Earth's Climate Part 1 Notes. Notes below are the things I highlighted in my book.

The syllabus says part 1 will take week 2 and 3 to complete. I completed the second half of chapter 2 and the entire chapter 3 for week 3.

Chapter 2 second half

2.8 Deep-Ocean Circulation

• The poleward flow of warm water that counters some of Earth’s heat imbalance occurs above the thermocline, a zone of rapid temperature change between warm upper layers and cold water filling the deeper ocean basins.
  • two thermoclines exist: a deeper permanent portion that is maintained throughout the year and  a shallower portion that changes as a result of seasonal heating by the Sun
• This overturning circulation is called the thermohaline flow and refers to temperature and salinity.
• The density of ocean water can be increased at lower latitudes when the atmosphere evaporates freshwater as water vapor, leaving the remaining water saltier (denser). 
• Another way to increase the density of ocean water is by cooling it.
• Most of the deepest ocean of the world is filled by water delivered from just two regions, the high latitude North Atlantic Ocean and the Southern Ocean, near Antarctica
  • North Atlantic deep water and Antarctic bottom water
• The oldest and slowest-moving deep water is found in the Pacific Ocean, while the Atlantic has younger, faster-moving water.
• upwelling, rapid upward movement of subsurface water from intermediate depths, occurs in two other kinds of ocean regions.
  • When surface winds in the Northern Hemisphere blow parallel to coastlines along the path. They push water away from the land. To replace surface water pushed offshore, water rises from below. The upwelling water is cooler than the nearby surface water that has remained at the surface and has been warmed by the Sun.
  • A second kind of upwelling occurs along the equator, especially in the eastern end of ocean basins. Trade winds push surface waters away from the equator. Warm surface water is driven northward in the Northern Hemisphere and southward in the Southern Hemisphere by the opposing Coriolis deflections north and south of the equator. The movement of warm water away from the equator causes upwelling of cooler water from below.

2.9 Sea Ice

• typical seawater resists freezing until it is cooled to 21.98C. As sea ice forms, it rejects almost all the salt in the seawater. Because sea ice is less dense than seawater, it floats on top of the salty ocean.
• When sea ice forms, it seals off the underlying ocean from interaction with the atmosphere. This change is vital to regional climates. Without an ice cover, high-latitude oceans transfer large amounts of heat to the atmosphere, especially in winter, when air temperatures are low. This heat transfer keeps temperatures in the lower atmosphere close to those of the ocean surface (near 0C).
• The formation and melting of sea ice are driven mainly by seasonal changes in solar heating.
• Seasonal extremes in sea-ice cover lag well behind the seasonal extremes of heating by solar radiation. The maximum extent of sea ice is usually reached in the spring, the minimum extent in the autumn.

2.10 Glacial Ice

• Glacier ice occurs mainly on land, in two forms.
  • Mountain glaciers are found in mountain valleys at high elevations. mountain glaciers near the equator are restricted to elevations above 5 kilometers. 
  • Continental ice sheets are a much larger form of glacier ice, typically hundreds to thousands of kilometers in horizontal extent and 1 to 4 kilometers in thickness. 
    • The highest regions on the ice sheets are rounded ice domes, with the elevations sloping gently away in all directions. Domes may be connected by high broad ridges with gentle sags called ice saddles. On the sides of ice sheets, ice flows in fast-moving ice streams from which ice lobes protrude beyond the general ice margins.
• Climate scientists analyze the conditions over present glaciers and ice sheets in terms of their mass balance, the average rate at which ice either grows or shrinks every year.
• mass balances are positive at upper elevations, where accumulation of snow and ice dominates, but negative at lower elevations, where rapid ablation (loss of ice) occurs.
• Carbon moves among and resides in several major reservoirs. The amount of carbon in each reservoir is typically quantified in gigatons of carbon
• Relatively small amounts of carbon reside in the atmosphere, the surface ocean, and vegetation; a slightly larger reservoir resides in soils, a much larger reservoir in the deep ocean, and a huge reservoir in rocks and sediments. In general, the sizes of the reservoirs are inversely related to their rates of carbon exchange.
• Carbon takes different chemical forms in these different reservoirs.
  • In the atmosphere, it is a gas (CO2). Carbon in land vegetation is organic, as is most carbon in soils, while that in the ocean is mostly inorganic, occurring as dissolved ions
• Plants grow on land if the conditions necessary for photosynthesis are met sunlight is needed to provide energy, and nutrients provide food for plant growth. With these conditions satisfied, plants draw CO2 from the air and water and from the soil to create new organic matter, while oxygen is liberated to the atmosphere
• Transpiration is a highly efficient way to return water vapor to the atmosphere, and it can occur at much faster rates than ordinary evaporation from vegetation-free ground.
  • After plants die oxygen is consumed in destroying their organic matter during oxidation.

2.11 Response of the Biosphere to the Physical Climate System

• Biomass is the total amount of organic matter present
• predominant types of vegetation and associated organisms, called biomes
  • savanna, hardwood forest, conifer forest, tundra
• With all these conditions favorable to photosynthesis, why isn’t the surface ocean an enormous photosynthesis machine? The answer is a lack of the nutrients nitrogen (N) and phosphorus (P). Nutrient food sources are scarce in most parts of the surface ocean.
• phytoplankton lives in the surface layers of the ocean and uses sunlight for photosynthesis
• In the few parts of the surface ocean where upwelling occurs, nutrients are more plentiful, and they result in greater productivity, or rates of photosynthesis
• surface waters near continental margins tend to be relatively productive. Upwelling in the eastern equatorial Pacific and Atlantic oceans also increases rates of photosynthesis and productivity in those regions.

2.12 Effects of the Biosphere on the climate system

• Life affects climate in many ways.
  • One way is by providing positive feedback to physical processes that affect climate.
  • A second way is through changes in the amount of greenhouse gases in the atmosphere, especially carbon dioxide (CO2) and methane (CH4). oscillation reflects cycling of vegetation in the Northern Hemisphere: CO2 is taken from the air by plant photosynthesis every spring and released by oxidation every autumn.
•  Example of vegetation climate feedback page 52
• The rapid increase in consumption of fossil fuels by humans over the last two centuries has tapped into huge reservoirs of coal, oil, and gas in rocks that naturally release their carbon at slow rates and has greatly accelerated these rates
• Methane (CH4) is a second important atmospheric greenhouse gas, although far less plentiful than CO2.
• By the early 21st century, methane concentrations in the atmosphere had risen by well over a factor of 2 above their natural (preindustrial) level to above 1,700 parts per billion
• As we noted earlier, both CO2 and CH4 trap part of Earth’s back radiation, keep the heat in the atmosphere, and make Earth warmer than it would otherwise be. This warming in turn activates the positive feedback effect of water vapor (H2O), the most important greenhouse gas.

Chapter 3: Climate Archives, Data, and Models

• Much of this history is recorded in four archives: sediments, ice, corals, and trees
• The interpretation of climate data is complemented by the use of climate models.
• Hypotheses of climate change can be tested by comparing results from models against “ground-truth” climatic data.

3.1 Types of Archives

• sedimentary debris deposited by water is the major climate archive on Earth for over 99% of geologic time.
• The sediments can deposited in quieter waters where layers of sediment are laid down in undisturbed succession or carried to the ocean and sediment delivered to the seafloor may persist there for tens of millions of years until destroyed by tectonic processes.
•  Two processes, erosion (wind and ice) and tectonic activity, decreases the likelihood of preservation of older sedimentary records as time passes.
• After ice sheets reach maximum size and then begin to retreat, they leave behind long curving ridges called moraines.
• In regions where the winds weaken, the silt is deposited in sequences called loess. Loess deposits can be excellent climate repositories of the last 3 million years
• For the portion of geologic time younger than 150 million years, climate scientists have access to an additional climate archive: sediments preserved in ocean basins.
• deep ocean is generally a quiet place with relatively continuous deposition, it usually yields climate records of higher quality than records from land
• Ice core can provide useful information for that geographic area
• In areas of sufficient rainfall, groundwater percolating through soil and bedrock dissolves and redeposits limestone (calcite, or CaCO3) in caves.
  • These deposits contain records of climate over intervals that can extend back several hundred thousand years
• Trees are valuable climate archives for the interval of the last few tens, hundreds, or (in exceptional cases) thousands of years.
  • The outer softwood layers of many kinds of trees are deposited in millimeter thick layers that soon turn into hardwood.
• You can also use coral as an archive by examine the calcium carbonate or magnesium carbonate band

3.2 Dating Climate Records

• Climate records in older sedimentary archives are dated by a two-step process.
  • First, scientists use the technique of radiometric dating to measure the decay of radioactive isotopes in rocks. 
  • In the second step, dates obtained from the igneous rocks provide constraints on the ages of sedimentary rocks that occur in layers between the igneous rocks and form the main archives of Earth’s early climate history.
• Radiometric dating is based on the radioactive decay of a parent isotope to a daughter isotope.
  
  • This decay occurs at a known rate, the decay constant. This rate of decay in effect forms a clock with which to measure age.
• sediment sequences are dated by a combination of radiometric dating followed by correlation using fossils or other features in the sediments.
  • The fossil correlation method relies on the fact that a unique and unrepeated sequence of organisms has appeared and disappeared through Earth’s entire history and has left fossilized remains.
• In the younger geologic record, radiocarbon dating is widely used to date lake sediments and other kinds of carbon-bearing archives.
• The death of the plant or animal then closes off carbon exchanges with the atmosphere and starts the decay clock ticking.
• Some climate repositories contain annual layers that form because of seasonal changes in the accumulation of distinctive materials.
  • Ice can have different color layer depending on the sediment during that time period, Lakes contain annual couplets called varves, Tree rings, and coral band.
• You can correlate records with orbital cycles
  • The two most prominent examples are changes in the strength of low-latitude monsoons and the cyclical growth and decay of high latitude ice sheets.
• some climate archives contain internal chronometers with which climate scientists can measure elapsed time

3.3 Climatic resolutions

• The extent to which information in climatic archives can be resolved depends mainly on the interplay between two factors:
  • the amount of disturbance of the sedimentary record by various processes soon after deposition
  • the rate at which the record is buried beneath additional sediments and thereby protected from further disturbance.
• Most sedimentary archives used for climate studies form in low-energy marine environments undisturbed by turbulent waves and storms.
• Typical rates of sediment deposition range from as much as meters per year in coastal marine sequences to millimeters per year in lakes to millimeters per thousand years in some deep-sea sediments.
• Annual layers of snow are visible at the surfaces of many mountain glaciers and rapidly deposited ice sheets
• As the snow is buried and slowly recrystallized into ice, annual layers remain resolvable to a depth that depends on their initial thickness at the time of deposition.
• At middle and high latitudes where trees produce annual layers, tree rings become a permanent record of annual climate change
• CaCO3 bands in corals form a permanent record of seasonal to annual climate change.
• The types of climate archives, the maximum time span of the records they contain, and the highest resolution achievable in each archive are summarized in Figure 3-11 on page 65
• Climate archives contain indicators of past climate referred to as climate proxies
  • Indirect indicators
•  The two climate proxies that are most commonly used are biotic proxies, which are based on changes in composition of plant and animal groups, and geological-geochemical proxies, which are measurements of mass movements of materials through the climate system, either as discrete (physical) particles or in dissolved (chemical) form.

3.4 Biotic Data

• fossil remains from the continents are the main climate proxy for older tectonic-scale intervals.
• to reconstruct past climates over longer tectonic time scales often requires a reliance on the general resemblance of past forms to their modern counterparts
• plant fossils are most common, we can example plant sensitive to temperature to get an idea about the climate
• pollen can be found and used in sediment to help understand the continental record
• larger macrofossils include cones, seeds, and leaves.
• In the oceans, four major groups of shell-forming animal and plant plankton are used for climate reconstructions
  • Two groups form shells made of calcite (CaCO3).
    • Globular sand-sized animals called planktic foraminifera inhabit the upper layers of the ocean.
    • Much smaller spherical algae called Coccolithophoridae secrete tiny plates called coccoliths in sunlit waters.
  • Two other groups of hard-shelled plankton secrete shells of opaline silica (SiO2 * H2O) and tend to thrive in productive, nutrient-rich surface waters.
    • Diatoms are silt-sized plant plankton shaped like pillboxes or needles.
    • Radiolaria (lower right) are sand-sized animals with ornate shells often resembling pre-modern (Prussian) military helmets.
•  Plankton and pollen are widely distributed making them useful as climate proxies.
• modern climate preferences can then be used to reconstruct past climates from fossil assemblages with great accuracy in sediment archives as old as a few million years or more.

3.5 Geological and Geochemical data

• Mass movements of materials through the climate system are tied to processes of erosion, transport, and deposition, mainly by water but also by ice and wind.
• We rely on physical debris deposited in sedimentary archives on the continents as the main proxy for inferring past climates.
• ocean sediment deposition in many areas spanning the last 170 million years was relatively continuous and is better dated.
• The second major way of removing sediments from the land is by chemical weathering and subsequent transport of dissolved ions  to the oceans in rivers.
• Chemical weathering occurs mainly in two ways: 
  • by dissolution, in which carbonate rocks (such as limestone, made of CaCO3) and evaporite rocks (such as rock salt, made of NaCl) are dissolved in water
  • by hydrolysis, in which the weathering process adds water to minerals derived from continental rocks made of silicates, such as basalts and granites.
• important proxies in ice cores, cave deposits, trees record, and annual coral bands contain a wide range of valuable data
• two kinds of numerical (computer) models used by climate scientists.
  • Physical climate models emphasize the physical operation of the climate system, particularly the circulation of the atmosphere and ocean but also interactions with global vegetation (biology) and with atmospheric trace gases (chemistry).
  • Geochemical models track the movement of distinctive chemical tracers through the climate system.

3-6 Physical Climate Models

• Most physical models are constructed to simulate the operation of the climate system as it exists today.
• Simulations of past climates occur in a three-step process: choose the experiment, actual operation of the model and analyze the climate data output.
• Often climate data output can be tested against independent geologic data that played no part in the experimental design
• One-dimensional “column” models are the simplest kind of physical model of the atmosphere. They simulate a single vertical column of air that represents the average structure of the atmosphere of the entire planet.
  • offer a way of gaining an initial understanding of climatic effects of changes in concentrations of greenhouse gases and of airborne particles (aerosols)
• 2-D model includes an atmosphere with many vertical layers but adds a second horizontal dimension that represents Earth’s physical properties averaged by latitude.
  • models can simulate processes that vary from pole to equator because of the presence of snow and ice at higher latitudes and can simulate long intervals of time quickly and inexpensively
• Three-dimensional atmospheric general circulation models provide more complete numerical representations and simulations of the climate system.
  • incorporate many key features of the real world: the spatial distribution of land, water, and ice; the elevation of mountains and ice sheets; the amount and vertical distribution of greenhouse gases in the atmosphere; and seasonal variations in solar radiation
• The operation of A-GCMs incorporates the physical laws and equations that govern the circulation of Earth’s atmosphere: the fluid motion of air; the conservation of mass, energy, and other properties; and gas laws covering the expansion and contraction of air.
• Models have a start up phase to calibrate itself and once stabilized the values will be used for climate output data (15y for spin up equilibrium and 5y for climate output)
• Ocean general circulation models (O-GCMs) are less development than atmospheric GCMs (A-GCMs).
• climate researchers know much less about the modern circulation of the ocean because scientists do not have as well defined a modern target for ocean models to reproduce
• O-GCMs uses boxes too and typical climate data output from O-GCM experiments includes ocean temperature, salinity, and sea-ice extent.
• Hard to use both atmospheric and oceanic models because of different reaction timescales
  • Reduce issues by asynchronous coupling. This procedure involves an ongoing series of alternating runs, first using the atmosphere to drive the ocean, then using the ocean to drive the atmosphere, and then back again.
  • Recent models have coupled the ocean more directly to a simplified version of the circulation of the atmosphere. Coupled models are called A/O-GCMs.
•  A-GCMs can simulate the instantaneous effects that these high, broad, reflective masses of ice have on the rest of the climate system
• Hybrid models have also been developed to simulate longer-term climatic responses, particularly those of the ice sheets.
  • These models are called EMICs (Earth system Models of Intermediate Complexity).
    • Sacrifice certain computationally demanding aspects from A-GCMs to get much longer simulations spanning hundreds to thousands or tens of thousands of years.
    • include submodel for the atmosphere, ocean, sea ice, land surfaces, and the biosphere. Can also attempt to simulate features like the growth or melting of ice sheets.
    • Vegetation is an active component within the climate system, and the representation of vegetation in climate models has progressed through several stages.
• Recent models incorporate vegetation in an interactive way. First, climate data derived as output from an A-GCM or A/O-GCM experiment are used as input to a vegetation model that simulates the resulting changes in vegetation. Then the simulated changes in vegetation are used as input to another A-GCM or A/O-GCM experiment that simulates the additional climatic feedback effects caused by the changes in vegetation

3.7 Geochemical Models

• Geochemical models are used to follow the movements of Earth’s chemical materials (called geochemical tracers) through the climate system.
• most geochemical models do not reproduce the physical processes that govern the flow of air and water.
  • focus on the sources, rates of transfer, and ultimate depositional fate of two major sedimentary components: those particles that result from physical weathering and the dissolved ions produced by chemical weathering
• Movements of these tracers can be evaluated as long as they are not created or destroyed by radioactive decay along the way.
• Geochemical models can also trace exchanges of biogeochemical materials such as carbon or oxygen isotopes that cycle back and forth among the atmosphere, ocean, ice, and vegetation.
• The most basic kind of geochemical model tracks transfers of material from their sources to their ultimate sites of deposition, such as debris eroded from the land and deposited in ocean sediments.
  • If deposited material has distinctive geochemical characteristics, it can be analyzed and its abundance can be quantified in terms of a flux rate, defined as its rate of burial in that sedimentary archive.
• The models that trace these movements among reservoirs are called mass balance models. The ocean is the most important reservoir because it receives almost all of the erosional products from the continents, it interacts with all of the other reservoirs, and it eventually deposits the tracers in well-preserved sedimentary archives.
• If the flux rates of a tracer into and out of a particular reservoir (the ocean) are equal, the system is said to be at steady state, with no net gain or loss of the tracer in the reservoir.
• The residence time is the average time it takes for a geochemical tracer to pass through a reservoir
• For a reservoir at steady state, the residence time is Residence time = Reservoir size / Flux rate (in or out)
• Another important exchange is the movement of a geochemical tracer back and forth between two (or more) reservoirs.
  • The tracer never comes permanently to rest in either reservoir. The process that interests’ scientists is the amount of movement between the reservoirs through time. The history of exchanges is usually detected in the sediment record recovered from the larger reservoir (the ocean), but the goal is to monitor changes in size of the smaller reservoirs (the volume of ice or the amount of vegetation).
• Exchanges of water between the relatively small reservoir stored in ice sheets on land and the much larger reservoir left behind in the ocean can be tracked by using the fact that the isotopic composition of oxygen in the H2O molecules that are deposited in ice sheets differs from the average composition of the molecules left in the ocean.
• Measurements of the oxygen isotope composition of the ocean in shells of plankton provide a way to estimate past changes in the volume of ice stored on land