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AP3A90/APMA90 Climate Change and Food Systems
Observations and projections of
climate change
Dan Hodson
[email protected]
27/1/2011
Observations and projections of Climate Change
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Observations and projections of climate
change
By the end of this lecture you should be able to engage
in a discussion about:
 The Observations of a changing Climate.
 Climate modelling and Projections of future Climate
Change
 Uncertainty in Climate modelling and projection.
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Observations and projections of Climate Change
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What is Climate?
 “Climate is what you expect, Weather is what you
get”.
 Climate is the statistics of weather, e.g. the average
of weather conditions over some period of time.
 Expect :
Maldives to
be Warm
Antarctica
to be Cold
Atacama
Desert Dry
Bergen
Wet
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Observations and projections of Climate Change
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What is the Climate System?
Ice
Atmosphere
Land
Vegetation
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Observations and projections of Climate Change
Ocean
4
Observations of
Climate Change
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Climate Change
 Climate can be defined as the average of weather.
 Climate is what we Expect.
 A Change in Climate means e.g. a change in the
average weather conditions & change in what we
expect e.g.:
– Warmer summers
– Wetter winters
 But also can talk about the Climate in terms of other
system components e.g.
– The height of the sea.
– The number of forest fires each summer.
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Observing the Weather
 People have always watched and noted the weather,
but Objective measurements using scientific
instruments began only ‘recently’:
– Thermometers around since 1600’s
– Early 1700’s Daniel Gabriel Fahrenheit manufactured first
reliable mercury thermometers.
 The Central England Temperature record is the
Oldest instrumental record of temperature in the
world.
– Monthly measurements back to 1649.
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CET
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Observing the Weather
 Observations continued in this uncoordinated way for
many years.
– Individual Weather diaries
– Marine weather logbooks
 Once weather forecasting began, it was realised that
coordinated, reliable, regular measurements of the
weather were required.
 World Meteorological Organisation (1950) set up to
coordinate this observation across the World.
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Global Observing Network
Locations of land, ship and buoy observations across
the world at 6am 14 January 2008
1970
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Land observations concentrated
in inhabited areas and mainly in
the Northern Hemisphere
Observations and projections of Climate Change
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Has the Earth Surface Warmed?
Different
estimates
(smoothed)
~ 0.8ºC
IPCC AR4
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Observations and projections of Climate Change
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Is the rate of warming increasing?
IPCC AR4
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Observations and projections of Climate Change
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Global Trends
Temperature trends at each point on the Earth
IPCC AR4
 More warming over land than oceans
 Some regions have cooled.
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The ten warmest years on record
1. 1998
2. 2005
2010
3. 2003
2005
4. 2002
2003
5. 2009
2002
6. 2004
2009
7. 2006
2004
8. 2007
2006
9. 2001
2007
10.2001
10.1997
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 Nine
Eight of
of these
these are
are from
from
the last decade
 All are from the last 13
years
(Data taken from the Hadley Centre)
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Is it unusual?
 Global average temperature rose in the
20th Century
 Is this unusual?
 Have temperatures changed like this in
the past?
 Problem: Very few temperature
measurements before 1900.
 How can we measure temperatures
before the invention of the
thermometer?
 Natural Thermometers!
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Tree Rings
 Trees grow outwards and lay
down a new ring of wood every
year.
 More vigorous growth = thicker
ring.
 Growth dependent on
temperature, rainfall etc.
 Can estimate temperature from
the width of rings.
 ~1000 years.
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Estimates of past Northern Hemisphere Temperature
IPCC AR4
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Recent warming
unprecedented
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Have other things changed?
 Arctic Sea Ice Area
 Glacier Lengths
 Sea level height
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Have other things changed?
 Oceans are Warming:
Three Estimates of the Amount of heat in the upper ocean.
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Greenhouse Gases?
 Carbon Dioxide (CO2)
 Nitrous Oxide (N2O)
 Methane (CH4)
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Ice Cores
 Ancient gases trapped in
bubbles in Antarctic Ice.
 Can recover ice & gases that
have been stored for 10 000s
of years.
 Can measure levels of
Greenhouse gases in
Ancient atmosphere.
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Unprecedented?
 Rate of increase of
greenhouse gases
unprecedented in
last 20 000 years.
IPCC AR4
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Observations of Climate Change: Summary
 Many 1000s of measurements form estimate of
changing climate.
 Global average (mean) surface temperature rose by
~0.8ºC during the 20th Century.
 9/10 of the last 10 years were the warmest on record
globally.
 These warm temperature are likely highest in the
past ~1000 years.
 Also
– Sea level & Upper ocean heat content rose
– Arctic sea ice and Glaciers melted
 Concentrations of Major Greenhouse gases risen
over 20th Century.
– Largest seen in last 20 000 years.
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Modelling the
Climate
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Climate System Components
Ice
Atmosphere
Land
Vegetation
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Ocean
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Why?
 Why do we need a model of the Climate System?
 Most of Experimental Science
– Take some part of the World.
– Make some change.
– Measure any Effect.
 The Climate System is the World.
 Can’t do real experiments on the
Whole Climate System.
 Need Climate Models.
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Modelling the Climate System
 Do experiments to find out how Climate components
(e.g. water) behave.
 Write down a mathematical description of this
behaviour.
 Convert this into a numerical form for use in a
computer.
Physics
 Can then use model Earth climate to perform
experiments not possible with the real Earth Climate.
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Is the model Correct?
 If we use our Climate Model to simulate 20th Century
climate we can compare to our past measurements
of the real 20th Century climate.
– Model Validation
Real World
Annual Rainfall
Model
IPCC AR4
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Is the model Correct?
 Models simulate some variables better than others.
– Temperature is predicted accurately
– Rainfall less so
 Climate models reproduce many of the features of
the real climate
– Global Patterns temperature & rainfall
– Seasonal retreat of ice caps
– North Atlantic Gulf Stream
these are not explicitly built into the model, they
emerge from the physics.
– More confidence that models are reliable.
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Two Experiments
 EXPT1: Use our climate models to simulate 20th
Century climate as it was.
– Known Increases in Greenhouse gases
– Known changes in Natural Forcings
• Volcanoes
• The slight variations of light from the Sun.
 EXPT2: Use our climate models to simulate 20th
Century climate as it might have been
– No changes in Greenhouse gases
– Known changes in Natural Forcings
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Results
 EXPT1: Greenhouse
gases + Natural Forcings.
– Black line: measured 20th
Century Global average
Temperature.
– Red Line: Average of same
experiment with many
different climate models.
 EXPT2: Natural Forcings
Only.
 Implication: Increases in
Global average
Temperature due to
increases in Greenhouse
gas emissions.
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Observations and projections of Climate Change
IPCC AR4
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Projections of
future Climate
Change
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Projected global temperatures
 A2/A1B/B1 – different
estimates of future GHG
emissions.
– A2 = High Emissions
– B1 = Low Emissions
 Large range by end of
century.
 Some warming even if we
stopped emitting all GHG
today.
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Projected regional temperature changes
2090-2099
 Northern latitudes warm B1 (low)
more.
– Melting sea ice –
feedbacks
 Land warms more than
oceans.
A2 (high)
IPCC AR4
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Projected regional Rainfall Changes
2090-2099
 Different regions show
different rainfall changes.
 Northern high latitudes:
increased rainfall in N.
Hemisphere winter.
Northern Winter
– Wetter, more flooding.
 Southern Africa &
Mediterranean reduced
rainfall in N. Hem. Summer
Northern Summer
– Drier, more droughts.
 Grey shading – regions
where we are confident.
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Extremes
 These are projections of the increase in the average
temperatures and rainfall.
 If the average temperature increases, then the
chance of heat waves occurring increases.
 2003 European heatwave
– Probably hottest EU summer in last
500 years.
– Stott et al (2004) likely to be
considered a cool summer in
2100.
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Uncertainty in
Climate modelling
and projection.
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Climate Projections: Sources of Uncertainty
In Science it’s important to know what you don’t know.
Three sources of uncertainty in climate projections:
 Uncertainty in future Greenhouse gas levels
(Scenario Uncertainty)
 Uncertainty in the formulation of the Climate models
(Model Uncertainty)
 Intrinsic uncertainty in the climate system.
(Internal Variability)
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Scenario Uncertainty
 We don’t know what future Greenhouse gas emissions
will be.
 Produce ‘Scenarios’ – estimates of future emissions
based on estimates of future
– Population Projections
– Economic Development
– Structural and Technological Change
 Vary from High growth (A1) to Low
growth (B2).
Emissions Scenarios IPCC,2000
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Model Uncertainty
Climate models are only approximate models of the real world.
 They have low resolution
– A region of 200km x 200km represented by a single point.
 They cannot represent small scale processes directly
– Formation of rain represented by statistical rather than physical model.
 They cannot include all processes in the Climate System
– Ocean biology, Dynamic vegetation, Agriculture, Economics are only
just beginning to be included.
 There are many Climate models (17 used in last IPCC report)
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Intrinsic Uncertainty (Internal Variability)
 If we start our Climate model
and watch how the
temperature evolves.
 If we start the model again,
but from a slightly different
point
 The temperature evolves
differently.
 Butterfly effect. Sensitivity to the initial conditions.
 Because we do not know the exact state of the Climate
system now, we cannot predict the exact future state of
the climate.
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Working with uncertainty
 Although these sources of uncertainty exist we can
assess and quantify the scale of uncertainty.
 This allows us to assign a level of confidence to
climate projections.
 This allows the IPCC AR4 to say:
– “Most of the observed increase in global average
temperatures since the mid-20th century is very likely due to
the observed increase in anthropogenic greenhouse gas
concentrations”.
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Summary
In this lecture we have discussed:
 The Observations of a changing Climate.
 Climate modelling and Projections of future Climate
Change
 Uncertainty in Climate modelling and projection
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Next Lecture
The next lecture:
The climate system and the
IPCC
March 3rd
 Examine the Climate System in detail
 Discuss the mechanisms for a changing climate
 Discuss the IPCC & Kyoto Protocol
Dan Hodson
[email protected]
27/1/2011
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