Increased Winter Precipitation Research Articles

Responses of insect pests, pathogens, and invasive plant species to climate change in the forests of northeastern North America: What can we predict?This 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.

Climate models project that by 2100, the northeastern US and eastern Canada will warm by approximately 3–5 °C, with increased winter precipitation. These changes will affect trees directly and also indirectly through effects on “nuisance” species, such as insect pests, pathogens, and invasive plants. We review how basic ecological principles can be used to predict nuisance species’ responses to climate change and how this is likely to impact northeastern forests. We then examine in detail the potential responses of two pest species (hemlock woolly adelgid ( Adelges tsugae Annand) and forest tent caterpillar ( Malacosoma disstria Hubner)), two pathogens (armillaria root rot ( Armillaria spp.) and beech bark disease ( Cryptococcus fagisuga Lind. + Neonectria spp.)), and two invasive plant species (glossy buckthorn ( Frangula alnus Mill.) and oriental bittersweet ( Celastrus orbiculatus Thunb.)). Several of these species are likely to have stronger or more widespread effects on forest composition and structure under the projected climate. However, uncertainty pervades our predictions because we lack adequate data on the species and because some species depend on complex, incompletely understood, unstable relationships. While targeted research will increase our confidence in making predictions, some uncertainty will always persist. Therefore, we encourage policies that allow for this uncertainty by considering a wide range of possible scenarios.

Changes in snowpack and snowmelt runoff for key mountain regions

AbstractMountain snowpack and spring runoff are key components of surface water resources, and serve as important, regionally integrated indicators of climate variability and change. This study examines whether mountain snowpack and snowmelt have manifested a consistent hydrologic response to global climatic changes over the past several decades. Prior findings are compared to identify spatial and temporal patterns of trends in the volume, extent, and seasonality of snowpack and melt for key mountain regions. Evidence suggests that both temperature and precipitation increases to date have impacted mountain snowpacks simultaneously on the global scale; however, the nature of the impact is, among other factors, strongly dependent on geographic location, latitude, and elevation. Warmer temperatures at mid‐elevations have decreased snowpack and resulted in earlier melt in spite of precipitation increases, while they have not affected high‐elevation regions that remain well below freezing during winter. At high elevations, precipitation increases have resulted in increased snowpack. Not all local responses are consistent with the general findings, possibly because of local climatic trends, atmospheric circulation patterns, record lengths, or data quality issues. With continued warming, increasingly higher elevations are projected to experience declines in snowpack accumulation and melt that can no longer be offset by winter precipitation increases. There is a continued research need for hydroclimatic trend detection and attribution in mountains owing to the length, quality, and sparseness of available data from monitoring stations not directly impacted by human activity. Copyright © 2008 John Wiley & Sons, Ltd.

Response of a montenegro glacier to extreme summer heatwaves in 2003 and 2007

.The Debeli Namet glacier in the Durmitor massif, Montenegro, is one of the lowest altitude glaciers (2050–2300 m) at this latitude (42–44°N) in the northern hemisphere. The glacier survives well below the climatological equilibrium line altitude because of substantial inputs from avalanching and windblown snow. The glacier survived two of the hottest summers on record in 2003 and 2007, although it experienced significant retreat. However, during the intervening years (2004–2006) the glacier increased in size and advanced, forming a new frontal moraine. This rapid advance was primarily in response to much cooler summer temperatures, close to or cooler than average, and a marked increase in winter precipitation. The rapid growth and decay of the Debeli Namet glacier in response to inter‐annual climate variability highlights the sensitivity of small cirque glaciers to short‐term climate change.

Holocene climate and glacier variability at Hallet and Greyling Lakes, Chugach Mountains, south-central Alaska

Evidence from lake sediments and glacier forefields from two hydrologically isolated lake basins is used to reconstruct Holocene glacier and climate history at Hallet and Greyling Lakes in the central Chugach Mountains of south-central Alaska. Glacial landform mapping, lichenometry, and equilibrium-line altitude reconstructions, along with changes in sedimentary biogenic-silica content, bulk density, and grain-size distribution indicate a dynamic history of Holocene climate variability. The evidence suggests a warm early Holocene from 10 to 6 ka, followed by the onset of Neoglaciation in the two drainage basins, beginning between 4.5 and 4.0 ka. During the past 2 ka, the glacial landforms and lacustrine sediments from the two valleys record a remarkably similar history of glaciation, with two primary advances, one during the first millennium AD, from ~500 to 800 AD, and the second during the Little Ice Age (LIA) from ~1400 to 1900 AD. During the LIA, the reconstructed equilibrium-line altitude in the region was no more than 83 ± 44 m (n = 21) lower than the modern, which is based on the extent of glaciers during 1978. Differences between the summer temperature inferred from the biogenic-silica content and the evidence for glacial advances and retreats suggest a period of increased winter precipitation from 1300 to 1500 AD, and reduced winter precipitation from 1800 to 1900 AD, likely associated with variability in the strength of the Aleutian Low.

Increased winter precipitation over the North Pacific from 1984–1994 to 1995–2005 inferred from the Global Precipitation Climatology Project

An analysis of the results from the Global Precipitation Climatology Project (GPCP) reveals a trend of increased wintertime precipitation over the North Pacific from 1984 to 2005. The precipitation over the North Pacific increases by about 1.5 mm/year in winter during the period of 1984 to 2005. Statistic analyses are performed to validate the observed precipitation trend, showing the f‐test and t‐test significances of greater than 99%. A cloud‐resolving weather research and forecast (CR‐WRF) model with a two‐moment bulk microphysical scheme is employed to consider the effects of an increasing Asian pollution outflow on wintertime precipitation over the North Pacific. The CR‐WRF simulations demonstrate more precipitation with elevated polluted continental aerosols than that with typical maritime aerosols, reproducing the observed precipitation trend. The results suggest that the increasing Asian pollution outflow represents one of the most plausible explanations for the increased precipitation trend observed in winter over the North Pacific, likely with important climate implications.

Urbanization Impacts on the Climate in Europe: Numerical Experiments by the PSU–NCAR Mesoscale Model (MM5)

Abstract The objective of this study is to investigate the effects of urban land on the climate in Europe on local and regional scales. Effects of urban land cover on the climate are isolated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) with a modified land surface scheme based on the Town Energy Budget model. Two model scenarios represent responses of climate to different states of urbanization in Europe: 1) no urban areas and 2) urban land in the actual state in the beginning of the twenty-first century. By comparing the simulations of these contrasting scenarios, spatial differences in near-surface temperature and precipitation are quantified. Simulated near-surface temperatures and an urban heat island for January and July over a period of 6 yr (2000–05) agree well with corresponding measurements at selected urban areas. The conversion of rural to urban land results in statistically significant changes to precipitation and near-surface temperature over areas of the land cover perturbations. The diurnal temperature range in urbanized regions was reduced on average by 1.26° ± 0.71°C in summer and by 0.73° ± 00.54°C in winter. Inclusion of urban areas results in an increase of urban precipitation in winter (0.09 ± 00.16 mm day−1) and a precipitation reduction in summer (−0.05 ± 0.22 mm day−1).

The impacts of climate change on hydrology in Ireland

A study of nine Irish catchments was carried out to quantify the expected impact of climate change on hydrology in Ireland. Boundary data from the European Centre Hamburg Model Version 5 (ECHAM 5) general circulation model were used to force the Rossby Centre Atmosphere Model (RCA3) regional climate model, producing dynamically downscaled precipitation and temperature data under past and future climate scenarios. This data was used to force the HBV-Light conceptual rainfall-runoff model to simulate stream flow in the reference period (1961–2000) and in the future (2021–2060) under the Special Report on Emissions Scenarios (SRES) A1B scenario. A Monte-Carlo approach to calibration was used to obtain 100 parameter sets which reproduced observed stream flow well. Use of an ensemble provided results in terms of a range rather than a single value. Results suggested an amplification of the seasonal cycle across the country, driven by increased winter precipitation, decreased summer precipitation and increased temperature. The expected changes in mean winter and summer flows as well as annual maximum daily mean flow varied depending on catchment characteristics and the timing and magnitude of expected changes in precipitation in each catchment.

Climate dynamics in the borderlands of the Aegean Sea during formation of sapropel S1 deduced from a marine pollen record

To unravel the climatic and environmental dynamics in the borderlands of the Aegean Sea during the early and middle Holocene, and notably for the interval of sapropel S1 (S1) formation, we have analysed terrestrial palynomorphs from a marine core in the northern Aegean Sea. The qualitative results were complemented by quantitative pollen-based climate reconstructions. A land–sea correlation was established based on pollen data and sediment lightness measurements from the same core, and previously published benthic foraminifer data from a nearby core.The borderlands of the Aegean Sea underwent a transition from an open vegetation to oak-dominated woodlands between ∼10.4 and ∼9.5kacalBP. A coeval increase in winter precipitation suggests that moisture availability was the main factor controlling Holocene reforestation. The ∼50% higher winter precipitation during S1 formation relative to “pre-sapropelic” conditions suggests a strong contribution from the borderlands of the Aegean Sea to the freshwater surplus during S1 formation. The humid and mild winter conditions during S1 formation were repeatedly punctuated by short-term climatic events that caused a partial deforestation and a reorganisation within the broad-leaved arboreal vegetation. In the marine realm, these events are documented by improved benthic oxygenation. The strongest event represents the regional expression of the 8.2ka cold event and led to an interruption in S1 formation. Except for the interval of S1 formation, the pollen-derived winter temperatures correlate with the smoothed GISP2 K+ series. They support the previously published, marine-based concept that the intensity of the Siberian High strongly controlled the winter climate in the Aegean region. During S1 formation in the Aegean Sea, however, climate conditions in the borderlands were more strongly affected by the monsoonally influenced climate system of the lower latitudes.

Deepened snow alters soil microbial nutrient limitations in arctic birch hummock tundra

Microbial activity in the long arctic cold season is low but cumulatively important. In particular, the size of the microbial biomass and soil solution nutrient pool at the end of winter may control the quantity of nutrients available to plants in the following spring. Microbial starvation and lysis as a result of increasingly severe soluble carbon (C) shortages over winter has been hypothesized as a potential mechanism for microbial nutrient release at thaw. These C shortages may be exacerbated by the warmer temperatures and increased winter precipitation that are consistently predicted for a large part of the low Arctic. In particular, warmer soil temperatures due to deeper snow may increase wintertime microbial activity and organic matter decomposition over the winter, potentially resulting in enhanced nutrient availability to plants in the following growing season. In this study, we investigated nutrient limitations to soil microbial growth and activity in late winter under ambient and experimentally deepened snow (∼0.3 and 1 m respectively) in birch hummock tundra within the Canadian low Arctic. We hypothesized that the build-up of moderately deeper snow over winter would exacerbate soluble C-limitation to microbial growth and activity and increase soluble N accumulation, and thus stimulate the growth of bacteria relative to fungi. We measured the in situ response of the soil microbial biomass and soil soluble pools in control and snow-fenced plots at the end of winter, and then incubated soils from these plots with added C, nitrogen (N) and phosphorus (P) (at 0–15 °C) to characterize nutrient limitations to microbial growth and activity. In late winter, deepened snow increased the microbial pool of N, yet decreased soil pools of dissolved organic N and C, and decreased bacterial counts. Fungal mass and hyphal lengths did not change, but remained dominant under both ambient and deepened snow. Deepened snow exacerbated the soluble C-limitation to microbial growth and reduced the P-limitation for microbial respiration. Fungal mass and hyphal length responses to nutrient addition were larger than the bacterial mass or abundance responses and fungi from under deepened snow responded more than those from under ambient snow, indicating a different potential structural and physiological response to substrate availability for these two soil microbial communities. Our results indicate that deeper snow may increase microbial nutrient pools and can alter the physiological functioning of the soil microbial community in late winter, suggesting that microbial N release and its availability to plants during spring thaw may be enhanced.

Effects of warmer world scenarios on hydrologic inputs to Lake Mälaren, Sweden and implications for nutrient loads

A simple, rapid, and flexible modelling approach was applied to explore the impacts of climate change on hydrologic inputs and consequent implications for nutrient loading to Lake Malaren, Sweden using a loading function model (GWLF). The first step in the process was to adapt the model for use in a large and complex Swedish catchment. We focused on the Galten basin with four rivers draining into the western region of Malaren. The catchment model was calibrated and tested using long-term historical data for river discharge and dissolved nutrients (N, P). Then multiple regional climate model simulation results were downscaled to the local catchment level, and used to simulate possible hydrological and nutrient loading responses to warmer world scenarios. Climate change projections for the rivers of Galten basin show profound changes in the timing of discharge and nutrient delivery due to increased winter precipitation and earlier snow melt. Impacts on total annual discharge and load are minimal, but the alteration in river flow regime and the timing of nutrient delivery for future climate scenarios is strikingly different from historical conditions.

Strong increases in flood frequency and discharge of the River Meuse over the late Holocene: impacts of long-term anthropogenic land use change and climate variability

Abstract. In recent years the frequency of high-flow events on the Meuse (northwest Europe) has been relatively great, and flooding has become a major research theme. To date, research has focused on observed discharge records of the last century and simulations of the coming century. However, it is difficult to delineate changes caused by human activities (land use change and greenhouse gas emissions) and natural fluctuations on these timescales. To address this problem we coupled a climate model (ECBilt-CLIO-VECODE) and a hydrological model (STREAM) to simulate daily Meuse discharge in two time-slices: 4000–3000 BP (natural situation), and 1000–2000 AD (includes anthropogenic influence). For 4000–3000 BP the basin is assumed to be almost fully forested; for 1000–2000 AD we reconstructed land use based on historical sources. For 1000–2000 AD the simulated mean annual discharge (260.9 m3 s−1) is significantly higher than for 4000–3000 BP (244.8 m3 s−1), and the frequency of large high-flow events (discharge >3000 m3 s−1) is higher (recurrence time decreases from 77 to 65 years). On a millennial timescale almost all of this increase can be ascribed to land use changes (especially deforestation); the effects of climatic change are insignificant. For the 20th Century, the simulated mean discharge (270.0 m3 s−1) is higher than in any other century studied, and is ca. 2.5% higher than in the 19th Century (despite an increase in evapotranspiration). Furthermore, the recurrence time of large high-flow events is almost twice as short as under natural conditions (recurrence time decreases from 77 to 40 years). On this timescale climate change (strong increase in annual and winter precipitation) overwhelmed land use change as the dominant forcing mechanism.

Snow loads in a changing climate: new risks?

Abstract. In January/February 2006, heavy snowfalls in Bavaria (Germany) lead to a series of infrastructural damage of catastrophic nature. Since on many collapsed roofs the total snow load was not exceptional, serious engineering deficiencies in roof construction and a sudden rise in the total snow load were considered to be the trigger of the events. An analysis of the then meteorological conditions reveals, that the early winter of 2005/2006 was characterised by an exceptional continuous snow cover, temperatures remained around the freezing point and no significant snowmelt was evident. The frequent freezing/thawing cycles were followed by a general compaction of the snow load. This resulted in a re-distribution and a new concentration of the snow load on specific locations on roofs. With respect to climate change, the question arises as to whether the risks relating to snow loads will increase. The future probability of a continuous snow cover occurrence with frequent freezing/thawing cycles will probably decline due to predicted higher temperatures. However, where temperatures remain low, an increase in winter precipitation will result in increased snow loads. Furthermore, the variability of extremes is predicted to increase. If heavy snowfall events are more frequent, the risk of a trigger event will likely increase. Finally, an attempt will be made here in this paper to outline a concept for an operational warning system for the Bavarian region. This system envisages to predict the development and risk of critical snow loads for a 3-day time period, utilising a combination of climate and snow modelling data and using this together with a snow pillow device (located on roofs) and the results of which.

Impact of Climate Change on North Cascade Alpine Glaciers, and Alpine Runoff

Analysis of key components of the alpine North Cascade hydrologic system indicate significant changes in glacier mass balance, terminus behavior, alpine snowpack, and alpine streamflow from 1950 to 2005. North Cascade glacier retreat is rapid and ubiquitous. All 47 monitored glaciers are currently undergoing a significant retreat and four of them have disappeared. Annual mass balance measured on ten glaciers, averaging 30–50 m in thickness, yields a mean cumulative annual balance for the 1984–2006 period of −12.4 m water equivalent (m we), a net loss of 14 m in glacier thickness and 20–40% loss of their total volume in two decades.The data indicate broad regional continuity in North Cascades glacial response to climate. The substantial negative annual balances have accompanied significant thinning in the accumulation zone of 75% of North Cascade glaciers monitored. This is indicative of glacier disequilibrium; a glacier in disequilibrium will not survive the current warmer climate trend. Alpine snowpack snow water equivalent (SWE) on April 1 has declined 25% since 1946 at five USDA Snow Course sites. This decline has occurred in spite of a slight increase in winter precipitation. The combination of a decline in winter snowpack and a 0.6° increase in ablation season temperature, during the 1946–2005 period in the North Cascades, has altered alpine streamflow in six North Cascade basins. Observed changes in streamflow are: increased winter streamflow, slightly declining spring streamflow and a 27% decline in summer streamflow. Only in the heavily glaciated Thunder Creek Basin (> 10% glaciated) has summer streamflow declined less than 10%; this is attributable to enhanced glacier melting.

Western juniper and ponderosa pine ecotonal climate–growth relationships across landscape gradients in southern Oregon

Forecasts of climate change for the Pacific northwestern United States predict warmer temperatures, increased winter precipitation, and drier summers. Prediction of forest growth responses to these climate fluctuations requires identification of climatic variables limiting tree growth, particularly at limits of tree species distributions. We addressed this problem at the pine–woodland ecotone using tree-ring data for western juniper ( Juniperus occidentalis var. occidentalis Hook.) and ponderosa pine ( Pinus ponderosa Dougl. ex Loud.) from southern Oregon. Annual growth chronologies for 1950–2000 were developed for each species at 17 locations. Correlation and linear regression of climate–growth relationships revealed that radial growth in both species is highly dependent on October–June precipitation events that recharge growing season soil water. Mean annual radial growth for the nine driest years suggests that annual growth in both species is more sensitive to drought at lower elevations and sites with steeper slopes and sandy or rocky soils. Future increases in winter precipitation could increase productivity in both species at the pine–woodland ecotone. Growth responses, however, will also likely vary across landscape features, and our findings suggest that heightened sensitivity to future drought periods and increased temperatures in the two species will predominantly occur at lower elevation sites with poor water-holding capacities.

Late Quaternary paleosol–sediment-sequences and landscape evolution along the Andean piedmont, Bolivian Chaco

As part of the Bolivian Chaco, the Andean piedmont is situated in the transitional zone between the tropical-humid climate regime to the north (Amazonia) and the subtropical-semiarid regime to the south (Gran Chaco). In this zone, which is sensitive to climatic changes, late Quaternary landscape and climate history was reconstructed based on detailed sedimentological, pedological and geochemical analyses of paleosol–sediment-sequences along the Andean piedmont. The determination of characteristic lithofacies allowed correlation between the investigated profiles and their interpretation in terms of paleoenvironmental conditions, i.e. geomorphic stability (soil formation) or geomorphic activity (fluvial or fluvio–aeolian sedimentation, erosion, etc.). Coarse fluvial deposits (gravels, sands) form the base of the investigated profiles and have likely been deposited in a braided-river environment with low vegetation cover and intensive discharge events before and during the Last Glacial Maximum. Well-developed paleosols formed in the upper part of these sediments during the Lateglacial, documenting geomorphic stability (high vegetation cover) and wet conditions. Overlying palustrine sediments, dated to the Early Holocene, indicate a dramatic change in seasonality, i.e. increased winter precipitation and decreased summer precipitation. Subsequent erosion, followed by accumulation of fluvial and fluvio–aeolian sands, points to much drier conditions during the Mid-Holocene. Since ∼ 2.9 cal ka BP soil formation indicates a return to geomorphic stability and wetter conditions (characteristic of the present day climate) interrupted by short arid intervals.

Northern hemisphere winter storm tracks of the Eemian interglacial and the last glacial inception

Abstract. Climate simulations of the Eemian interglacial and the last glacial inception have been performed by forcing a coupled ocean-atmosphere general circulation model with insolation patterns of these periods. The parameters of the Earth's orbit have been set to conditions of 125 000 and 115 000 years before present (yr BP). Compared to today, these dates represent periods with enhanced and weakened seasonality of insolation in the northern hemisphere. Here we analyse the simulated change in northern hemisphere winter storm tracks. The change in the orbital configuration has a strong impact on the meridional temperature gradients and therefore on strength and location of the storm tracks. The North Atlantic storm track is strengthened, shifted northward and extends further to the east in the simulation for the Eemian at 125 kyr BP. As one consequence, the northern parts of Europe experience an increase in winter precipitation. The frequency of winter storm days increases over large parts of the North Atlantic including the British Isles and the coastal zones of north-western Europe. Opposite but weaker changes in storm track activity are simulated for 115 kyr BP.

Strength and spatial patterns of the Holocene wintertime westerlies in the NE Atlantic region

Two records of July temperature and two records of reconstructed winter precipitation along the western coast of Norway have been combined to examine regional patterns in glacier activity and winter precipitation during the Holocene. The maritime glaciers in western Norway are mainly controlled by winter precipitation. Hence, fluctuations in magnitude and equilibrium-line altitude (ELA) of these glaciers reflect variability in wintertime atmospheric circulation patterns in the northeast Atlantic region. By combining an independent proxy for summer temperature with reconstructed ELAs during the Holocene, it is possible to reconstruct former winter precipitation. We track the dominant position of the westerlies during the Holocene by comparing records of winter precipitation along a 2000 km south–north coastal transect in Norway. Analyses of modern data on the evolution of the polar vortex indicate that there is a strong relationship between polar vortex and storm-track variability over the North Atlantic. Periods with increased winter precipitation along the coast of Norway are associated with a stronger effect of the westerlies, where differences in the distribution of precipitation are assumed to reflect changes in the position of the westerlies. The largest precipitation anomalies caused by a dominant southerly position of the westerlies are found around 2.8, 1.2 and 0.4 ka yrs BP. However, a general humid phase prevailed between 2.3 and 0.9 ka yrs BP.

Snow cover variability in Bulgarian mountainous regions, 1931–2000

AbstractPrincipal component (PC) analysis was carried out on annual snow cover series from 15 mountain climate stations in Bulgaria to characterize the spatial and temporal variability in snow cover over the 1931–2000 period. Three distinct snow cover response regions were identified: (1) high elevation sites above 1500 m in the Vitosha and Rila Mountains; (2) low elevation sites below 1000 m in the eastern Rhodope and southern Pirin Mountains and (3) a mid‐mountain zone of 1000–1500 m elevation including stations from several mountain ranges. Snow cover exhibited evidence of significant decadal‐scale variability over the 1931–2000 period but little evidence of long‐term trends linked to climate warming. Over the more recent 1971–2000 period, the mid‐mountain zone has increased in importance and shows a significant trend toward a later start to the snow cover season.Composite analysis of high and low snow accumulation winters showed high accumulation years associated with a negative North Atlantic Oscillation (NAO)/East Atlantic (EA)‐like pattern that favored increased winter precipitation over the Mediterranean and Balkans and anomalous easterly flow across the Black Sea into Bulgaria. Low snow accumulation years were associated with the positive mode of a Scandinavian (SCA)‐like pattern that generated fall warming, a delay to the start of the snow cover season and reduced winter precipitation. Correlation of annual snow cover series with these modes of atmospheric variability revealed few significant correlations but evidence of considerable secular variability in the strength and sign of the relationships. This reflects the complex climate regime of Bulgaria, which is located at the cross‐roads of Mediterranean and Continental climate influences and on the periphery of the main nodes of influence of Northern Hemisphere circulation patterns that play important roles in European snow cover variability. Copyright © 2007 Royal Meteorological Society

Regional changes of precipitation characteristics in Northern Eurasia from simulations with global climate model

Characteristics of precipitation including mean precipitation, rain intensity, rain event probability and an index of extreme events from different global climate model simulations are analyzed. Results of numerical experiments with the ECHAM5/MPI-OM general circulation model (GCM) using anthropogenic greenhouse gas emission scenarios SRES-A2 and SRES-B1 have been used. Simulations with the ECHAM5/MPI-OM are compared to the observational data and ERA-40 reanalysis. The basins of major Eurasian rivers such as Volga, Ob, Yenisei and Lena rivers have been chosen for a detailed analysis. Scenario SRES-B1 and SRES-A2 simulations with the ECHAM5/MPI-OM for the 21st century demonstrate an increase of winter precipitation, its intensity and rain event probability as well as heavy precipitation in the catchments areas of Volga, Ob, Yenisei and Lena rivers. Trends for the summer rain intensity and heavy precipitation in the 21st century are also positive but less significant than for winter trends. This is in contrast to the summer rain event probability, which has a negative trend in all analyzed river basins.

世界における降水量と降水特性の長期変化

Recent advances in studies on long-term changes in precipitation amount and its characteristics in the world and its regional aspects are reviewed. According to the IPCC Fourth Assessment Report (IPCC, 2007), global annual land precipitation has increased about 1.08-1.10 mm per decade since 1901. Precipitation amount mainly increased in middle and high latitudes. Increases in winter precipitation are obvious in Eurasia, North America and northern Europe. On the other hand, decreasing tendency are observed in southern Europe, northern Africa, and South and Southeast Asia. There is no evident trend in Oceania and Central America. Although a long-term increasing trend is seen in the global land average precipitation, there are distinct decadal-scale temporal variations in precipitation time series. It is expected that precipitation characteristics, such as precipitation frequency, and intensity, will increase because of an increase in the water holding capacity of the atmosphere in a future warmer world. Changes in precipitation characteristics have been investigated in many regions around the world. The frequency of heavy precipitation has tended to increase in several countries in the middle and high latitudes during recent decades. In Central America and the Caribbean region, interannual variations related to SST variations in the tropical Pacific and the tropical Atlantic were observed. The contribution of very wet days to annual total precipitation across the globe has sharply increased in recent decades. However, daily precipitation data used for investigating on the precipitation characteristics are very limited, particularly, in African and Southeast Asian regions. Thus, the long-term changes in precipitation characteristics presented in a number of recent global and regional reports are not very reliable for these regions. Data rescue activity, particularly, for the first half of the 20th century is urgently needed in such sparse data regions, as Africa and Southeast Asia.