Anthropogenic Climate Change

1
Anthropogenic Climate Change

• The Greenhouse Effect that warms the surface of
  the Earth occurs because of a few minor
  constituents of the atmosphere (GHGs) that absorb
  IR radiation very efficiently.
• As a result of human activities, the
  concentrations of GHGs are increasing.
• This will lead to a warmer Earth, the amount
  depending on forcing and feedback effects.

2
The Historical Data
3
Climate Forcing from GHGs
Temperature Forcing for uniform increase in
GHGs(oC/ppb)
4
Radiative Forcing of 1990 Emissions
5
The 2xCO2 Experiment

• A standard experiment with global climate models
  is to calculate the equilibrium climate for the
  present and for 2xCO2 conditions, and examine the
  difference between the two states.
• Doubled CO2 is used as a surrogate for an
  equivalent climate forcing from a mix of
  contributions from many different GHGs.

6
Equilibrium Modelling

• In equilibrium studies, the time-dependent nature
  of CO2 increase and the resulting climate
  response are ignored.
• We only consider the state of the climate when it
  is in balance with the radiative forcing
  associated with a particular level of CO2.
• As long as GHGs continue to increase, climate
  will never reach equilibrium.

7
2xCO2 Equilibrium

• The equilibrium climate is much easier to
  compute, since it is not necessary to
• choose a particular scenario for GHG increases
• account for the thermal capacity (lag) of the
  deep ocean
• The heat capacity of the atmosphere is equal to
  less than 3 metres depth of ocean.
• The lag in the system means that climate change
  will be out of phase with atmospheric change

8
Realised vs Committed Change

• Realised change is that already showing up in
  instrumental and proxy records of climate,
  relative to some baseline condition.
• Committed change is the equilibrium response to
  an increase in radiative forcing from the
  increase in GHGs.
• Equilibrium simulations cannot be used as actual
  forecasts of future climate but they can be used
  to estimate the committed change

9
Prediction versus Projection

• Prediction
• Forecast, What will be
• Projection
• Scenarios, What could be
• Climate vs Weather
• Climate change is a shift in the average weather
  that a region experiences

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Canada in the 20th Century

• The climate of Canada became warmer and wetter in
  the 20th Century
• The annual mean temperature increased by about
  0.8oC, with 1998 being the warmest year of the
  century
• almost 2oC above the 1961-90 normal.
• The warming has been greatest in (north) western
  Canada, and concentrated in the winter and spring
  seasons.

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Night and Day

• Night-time (minimum) temperatures have warmed
  more than daytime (maximum) temperatures,
  reducing the diurnal range.
• This is consistent with climate forcing due to an
  enhanced greenhouse effect.

12
Its also Getting Wetter

• Annual precipitation has increased by 10 across
  the country
• the last 20 years were the wettest of the 20th
  century
• over parts of Arctic Canada, annual precipitation
  has increased by 25
• For more details visit
• http//www.cru.uea.ac.uk/mikeh/research/wwfscenar
  ios.html

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Climate Projections

• CO2 Scenarios

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Scientific Uncertainties

• Two major sources of uncertainty
• Magnitude (and direction) of feedback effects
• Lack of empirical data to accurately represent
  processes in models
• Mixing time of oceans (thermal inertia)
• Realised vs committed change
• Other sources of climate change and variation

15
CO2 Scenarios

• There are four future emissions pathways defined
  in IPCC/SRES
• A1, A2, B1 and B2
• relating to four different possible future
  worlds.
• The four scenarios lead to different atmospheric
  carbon dioxide concentrations over the next
  century, but concentrations rise in all cases.

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Are they realistic?

• Assumptions about future greenhouse gas emissions
  have been selected so as to span about 95 per
  cent of the range of emissions
  scenarios published in the
  current literature.

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Range of Scenarios

• The four emissions scenarios were combined with
  low, medium and high levels of "climate
  sensitivity". (The level of warming resulting
  from a doubling in CO2).
• The combinations of low emissions low climate
  sensitivity through to high emissions high
  climate sensitivity produce a range of future
  global warming and sea-level rise that span
  perhaps 90 per cent of likely future climates.

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CO2 Scenarios and Global Climate Change
The change in annual carbon emissions from
energy/industrial sources by 2100 for these four
scenarios ranges from a decrease of 4 per cent
(B1) - compared to estimated year 2000 emissions
- to an increase of about 320 per cent (A2).
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How accurate are the projections likely to be?

• Climate scenarios are not predictions of the
  future
• they are descriptions of one or more possible
  future climates.
• One important reason for uncertainty is that we
  do not know how rapidly emissions of global
  warming gases will grow in the future,
• nor do we know exactly how natural climate
  variability will evolve.

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Temperature changeCanada and the Globe
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With and Without Aerosols
Global TemperatureB-as-U scenarios for GHGs.
22
Equilibrium results from two GCMs for CO2 doubling
23
Canada Temperature Changes
24
High CO2 Scenarios
25
Canada warms .

• Canada warms substantially more in the future
  than the global average. This is particularly
  true of the Arctic region
• Annual warming north of 60oN reaches 0.2oC/decade
  even under the B1-low scenario
• This rate increases to 0.7oC/decade under the
  A2-high scenario
• Winter warming reaches 1.0oC/decade for the
  A2-high scenario
• These rates are 50 higher than those south of 60N

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Polar Ice Feedback
The extent of snow and ice affects the global
albedo.Also, the thinning of Arctic Ocean ice
releases ocean heat to the atmosphere
27
Precipitation Changes
28
The High CO2 Scenarios
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Arctic Sea Ice

• Sea ice extent has been declining since the 1970s
  and there has been an increase in the length of
  the summer melt season.

30
Change in Arctic Ocean Summer Ice Cover
31
Historic Seal Level Rise
32
Rising Seas The Future

• One of the most striking consequences of global
  warming will be the associated rise in global
  mean sea level.

33
Global Sea Level Rise

• The largest contribution to sea-level rise comes
  from the expansion of warmer ocean water - a
  process that will continue for centuries to come.
  Melting of land glaciers account for about 20.
• Complete melting of all land-based glacier ice
  would ultimately raise sea levels worldwide by
  about 60 metres!

34
The last 1000 years

• Recent reconstruction of NH air temperature based
  on tree-ring, ice core, coral and historical
  evidence.

35
Recent Historical Changes
Climate change is not uniform over the
Earth Magnitude varies with location
36
A cooler upper atmosphere

• Data series are much shorter for upper air
  temperatures but measurements taken since 1960
  suggest the upper atmosphere has cooled by about
  0.5oC/decade.
• This pattern is also consistent with an enhanced
  greenhouse effect.

37
What does it all mean?

• The warming clearly observed in the 20th Century
  has been historically unusual
• 1998 was probably the warmest year in the last
  1000 years
• The balance of evidence suggests a discernible
  human influence on the global climate

38
Humans and Climate Change

• Although the precise contribution of human
  activities to global warming cannot yet be stated
  with confidence, it is clear that the planet
  would not be warming as rapidly if humans were
  not currently emitting about 7 billion tonnes of
  carbon into the atmosphere each year.