Lecture notes for Lecture 2. This material is Chapters 2 and 3 in Cain et al.
The Physical Environment (chapter 2)
- The case study for this chapter is about the Pacific Decadal Oscillation (PDO), a fairly recently
discovered climate pattern that occurs over several years.
- It was discovered when scientists looked at patterns in the number of fish caught in the North Pacific.
They saw that for many years (~15-20 years) in a row the catch would be low. Then it would suddenly
increase and again be relatively stable for another long period.
- Interestingly, when catch was high in one place (Alaska), it would be low in another place (Washington),
suggesting that this pattern was occurring over a large area.
- Eventually this pattern in fish was linked to a multi-decadal (more than 10 year period, average 18 year)
shift in the temperature of the sea surface of the North Pacific.
- We are currently in a phase of the PDO that is cool, and some scientists think this is lowering global
temperatures and disguising (hiding) the effects of global warming.
- We take away from this case study the idea that even basic information about the physical
environment is still being learned.
- The chapter then focuses on the climate. The climate is a long-term description of the weather at any
one location.
- The textbook makes the important point that it is not necessarily the average climate at a particular
place but also the extremes that matter. Because the extremes – very hot or very cold and freezing
temperatures, or many days without rain – may kill many organisms.
- The most important determinate of climate is irradiance from the sun. Much of the energy of the sun
(49%) is absorbed by the earth. The rest is reflected back away from the earth, or absorbed by the
atmosphere.
- The earth then loses that energy by reemiting it mostly as infrared radiation (“radiant heat” … think of
a road on a hot day). Some energy is lost due to convection (heat lost to air by wind). See figure on Slide
7.
- Much of this outgoing energy is then reradiated back to the earth by the layer of gases (carbon dioxide,
water vapor, methane and nitrous oxide) in the atmosphere. We are now adding to these “greenhouse
gases” which means we are trapping more warm air near the earth.
- The difference between the tropics and the temperate areas of the world is created by the sun’s
energy hitting directly (perpendicularly) at the tropics, but spreading the same amount of energy over a
larger area towards the poles. See figure on Slide 8.
- This high amount of heat rises above the tropics producing a zone of low atmospheric pressure
(atmospheric pressure is the force exerted on air by the molecules above it; here air is pushing up and
hence lowering this pressure). It flows outwards towards the poles and at about 30 degrees latitude this
air has cooled and comes back towards earth, creating high pressure zones. This movement of air is
called a Hadley atmospheric circulation cell.
- These Hadley cells influence precipitation, which is high in the tropics, but very low near ~30 degrees
latitude. Most of the world’s deserts are near 30 degrees latitude. Other cells closer to the poles also
exist, and the result is that there are bands of high precipitation or low precipitation at different
latitudes (slide 9).
- Air moves between high pressure zones and low pressure ones, creating winds. The rotation of the
earth makes these wind patterns curved in the way depicted on slide 10. These winds are predictable in
their directions (e.g. “trade winds”, “westerlies”) and knowledge of winds was used for many years
when much international travel was by sailboats.
- Winds in turn drive ocean currents. Ocean currents are particularly important because they bring warm
or cold water a long distance and hence warm or cool the landmasses they are near. For example, the
Gulf Current that runs along North America’s Atlantic coast and goes north, eventually ending in
Northern Europe, brings warm climates to high latitude areas like England. Whereas a cool current runs
south on the Pacific coast, bringing cold waters from Alaska and making cities at low latitudes like San
Diego relatively cool.
- In some places the prevailing winds run parallel to the coast, and this sort of circulation drives water
from the bottom of the ocean upwards, called an “upwelling”. Because nutrients accumulate in deep
ocean waters (falling from above), upwelling brings nutrient-rich water to the surface and provides a
rich environment for life. Some of the world’s greatest fishing areas are in regions of upwelling.
- So climate is influenced by latitude and ocean climates. It is also influenced by elevation. Remember
that infrared radiation is coming from the ground, so at higher distances from the ground there is less
energy. Air near mountains mixes directly with this cold air, and also as one gets further from the earth
the air has fewer gas molecules that can absorb heat. These are reasons why temperature declines as
elevation increases.
- There are also regional influences on climate produced by the positions of mountains, or the distance
to the ocean. Water retains heat well and stabilizes temperatures so that day and night
temperatures are relatively close together near the ocean, whereas areas in the interior of continents
can get very cold at night and very hot during the day. Mountains block clouds, meaning that rain all
falls to one side of them and the other side is very dry (the mountain’s “rain-shadow”).
- The vegetation at a particular place also influences the weather. Summarizing a rather complex figure
on slide 16, we see forest (dark in color) absorbs more heat, and reflects back less, than grassland (light
in color), keeping those areas warmer. But this effect is outweighed by the fact that forest has a lot of
heat lost due to evaporation from plants (known as evapotranspiration; this also creates clouds that
then precipitate). Hence, deforestation -- the loss of forest -- creates a warmer, drier climate.
- We then turn to what controls the variation in the climate over time. First we discuss seasonal
variation. The earth is tilted on its rotation so that irradiance at high latitudes changes seasonally
because the sun is either directly shining on it, or away from it. See Figure on page 17, and convince
yourself that this occurs by using a ball as a model of earth and moving it around a flashlight. During the
summer the sun never sets at the poles.
- As said before, water retains heat better than land, and it does not get as cold during the winter as the
land does. Water is stratified, with warm layers of water (nearer the sun) sitting above cold layers. In a
lake, for example, the cold deep part of the waters will not circulate with the top parts during the
summer. Dead organisms will sink into this lower layer of water making it resource rich. In the fall the
surface water will become less warm and will begin to sink, mixing the surface and lower waters. This
“turnover” event is important because the nutrients trapped in the bottom of the lake in the summer
are redistributed throughout the lake.
- Beyond seasonal variation, climate also changes over several years. The best known example of this is
the El Niño oscillation. El Niño occurs every three to six years and alternates with La Niño, which has the
opposite characteristics. The high pressure cells change in their location in the south Pacific, west of
South America. This disrupts the normal trade winds and increases surface temperatures of the ocean
and decreases upwelling. The most apparent symptom of El Niño is poor fish numbers from the lack of
upwelling, but other effects are seen as far away as Indonesia, where fires may be more frequent and
mass-fruiting trees may use the weather changes as a cue to start flowering. PDO is another oscillation
that we don’t understand as well as El Niño.
- And finally there is the long-scale of time variation. For example in the 10,000 year scale, temperatures
go up and down leading to glaciation events. The most recent glacial maximum was 18,000 years ago.
And then in the millions of years there have been regular oscillations in temperature. See Slide 21.
- What causes these long time-scale variations in climate? As we talked about earlier, earth’s axis is
tilted, and the angle of this tilt changes between 24.5 degrees and 22 degrees every 41,000 years. Over
a period of 100,000 years earth’s axis changes in shape from elliptical to more circular. And Earth’s
orientation relative to other celestrial bodies (e.g. stars) also changes. All of these changes, collectively
known as Milankovitch cycles after the scientist who discovered them, probably produce the oscillations
in climate that we’ve discussed.
- Factors other than climate also influence life on earth. We discuss three briefly: salinity, acidity and
oxygen.
- Water varies highly in the amount of solute, particularly salt, dissolved in it. Most freshwater has very
little salt, but some lakes without outlets (e.g. Dead Sea, Great Salt Lake) accumulate salt that enters
from dissolving out of rock and concentrates due to evaporation. Salinity is also a big problem for
farming as it kills plants. Areas that are at risk for salinity include areas near the sea and also hot regions
where irrigation is used a lot. Irrigation sometimes is pumped up from underground water reserves,
bringing with it salts, and high evaporation rates concentrate the salt in soils.
- Acidity is measured by pH, a log scale that measures how many H+ ions are present in the water, with a
neutral rating of 0 (distilled water), acidic readings below this and base readings above this. pH is very
important to living organisms as must organism’s enzymes only work under a limited range of pH. The
pH of freshwater can vary quite strongly; in contrast the huge amounts of water in the oceans mean that
it is quite constant there. However, there is concern that CO2, building up due to human emissions,
could change the acidity of the oceans: CO2 makes water acidic (think of a carbonated beverage). Corals
are known to be very sensitive to acidity of the oceans, and these changes in CO2 levels may be related
to mass die offs of corals known as coral bleaching.
- Oxygen doesn’t vary much today in the atmosphere, but did historically. In the ancient earth, oxygen
was very rare… it built up over billions of years due to the activity of plants. Today there are a few
ecosystems like deep sea trenches or soil in peat bogs that are low in oxygen and special organisms that
thrive in anoxic conditions are plentiful there.
Chapter 3 of the textbook concentrates on the different biomes found on earth.
- The book defines a biome as a “large-scale biological community shaped by the physical
environment”.
- The basic terrestrial (land-based) biomes the book describes have different combinations of
temperature and precipitation. See figure on Slide 28.
- Biomes are characterized by the dominant life forms of plants (for example, broad leaf deciduous
trees; or grasses and sedges). This is because plants can’t move (as adult individuals) and are therefore
particularly sensitive to climate.
- Note the climate diagrams on the biome slides and what information they give (principally
temperature and precipitation).
- We then toured the primary terrestrial biomes:
- Tropical rainforests. High temperatures and a lot of precipitation all year round. Huge
diversity: perhaps 50% of earth’s species in 11% of terrestrial vegetation cover. Poor soils
because of high decomposition. Threatened by logging and deforestation.
- Topical seasonal forests. High temperature; seasonal precipitation. Characterized by
deciduous trees, open forests, often with regular fires. One of the most threatened biomes
because of heavy population pressure in these areas.
- Deserts. Hot and little precipitation. Often in the 30 degree N and S latitude. May bloom for
short wet periods. Deserts may expand in areas where humans have overused (unsustainable
grazing by livestock).
- Temperate grasslands. Warm moist summers and cold dry winters. Grassland may be
maintained by fires of grazing animals. Threatened because of usefulness of areas for
agriculture.
- Temperate shrublands and woodlands. Cool wet winter and hot dry summers lead to limited
growth of plants. Often called Mediterrean climates, can be highly diverse, and highly
threatened because of human population and development.
- Temperate deciduous forests. More rainfall than temperate grasslands. Trees lose leaves
seasonally. Soild are rich because of leaf loss and lower rates of decomposition. Historically a
lot of agriculture, recently beginning to be reforested.
- Temperature evergreen forests. Areas of high rainfall and dominated by coniferous trees.
Sometimes called temperate rainforests. Threatened by logging and development.
- Boreal forests. Generally above 50 degrees latitude where soils freeze in winter. Waterlogged soils because of permafrost (layer of soil frozen continuously)and abundant
precipitation. Mostly coniferous forests with low diversity. Historically untouched but recently
have begun to be logged.
- Tundra. Generally above 65 degrees latitude, where trees can no longer exist because of cold
temperatures. Quite dry, blooms in short warm period.
- The book stresses that habitats in mountainous areas are quite different from what surrounds them
because of the temperature decline as one climbs (1000 m up, - 6.5 C). High alpine zones resemble
tundra, and coniferous forests resembling boreal forests are found at somewhat lower elevations.
Hence there are dramatic changes in vegetation in relatively short distances.
- There are also aquatic biomes, which the book summarizes quickly.
- Freshwater systems consist of rivers and streams and lakes.
- Rivers and streams are classified by how close they are to the headwaters (where they start). Large
differences are seen due to the speed of the water movement (greater speed gives more dissolved
oxygen in the water). Rivers have been dramatically disturbed by humans through pollution and
damming.
- In lakes, large differences are seen between the area near the surfaces, where most life exists, and
the lower layers, generally with little light penetration or light, and the bottom substrate. They are
threatened by pollution and irrigation, which subverts their water.
- Oceanic systems can be divided into three zones: near-shore, shallow oceans, and pelagic areas.
- Of near-shore biomes, some of the most familiar include mangroves (generally found in tropical
areas), and saltwater marches (generally found in temperate areas). These areas as well as estuaries
(where freshwater meets saltwater) can be highly productive, as measured by the amount of biomass
produced per year.
- The shore area itself is characterized by tidal changes, driven by the gravitational forces between the
Earth, the moon and the sun. Rocky intertidal zones show very distinct communities depending on
whether they are relative to the tidal maximum and minimum. Sandy shores are less biodiverse as
there is little for organisms to hold onto.
- In shallow seas the primary biomes are coral reefs (most often found in the tropics) and kelp or
seagrass beds (mostly temperate). Coral reefs are the rainforests of the seas, with immense
biodiversity. They are highly threatened by overharvesting and climate change. Kelp beds are also very
productive in terms of their biomass.
- The open oceans (pelagic biome) has a thin layer of life between 0 and 200 m where light penetrates.
Photosynthetic plankton near the surface are the foundation of the oceanic food chain. The bottom of
the oceans would seem a very harsh environment with low temperatures and high pressures, but we
are finding through the use of submersibles more biodiversity there than we expected.