Moisture Sources Research Articles

СОДЕРЖАНИЕ ИЗОТОПОВ КИСЛОРОДА В ОСАДКАХ В ТЕНЬКИНСКОМ РАЙОНЕ МАГАДАНСКОЙ ОБЛАСТИ

A review of the isotopic composition of precipitation sampled from October 2020 to January 2022 at four locations in the Magadan oblast at a distance to 300 km from the shore of the Sea of Okhotsk is presented. It is shown that the isotopic composition of precipitation on the coast (the city of Magadan) differs significantly from the inland one in both values and δ18O/δD and δ18O/ Т ratios. The air-mass back-trajectory analysis revealed that these differences are primarily caused by varying roles of different moisture sources forming precipitation.

Large-scale moisture transport and local-scale convection patterns associated with warm-sector heavy rainfall over South China

Warm-sector heavy rainfall (WSHR) frequently occurs in South China, causing severe floods and posing serious threats to human health and societal systems there. However, its moisture sources, transport pathways, and triggering mechanisms still warrant in-depth investigations. Here we identify three different clusters of moisture sources and transport pathways of WSHR in South China during 1982–2015 and reveal their associated atmospheric circulations and local-scale convection patterns. In the case of cluster 1, a strong anomalous high-pressure system and an anomalous low-pressure system jointly induce strong southwesterly air-flows, and a local convergent flow and ascending motions appear over the west of South China, thereby providing sufficient water vapor from the Indian Ocean and favorable dynamical conditions for triggering WSHR events in the west of South China. In contrast, the trajectories of cluster 2 are characterized by strong easterly air-flows that supply a large amount of water vapor from the Northwest Pacific and induce a strong convection, favoring the WSHR occurrences in the eastern coastal area of South China. In the case of cluster 3, a high-pressure anomaly and a shallow low-pressure system respectively cover the southeast and northwest portions of South China. This circulation pattern causes a strong anomalous southerly wind and induces an anomalous convergent flow over entire South China, thereby transporting sufficient water vapor from the South China Sea and triggering WSHR events over South China. Differing from WSHR, the moisture of frontal heavy rainfall events over South China is not only contributed by the three dominant ocean-originating moisture transport paths but also by the continent-originating paths. The findings reported here highlight that the atmospheric trajectories and synoptic systems jointly modulate the spatial variation of heavy rainfall events in South China by dominating large-scale transport pathways of water vapor and local-scale convection patterns.

Isotopic measurements in water vapor, precipitation, and seawater during EUREC4A

Abstract. In early 2020, an international team set out to investigate trade-wind cumulus clouds and their coupling to the large-scale circulation through the field campaign EUREC4A: ElUcidating the RolE of Clouds-Circulation Coupling in ClimAte. Focused on the western tropical Atlantic near Barbados, EUREC4A deployed a number of innovative observational strategies, including a large network of water isotopic measurements collectively known as EUREC4A-iso, to study the tropical shallow convective environment. The goal of the isotopic measurements was to elucidate processes that regulate the hydroclimate state – for example, by identifying moisture sources, quantifying mixing between atmospheric layers, characterizing the microphysics that influence the formation and persistence of clouds and precipitation, and providing an extra constraint in the evaluation of numerical simulations. During the field experiment, researchers deployed seven water vapor isotopic analyzers on two aircraft, on three ships, and at the Barbados Cloud Observatory (BCO). Precipitation was collected for isotopic analysis at the BCO and from aboard four ships. In addition, three ships collected seawater for isotopic analysis. All told, the in situ data span the period 5 January–22 February 2020 and cover the approximate area 6 to 16∘ N and 50 to 60∘ W, with water vapor isotope ratios measured from a few meters above sea level to the mid-free troposphere and seawater samples spanning the ocean surface to several kilometers depth. This paper describes the full EUREC4A isotopic in situ data collection – providing extensive information about sampling strategies and data uncertainties – and also guides readers to complementary remotely sensed water vapor isotope ratios. All field data have been made publicly available even if they are affected by known biases, as is the case for high-altitude aircraft measurements, one of the two BCO ground-based water vapor time series, and select rain and seawater samples from the ships. Publication of these data reflects a desire to promote dialogue around improving water isotope measurement strategies for the future. The remaining, high-quality data create unprecedented opportunities to close water isotopic budgets and evaluate water fluxes and their influence on cloudiness in the trade-wind environment. The full list of dataset DOIs and notes on data quality flags are provided in Table 3 of Sect. 5 (“Data availability”).

Moisture Sources and Pathways Determine Stable Isotope Signature of Himalayan Waters in Nepal

AbstractThe Himalayan mountain range produces one of the steepest and largest rainfall gradients on Earth, with >3 m/yr rainfall difference over a ∼100 km distance. The Indian Summer Monsoon (ISM) contributes more than 80% to the annual precipitation budget of the central Himalayas. The remaining 20% falls mainly during pre‐ISM season. Understanding the seasonal cycle and the transfer pathways of moisture from precipitation to the rivers is crucial for constraining water availability in a warming climate. However, the partitioning of moisture into the different storage systems such as snow, glacier, and groundwater and their relative contribution to river discharge throughout the year remains under‐constrained. Here, we present novel field data from the Kali Gandaki, a trans‐Himalayan river, and use 4‐year time series of river and rain water stable isotope composition (δ18O and δ2H values) as well as river discharge, satellite Global Precipitation Measurement amounts, and moisture source trajectories to constrain hydrological variability. We find that rainfall before the onset of the ISM is isotopically distinct and that ISM rain and groundwater have similar isotopic values. Our study lays the groundwork for using isotopic measurements to track changes in precipitation sources during the pre‐ISM to ISM transition in this key region of orographic precipitation. Specifically, we highlight the role of pre‐ISM precipitation, derived from the Gangetic plain, to define the seasonal river isotopic variability across the central Himalayas. Lastly, isotopic values across the catchment document the importance of a large well‐mixed groundwater reservoir supplying river discharge, especially during the non‐ISM season.

Dominant Contribution of South Asia Monsoon to External Moisture for Extreme Precipitation Events in Northern Tibetan Plateau

Numerous previous studies have pointed out that the South Asia monsoon (SAM) contributes most moisture to the southern Tibetan Plateau, whilst the moisture over the Northern Tibetan Plateau (NTP) is supplied by the westerlies, but the moisture sources for extreme precipitation events remain unclear. In this study, the tracking of external moisture sources was performed on ten extreme precipitation events over each of six target subregions of the NTP during the summer of 2010–2018. We found that the SAM provided most of the external moisture for extreme precipitation events in the NTP, except for the largest contribution from East Asia to extreme precipitation in the easternmost subregion. The moisture carried by westerly winds is the second foreign source over the western NTP. In addition, more than 40% of the NTP extreme precipitation events occurred under the synergy of weak westerlies and enhanced SAM, and these events have a longer duration than others. Thus, SAM plays a key role in moisture transport for the extreme precipitation events over the NTP, even though its contribution to the climatological moisture is not significant.

The influence of the precipitation recycling process on the shift to heavy precipitation over the Tibetan Plateau in the summer

On the Tibetan Plateau (TP), precipitation intensity has shifted to heavy precipitation due to global warming. However, the influence of the precipitation recycling process on this phenomenon remains unknown. Using the Water Accounting Model-2layers (WAM2layers) model and ERA5 reanalysis, this study investigates the contributions of the precipitation recycling process to precipitation shifts over the TP during 1979–2019. The precipitation shift rate was proposed to quantify this process, and the results reveal that the positive precipitation shift (1.384 mm/41 years) over the TP consists of a positive shift over the western TP (5.666 mm/41 years) and a negative shift (−3.485 mm/41 years) over the eastern TP. Considering the source of moisture, either a local source or a remote source, precipitation was decomposed into internal and external cycles of the precipitation recycling process based on the WAM2layers model. Further analysis indicates that the internal cycle (87.2%) contributes more to the shift than the external cycle (12.8%) over the TP. The contributions of the precipitation recycling ratio (PRR) and precipitation amount to the precipitation shift rate induced by the internal cycle were further investigated. The results indicate that PRR changes contribute more to heavy precipitation over the TP, while precipitation amount changes contribute more to light precipitation. The precipitation recycling process contributes to the shift by increasing atmospheric moisture and increasing (decreasing) the dependency on local evaporation in heavy (light) precipitation. Increased dependence of heavy precipitation on evaporation increases the risk of extreme precipitation, and the government should take preventative actions to mitigate these adverse effects.

Oceanic SACZ produces an abnormally wet 2021/2022 rainy season in South America

The oceanic South Atlantic Convergence Zone (SACZ) has played a major role during South America’s 2021/2022 summer extreme rainy season, being responsible for more than 90% of the precipitation in some regions of Southeast Brazil and in some regions of the Southwestern Atlantic Ocean (SWA). The summer of 2021/2022 was unique and rare and considered an abnormally humid season as verified by official Brazilian Institutes. First, the unusual number of cases of SACZ episodes (seven), was the highest recorded in the last decade. Second, all the cases that occurred were oceanic SACZ that assumed characteristics of an Atmospheric River and produced an excessively anomalous amount of precipitation during this period. Excess precipitation along with the regions located in mountainous and very uneven relief, which by orographic effects favors high precipitation volumes, were responsible for amplifying the observed impacts, such as landslides and floods that caused several losses to society. We also showed the main effects of coupling and interaction between the waters of the surface layer of the SWA and the atmosphere. Our learning from this study ends with the unprecedented results of how the marine atmospheric boundary layer (MABL) is locally modulated by the sea surface temperature (SST) that lies just below it. Until the present moment, we emphasize that this important mechanism has not been widely highlighted in the literature, showing that even though the ocean is colder than before oceanic SACZ is established, it is still warmer than the overlying air, thus, the ocean continues to be an active source of heat and moisture for the atmosphere and enhances the MABL instability process.

Inverse Isotopic Altitude Effect of Surface Water across the Upper Reaches of the Jinsha River, China: A Consideration of Moisture Sources

Characterization of the stable isotopic composition of modern surface water is essential to understand the complex hydrological processes in high-altitude mountainous areas. As one of the largest rivers draining from the Tibetan Plateau, there are large gaps in the knowledge of the isotopic variations of surface water across the Jinsha River. Here, we present new isotopic data from surface water along the upper reaches of the Jinsha River in June 2019. An inverse isotopic altitude effect was found with an average altitude gradient of 1.04‰/100 m for δ2H and 0.10‰/100 m for δ18O. The effect of the inflow of the tributary waters on the isotopic characteristics of the main stream was clearly observed. The history of the moisture transportation was evaluated at five major local hydrological stations along the main stream using the HYSPLIT model. A total of three moisture source classifications (southwest monsoon, recycled moisture, and westerly) were summarized, and a gradual increase in the percentage of the westerly was observed with increasing geographical altitude and latitude. A hypothesis was proposed that the moisture sources affected by different monsoons determine the isotopic characteristics of precipitation within the basin, and the rainfall runoff further affects the spatial distribution of surface water isotopes, responsible for the inverse isotopic altitude effect. These findings contribute to a better understanding of the spatial complexity of the isotopic altitude effect of surface water and the hydrological dynamics in the monsoon-influenced rivers around the Tibetan Plateau.

Comparison of wind-driven rain load on building facades in the urban environment and open field: A case study on two buildings in Zurich, Switzerland

Wind-driven rain (WDR) is one of the largest moisture sources affecting the hygrothermal performance and durability of building envelopes. Accurate estimation of WDR loads on building facades as boundary condition is important for the hygrothermal analysis of building envelopes. In this work, computational fluid dynamics (CFD) simulations and semi-empirical models are used to compare WDR load on building facades in the urban environment and open field. The research focuses on two historical buildings in the city center of Zurich, Switzerland. There is a very large difference in terms of the local wind-flow fields around buildings in the urban environment and open field. Raindrop trajectories in the open field are more horizontal due to higher wind speeds around the building. In the urban environment, the wind speed is lower yielding droplets following trajectories that are more parallel to the facades. The building overhang is more effective at rain protection in the urban environment compared to the building in the open field due to a larger rain sheltering effect, since the rain trajectories are more parallel to the facades. There is a very large difference in WDR load and distribution on the facade of buildings in the urban environment and open field. WDR loads on buildings in the urban environment are much smaller than in the open field. When semi-empirical models are used for this case study, there is large uncertainty in the prediction and the predicted WDR load can be many times larger compared to the load predicted by CFD.

Cenozoic topographic evolution of the Southern Central Andes foreland as revealed by hydrogen stable isotopes in hydrated volcanic glass

We use the “isotope-paleotopography” method to resolve the topographic evolution of the Southern Central Andes and adjacent foreland to the east, at the latitude of 35°S. Our analysis is based on δ2H measurements from hydrated volcanic glass from 107 samples collected from a 55 to 10 Ma stratigraphic section in the Malargüe basin, and from an additional 11 samples of Quaternary tuffs. The results are supported by an analysis of a large dataset of modern meteoric water samples (n=197), which are used to characterize the relationship between orographic lifting and precipitation isotopes in the modern. Our interpretations are guided by the Orographic Precipitation and Isotopes (OPI) programs, which provide a full simulation of the flow of moist air over a specified 3D topography and the resulting condensation and fall out of rain and snow, and the isotopic fractionation associated with these processes. The OPI model is fit, in a least-squares sense, to the modern isotope data, which provides a way to test for moisture sources and to calculate how precipitation isotopes would be influenced by variations in climate.There are three important conclusions from these data: 1) OPI modeling of modern water isotopes shows that precipitation at the Malargüe study area is derived solely from the moist northeasterly winds. The isotopic fractionation along this wind path occurs by lifting over the Córdoba and San Luis basement highs, and during the initial rise up the east side of the range. Westerly moist air is able to pass over the range, but downslope flow over the east side means that this source becomes strongly undersaturated, so precipitation from this source is suppressed in this area. 2) Our volcanic glass data indicate that the hydrogen isotopic composition of precipitation, δ2H, has been strongly depleted, by −50 to −90 per mil, since 55 Ma to present. The only way to produce this depletion is by upslope flow of moist winds and associated precipitation over the eastern side of the range. The amount of isotopic fractionation remains fairly constant and similar to modern for the interval from 55 to 15 Ma, which indicates that the topography to the east of Malargüe has been fairly steady during that time interval. 3) Our δ2H record indicates that between 15 and 10 Ma, the topography upwind of Malargüe decreased by about 50%, and then increased by the same amount between 10 and 0 Ma. This subsidence event coincides with the Paranense marine transgression, and is also predicted by numerical modeling of mantle flow and dynamic topography associated with subduction of the Nazca plate.

The South Atlantic Ocean as a moisture source region and its relation with precipitation in South America

The South Atlantic Ocean as a moisture source region and its relation with precipitation in South America

Millennial‐Scale Precipitation Variability in the Indo‐Pacific Region Over the Last 40 Kyr

AbstractDeep atmospheric convection in the Indo‐Pacific Warm Pool (IPWP) represents a major source of heat and moisture, thereby affecting the global climate, but its past changes remain debated. Here, we present sub‐millennial clay mineralogy and elemental records spanning the last 40 Kyr from within the IPWP. From these data, we infer millennial‐scale fluctuations in precipitation, with generally lower precipitation during Heinrich Stadials 1–4, corresponding to El Niño‐like conditions. Higher precipitation coincided with the warm interstadials, accompanied by La Niña‐like conditions. Moreover, our record indicates the lowest precipitation occurred during the late Holocene, supporting the hypothesis of a stronger‐than‐modern Walker circulation during the Last Glacial Maximum. In combination with other proxy records and TraCE‐21 modeling results, we recognize a distinct spatial heterogeneity of precipitation within the IPWP, resulting from the dominant influences of the ENSO‐like system and migration of the Intertropical Convergence Zone in the eastern and western IPWP, respectively.

Interdecadal to Centennial Climate Variability Surrounding the 8.2 ka Event in North China Revealed Through an Annually Resolved Speleothem Record From Beijing

AbstractThe 8.2 ka event has been extensively studied, whereas its structure is ambiguous in North China. Here we present a high‐resolution (∼1 year) δ18O record from annual laminated speleothem from Beijing to characterize the detailed variability across this event in North China. Our record indicates a dry 8.2 ka event spanning 8.254–8.107 ka BP with a two‐stage structure superimposed by three prominent high δ18O excursions. The identical structure of speleothem δ18O records between North and central China during the event suggests a common forcing/response in East China, whereas the progressively increased offset between their average values may reflect changes in moisture source or rainout effect. A close comparison with the Greenland ice core records suggests a strong linear response of the Asian summer monsoon to the North Atlantic climate changes across the early and middle stages of the event, but a different mechanism in the termination processes.

Isotopic compositions (δD, δ18O) and end-member mixing for the control interface in a complex tidal region

Identifying the mixing processes of waters and currents in tidal reach is an important aspect of environmental management to protect freshwater resources and prevent water pollution. In this study, three field investigations conducted in a typical tidal reach in August, November and the following April focused on two isotopes (δD and δ18O) and salinity. A salinity-isotope conservative mixing model was established to differentiate water flows of the important control interface (CI) from freshwater, transition zone and saltwater end-members. Results suggested that the average δD and δ18O values during the ebb and flood tides depleted from August to November, then enriched significantly in the following April and were even higher than those in August. The δD and δ18O values in the saltwater zone enriched markedly compared with those in freshwater zone and transition zone due to the stronger evaporation occurring in the saltwater zone. Based on the revised model, the average contributions of freshwater end-member, transition zone end-member and saltwater end-member in three months were, respectively, 51.50 %, 36.93 % and 11.57 %. However, the contributions of freshwater and transition zones in April end-member were equivalent (47.45 % vs 44.31 %). Meanwhile the largest contribution of saltwater end-member was 20.56 % and occurred in August. The proportions of three end-members that contributed to CI changed with different evaporation scenarios and moisture sources of precipitation. Our research provides important information that furthers our understanding of the isotopes and their applications to environmental management in estuarine regions.

Interannual snowfall variations in Central Asia and their association with ENSO and stratospheric polar vortex during winter

As a major source of moisture in Central Asia (CA), snowfall may significantly impact agriculture and economics in CA. The study has investigated the dominant modes of snowfall frequency during winter over CA and associated mechanisms. The first EOF mode (EOF1) of snowfall frequency corresponds to a homogeneous pattern over CA. In contrast, the second EOF mode (EOF2) is characterized by reversed anomalies over northern and southern CA. The mechanisms of the interannual variation of the two leading modes are different. EOF1 is influenced by the sea surface temperature anomalies (SSTA) over the North Atlantic and eastern tropical Pacific. Positive SSTA in the eastern tropical Pacific may stimulate a zonal wave train that propagates eastward and induce an anomalous cyclone in CA. The anomalous cyclone associated with ascending motions and water vapor transport convergence can contribute to increased snowfall frequency over CA. Besides, the interaction between the North Atlantic Oscillation and North Atlantic triple SSTA may also strengthen the zonal wave train. EOF2 is affected by the stratospheric polar vortex which is related to the wave reflections in winter. The wave reflections may strengthen the coupling of atmospheric circulation in the stratosphere and the troposphere, inducing a positive (negative) geopotential height anomaly over southern (northern) CA. These geopotential height anomalies may contribute to increased and decreased synoptic-scale wave activity over northern and southern CA which is conducive to the dipole mode of snowfall frequency over CA.

БИОИНДИКАЦИОННЫЕ ПРИЗНАКИ АТМОСФЕРНЫХ АНОМАЛИЙ В РАЗРЕЗАХ ТОРФЯНИКОВ НА ВОСТОЧНОМ МАКРОСКЛОНЕ СИХОТЭ-АЛИНЯ В ПОЗДНЕМ ГОЛОЦЕНЕ

Studies of peat stratigraphy in the upper reaches of the Milogradovka River made it possible to restore changes in humidity as a source of moisture in the middle Sikhote-Alin over the last 3400 years. High-resolution reconstructions were based on the data from biostratigraphic analyses (botanical and diatom); the ash content of peat was also studied. Radiocarbon dating of the events was performed using an age-depth model. In the upper part of the section, the B-Tm cryptotephra of the Baitoushan caldera-forming volcano was found, which was identified by the volcanic glass chemical composition. Prolonged rains are assumed to cause a massive landslide with the formation of a dam and a shallow reservoir turning the river valley into a swamp. Bioindicators of changes in humidity in the river valley and the occurrence of floods were identified. Events were correlated with the data for the main watershed of the Sikhote-Alin, mountainous plateaus, lakes on the slopes of the ancient Solontsovsky volcano and the eastern Primorye coast. The peat section in the valley is shown to record in more detail the hydrological changes associated with the nature of cyclogenesis. A relationship was revealed between paleoevents and anomalies of large-scale atmospheric circulation. Long dry phases occurred during global cooling and were accompanied by aridization in East Asia, caused by a decrease in the intensity of the East Asian summer monsoon. Studies show that floods occurred in the summer to autumn season even in dry phases. The phases of flooding were associated with the activation of the summer monsoon, as well as more intense cyclogenesis. The most complex climate rhythm was established for the last 1340 years.

Moisture Sources and Their Contributions to Summer Precipitation in the East of Southwest China

Moisture Sources and Their Contributions to Summer Precipitation in the East of Southwest China

Stable-isotope ratios (δ18O and δD) in a firn core from West Antarctica.

A 22.48 m long ice core (BR-IC-4) was collected in the West Antarctic Ice Sheet (at 83°58'59.4" S, 80°07'01.4" W, 1,295 m above the sea level) during the Austral summer of 2004-2005, as a contribution to the International Trans-Antarctic Expedition program. The isotopic composition (δD and δ18O) of 599 samples, corresponding to the upper 12.98 m of the ice core, was determined by gas source mass spectrometry and cavity ring-down spectroscopy. Relative dating was based on the isotopic ratios and major ions (MS-, Na+, nssSO4 2-) and trace elements (Na, S, Sr) concentrations. The record covers approximately 13 years - from 1990 to 2003. The mean accumulation rate of 0.48 ± 0.09 m water equivalent per year (m eq H2O a1) is relatively high for the geographical area and possibly results from snowdrifting from near areas, as attested by ice glaze surfaces in other sites in the region. The stable isotope δD content varies between -367.90‰ and 256.30‰ (mean -314.42 ± 19.01‰); and δ18O ranges from -44.96‰ to 35.08‰ (mean -39.95 ± 2.05‰). Deuterium excess values (mean 3.70 ± 1.54‰) indicate episodic intense oceanic evaporation and high relative humidity in the moisture sources.

A new contribution ratio for indoor climate (CRISE) for the contribution evaluation of each heat and moisture source to the specific enthalpy in a closed space

The indoor thermal environment comprises temperature, humidity, wind velocity, and radiant temperature. The spatial distribution of these elements is a combination of the individual responses of the heat and water vapor transported into the space from each source. The contribution ratio for indoor climate (CRI) proposed by Kato, Kobayashi, and Murakami is a single index that evaluates the effect of each heat source on the temperature by breaking down the temperature increase at position x in a room into the effect of each heat source. We extended the CRI concept to three single indices to improve its applicability. Previously, thermal environment evaluation was often performed using a composite index rather than a single index. To evaluate the combined effects of heat and moisture sources, decomposing multiple elements individually and then combining them is desirable. In this study, we propose CRISE to evaluate the contribution of a heat or moisture source to the specific enthalpy at a given point in a closed space. The index was calculated by combining a single index of dry-bulb temperature and absolute humidity. The CRISE is a useful index for evaluating the indoor thermal environment and HVAC load.

Beautiful Days in the Neighborhood: Land–Atmosphere Interactions as Drivers of Forest Expansion

Abstract We explore the possible role of plant–atmosphere feedbacks in accelerating forest expansion using a simple example of forest establishment. We use an unconventional experimental design to simulate an initial forest establishment and the subsequent response of climate and nearby vegetation. We find that the forest’s existence produces favorable nearby growing-season conditions that would promote forest expansion. Specifically, we consider a hypothetical region of forest expansion in modern Alaska. We find that the forest acts as a source of heat and moisture for plants to the west, leading them to experience earlier springtime temperatures, snowmelt, and growth. Summertime cooling and cloud formation over the forest also drive a circulation change that reduces summertime cloud cover south of the forest, increasing solar radiation reaching plants there and driving warming. By isolating these vegetation–atmosphere interactions as the mechanisms of increased growth, we demonstrate the potential for forest expansion to be accelerated in a way that has not been highlighted before. These simulations illuminate two separate mechanisms that lead to increased plant growth nearby: 1) springtime heat advection and 2) summertime cloud feedbacks and circulation changes; both have implications for our understanding of past changes in forest cover and the predictability of biophysical impacts from afforestation projects and climate change–driven forest-cover changes. By examining these feedbacks, we seek to gain a more comprehensive understanding of past and potential future land–atmosphere interactions. Significance Statement This study investigates whether the emergence of a high-latitude forest could influence the way water and energy are exchanged between the land and atmosphere in a way that impacts nearby growing conditions and subsequent forest expansion. We use a computer model to simulate a climate with and without forest establishment in the high latitudes and test the response of plants surrounding the forest to the two different climates. We find that a forest is indeed able to spur neighboring plant growth by modifying regional climate and producing more favorable growing conditions for surrounding vegetation. Specifically, forest establishment can bring better growing conditions to plants adjacent to it by warming the air and altering nearby circulation and cloud cover.