Transcript 45.315

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.
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The Historical Data
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Climate Forcing from GHGs
Temperature Forcing for uniform increase in GHGs
(oC/ppb)
CO2
CH4
N2O
CFC-11
CFC-12
CFC-13
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0.000004
0.0001
0.001
0.07
0.08
0.1
Radiative Forcing of 1990
Emissions
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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.
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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.
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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
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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
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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.
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It’s 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/ww
fscenarios.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
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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
change:
Canada and
the Globe
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With and Without Aerosols
Global Temperature:
B-as-U scenarios for
GHGs.
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Equilibrium results
from two GCMs for
CO2 doubling
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Canada: Temperature Changes
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High CO2 Scenarios
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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 A2high 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
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Precipitation Changes
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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.
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Change in Arctic Ocean:
Summer Ice Cover
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Historic Seal Level Rise
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Rising Seas: The Future
• One of the most
striking
consequences of
global warming
will be the
associated rise in
global mean sea
level.
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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!
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The last 1000 years
• Recent reconstruction of NH air temperature based
on tree-ring, ice core, coral and historical evidence.
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Recent Historical Changes
Climate change is not uniform over the Earth
Magnitude varies with location
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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.
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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
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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.
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