Extreme events,trends, and variability in Northern Hemisphere lake-ice phenology (1855–2005)

Often extreme events, more than changes in mean conditions, have the greatest impact on the environment and human well-being. Here we examine changes in the occurrence of extremes in the timing of the annual formation and disappearance of lake ice in the Northern Hemisphere. Both changes in the mean condition and in variability around the mean condition can alter the probability of extreme events. Using long-term ice phenology data covering two periods 1855–6 to 2004–5 and 1905–6 to 2004–5 for a total of 75 lakes, we examined patterns in long-term trends and variability in the context of understanding the occurrence of extreme events. We also examined patterns in trends for a 30-year subset (1975–6 to 2004–5) of the 100-year data set. Trends for ice variables in the recent 30-year period were steeper than those in the 100- and 150-year periods, and trends in the 150-year period were steeper than in the 100-year period. Ranges of rates of change (days per decade) among time periods based on linear regression were 0.3−1.6 later for freeze, 0.5−1.9 earlier for breakup, and 0.7−4.3 shorter for duration. Mostly, standard deviation did not change, or it decreased in the 150-year and 100-year periods. During the recent 50-year period, standard deviation calculated in 10-year windows increased for all ice measures. For the 150-year and 100-year periods changes in the mean ice dates rather than changes in variability most strongly influenced the significant increases in the frequency of extreme lake ice events associated with warmer conditions and decreases in the frequency of extreme events associated with cooler conditions.     

Gender and occupational perspectives on adaptation to climate extremes in the Afram Plains of Ghana

Although sub-Saharan Africa does not contribute significantly to greenhouse gas emissions, significant adverse impacts of climate change are anticipated in this region. Countries in West Africa, which are heavily dependent on rain-fed agriculture, are projected to experience more frequent and intense droughts, altered rainfall patterns and increases in temperature through the end of this century. Changes in hydrology and temperature are likely to affect crop yields, thereby placing pressure on scarce resources in a region that is characterised by limited social, political, technical and financial resources. The success with which communities cope with the impacts of climate change is influenced by existing conditions, forces and characteristics which are peculiar to each of these communities. This paper assesses the preferred adaptation strategies during floods and droughts of males and females in three different occupations (farming, fishing, and charcoal production). Findings are based upon an analysis of focus group discussions and a ranking of preferred adaptation options in three communities in the Afram Plains of Ghana. Assessments of this nature should aid in the selection and implementation of adaptation options for communities and households, which is the level at which climate change adaptation is likely to occur in West Africa.     

天文光度函数

至少从托勒玫时代以来，天体的计数已成为天文学的一部分。光度函数是作为某一波段的光度或亮度的函数的特定种类的天体数目。而视亮度的相应概念通常叫作计数。虽有然点夸张，但这些工具仍是检验天体物理假设和检验天体物理新概念的强有力的手段。     

Origin of the valley networks on Mars: a hydrological perspective

The geomorphology of the martian valley networks is examined from a hydrological perspective for the compatibility with an origin by rainfall, globally higher heat flow, and localized hydrothermal systems. Comparison of morphology and spatial distribution of valleys on geologic surfaces with terrestrial fluvial valleys suggests that most martian valleys are probably not indicative of a rainfall origin, nor are they indicative of formation by an early global uniformly higher heat flow. In general, valleys are not uniformly distributed within geologic surface materials as are terrestrial fluvial valleys. Valleys tend to form either as isolated systems or in clusters on a geologic surface unit leaving large expanses of the unit virtually untouched by erosion. With the exception of fluvial valleys on some volcanoes, most martian valleys exhibit a sapping morphology and do not appear to have formed along with those that exhibit runoff morphology. In contrast, terrestrial sapping valleys form from and along with runoff valleys. The isolated or clustered distribution of valleys suggests localized water sources were important in drainage development. Persistent groundwater outflow driven by localized, but vigorous hydrothermal circulation associated with magmatism, volcanism, impacts, or tectonism is, however, consistent with valley morphology and distribution. Snowfall from sublimating ice-covered lakes or seas may have provided an atmospheric source of water for the formation of some valleys in regions where the surface is easily eroded and where localized geothermal/hydrothermal activity is sufficient to melt accumulated snowpacks.     

Light attenuation and submersed macrophyte distribution in the tidal Potomac River and estuary

Changing light availability may be responsible for the discontinuous distribution of submersed aquatic macrophytes in the freshwater tidal Potomac River. During the 1985–1986 growing seasons, light attenuation and chlorophylla and suspended particulate material concentrations were measured in an unvegetated reach (B) and in two adjacent vegetated reaches (A and C). Light attenuation in reach B (the lower, fresh to oligohaline tidal river) was greater than that in reach A (the recently revegetated, upper, freshwater tidal river) in both years. Reach B light attenuation was greater than that in reach C (the vegetated, oligohaline to mesohaline transition zone of the Potomac Estuary) in 1985 and similar to that in reach C in 1986. In reach B, 5% of total below-surface light penetrated only an average of 1.3 m in 1985 and 1.0m in 1986, compared with 1.9 m and 1.4 m in reach A in 1985 and 1986, respectively. Water column chlorophylla concentration controlled light availability in reaches A and B in 1985, whereas both chlorophylla and suspended particulate material concentrations were highly correlated with attenuation in both reaches in 1986. Reach C light attenuation was correlated with suspended particulate material in 1986. The relationship between attenuation coefficient and Secchi depth was K_PAR=1.38/Secchi depth. The spectral distribution of light at 1 m was shifted toward the red portion of the visible spectrum compared to surface light. Blue light was virtually absent at 1.0 m in reach B during July and August 1986. Tidal range is probably an important factor in determining light availability for submersed macrophyte propagule survival at the sediment-water interface in this shallow turbid system.     

A benthic index of environmental condition of Gulf of Mexico estuaries

An index was developed for estuarine macrobenthos in the Gulf of Mexico that discriminated between areas with degraded environmental conditions and areas with undegraded or reference conditions. Test sites were identified as degraded or reference based on criteria for dissolved oxygen levels, sediment toxicity tests, and sediment contamination. Discriminant analysis was used to identify a suite of measures of benthic community composition and diversity that would most successfully distinguish degraded from undegraded sites. The resultant benthic index was composed of a linear combination of three factors: the Shannon-Wiener diversity index, the proportion of total benthic abundance as tubificid oligochaetes, and the proportion of total benthic abundance as bivalve molluscs. This index was used to evaluate the spatial patterns of degraded benthic resources in the Gulf of Mexico.     

Role of weather and water quality in population dynamics of submersed macrophytes in the tidal Potomac River

Weather and water-quality data from 1980 to 1989 were correlated with fluctuations in submersed macrophyte populations in the tidal Potomac River near Washington, D.C., to elucidate causal relationships and explain population dynamics. Both reaches were unvegetated in 1980 when mean growing-season Secchi depths were <0.60 m. Macrophyte resurgence in the upper tidal river in 1983 was associated with a growing-season Secchi depth of 0.86 m, total suspended solids (TSS) of 17.7 mg l^?1, chlorophyll a concentrations of 15.2 μg l^?1, significantly higher than average percent available sunshine, and significantly lower than average wind speed. From 1983 to 1989, mean seasonal Secchi depths <0.65 m were associated with decrease in plant coverage and mean seasonal Secchi depths >0.65 were associated with increases in plant coverage. Changes in mean seasonal Secchi depth were related to changes in mean seasonal TSS and chlorophyll a concentration; mean Secchi depths >0.65 generally occur when seasonal mean TSS is <19 mg l^?1 and seasonal mean chlorophyll a concentration is ≤15 μg l^?1. Secchi depth is highly correlated with plant growth in the upper tidal river and chlorophyll a and TSS with plant growth in the lower tidal river. Wind speed is an important influence on plant growth in both reaches.     

Parallel, similar and constrained folds

Theoretical analysis of folding of viscous multilayers with free slip or bonding at layer contacts indicates that folds in such multilayers can be described in terms of three end-members:parallel, in which orthogonal thicknesses of layers are largely constant;similar, in which vertical thicknesses of layers and shapes of successive interfaces are essentially constant; andconstrained, in which amplitudes of anticlines and synclines decrease to zero at upper and lower boundaries. Constrained,internal folds form if the multilayer is confined by rigid media; parallel,concentric-like folds form if the multilayer is confined by soft media, provided soft interbeds are sufficiently thin for the stiff layers to fold as an ensemble. Similar,sinusoidal orchevron folds form throughout much of the thickness of a multilayer, for any stiffness of confining media, provided wavelengths of folds are short relative to the thickness of the multilayer or soft interbeds are sufficiently soft and thick for the stiff layers to act independently. The analysis shows that multilayer folds may have the same form regardless of whether the layer contacts are freely slipping or bonded.

The forms of folds in multilayers confined by media with different viscosities above and below depend on the viscosity contrast of the media. For no medium above and a rigid medium below, the forms are concentric-like in the upper part and internal in the lower part of the multilayer. For no medium above and a soft medium below, the folds are concentric-like throughout the multilayer.

The theory indicates that a useful way to analyze forms of folds in rocks or in experiments is in terms of component waveforms, as defined, for example, by Fourier series. The distributions of amplitudes of component waveforms throughout the multilayer appears to be diagnostic, reflecting contrasts in properties of the multilayer and its confining media. Analysis of a large fold in the central Appalachians, Pennsylvania, and of a smaller fold in the Huasna syncline, California, indicates that at least three component waveforms are required to produce the gross forms of those folds.

The theory closely predicts wavelengths and shapes of folds produced in analogous elastic multilayers, indicating that nonlinearities in material behavior, which are inherent in the elastic material but are absent in the viscous material, are less significant than nonlinearities in the boundary conditions, which are the same in elastic and viscous materials.     

Resurgence of submersed aquatic macrophytes in the tidal Potomac River,Maryland, Virginia,and the District of Columbia

A 1978–81 survey of submersed aquatic macrophytes in the tidal Potomac River showed that there were virtually no plants in the freshwater tidal river between Chain Bridge and Quantico, Virginia, decades after the disappearance of plants in the late 1930’s. Plant populations were monitored in subsequent years (1983–85) using qualitative shoreline surveys and quantitative resampling of the original 1978–81 transects. In 1983, 12 species of submersed aquatic macrophytes were found in the tidal river. Population increases were dramatic; by fall 1985, plants had colonized all shallow areas between Alexandria and Gunston Cove, Virginia.Hydrilla verticillata dominated in Dyke Marsh-Hunting Creek and Swan Creek. Most other areas contained a variable mixture ofHeteranthera dubia, Myriophyllum spicatum, Ceratophyllum demersum, Vallisneria americana, Najas guadalupensis andHydrilla verticillata. No plants were found along the main river or in tidal embayments in the reach between Gunston Cove and Quantico, Virginia. Total dry weight collected in the upper tidal river in fall 1985 was 14.5 times that of spring 1985, and four times that of fall 1984.