The Physics of Climate and Climate Change

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The Physics of Climate andClimate Change

• A/Professor Michael Box
• Dr. Gail Box
• School of Physics, UNSW

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• Radiation and Climate
• Thermal radiation laws
• The Greenhouse Effect
• Global energy budget
• Simple models
• Climate Forcing
• Aerosol and gas forcings
• Feedback Mechanisms
• Ice-albedo feedback
• Global Climate Models
• Strengths and weaknesses
• Climate Prediction
• Scenarios and uncertainties

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Radiation and Climate

• The Earths climate, at both global and regional
  scales, is the result of dynamic balances
  (equilibrium) in the flows of energy (heat), when
  averaged over sufficiently large time and space
  scales.
• The only energy exchange mechanism between the
  Earth and space is via thermal (electromagnetic)
  radiation, so well start our physics lesson
  here.

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Radiation laws 1.

• In order to understand radiation exchange, we
  need to know the laws of (thermal) radiation.
• Law 1 All bodies with temperatures above 0 K
  (absolute zero) emit electromagnetic radiation.
• A black body is an ideal body which absorbs all
  radiation incident on it, and reflects none.
  (Thus it appears black!) A black body is also the
  most efficient emitter of radiation. We will
  start be examining the physics of blackbody
  radiation.

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Radiation laws 2.

• Law 2 A blackbody at temperature T (K) emits
  radiation from its surface at the rate
• Watts per square metre
• Here is the
  Stephen-Boltzmann constant.
• For a body which is not black, we may interpret
  this temperature as an effective temperature.

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Energy balance

• The Earths climate is governed by the balance
  between
• incoming solar radiation, S, minus the fraction,
  a, which is reflected (both are measured by
  satellite)
• and the emission of terrestrial radiation.
• If we assume that the Earth is a blackbody with
  an unknown (effective) temperature T, then we can
  determine T by ensuring this balance

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Radiative equilibrium
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Energy balance

• To determine the effective temperature of the
  Earth, we balance these two terms
• where we have used F 1368 Wm-2 and a 0.3.
  
• Seems a bit cold! (Average surface temp is 14.5
  C.)
• Is the physics wrong? No, its just incomplete.

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Radiation laws 3.

• Law 3. The emission spectrum of blackbody
  radiation follows Plancks law. (This law has a
  very interesting history, and was in fact the
  first step in the development of Quantum
  Physics.)
• Law 4. The wavelength at which this spectrum
  peaks is inversely proportional to temperature.
  (This is known as Wiens law, and was actually
  discovered some years before Plancks law.)

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Planck curve for different T
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The Greenhouse Effect

• An examination of the spectra for 5750 K (the
  suns temperature ), and 250 K (the Earths
  effective temperature), shows quickly that
• 99 of sunlight has wavelength less than 4.0 µm
  known as shortwave radiation
• 99 of earthlight has wavelength more than 4.0
  µm known as longwave radiation.
• Our atmosphere contains a number of gases which
  absorb in the longwave region these are
  greenhouse (or radiatively active) gases.
  These include H2O, CO2, CH4, N2O, O3, CFCs.

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Atmospheric absorption 1.
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Atmospheric absorption 2.
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Radiation laws 4.

• A body which is not black (any gas) will absorb
  a fraction, a? of the radiation incident upon it
  this usually varies (strongly) with wavelength,
  ?. It will also emit a fraction, e? of the
  radiation that a black body would emit at that
  wavelength.
• Law 5. Fractional absorptivity equals fractional
  emissivity, at all wavelengths (Kirchhoffs law)

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Consequences

• Because of its temperature, the Earths surface
  emits radiation in the 4.0 to 100.0 µm region.
• Most of this is absorbed by greenhouse gases.
• But the atmosphere is at a similar temperature,
  so by Kirchhoffs law these gases will re-emit
  much of this radiation, some to space, but more
  back to the surface, making the surface warmer.
• This is known as the greenhouse effect, or more
  correctly, the atmosphere effect. I now display
  it both qualitatively, and quantitatively.

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Measuring the greenhouse effect

• There are two ways of measuring the greenhouse
  effect.
• The first is the 33 difference between the
  effective temp (-18C) and the actual (average)
  surface temp (15C).
• The second is the difference between the 390 Wm-2
  surface emission and the 237 Wm-2 emission to
  space.
• Of this 153 Wm-2, H2O accounts for about 95, CO2
  for about 50, and N2O, CH4, O3 and CFCs about 2
  each.
• The interesting question which now confronts us
  is how are these numbers changing, as a result
  of our actions?

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A simple one-layer model

• We can construct a very simple model of an
  absorbing atmosphere as follows
• Assume that the incoming shortwave radiation
  (after removing the reflected component) is
  transmitted by the atmosphere, and is all
  absorbed at the ground.
• Assume that the ground emits as black body with
  Tg.
• Assume the atmosphere absorbs all of this energy,
  and re-emits energy, as a black body with Ta,
  from both surfaces i.e. to space and back to
  ground.

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Energy balance at the surface, and at the
top-of-atmosphere, givesWhen these equations
are solved for the two temperatures we
obtain Ta 255 K Tg 300 K 27 CThis time
it is a little too warm, but it is an improvement.
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More realistic models

• For teaching purposes we use a model which allows
  some solar radiation to be absorbed in the
  atmosphere, and also allows some longwave
  radiation to pass right through the atmosphere
  (i.e. fractional emissivity lt 1.0).
• A radiative-convective model allows for an
  atmosphere with many layers, each with its own
  temperature and gas concentration (and hence
  fractional emissivity). This model can only be
  solved iteratively, but it serves as a first step
  in realistic modelling of radiation flows.

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Modified one layer greenhouse model

• Solar Radiation
• ? reflected
• a absorbed in atmosphere
• (1 a - ?) absorbed at surface

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Modified one layer greenhouse model

• Terrestrial Radiation
• e? Tg4 absorbed in atmosphere
• (1 e)? Tg4 emitted to space

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Modified one layer greenhouse model

• Terrestrial Radiation
• e? Tg4 absorbed in atmosphere
• (1 e)? Tg4 emitted to space
• Atmosphere
• e? Ta4 emitted to space AND to ground

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• For radiative balance
• Incoming absorbed Outgoing emitted
• In atmospheric layer
• aE e?Tg4 2e?Ta4
• Ground
• (1 a ?)E e?Ta4 e?Tg4
• Solve the simultaneous equations for two unknowns.

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Climate Forcing

• Any change in the radiation balance (at TOA)
  caused by changes in atmospheric composition,
  etc., is a called a radiative forcing.
• We can evaluate radiative forcings with a very
  high precision by running a 1D radiative-convectiv
  e model before and after.
• IPCC 4AR very high confidence (gt90).
• What forcings have been identified? The major
  ones are greenhouse gases and aerosols.

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Atmospheric aerosols

• Atmospheric aerosols are small particles with
  sizes ranging from 10nm to 10µm. They have
  atmospheric residence times 1 week.
• They may be produced by both natural and
  anthropogenic processes (or a combination).
• Primary particles are directly injected into the
  atmosphere (e.g. dust, sea salt, soot).
• Secondary particles are created by
  gas-to-particle conversion, from precursor gases
  (e.g. SO2 to sulphate aerosols).

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Aerosol forcing 1.

• Aerosols are very efficient light scatterers, and
  will reflect (some) sunlight back to space.
• Increasing levels of (anthropogenic) aerosols
  provide a negative forcing (cooling the surface).
  This is known as the aerosol direct effect.
• Some particles, mainly soot, but also mineral
  dust, are efficient absorbers. They may affect
  the vertical heating rate in the atmosphere.

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Aerosol forcing 2.

• At the heart of every cloud droplet is an aerosol
  particle (CCN), which is essential for its
  startup.
• Increasing levels of aerosols may lead to more,
  but smaller, cloud droplets (for fixed l.w.c.).
• Such a cloud will be more reflective (brighter)
  this is the first aerosol indirect effect.
• Smaller droplets also take longer to grow large
  enough to precipitate, so a longer-lived cloud
  this is the second aerosol indirect effect.

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These shiptracks, seen from space, are an
example of the indirect effect. Ships sailing
beneath these clouds have released particles
which have seeded them with more CCN, creating
lines of enhanced reflectivity.
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Feedback Mechanisms

• A radiative-convective model is only a first step
  in understanding climate change, as we must now
  allow the climate system to respond. This
  involves simple dynamics, plus feedbacks.
• Feedbacks can be positive, enhancing any initial
  warming (or cooling), or negative, damping out
  any initial climatic change.
• Unfortunately, many of the feedbacks which have
  been identified are positive.

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Feedback examples

• The simplest feedback involves water vapour.
  Warmer ocean temperatures lead to increased
  evaporation, hence more water vapour in the
  atmosphere. This is a powerful greenhouse gas,
  which leads to more warming, which leads to.
• Other feedbacks involve the carbon cycle and the
  biosphere both positive and negative.
• As the oceans warm, their ability to dissolve CO2
  decreases, so more will stay in the atmosphere.

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Ice-albedo feedback

• Surface warming at high latitudes leads to the
  melting of ice and snow.
• Ice has a much higher albedo (reflectivity) than
  ocean 80 vs. 5. Less snow cover means more
  solar energy is absorbed, causing more warming,
  and hence more ice melting, etc.
• This is the reason polar regions are warming
  faster than the rest of the globe.
• It is also a key to the glacial/interglacial
  cycle. (The Milankovitch ice-age mechanism.)

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Climate Models

• Climate models are an attempt to encapsulate
  everything we know about the Earth System.
• This involves the atmosphere, the oceans and sea
  ice, vegetation, biogeochemistry, aerosols and
  atmospheric chemistry. along with all of the
  interconnections and feedbacks involved.
• The growth of computer power, plus of our
  knowledge of planetary systems, has allowed these
  models to become increasingly powerful.

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Atmospheric models

• An atmospheric General Circulation Model (GCM),
  like a numerical weather model, solves the
  equations of motion for the fluid, plus equations
  for conservation of energy (including radiative
  transfer), mass and water vapour.
• To do this the (continuous) atmosphere is
  replaced by a collection grid-boxes maybe
  20 vertical layers, and a horizontal spacing of
  around 100 km (or more).

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Climate models impossible dream?
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Sub-grid-scale phenomena

• All processes which take place on scales smaller
  than the grid scale must be parameterized.
  This is one of the major sources of uncertainty
  in using these models.
• Major examples include clouds (still the main
  problem), topography and coastlines.
• This is one reason why global predictions are
  more reliable than regional predictions.
• Sometimes we run nested models.

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Planetary heat transport

• Both the atmosphere and ocean act to transport
  heat from equatorial regions to polar regions.
• Temperature gradients drive the weather.
• For day-to-day weather forecasting, we can ignore
  the ocean, as its conditions will not change in
  the next week.
• For longer time-scales we need to understand how
  the atmosphere affects the oceans, and how
  changes in ocean circulation (e.g. El Nino) can
  feed back to affect weather patterns.

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Climate Prediction

• To predict the climate in the year 2100, we run
  the best climate models available.
• To do this, we need to decide on the conditions
  which are significant e.g. the composition of
  the atmosphere over the next 100 years, and run
  the model for 100 years of computer time.
• Since we cant know in advance what will be the
  atmospheric conditions, we use scenarios.

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Climate scenarios

• The CO2 content of the atmosphere in 2050 depends
  on inputs and outputs between now and 2050. Thus
  we need emissions scenarios, and a good
  understanding of the carbon cycle.
• The IPCC asks modelers to run their models for a
  range of emissions scenarios, which are based on
  assumptions about technological changes and
  economic decisions.
• The main focus is usually on what used to be
  called the business-as-usual scenario.

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Changing climate statistics

• What do we look for in model predictions?
• A major focus is on global mean temperature.
• However other, statistical, predictions are
  studied (we extract both means and variation from
  model runs)
• Rainfall how is it distributed spatially and
  seasonally does it come as more intense
  downpours is it likely to rapidly re-evaporate
  due to higher temperatures?
• Changes in winter storms or tropical cyclones?
• Temperature will there be more heatwaves
  (periods of several days that are too hot) or
  other extremes?

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Prediction uncertainties

• Predictions of our climatic future naturally
  contain many uncertainties.
• Emissions scenarios are clearly an uncertainty,
  but one which we understand (and control).
• Models are never perfect for example,
  sub-grid-scale phenomena or simplified
  chemistry.
• There are always processes (and feedbacks) which
  are missing from the models, for different
  reasons.

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Whats missing?

• There will always be processes missing from the
  models, and for a variety of reasons
• Processes which are just too complex e.g. a
  full atmospheric chemistry/aerosol package.
• Feedbacks we are not sure just when theyll kick
  in e.g. icecaps being lubricated, and sliding
  off permafrost melting, releasing trapped
  methane.
• Processes that havent even entered our thinking
  yet. For this reason, we must always monitor as
  many aspects of the climate system as possible,
  and be on the lookout for the unexpected (eggs
  and baskets).