SAS025 Midterm

Weather vs climate

weather involves short term physical conditions like temperature, humidity, precipitation, wind speed, and direction; climate is long term patterns

List of proxy measures

tree rings, ice cores, isotopes, coral dating

Mann “hockey stick”

•  emissions of CO2 and greenhouse gases responsible for dramatic warming trend after industrial revolution

Why are seasonal variations in CO2 bigger in the northern hemisphere than the south?

The northern hemisphere has more landmass

How did snowball earth thaw?

• volcanic eruptions releasing CO2, and the weathering of silicate rock and photosynthesis (which decrease CO2) almost stopping due to freezing temperatures

• once ice at equator thawed, snowball effect of decreasing albedo

Processes that decrease atmospheric CO2

Weathering of silicate rock, burial of organic matter, photosynthesis

Isotopes are proxy measures for

• age, atmospheric CO2 concentration, photosynthetic activity, oceanic pH and temperature

Molecules

•  stable group of 2+ atoms held together by strong chem bonds

Ions

• molecules become ions when electrically charged through gain or loss of electorns

  • Tend to be water soluble, ex carbonic acid dissolving in water to make two ions, proton and bicarbonate ion

Isotopes

• form of chem element w diff numbers of neutrons in nucleus

  • Usually nucleus will have equal # of protons and neutrons, but not always

Stable isotopes

• most common isotopes stable unless subjected to “high-energy bombardment”, ex carbon isotopes 12C and 13C

Radioactive isotopes

•  many isotopes are unstable, meaning they breakdown or decay at a predictable rate, emit electromagnetic radiation (sub-atomic particles), and afterwards become a diff element

  • Original isotope = parent, element produced = daughter

Radioactive dating

• decay of long lasting radioisotopes is primary method for dating materials 

Radiocarbon dating

• age of rock as determined by relative amts of nitrogen (daughter) to 14C (parent)

Isotope discrimination

• some biochem reactions have stronger affinity for one isotope than another

• for example, photosynthetic reaction that uses CO2 discriminates against 13CO2, heavy form, for 12Co2, the light form

Oxygen isotopes

• Water that has the light oxygen isotope is less likely to evaporate

  • So water vapor has a lower ratio of 18O to 16O than liquid water left behind

  • Heavy water is more likely to condense than light water

Rainout Effect

• As cloud drifts to higher latitude, som of the water vapor turns into precipitation; this rainwater/snow is enriched in the heavy isotope (higher delta18O), and the water vapor that’s left in the cloud after the precipitation is more depleted (lower delta18O)

• If precipitation continues, the leftover vapor gets more and more depleted

• As temps rise, all water molecules become agitated; lot of heavy water evaporates, and the liquid becomes less depleted of light water

• So at higher temps, delta18O value of water vapor increases (value deviates less from zero); so does the value of the precipitation that forms when that same vapor melts or freezes

Hydrogen isotopes

• water w light hydrogen isotope is more likely to evaporate than the water composed of deuterium (heavy water)

  •  At higher temps the delta value of water vapor increases, as does value of precipitation from that vapor

  • More pronounced discrimination btwn light and heavy hydrogen than w light and heavy oxygen; differences in mass between the oxygen isotopes is much smaller

  • Delta value of heavy hydrogen ice cores is more sensitive temp proxy than that of heavy oxygen ice cores

physical evidence of series of severe glaciations

• Rock layer deposits after this time show sudden severe decline of delta value of 13C, suggesting the photosynthetic CO2 consumption stopped bc of low temps

• These layers have unoxidized iron deposits, prob bc atmospheric oxygen levels dropped bc of lack of photosynthesis

• The next layers have sediments deposited by glaciers, or glacial till, covered w carbonate

  • This is bc silicate rock weathered in an atmosphere w high CO2 concentration

Difference between snowball and slushball

• Snowball, in which earth is covered in total ice, would have resulted in extinction of a lot of life and only hardy bacteria would make it

• Slushball, in which there is still some open water, would let more complex life forms survive

Cryogenian

• period of severe glaciation after breakup of Rodinia 

• exposed organic matter to anaerobic respiration, creating atmosphere in which major GHG was methane; atmospheric methane concentrations decreased rapidly once the movement diminished and less organic matter was exposed

• As methane declined, earth cooled and cooler temps inhibited production of methane, continuing the cooling process

• Ice on landmasses reflected solar energy away, further reinforcing the cold temps (albedo)

• So temps sank and even some tropical areas were covered w ice

What ended the Cryogenian?

•  CO2 from volcanic eruptions, and the low temperatures inhibiting processes that decrease CO2, such as silicate rock weathering and photosynth

• Ice cracked at 1.2 ppm CO2, leading to albedo decreasing and domino effect of everything getting warmer again

Will snowball earth happen again?

No, probably not:

• our sun has matured since then, we’re getting 6% more solar energy

• the landmasses are mostly at very high latitudes now, and average CO2 levels and average temps are much higher than they were then

• if it did freeze over, the GHE would thaw it out again

Cambrian Explosion

•  570 mil years ago, climate became warm and glaciers retreated; rising sea levels covered 85% of earth

• Rapid expansion of biological diversity; 900+ animal species appeared

  • Cambrian animals were more shelled or hard-bodied so easier to leave marks that could be traced in the fossil record

Paleomagnetism

Iron in rock has magnetic orientation and intensity that reflect Earth’s magnetic field during time and place at which rock was formed

Ordovician-Silurian Extinction

• abt 85% of creatures often found in fossil record disappeared, prob due to a brief glacial period; ice accumulating at the poles of Gondwana led to falling sea level, higher albedo, cooler temps and eventually glaciation

• As before, inhibition of CO2 consuming processes let CO2 concentrations rise enough to cause a global warming to thaw the glaciation

• But none of this seems severe enough to have caused the extinctions - so what happened? Probably the sea level decline (it was massive) led to a lot of organisms dying, as it would have resulted in harsh climate at mid and low latitudes. Could’ve also activated deep ocean currents that brought up toxic materials from the depths

Diffusion of CO2 through water is

• roughly 9000x slower than through air on land

aftermath of OSE

• in mild climate after the extinction plants came to land

• Primary productivity levels rose and jungles grew (they were short due to slow movement of necessary materials tho)

• Carboniferous period

• Vascular plants abt 380 mil ago

• plants able to better extend their reach due to specialized tissues

• More plant life = more worldwide photosynthesis. Time w towering plants

What caused the Great Dying?

It was either meteor, volcanic eruption, or methane release; probably combo

Volcanic eruptions would emit ton of CO2, rapid global warming; heated oceans, triggering melting of glaciers and release of methane, more warming, less oxygen, eventual suffocation

Mesozoic Era

• age of dinosaurs

• angiosperms (flowering plants)

• Relatively stable, warm climate

Cretaceous Period

the climate was uniform across planet. the poles were ice free, and the deep oceans were at least 10C warmer than they are now. Deposits contain many fossils

K-T boundary

• end of Age of Dinosaurs. All non-avian dinosaurs went extinct (but not just dinosaurs went extinct - mammals and plants too)

Possible causes of K-T boundary

• Probably meteor bc of rock layers at KT boundary. More iridium, so could tell it was from space

• After impact, fallout/dust would block out the sun, planet would cool rapidly; photosynthesis stop; everything dies

• Could’ve also triggered volcanic eruptions, which would in turn cause warming

Late Paleocene Thermal Maximum

most extreme climate event of modern era, abt 55 mil ago; methane hydrates in ocean melted and release caused warming

Oi-1 glaciation

•  sudden growth of ice sheets on Antarctica at abt 34 mil yrs ago (Eocene -Oligocene boundary

Mi-1 glaciation

•  brief but intense glacial maximum abt 23 mil yrs ago (Oligocene-Miocene boundary)

  • By end, ocean productivity had increased again and CO2 levels diminished

• Ice coring

• As snow accumulates on top of glacier, weight of upper layers compacts porous snow of lower layers into ice, trapping air as bubbles within the ice

  • Bubbles are samples of the atmosphere at the time the ice was formed

• Dating a disk from an ice core entails keeping track of number of annual cycles from the very top of the ice sheet

Counting years in ice cores is difficult because

they often become compressed w depth or distorted with sideward flows

All 11 interglacial periods correspond with

• higher concentrations of GHGs nitrous oxide, methane, and CO2

• Tree rings - dendrochronology

• each year a tree adds new layer of wood; when growing conditions are favorable tree produces large cells

• If conditions worsen growth slows and cell size decreases

• Under bad conditions, growth and cell division stop

• Comparing ring patterns through diff trees in the same area and averaging them out establishes a chronology

• Width of growth rings, esp for trees found near their altitude/latitude limits, increases w air temp

• Temperatures during middle of growing season have strongest influence on the ring width

What does compilation of tree ring data from Europe suggest?

• tree ring data from Europe indicates air temperatures today are as warm as any time during past 2,500 years

Coral dating

•  coral have exoskeletons made of calcium carbonate; gather denser layers in months w severe weather, lighter in months w more benign weather

  • So they develop annual bands that can be counted for age

  • Ratio of heavy to light oxygen isotopes decrease with with temperature of the seawater

  • When in shallow water, their ratio also changes w amount of rainfall, evaporation and river input

  • So oxygen ratio record often supplemented w proxy measures of temp, like strontium-calcium ratio

Strontium-calcium ratio

• ratio of strontium to calcium in corals used as proxy measure for oxygen ratio in ocean

• Cores from great barrier reef show temps near the corals during last half of the century are are as warm as they have been in over 400 yrs

• Boreholes

Temps deep in ground respond to changes near surface; ex a surface heat wave will cause heat to move slowly downward, taking abt 100 yrs to reach a depth if 150 meters

So vertical dist of temps in boreholes gives idea of temperatures on the surface

It measures temp directly and isnt a proxy

Glacier length

• Can reconstruct temps near various glaciers based on isotope values, tree rings, soil cores, paintings, etchings, direct measurements, photos

• major glacial advances correlate strongly w multi-decadal periods of decreased summer temp

• Reconstructions indicate temps rose steeply worldwide in 19th

• Direct measurements

temp measurements from global network of land-based weather stations, plus readings from ships and fixed buoys

have been available since 1850

shows average global temperatures have significantly increased over last century; over 1 deg over land and 2 degs at poles

Why do sea levels rise?

• Global sea levels rise and fall depending on volume of ocean basins versus that of water in them

• changes in water volume may occur rapidly and depend on global temps

Factors that may have an impact on volume of ocean water

• Temperature

• How much fresh water is sequestered in lakes, rivers, glaciers

• Warmer global temperatures and melting of glaciers

Tide gauges

 monitor the height of the sea relative to a nearby geological marker

indicate that sea level has risen average of 2.0 mm per year over last century

Sea level satellite imaging

• Satellites TOPEX/Poseidon, Jason-1, Jason-2 and Jason-3 specifically designed for monitoring sea level

• Follow a set path, bouncing microwaves off ocean and monitor time it takes between sending and receiving the microwaves to judge distance from surface

• GPS receivers on satellites establish their orbital height; sea level is taken as difference between orbital height of satellite and its distance from sea surface

  • Very accurate measure

• Satellite data has shown that sea level has risen nearly 3.0 mm per year over last decade

Why is the sea level rising faster than predicted?

ice sheets in Antarctica and Greenland are melting faster than expected

Major tropical storms

 increasing in intensity bc/o global warming enhancing temp gradient between sea surface and upper atmos

Aerosol concentrations and storms

• Additional aerosols over oceans, which decrease warming and a lot of variability results in some obscure trends for storms; hard to tell w all the factors

• Expect that aerosol levels will remain level or decrease, and GHG emissions will continue to rise, resulting in more severe storms

Gaia Hypothesis

Life on earth has served as an active control system that stabilizes the physical environment and the chemical composition of the planet

Current concentration of oxygen in the atmosphere

41 ppm

Current concentration of CO2 in the atmosphere

420 ppm

What happens to sunlight striking an object?

Some absorbs, some is reflected (bounces back off), some is refracted (trajectory changes)

When electromagnetic radiation is absorbed:

• molecules start moving faster, temperature increases

External climate forcers

• things that operate outside the earth and its atmosphere

  • Galactic variations

  • Orbital variations

  • Solar variations

Galactic variations

 the galaxy rotating on its axis influences how much electromagnetic radiation gets from the sun to Earth

Orbital variations: cosine law

• Amount of radiation intercepted develops on angle of light source to the object its hitting

• How much the poles intercept energy - angle to sun is much more oblique, and sun it does get is spread over a larger area

  • Energy per unit area of sun that poles get is smaller 

• Equator - more energy per unit area due to equal beams of solar energy focused on a small area. Directly overhead, path is shorter, more energy strikes surface

Inverse Square Law

• Sun is point-source of light

• Distributes energy over surface of a sphere

• at a given distance from the sun, sun’s energy is distributed over surface area of a sphere; at twice the distance energy distributed over 4x the area, etc etc

  • Amount of solar energy striking earth decreases w the square of the distance from sun

• So further you are from sun, less intense light is

• In summer, sun is further away frim the earth than in winter; planet as a whole is further away, and amount of energy goes down 

Organisms altered climate through

photosynthesis, respiration, and carbon sequestration

Photosynthesis

• Takes a lower energy carbon atom in carbon dioxide and uses energy in sunlight to create a higher energy form of carbon, carbohydrates, like glucose

• Usually splits water and releases oxygen into the atmosphere

• CO2+H2O+sunlight = CH2O + O2

Aerobic Respiration

• Organisms generate the energy they need to sustain life by converting higher energy carbon in carbohydrates into lower energy carbon in CO2

  • If oxygen is available they can go aerobic respiration – CH2O + O2 = CO2 + H2O + energy

Anaerobic Respiration

• w/o oxygen, anaerobic respiration; breaks down carbohydrates into methane, which reacts with oxygen to make chemical energy

What happens when photosynthesis surpasses amount of respiration?

extra carbohydrates are converted into long-lived organic carbon compounds, ex cellulose

When surface of earth cooled to below boiling point of water, abt 3.8 bil yrs ago, atmosphere was mostly

• high concentrations of CO2, CO, water vapor, dinitrate gas, hydrogen fluoride and small amts of methane

Photosynthesis formula

•  CO2 + water + light = carbohydrates + O2

Greenhouse Gas

• gas that absorbs a sig amount of electromagnetic energy emitted by the earth and warms the planet

Proxy measures cannot provide an estimate of Earth’s climate during the big bang, but they can provide an estimate of

• the date, precipitation, location, and the composition of the atmosphere

• Changes thought to have occurred during the last 4 bil years:

• At times, the planet was completely or nearly covered with ice

• At times, climate shifts were responsible for the extinction of many species

• At times, there were far fewer continents than there are today

Things that serve as proxy measures for temperature

• tree rings, length of glaciers, relative amounts of oxygen isotopes in water, relative amounts of oxygen isotopes in marine shells. Magnetic orientation of iron in rocks does not show temperature

The K-T Boundary refers to

the event separating the Cretaceous and Paleogene periods; when dinosaurs went extinct

Ice cores from Antarctica…

• do not demonstrate that cosmic rays and hydrogen peroxide are correlated with greenhouse gas levels

  • They do:

  • provide a climate record that extends back for more than 100,000 years

  • indicate that earth has experienced a pattern of glacial and interglacial periods

  • those interglacial periods had higher GHG levels

  • indicate that current levels of GHGs exceed those found at any previous time during the history of the ice sheet

Annual bands in coral derive from

• differential growth during severe versus benign weather

Sea level may rise because

• icebergs floating on the oceans melt, the volume of water expands with a temperature over 4C

Over the last century, average sea level has risen by

2 meters

Over the last century, global average temps

have increased abt 0.6 Celsius

Sea levels depend on

• volume of the ocean basins vs the volume of the water that’s in them; changes in water volume may be short and have a strong climate component

How much of an iceberg’s volume is below the surface?

90%

does sea level ice melting affect sea levels?

no; Sea ice floating on ocean, ex arctic ice pack, has mass equal to water it displaces, so that melting doesn’t do much

does melting of glaciers in antarctica and greenland affect sea levels?

• Of earth’s water, antarctica ice = 2%, greenland ice = 0.2%, and changes in their mass alters sea levels

Satellite data shows sea levels

• rising abt 3 mm a year over last decade

Tide gauges and satellites both indicate

• antarctic and greenland ice are melting faster than expected, and sea level is rising more than 1.7 mm per year

What causes storms?

• evaporation from warm water in tropics

• rises and condenses at high altitudes

• updrafts

• winds

• water spray and low atmospheric pressure - more evaporation

• loops until passes over land

• solar system rotates around milky way

every 150-350 mil yrs

Lambert’s cosine law

The angle of impact influences the intensity of sunlight received on Earth. Sunlight hits different parts of the earth differently because of earth’s rotation around the sun and the wobble on its axis. Perpendicular ang

Explains differences in climate between equator and the poles, and seasonal differences

On the poles it’s dispersed pretty well, resulting in them being cold; equator gets hit straight on, so it’s hotter

Kepler’s second law

• Gravitational bodies rotate thru equal areas in equal time

• Earth moves faster when near the sun than when it’s far

• Aphelion - point where earth is furthest from the sun, perihelion = closest to sun

Inverse square law

• at a given distance from the sun, sun’s energy is distributed over surface area of a sphere; at twice the distance energy distributed over 4x the area, etc etc

  • Amount of solar energy striking earth decreases w the square of the distance from sun

At its farthest distance from the sun, earth receives only

• 93.5% of the solar energy that it gets when at its closest during winter in northern hemisphere

Together, cosine and square laws result in

• more sun at tropic of cancer in june/july than december/january

Obliquity

• the angle that earth’s axis is tilted with respect to Earth’s orbital plane

• the planet wobbles on its axis, so although the normal angle is 23.5 deg

• angle oscillates between 22.1 and 24.5 every 41,000 yrs

• At extremes, trop of canc would receive abt 4.5% change in solar energy from max to min tilt

Eccentricity

• measure of deviation of orbit from perfect circle 

• Earth’s orbit has eccentricity of 0.0167, but oscillates between 0.005 (almost circular) and 0.0617 (more elliptical) w average periodicity of abt 100,000 yrs

As eccentricity increases

• As eccentricity increases, fluctuations in solar energy reaching earth during diff seasons increase

• this is bc of distance between earth and sun, inverse square law, and time spent at perihelion and aphelion

when eccentricity is minimal

• seasonal variation depends on diffs in angle btwn axis of earth’s diurnal rotation and its orbital plane around sun

Precession

• alignment of its axis of diurnal rotation w its distance from the sun, oscillates w av. period of abt 21,000 yrs

• So 10,500 yrs ago earth closest to sun on july 4th, and and farthest on jan 3

Milankovitch Theory

• by combining obliquity, eccentricity and precesson, can reconstruct history of solar insollation (amt of energy reaching earth per unit area)

Sol. ins. Has greater effect on NH than SH bc

• NH has abt 65% of earth’s landmass, and land absorbs more sol energy than ocean

• So when NH gets more, the whole planet does