO2 (from 400 to 200 ppmv) and decreasing summer insolation includes a shift from boreal forest to tundra biomes, with implications for the surface energy balance. The expansion of tundra amplifies summer surface cooling and heat loss from the ground, leading to an expanded summer snow cover over Greenland. Atmospheric and land surface fields respond to forcing most prominently in late spring-summer and are more sensitive at lower Pleistocene-like levels of pCO2. We find cold boreal summer orbits produce favorable conditions for ice sheet growth, however simulated ice sheet extents are highly dependent on both background pCO2 levels and land-surface characteristics. As a result, late Pliocene ice sheet configurations on Greenland differ considerably from late Pleistocene, with smaller ice caps on high elevations of southern and eastern Greenland, even when orbital forcing is favorable for ice sheet growth.  相似文献   

18.
Brazilian strategic interest in the Madeira River basin, one of the most important of the southern Amazon tributaries, includes the development of hydropower to satisfy the country’s growing energy needs and new waterways to boost regional trade and economic development. Because of evidences that climate change impacts the hydrological regime of rivers, the aim of this study was to assess how global climate change and regional land cover change caused by deforestation could affect the river’s hydrological regime. To achieve this goal, we calibrated a large-scale hydrological model for the period from 1970–1990 and analyzed the ability of the model to simulate the present hydrological regime when climate model simulations were used as input. Climate change projections produced by climate models were used in the hydrological model to generate scenarios with and without regional land-use and land-cover changes induced by forest conversion to pasture for the period from 2011–2099. Although results show variability among models, consensus scenarios indicated a decrease in the low-flow regime. When the simulations included forest conversion to pasture, climate change impacts on low flows were reduced in the upper basin, while, in the lower basin, discharges were affected along the whole year due to the more vigorous land-use conversion in the Brazilian region of the basin.  相似文献   

19.
We examined if climate change in two dry ecosystems—Mediterranean (DME) and Semiarid (SAE)—would cause substantial reduction in the production of annual vegetation. Field measurements and computer simulations were used to examine the following six climate change scenarios: (1) rainfall amount reduction; (2) increases of 10 % in annual evaporation rate and 5 % in annual temperature; (3) increase in magnitude of rainfall events, accompanied by reductions in frequency and seasonal variation; (4) postponement of the beginning of the first rainfall event of the growing season; (5) long dry spells during the growing season; and (6) early ending of the growing season. The results revealed the following outcomes. a) Reduction by 5–35 % in annual rainfall amount did not significantly affect productivity in the DME, but a large (25–35 %) decrease in rainfall would change vegetation productivity in the SAE and lead to a patchier environment. b) Similar results were observed: when temperature and evaporation rate were increased; when the magnitude of rainfall events increased but their frequency decreased; and during a long mid-season dry spell. c) In both ecosystems, changes in the temporal distribution of rainfall, especially at the beginning of the season, caused the largest reduction in productivity, accompanied by increased patchiness. d) Long-term data gathered during the last three decades indicated that both environments exhibited high resilience of productivity under rainfall variability. These results imply that the response of dry ecosystems to climate change is not characterized by a dramatic decrease in productivity. Moreover, these ecosystems are more resilient than expected, and their herbaceous productivity might undergo drastic changes only under more severe scenarios than those currently predicted in the literature.  相似文献   

20.
Alaskan Arctic waters have participated in hemispheric-wide Arctic warming over the last two decades at over two times the rate of global warming. During 2008–13, this relative warming occurred only north of the Bering Strait and the atmospheric Arctic front that forms a north–south thermal barrier. This front separates the southeastern Bering Sea temperatures from Arctic air masses. Model projections show that future temperatures in the Chukchi and Beaufort seas continue to warm at a rate greater than the global rate, reaching a change of +4℃ by 2040 relative to the 1981–2010 mean. Offshore at 74°N, climate models project the open water duration season to increase from a current average of three months to five months by 2040. These rates are occasionally enhanced by midlatitude connections. Beginning in August 2014, additional Arctic warming was initiated due to increased SST anomalies in the North Pacific and associated shifts to southerly winds over Alaska, especially in winter 2015–16. While global warming and equatorial teleconnections are implicated in North Pacific SSTs, the ending of the 2014–16 North Pacific warm event demonstrates the importance of internal, chaotic atmospheric natural variability on weather conditions in any given year. Impacts from global warming on Alaskan Arctic temperature increases and sea-ice and snow loss, with occasional North Pacific support, are projected to continue to propagate through the marine ecosystem in the foreseeable future. The ecological and societal consequences of such changes show a radical departure from the current Arctic environment.  相似文献   

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