The implications of climate policy for the impacts of climate change on global water resources

This paper assesses the implications of climate policy for exposure to water resources stresses. It compares a Reference scenario which leads to an increase in global mean temperature of 4 °C by the end of the 21st century with a Mitigation scenario which stabilises greenhouse gas concentrations at around 450 ppm CO₂e and leads to a 2 °C increase in 2100. Associated changes in river runoff are simulated using a global hydrological model, for four spatial patterns of change in temperature and rainfall. There is a considerable difference in hydrological change between these four patterns, but the percentages of change avoided at the global scale are relatively robust. By the 2050s, the Mitigation scenario typically avoids between 16 and 30% of the change in runoff under the Reference scenario, and by 2100 it avoids between 43 and 65%. Two different measures of exposure to water resources stress are calculated, based on resources per capita and the ratio of withdrawals to resources. Using the first measure, the Mitigation scenario avoids 8-17% of the impact in 2050 and 20-31% in 2100; with the second measure, the avoided impacts are 5-21% and 15-47% respectively. However, at the same time, the Mitigation scenario also reduces the positive impacts of climate change on water scarcity in other areas. The absolute numbers and locations of people affected by climate change and climate policy vary considerably between the four climate model patterns.     

Lags in vegetation response to greenhouse warming

Fossil pollen in sediments documents vegetation responses to climatic changes in the past. Beech (Fagus grandifolia), with animal-dispersed seeds, moved across Lake Michigan or around its southern margin, becoming established in Wisconsin about 1000 years after populations were established in Michigan. Hemlock (Tsuga canadensis), with wind-dispersed seeds, colonized a 50,000 km² area in northern Michigan between 6000 and 5000 years ago. These tree species extended ranges northward at average rates of 20–25 km per century. To track climatic changes in the future, caused by the greenhouse effect, however, their range limit would need to move northward 100 km per °C warming, or about 300 km per century, an order of magnitude faster than range extension in the past. Yet range extension in the future would be less efficient than in the past, because advance disjunct colonies have been extirpated by human disturbance, and because the seed source is reduced due to reductions in tree populations following logging. Many species of trees may not be able to disperse rapidly enough to track climate, and woodland herbs, which have less efficient seed dispersal mechanisms, may be in danger of extinction.     

Coupling climate change with hydrological dynamic in Qinling Mountains, China

This study intends to disclose orographic effects on climate and climatic impacts on hydrological regimes in Qinling Mountains under global change background. We integrate a meteorological model (MM5 model, PSU/NCAR, 2005) and a hydrological model (SWAT model, 2005) to couple hydrological dynamic with climate change in Qinling Mountains. Models are calibrated and validated based on the simulation of different combined schemes. Following findings were achieved. Firstly, Qinling Mountains dominantly influence climate, and hydrological process in Weihe River and upper Hanjiang River. Results show that Qinling Mountains lead to a strong north–south gradient precipitation distribution over Qinling Mountains due to orographic effects, and it reduces precipitation from 10–25 mm (December) to 55–80 mm (August) in Weihe River basin, and adds 25–50 mm (December) or 65–112 mm (August) in upper Hanjiang River basin; evapotranspiration (ET) decrease of 21% in Weihe River (August) and increase 10.5% in upper Hanjiang River (July). The Qinling Mountains reduce water yields of 23.5% in Weihe River, and decrease of 11.3% in upper Hanjiang River. Secondly, climate change is responsible for the changes of coupling effects of rainfall, land use and cover, river flow and water resources. It shows that average temperature significantly increased, and precipitation substantially reduced which leads to hydrological process changed greatly from 1950 to 2005: temperature increased and precipitation decreased, climate became drier in the past two decades (1980–2005), high levels of precipitation exists in mid-1950, mid-1970, while other studied periods are in low level states. The inter-annual variation in water yield correlates with surface runoff with an R ² value of 0.63 (Weihe River) and 0.87 (upper Hanjiang River). It shows that variation of annual precipitation was smaller than that of seasonal precipitation.     

Glacier recession and human vulnerability in the Yanamarey watershed of the Cordillera Blanca, Peru

Glaciers in the Cordillera Blanca, Peru, are undergoing rapid retreat, in large part due to climate change. These changes are significantly altering water availability in the region and pose critical risks to local populations that are highly dependent on these resources for livelihoods. We examine these issues through an interdisciplinary and linked evaluation of hydrological change and livelihood vulnerability in the Yanamarey watershed. Physical observations of the Yanamarey glacier show acceleration in frontal retreat at a rate of 8 m decade^???1 since 1970, accompanied by total volume loss on the order of 0.022 km³. Hydrological and hydrochemical analyses document a possible transformation of stream flow over the past decade as the seasonal storage capacity of the glacier has degraded. Recent stream discharge measurements from the proglacial lake below the glacier are more coincident with the highly variable seasonal precipitation than they were during the 1998?C1999 hydrological year. Local household perceptions of glacier recession and seasonal hydrological variability agree with this trend, which is increasing human vulnerability in the watershed. Household case-study survey results demonstrate that shifting water resources, increasing weather extremes and climate-related threats to tourism are all new vectors of vulnerability for household livelihoods.     

Simulated Climate Change Effects on Year-Round Water Temperatures in Temperate Zone Lakes

A deterministic, one-dimensional model is presented to simulate daily water temperature profiles and associated ice and snow covers for dimictic and polymictic lakes of the temperate zone. The lake parameters required as model input are surface area (A_s), maximum depth (H_MAX), and Secchi depth (z_s), the latter, used as a measure of light attenuation and trophic state. The model is driven by daily weather data and operates year-round over multiple years. The model has been tested with extensive data (over 5,000 temperature points). Standard error between simulated and measured water temperatures is 1.4°C in the open water season and 0.5°C in the ice cover season. The model is applied to simulate the sensitivity of Minnesota lake water temperature characteristics to climate change. The projected climate changes due to a doubling of atmospheric CO₂ are obtained from the output of the Canadian Climate Center General Circulation Model (CCC GCM) and the Goddard Institute of Space Studies General Circulation Model (GISS GCM). Simulated lake temperature characteristics have been plotted in a coordinate system with a lake geometry ratio (A _s ^0.25 /H_MAX) on one axis and Secchi depth on the other. The lake geometry ratio expresses a lake's susceptibility to stratification. By interpolation, the sensitivity of lake temperature characteristics to changes of water depth and Secchi depth under the projected climate scenarios can therefore be obtained. Selected lake temperature characteristics simulated with past climate conditions (1961–1979) and with a projected 2 × CO₂ climate scenario as input are presented herein in graphical form. The simulation results show that under the 2 × CO₂ climate scenario ice formation is delayed and ice cover period is shortened. These changes cause water temperature modifications throughout the year.     

Variations in hydrological parameters of the Zeravshan River and its tributaries depending on meteorological conditions

According to the data of the Agency for Hydrometeorology of the Republic of Tajikistan, the volume of the Zeravshan glacier significantly reduced in 1927–1991 (by more than 2 km³), and its further degradation by 30–35% is expected by 2050. To monitor the meteorological conditions in the Zeravshan River basin, in particular, the area of the Zeravshan glacier, air temperature variations in 1931–1961 and 1981–2011 are analyzed. It was found that the period of 1931–1961 is characterized by stable air temperature and its significant rise began in 1981. The trend towards the decrease in water discharge of the Zeravshan River is observed in 1931–1961. It is demonstrated that the average long-term runoff decreased from 6.08 km³ in 1931–1961 to 5.36 km³ in 1981–2011. The similar measurements were carried out in the basin of the Yaghnob River being the tributary of the Zeravshan River. It was revealed that the difference in average long-term runoff between the periods of 1931–1961 and 1981–2011 is insignificant and makes up not more than 2%. Besides, it was found that the results of meteorological observations in 1931-1961 do not agree with the real picture of the Zeravshan glacier degradation.     

Testing a GCM land surface scheme against catchment-scale runoff data

A GCM land surface scheme was used, in off-line mode, to simulate the runoff, latent and sensible heat fluxes for two distinct Australian catchments using observed atmospheric forcing. The tropical Jardine River catchment is 2500 km² and has an annual rainfall of 1700 mm y^–1 while the Canning River catchment is 540 km², has a Mediterranean climate (annual rainfall of 800 mm y^–1) and is ephemeral for half the year. It was found that the standard version of a land surface scheme developed for a GCM, and initialised as for incorporation into a GCM, simulated similar latent and sensible heat fluxes compared to a basin-scale hydrological model (MODHYDROLOG) which was calibrated for each catchment. However, the standard version of the land surface scheme grossly overestimated the observed peak runoff in the wet Jardine River catchment at the expense of runoff later in the season. Increasing the soil water storage permitted the land surface scheme to simulate observed runoff quite well, but led to a different simulation of latent and sensible heat compared to MODHYDROLOG. It is concluded that this 2-layer land surface scheme was unable to simulate both catchments realistically. The land surface scheme was then extended to a three-layer model. In terms of runoff, the resulting control simulations with soil depths chosen as for the GCM were better than the best simulations obtained with the two-layer model. The three-layer model simulated similar latent and sensible heat for both catchments compared to MODHYDROLOG. Unfortunately, for the ephemeral Canning River catchment, the land surface scheme was unable to time the observed runoff peak correctly. A tentative conclusion would be that this GCM land surface scheme may be able to simulate the present day state of some larger and wetter catchments but not catchments with peaky hydrographs and zero flows for part of the year. This conclusion requires examination with a range of GCM land surface schemes against a range of catchments. Received: 9 June 1995 / Accepted: 4 April 1996     

Pollutant concentrations in fog and low cloud water at selected sites of the Czech Republic

In this paper, the basic composition of fog and low cloud water are presented, resulting from the analyses of water samples from 111 fog/cloud events. The samples were collected at five sites located in various regions of the Czech Republic. Two sampling sites are in mountainous regions and three sites represent various urban areas. The mountain stations are located in two regions of the Czech Republic with different industry types. At all the sites, active fog collectors were employed. In the water samples, the conductivity, acidity (pH), cations (H⁺, Na⁺, K⁺, NH₄⁺, Mg²⁺, Ca²⁺) and anions (F⁻, Cl⁻, NO₃⁻, SO₄²⁻) were determined.A mean pH value of about 4.5 was obtained at mountain sites whereas the measurements in urban areas showed mean pH values from 4.9 to 6.4. The mean conductivity values in the samples from the two mountain stations were 137 and 191.5 μS cm⁻¹. The samples from urban sites showed mean values between 127.7 and 654.4 μS cm⁻¹. The maximum concentration means for the three dominant pollutants (expressed by the ratio mountain sites/urban sites) are 32.9/99.6 mg l⁻¹ for NO₃⁻, 32.5/192.9 mg l⁻¹ for SO₄²⁻ and 18.5/52.7 mg l⁻¹ for NH₄⁺. As expected, we found higher ion concentrations in the northern part of the Czech Republic where larger numbers of lignite-burning power plants, chemical factories and opencast lignite mines are located. A decrease in ion concentrations was observed at higher altitude sites, probably reflecting at least in part higher liquid water contents at these locations.     

A multi-model ensemble of downscaled spatial climate change scenarios for the Dommel catchment, Western Europe

Regional or local scale hydrological impact studies require high resolution climate change scenarios which should incorporate some assessment of uncertainties in future climate projections. This paper describes a method used to produce a multi-model ensemble of multivariate weather simulations including spatial–temporal rainfall scenarios and single-site temperature and potential evapotranspiration scenarios for hydrological impact assessment in the Dommel catchment (1,350 km²) in The Netherlands and Belgium. A multi-site stochastic rainfall model combined with a rainfall conditioned weather generator have been used for the first time with the change factor approach to downscale projections of change derived from eight Regional Climate Model (RCM) experiments for the SRES A2 emission scenario for the period 2071–2100. For winter, all downscaled scenarios show an increase in mean daily precipitation (catchment average change of +9% to +40%) and typically an increase in the proportion of wet days, while for summer a decrease in mean daily precipitation (−16% to −57%) and proportion of wet days is projected. The range of projected mean temperature is 7.7°C to 9.1°C for winter and 19.9°C to 23.3°C for summer, relative to means for the control period (1961–1990) of 3.8°C and 16.8°C, respectively. Mean annual potential evapotranspiration is projected to increase by between +17% and +36%. The magnitude and seasonal distribution of changes in the downscaled climate change projections are strongly influenced by the General Circulation Model (GCM) providing boundary conditions for the RCM experiments. Therefore, a multi-model ensemble of climate change scenarios based on different RCMs and GCMs provides more robust estimates of precipitation, temperature and evapotranspiration for hydrological impact assessments, at both regional and local scale.     

Impacts of climate variability and human activities on decrease in streamflow in the Qinhe River,China

Under the impacts of climate variability and human activities, there are statistically significant decreasing trends for streamflow in the Yellow River basin, China. Therefore, it is crucial to separate the impacts of climate variability and human activities on streamflow decrease for better water resources planning and management. In this study, the Qinhe River basin (QRB), a typical sub-basin in the middle reach of the Yellow River, was chosen as the study area to assess the impacts of climate variability and human activities on streamflow decrease. The trend and breakpoint of observed annual streamflow from 1956 to 2010 were identified by the nonparametric Mann–Kendall test. The results showed that the observed annual streamflow decreased significantly (P?<?0.05) and a breakpoint around 1973 was detected. Therefore, the time series was divided into two periods: “natural period” (before the breakpoint) and “impacted period” (after the breakpoint). The observed annual streamflow decreased by 68.1 mm from 102.3 to 34.2 mm in the two periods. The climate elasticity method and hydrological model were employed to separate the impacts of climate variability and human activities on streamflow decrease. The results indicated that climate variability was responsible for 54.1 % of the streamflow decrease estimated by the climate elasticity method and 59.3 % estimated by the hydrological modeling method. Therefore, the climate variability was the main driving factor for streamflow decrease in the QRB. Among these driving factors of natural and anthropogenic, decrease in precipitation and increase in water diversion were the two major contributions of streamflow reduction. The finding in this study can serve as a reference for regional water resources management and planning.     

Using the SWAT Model for Studying the Hydrological Regime of a Small River Basin (the Komarovka River,Primorsky Krai)

The research describes the experience of using digital models (of terrain, soil, and vegetation) for the underlying surface of the catchment for developing the spatial structure of the open-source SWAT (Soil and Water Assessment Tool) hydrological model. The hydrological regime for the Komarovka River basin (616 km²) is simulated with a daily resolution using the data of Primorskaya water balance station and the modern observation network of Primorye Administration for Hydrometeorology and Environmental Monitoring. It is found that the calculated and measured runoff hydrographs are generally in good agreement, and the model is suitable for describing the hydrological regime of mid-latitude rivers where rainfall floods prevail. The model well reproduces average water years, underestimates the peaks caused by intense rainfall of the typhoon origin and overestimates baseflow.     

Long-term changes of water balance components in the basins of rivers fed by snow and ice

The variability of the main components of the annual water balance (precipitation, evaporation, glacial alimentation, and dynamic water reserves in the basin) for 1935–1990 is, for the first time, determined for the area where the Zeravshan runoff is formed, higher than hydrological post Dupuli is located. Long-term data on the annual Zeravshan River runoff from an area of 10 200 km² were derived from the measurements at Dupuli hydrological post. The other water balance components were determined with the help of computation methods. Comparison of the measured and calculated volumes of the annual runoff demonstrated that a relative difference between them is systematic, and as a whole for a computation period it is in the interval from ?0.31 to ?4.78%. The annual balance of accumulation and thawing of solid precipitation on glaciers and in the extraglacial area is also determined in the Zeravshan River basin. A new method for computing and mapping spatial variability of the maximum snowline altitude is developed.     

Impacts of carbon dioxide warming on climate and man in the semi-arid tropics

Tropical semi-arid climates occur between 10 and 35 deg latitude and are characterised by highly variable summer rainfall of between 300 and 750 mm in a rainy season of at least 4 months, generally adequate for rainfed cropping but with considerable drought risk. They support a mesic savanna vegetation. They have a land extent of 4.5 million km², mainly occupied by Third World nations with rapidly increasing populations which in the main are predominantly rural and largely agricultural with low per capita incomes, consequently vulnerable to climate change. A doubling of atmospheric CO₂ by the year 2030 is predicted to cause a rise in equilibrium mean temperature of 1–3 °C; however there is continuing uncertainty regarding the consequences for rainfall amount, variability and intensity, length of rainy season or the frequency of extreme rainfall events. Two scenarios are considered, with reduction and increase in rainfall respectively, involving corresponding latitudinal shifts in present climatic boundaries of about 200 km. Because of their variability, a clear signal of the greenhouse effect on these climates may be delayed, whilst regional responses may differ. Vegetational and hydrological responses under the alternative scenarios are considered. The possible consequences for rainfed and irrigated agriculture, water and energy supplies and disease and pest ecology are discussed. Lands of the semi-arid tropics are already extensively desertified, with consequent lowered productivity and heightened vulnerability to drought, and the possible impacts of greenhouse warming on desertification processes and on measures for land rehabilition to the year 2030 are reviewed. Measures to conserve the biological diversity of savanna lands in face of greenhouse warming are discussed.     

Simulating the hydrological response to predicted climate change on a watershed in southern Alberta, Canada

The current body of research in western North America indicates that water resources in southern Alberta are vulnerable to climate change impacts. The objective of this research was to parameterize and verify the ACRU agro-hydrological modeling system for a small watershed in southern Alberta and subsequently simulate the change in future hydrological responses over 30-year simulation periods. The ACRU model successfully simulated monthly streamflow volumes (r ²?=?0.78), based on daily simulations over 27 years. The delta downscaling technique was used to perturb the 1961?C1990 baseline climate record from a range of global climate model (GCM) projections to provide the input for future hydrological simulations. Five future hydrological regimes were compared to the 1961?C1990 baseline conditions to determine the average net effect of change scenarios on the hydrological regime of the Beaver Creek watershed over three 30-year time periods (starting in 2010, 2040 and 2070). The annual projections of a warmer and mostly wetter climate in this region resulted in a shift of the seasonal streamflow distribution with an increase in winter and spring streamflow volumes and a reduction of summer and fall streamflow volumes over all time periods, relative to the baseline conditions (1961?C1990), for four of the five scenarios. Simulations of actual evapotranspiration and mean annual runoff showed a slight increase, which was attributed to warmer winters, resulting in more winter runoff and snowmelt events.     

未来气候变化对黄河流域水文过程的影响

使用NASA/NCAR有限区域大气环流模型FvGCM结果驱动高分辨率区域气候模式RegCM3 (20 km),进行1961～1990年当代气候模拟(控制试验)和2071～2100年IPCC A2排放情景下未来气候模拟(A2情景模拟试验)。将RegCM3径流模拟结果同大尺度汇流模型LRM [分辨率0.25°(纬度)×0.25°(经度)]相连接,模拟预估未来气候变化对我国黄河流域水文过程的影响。结果表明:相对于当代气候,未来黄河流域呈现气温升高、降水增加(夏季7～8月降水减少)和蒸发增大的趋势,且空间分布极不均匀,造成河川径流在5～10月减少,加剧流域夏季的水资源短缺;未来气温升高使得融雪径流增加,可能导致更早和更大的春季径流,使径流过程发生季节性迁移,引起黄河流域水资源年内分配发生变化。     

Recent fluctuations in precipitation and runoff over the Nile sub-basins and their impact on main Nile discharge

Substantial fluctuations in precipitation and runoff have occurred over the Nile Basin in recent decades. Ten-year mean flows of the Blue Nile (Khartoum gauge) during the 20th century have ranged from 42.2 to 56.7 km³ and for the White Nile (Malakal gauge) from 25.5 to 36.9 km³. These fluctuations have been responsible for changes in decade-mean Main Nile discharge of up to ± 20% which have had important consequences for water resource management in both Egypt and Sudan.This paper provides a review of the Nile Basin hydrology incorporating, for the first time, analyses of the relationships between precipitation and runoff fluctuations in the instrumental period for each of the eight major sub-basins within the Nile Basin. These sub-basins possess very different physical, climatic and hydrological characteristics. Over 90% of the Main Nile discharge originates from only four of the sub-basins; Lake Victoria, Blue Nile, Atbara, and the Sobat. Interbasin correlations of 40 y (1945-84) precipitation and runoff annual time series identify two broadly homogeneous regions; the Ethiopian highlands (Blue Nile and Atbara) and the Lake Victoria and Equatorial Lakes (White Nile). These regions posses contrasting precipitation regimes whose interannual variations are uncorrelated in time and which are therefore associated with different atmospheric circulation anomalies. The observed relationships between catchment precipitation and runoff, however, are not straightforward and the sensitivity of runoff to precipitation fluctuations varies from basin to basin. Some of the water resource management implications of these fluctuations for Egypt are discussed. With water demand in Egypt alone set to increase 17% by the year 2000, it is critical that the role of future climate change in Nile water management is thoroughly assessed based on a correct modelling of the diverse hydrological characteristics of the various Nile sub-basins.     

Historical Trend in River Ice Thickness and Coherence in Hydroclimatological Trends in Maine

We analyzed long-term records of ice thickness on the Piscataquis River in central Maine and air temperature in Maine to determine whether there were temporal trends that were associated with climate warming. The trend in ice thickness was compared and correlated with regional time series of winter air temperature, heating degree days (HDD), date of river ice-out, seasonal center-of-volume date (SCVD) (date on which half of the stream runoff volume during the period 1 Jan. to 31 May has occurred), water temperature, and lake ice-out date. All of these variables except lake ice-out date showed significant temporal trends during the 20th century. Average ice thickness around 28 February decreased by about 23 cm from 1912 to 2001. Over the period 1900 to 1999, winter air temperature increased by 1.7 °C and HDD decreased by about 7.5%.Final ice-out date on the Piscataquis River occurred earlier (advanced), by 0.21 days yr^–1 over the period 1931 to 2002, and the SCVD advancedby 0.11 days yr^–1 over the period 1903 to 2001. Ice thickness was significantly correlated (P-value <0.01) with winter air temperature, HDD, river ice-out, and SCVD. These systematic temporal trends in multiple hydrologic indicator variables indicate a coherent response to climate forcing.     

Temperature stratification of the atmospheric boundary layer over the Deccan Plateau,India, during the summer monsoon

The vertical and horizontal temperature structure of the atmospheric boundary layer (ABL) were studied using aircraft observations made in the lowest 2.4 km above ground level during the summer monsoon.The vertical temperature structure of the ABL in the region may be classified into the following four categories.Category The ABL consisted of two layers of thickness 700–900 m separated by a thin transition layer. The lapse rates in the former two layers were dry adiabatic.Category The lowest layer of the ABL of thickness 400–600 m was adiabatically stratified and the overlying layer was stable with gradients of potential temperature 4–5°C km^–1. The stable layer contained a thin adiabatic stratified layer of 200–300 m thickness at a height of 1.5 km.Category The lowest 200–400 m layer of the ABL was adiabatically stratified and the overlying layer was stable with potential temperature gradients of 5–6 °C km¹.Category The ABL was mainly stable with potential temperature gradients of 6 °C km^–1 or greater. Occasionally thin layers with adiabatic stratification were found embedded in the ABL.The temperature distribution of the horizontal temperature at 900 m was mainly normal. The high-frequency portion of the spectra lying between 0.05 and 0.16 Hz (corresponding to wave length 1 km to 300 m) oscillated around the –\2/3 power law line. The spectral curve showed a significant peak at 0.011 Hz having a wave-length of 5 km.Department of Geoscience, North Carolina State University, Raleigh, NC, 27650, U.S.A.     

Light hydrocarbons in the surface water of the mid-Atlantic

During a cruise of RV Polarstern over the Atlantic in September/October 1988, C₂–C₄ hydrocarbons were measured in surface sea water. The ship passed through three different ocean regions divided by divergences at 8° N and 3° S. Hydrocarbon concentrations differed considerably in these regions. The highest values were obtained for ethene with mean concentrations of 246 pMol/l between 35° N and 8° N, 165 pMol/l between 8° N and 3° S, and 63 pMol/l between 3° S and 30° S. Low values were found for i- and n-butane and acetylene between 32 pMol/l and 1 pMol/l. The alkene concentrations were in general higher than the concentrations of their saturated homologs. Concentrations decreased with increasing carbon numbers. The various alkenes were well correlated with one another as were the various alkanes. Oceanic emission rates of the light hydrocarbons were calculated from their sea water concentrations using an ocean atmosphere exchange model. The averaged fluxes ranged from about 10⁸ molec cm^-2 s^-1 for the alkenes and ethane to less than 10⁷ molec cm^-2 s^-1 for the C₄ alkanes. Acetylene emissions were below 3×10⁶ molec cm^-2 s^-1. Based upon these rates budget estimates of NMHC in the ocean surface layer were made with a simple model considering production and destruction processes in the water. The emissions to the atmosphere appear to be the dominant loss process between 35° N and 8° N, whereas destruction in the water seems to be dominant in the latitude ranges 8° N-3° S and 3° S-30° S.     

Projections of Climate Change Effects on Water Temperature Characteristics of Small Lakes in the Contiguous U.S.

To simulate effects of projected climate change on water temperature characteristics of small lakes in the contiguous U.S., a deterministic, one-dimensional year-round water temperature model is applied. In cold regions the model simulates ice and snow cover on a lake. The lake parameters required as model input are surface area, maximum depth, and Secchi depth as a measure of radiation attenuation and trophic state. The model is driven by daily weather data. Weather records from 209 stations in the contiguous U.S. for the period 1961–1979 were used to represent present climate conditions. The projected climate change owing to a doubling of atmospheric CO₂ was obtained from the output of the Canadian Climate Center General Circulation Model. The simulated water temperature and ice characteristics are related to the geometric and trophic state lake characteristics and to geographic location. By interpolation, the sensitivity of lake water temperature characteristics to latitude, longitude, lake geometry and trophic status can therefore be quantified for small lakes in the contiguous U.S. The 2× CO₂ climate scenario is projected to increase maximum and minimum lake surface temperatures by up to 5.2°C. (Maximum surface water temperatures in lakes near the northern and the southern border of the contiguous U.S. currently differ by up to 13°C.) Maximum temperature differences between lake surface and lake bottom are projected to increase in average by only 1 to 2°C after climate warming. The duration of seasonal summer stratification is projected to be up to 66 days longer under a 2×CO₂ climate scenario. Water temperatures of less than 8°C are projected to occur on lake bottoms during a period which is on the order of 50 days shorter under a 2×CO₂ climate scenario. With water temperature change projected to be as high as 5.2°C, ecological impacts such as shifts in species distributions and in fish habitat are most likely. Ice covers on lakes of northern regions would also be changed strongly.