EAS 212

• Einstein: Energy cannot be created or destroyed (it is conserved)
• Earth's Energy Balance


  1. Energy input = solar radiation absorbed

  2. Energy output = solar and longwave radiation lost to space

  3. If the stored energy is not changing then Energy input = Energy output

  
• Incoming energy = incoming solar radiation (over the area of a circle)

  Outgoing energy = reflected solar radiation and outgoing longwave radiation

  Any object that contains heat (i.e. is above absolute zero -273.15 degrees C) emits longwave radiation (infrared heat energy) at a rate proportional to its temperature: L = σT4 over it's entire area (area of a sphere for Earth)

  
• Steady state global energy balance (assumes energy stored is not changing):

  RE: Earth's radius (6370 km)

  α: Earth's albedo (0.3)

  S0 = solar constant (~1367 W m-2 )

  L = outgoing longwave radiation;

  L = σT4

  σ= Stefan Boltzman constant (5.67 × 10–8 W m–2 K–4)

  T = average temperature of the Earth (in Kelvin [K])

  
• We can find the temperature required for the Earth's outgoing radiation to balance the incoming solar radiation = 255K, or –18°C

  But in fact the Earth's average temperature if +15°C

  Why is the Earth 33°C warmer than it should be?

  
• Greenhouse Effect


  The atmosphere easily transmits solar radiation but almost completely absorbs Earth's longwave radiation

  Earth's surface emits longwave radiation that is absorbed by certain atmospheric gases and clouds

  The atmosphere emits longwave radiation in all directions—including downwards to Earth's surface (warms the Earth)

  
• Primary gases that absorb Earth's outgoing longwave radiation

  • H2O: Water vapour
  • CO2: carbon dioxide
  • CH4: methane
  • N2O: nitrous oxide
  • O3: ozone

  
• Result is more longwave radiation is absorbed by the atmosphere, and insufficient heat energy is emitted to space, so the Earth's temperature increases

  
• Climate


  Average atmospheric conditions that prevail in a region over extended time span (usually minimum 30yr average)
• Components of the Earth's climate system

  • Atmosphere
  • Hydrosphere
  • Cryosphere
  • Biosphere
  • Lithosphere

  
• Climate system


  • When one component of the system changes, the other ones react

  Change in these components are analyzed in terms of cause (forcing) and effect (response)

  
• Climate forcings


  • Changes in incoming solar radiation (S0)

  Changes in Earth's albedo

  Tectonic processes

  Changes in atmospheric composition

  Anthropogenic climate forcing

  
• Climate has always varied

  To study climate of the past we use climate archives

  Paleoclimatology

  
• Feedbacks


  Processes that alter climate changes that are already underway
• Time scales of climate change

  Faster changes in climate at shorter timescales are embedded in and superimposed on the slower changes at the longer time scales

  As you go back in time, the resolution and certainty of climate records decrease
• What is global warming: Increase in AVERAGE global temperature

  It does not mean it is getting warmer in every single point of the Earth

  It does not mean that each year is successively warmer than the previous one (interannual variability)
• Greenhouse gases and temp.

  Concentrations of the atmospheric greenhouse gases (CO2, CH4, N2O) in 2011 exceed the range of concentrations recorded in ice cores during the past 800 kyr
• “It’s the Sun”

  Current research shows an imbalance at the top of the atmosphere

  0.9 W m–2 of additional energy is being absorbed

  This is an additional 1022 J of energy per year

  Less longwave radiation is being emitted than is required to balance incoming solar radiation

  The overall consequence is the that the average temperature of the Earth is increases

  
• Natural vs Anthropogenic forcing

  Global climate models – based on our best understanding of the physics/biology/chemistry of the Earth system simply cannot simulate the recent observed increase in global temperatures by natural forcings alone

  Anthropogenic (human) emissions and land use changes are required

  
• Impacts of climate change on the oceans

  • Increase in ocean temperature

  Increase in ocean stratification

  Changes in ocean circulation

  Sea level rise

  Ocean acidification

  Harmful algal blooms

  Fisheries declines

  Change in precipitation/evaporation patterns

  Changes in diversity/abundance of species

  More frequent extreme El Nino events

  Expansion of oxygen depleted zones
• Sun cycles (internal variations in

  the strength of the Sun)
• Earth-orbital changes

  o Alter the amount of solar radiation

  received on Earth by season and by

  latitude

  o Occur over tens to hundreds of

  thousands of years

  o Contribute to alternating glacial and

  interglacial periods
• Changes in Earth’s albedo

  • Distribution of land masses

  • Snow, ice, cloud cover

  • Ecosystem distribution (e.g. deserts vs forests)
• Tectonic processes

  o Moving landmasses → alter ocean circulation

  and distribution of heat

  o Volcanism (aerosols)

  o Tectonic control of carbon cycle (CO2)
• Changes in atmospheric composition

  o Addition or removal of greenhouse gases

  o E.g. Early evolution of plant life, volcanic

  outgassing
• Anthropogenic climate forcing

  o Changes in atmospheric composition (CO2,

  CH4, N2O, O3) from emissions

  o Land use changes (albedo)
• Long-term Natural Climate Variations

  • Climate has always varied

  • To study climate of the past we use climate archives

  • Paleoclimatology

  • Dating: radioactive isotopes, fossils, annual layers
• Climate system responses

  Different components of the climate system have different response

  time (time it takes to fully react to the imposed change)

  o Hours-days up to thousands of years

  • Atmosphere has a very fast response time

  • Land surface reacts more slowly, but still shows heating and cooling

  changes on time scales of hours to weeks

  • Liquid water has a slower response because it can hold much more

  heat

  o Upper ocean: weeks to months

  o Deeper ocean: decades to centuries
• Feedbacks

  Processes that alter climate changes that are already underway

  
• Long-term carbon exchanges

  -Volcanic input of carbon from rocks to atmosphere

  -Removal of CO2 from the atmosphere by chemical weathering-> influenced by:

  • temperature: ^T, ^W
  • precipitation: ^P, ^W
  • vegetation: ^V, ^W

  -removal of carbon via chemical weathering must balance changing volcanic inputs over long timescales ~1000 year time scale
• Chemical Weathering as a thermostat

  • During warm periods, weathering removes CO2 from the atmosphere faster than volcanos can build it up. Causes the total volume of co2 to go down, cooling the planet
  • During the cold periods, volcanos add CO2 faster than can be removed, harming the planet

  
• Climate Change ≠ Climate Variability

  • Climate variability: the way climate fluctuates yearly above or below a long term average value
  • Climate change: Long term continuous change (increase or decrease) from average weather conditions or the range of weather. Slow and Gradual, unlike year to year variability. It is difficult to perceive without scientific records
• Recent Climate Change


  • 1930s – Hot, dry interval produced the

  Dust Bowl

  • A century earlier – air temperatures

  were cooler than now
• Climate Change: LGM

  • 21,000 years ago – climate was so cold

  that huge ice sheets covered Canada

  and northern Europe (sea level ~120

  lower than present)
• Climate Change: Further Back

  • 100 million years ago – warmer

  conditions had eliminated ice from the

  face of the Earth
• IPCC Projections

  • RCP2.6*: mitigation scenario (stabilizes and then slowly

  reduces radiative forcing after mid-21st century)

  • RCP8.5: continue emissions

  • Mitigation actions starting now do not produce

  discernibly different climate change outcomes for the

  next 30 years or so, whereas long-term climate change

  after mid-century is appreciably different across the

  RCPs

  *RCP = representative concentration pathway (for greenhouse gases)

  
• Radiative Forcing


  Total Anthropogenic radiative forcing has been increasing
• CO2 lags temp.

  • feedbacks are very important
  • CO2 acts sometimes as a forcing, sometimes as a positive feedback on climate change