Chapter 2

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Biological Sciences
Marc Cadotte

Chapter 2: The Physical Environment CLIMATIC VARIATION AND SALMON ABUNDANCE: A CASE STUDY  Hare and Francis hypothesized that the abrupt shifts in salmon production were associated with long term climatic variation in the North Pacific  Munta found good correspondence between the multi-decadal shifts in salmon production and changes in sea surface temperatures in the North Pacific INTRODUCTION  The physical environment is the ultimate determinant of where organisms live, the resources that are available to them, and the rate at which their populations can grow  The physical environment includes climate, which consists of long term trends in temperature, wind and precipitation  Radiation from the sun ultimately drives the climate system as well as biological E production  The chemical environment, which includes salinity, acidity, and concentrations of gases in the atmosphere and dissolved in water  Soil is an important component of the physical environment because it is where microorganisms, plants and animals live Concept 2.1 Climate is the Most Fundamental Component of the Physical Environment CLIMATE  Weather - the current temperature, humidity, precipitation, wind and cloud cover  Climate - is the long term description of weather at a given location, based on averages and variation measured over decades  Climatic variation includes the daily and seasonal cycles associated with changes in solar radiation as Earth rotates on its axis and orbits the sun - Can include changes over years or decades - Longer term climate change occurs as a result of changes in the intensity and distribution of solar radiation reaching the Earth‟s surface  Earth‟s climate is currently changing due to the changes in concentrations of gases as a result of human activity  Gases absorb and radiate E back to the Earth‟s surface, creating a greenhouse effect CLIMATE CONTROLS WHERE AND HOW ORGANISMS LIVE  Where organisms live and how they function are determined by climate  FW organisms are dependent on precipitation for the maintenance and quality of their habitats  Marine organisms depend on ocean currents that influence the temperature and chemistry of the water they liv in  The distributions of organisms often reflect extreme conditions more than average conditions because extreme events are important determinants of mortality  The physical environment must also be characterized by its variability over time, not just by average conditions, if we are to understand its ecological importance  The timing of changes in the physical environment is also ecologically important  The seasonality of rainfall, for example, is an important determinant of water availability for terrestrial organisms  Climate also influences the rates of abiotic processes that affect organisms - The rate at which rocks and soil are broken down to supply nutrients to plants and microorganisms is determined by climate  Climate can also influence the rates of periodic disturbances, such as fires, rockslides, and avalanches  These events kill organisms and disrupt biological communities, but subsequently create opportunities for the establishment and growth of new organisms and communities GLOBAL ENERGY BALANCE DRIVES THE CLIMATE SYSTEM  The E that drives the global climate system is ultimately derived from solar radiation  If Earth‟s temperature is to remain the same, these E gains from solar radiation must be offset by E losses  Much of the solar radiation absorbed by the Earth‟s surface is emitted to the atmosphere as infrared radiation  Earth‟s surface also loses E when water evaporates  Latent Heat Flux: the heat loss due to evaporation  E is also transferred through conduction and convection  Sensible Heat Flux - E transfer from the warm air immediately above Earth‟s surface to the cooler atmosphere by convection and conduction  Earth‟s surface actually releases more E than it receives by direct solar radiation  The atmosphere absorbs and reradiates much of the infrared radiation emitted by Earth  The atmosphere contains several gases called greenhouse gases that absorb and reradiate that infrared radiation - Carbon dioxide - Methane - Nitrous oxide  Without the greenhouse gases the Earth „s climate would be considerably cooler  Not every location on Earth receives the same amount of E from the sun Concept 2.2 Winds and Ocean Currents Result from Differences in Solar Radiation Across Earth‟s Surface ATMOSPHERIC AND OCEANIC CIRCULATION  When solar radiation heats Earth‟s surface, the surface warms and emits infrared radiation to the atmosphere, warming the air above it  The heating of Earth‟s surface varies with latitude, and it can also vary with topography  Such differential warming creates pockets of warm air surrounded by cooler air  Warm air is less dense than cool air, so as long as a pocket of air remains warmer than the surrounding air, it will rise (uplift)  Atmospheric Pressure: results from the force exerted on a packet of air (or on Earth‟s surface) by the air molecules above it, so it decreases with increasing altitude  As warm air rises higher, it expands  Cool air cannot hold as much water vapor as warm air, so as the air continues to rise and cool, the water vapor contained within it begins to condense into droplets and form clouds  When there is a substantial heating of the Earth‟s surface and a progressively cooler atmosphere above the surface, the uplifted air will form clouds with wedge-shaped tops  The clouds reach to the boundary between the troposphere (cool) and stratosphere (warm)  The air pockets ceases to rise once it reaches the warmer temperatures of the stratosphere  The tropics receive the most precipitation of any area on Earth because they receive the most solar radiation and thus experience the greatest amount of surface heating, uplift of air, and cloud formation  The uplift of air in the tropics creates a band of low atmospheric pressure relative to zones to the north and south  The pole-ward moving air cools as it exchanges heat with the surrounding air and meets cooler air moving from the poles to the equator  Once the air reaches a temperature similar to that of the surrounding atmosphere, it descends towards the Earth‟s surface, a process known as subsidence  This subsidence of air creates regions of high atmospheric pressure at latitudes 30° N and S, which inhibits the formation of clouds (major deserts are found at these latitudes)  Hadley Cell - The tropical uplift of air creates a large scale pattern of atmospheric circulation in each hemisphere  Polar Cell – occurs at the N and S poles  Cold, dense air subsides at the poles and moves toward the equator when it reaches the Earth‟s surface  The descending air at the poles is replaced by air moving through the upper atmosphere from lower altitudes  Subsidence at the poles creates an area of high pressure, so the polar regions, despite the abundance of ice and snow on the ground, actually receive little precipitation, and are known as “polar deserts”  Ferrell Cell – exists at mid-latitudes between the Hadley and polar cells  The Ferrell cell is driven by the movement of the Hadley and polar cells and by the exchange of E between tropical and polar air masses in a region known as the polar front  Major climatic zones: 1. Tropical Zone: between 30° N and S 2. Temperate Zones: between 30° and 60° N and S 3. Polar Zones: above 60° N and S ATMOSPHERIC CIRCULATION CELLS CREATE SURFACE WIND PATTERNS  Winds flow from areas of high pressure to areas of low pressure  Atmospheric circulation cells give rise to consistent patterns of air movement at Earth‟s surface, known as prevailing winds  Coriolis Effect – the prevailing winds appear to be deflected to the right (clockwise) in the Northern Hemisphere - Left (counterclockwise) in the Southern Hemisphere - This apparent deflection is associated with the rotation of Earth on its axis - However there is no apparent deflection in the direction of the wind  As a result of the Coriolis effect, surface winds blowing toward the equator from the high pressure zones at 30° N and S appear to be deflected to the W  trade winds  Winds blowing toward the poles from the zones of high pressure, called westerlies are deflected to the E  Water has a higher heat capacity than land; water can absorb and store more E without its temperature changing than land can  Seasonal temperature changes over the oceans are smaller and less extreme than those on land  In summer, air over the oceans is cooler and denser than that over land, in the winter, the opposite situation exists  Seasonal shifts in pressure cells influence the direction of prevailing winds  Semi-permanent pressure cells is more pronounced in the Northern Hemisphere than in the Southern Hemisphere OCEAN CURRENTS ARE DRIVEN BY SURFACE WINDS  Wind moving across the ocean surface creates a frictional drag that moves the surface water  As a result of the Coriolis effect, the water appears to move at an angle to the wind  Like atmospheric circulation, oceanic circulation has a vertical dimension  The surface and deep layers of ocean water do not mix because of differences in their temperature and salinity  Surface waters are warmer and less saline and therefore is less dense than deeper, cooler waters  Upwelling – where deep ocean water rises to the surface  Upwelling occurs where prevailing winds blow nearly parallel to a coastline  The force of the wind, in combination with the Coriolis effect, causes surface waters to flow away from the coast, and deeper, colder ocean water rises to replace them  As a result of the Coriolis effect, water just to the N and S of the equator is deflected slightly away from the equator, causing divergence of surface water and a zone of upwelling  Upwelling has important consequences for the local climate, creating a cooler, moister environment  Also has a strong effect on biological activity in the surface waters - Dead organisms in the surface waters fall into deeper water - Nutrient thus accumulate in deep water, upwelling brings these nutrients back to the photic zone (the layer of surface water where there is enough light to support photosynthesis)  Upwelling zones are among the most productive open ocean ecosystems because these nutrients increase the growth of phytoplankton which provide food for zooplankton, which in turn support the growth of their consumers  Ocean currents influence the climates of the regions where they flow  Ocean currents are responsible for about 40% of the heat exchanged between the tropics and the polar regions; the remaining 60% is transferred by winds  Ocean currents are sometimes referred to as the “heat pumps” or “thermal conveyers” of the planet  A large system of interconnected currents that links the Pacific, Indian and Atlantic oceans, sometimes called the “great ocean conveyer belt”, is an important means of transferring heat to the polar regions Concept 2.3 Large-scale Atmospheric and Oceanic Circulation Patterns Establish Global Patterns of Temperature and Precipitation GLOBAL CLIMATIC PATTERNS  Climatic averages and climatic variation are influenced by prevailing winds and ocean currents OCEANIC CIRCULATION AND THE DISTRIBUTION AND TOPOGRAPHY OF CONTINENTS INFLUENCE GLOBAL TEMPERATURES  Temperatures at Earth‟s surface becomes progressively cooler from the equator toward the poles  The changes in temperature are not exactly parallel with the changes in latitude  3 major influences alter the global pattern: 1) Ocean currents 2) Distribution of land and water 3) Elevation  Air temperatures over land show greater seasonal variation, with warmer temperatures in summer and colder temperatures in winter, than those over the oceans  Elevation above sea level has an important influence on continental temperatures  2 factors contribute to the colder climates found at higher elevations 1) Atmospheric pressure, the density of air decreases with increasing elevation o Fewer air molecules to absorb the infrared E radiating from the Earth‟s surface 2) Highlands exchange air more effectively with cooler air in the surrounding atmosphere o The temperature of the atmosphere decreases with increasing distance from the ground o Wind velocity increases with increasing elevation because there is less friction with the ground surface o The decrease in air temperature with increasing elevation tends to follow the lapse rate  Lapse Rate – decrease in temperature with increasing height above the surface PATTERNS OF ATMOSPHERIC PRESSURE AND TOPOGRAPHY INFLUENCE PRECIPITATION  The locations of the Hadley, Ferrell and polar circulation cells suggest that precipitation should be highest in the tropical latitudes between 23.5° N and S and in a band at about 60° N and S and lowest in zones around 30° N and S  The African continent displays the pattern closest to this idealized precipitation distribution  However there are substantial deviations from the expected latitudinal precipitation patter in other areas  Deviations are associated with the semi-permanent high-pressure and low-pressure cells as well as with large mountain chains  Pressure cells influence the movement of moist air from oceans to continents as well as cloud formation  Mountains also influence precipitation patterns by forcing air moving across them to rise, which enhances local precipitation Concept 2.4 Regional Climates Reflect the Influence of the Distribution of Oceans and Continents, Mountains, and Vegetation REGIONAL CLIMATIC INFLUENCES  Climatic differences result from the effects of oceans and continents on regional E balance and the influence of mountain on air flow and temperature  Vegetation often reflects these regional climatic differences  Vegetation also has important feedbacks to the climate through its influence on E and water balance PROXIMITY TO OCEANS INFLUENCES REGIONAL CLIMATES  Seasonal temperature changes are smaller over oceans than over continental areas  Oceans provide a source of moisture for clouds and precipitations  Maritime Climate – coastal terrestrial regions that are influenced by an adjacent ocean  Continental Climate – areas in the middle of large continental land masses  The maximum and minimum temperatures occur slightly later in the year in the maritime climate, another reflection of the high heat capacity of the ocean and its effect on local climate MOUNTAINS INFLUENCE WIND PATTERNS AND GRADIENTS IN TEMPERATURE AND PRECIPITATION  The effects of mountains on climate are visually apparent in the elevational patterns of vegetation, particularly in arid regions  Abrupt shifts in vegetation patterns reflect the rapid changes in climate that occur over short distances in mountains as temperatures decrease, precipitation increases, and wind speed increases with elevation  When air moves across Earth‟s surface and encounters a mountain range, the slopes of the mountains force it upward  This uplifted air cools as it rises, and water vapor condenses to form clouds and precipitations  As a result, the amount of precipitations increases with elevation  The loss of moisture, as well as the warming of the air as it moves down the eastern slopes, dries the air mass  This rain-shadow effect results in lower precipitations and soil moisture on the slopes facing away from the prevailing wind and higher precipitation and soil moisture on the windward slope  The rain-shadow effect influences the types and amount of vegetation on mountain ranges  Mountains can also generate local wind and precipitation patterns  Differences in the direction that mountain slopes face can cause differences in the amounts of solar radiation the slopes and surrounding flatlands receive  Depending on the moisture content of the air and the prevailing winds at higher elevations, clouds may form on the eastern flanks of the mountains  These clouds can generate local thunderstorms that may move off the mountains and into surrounding lowlands, increasing local precipitation  At night, the ground surface cools, and the air above it becomes denser  Nighttime cooling is more pronounced at high elevations because the thinner atmosphere absorbs and
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