Download Climate Change: Is It Becoming A Reality In South Africa?

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CLIMATE CHANGE:
IS IT BECOMING A
REALITY IN
SOUTH AFRICA?
Hannes Rautenbach
Department of Geography, Geoinformatics and Meteorology,
University of Pretoria
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What planetary forces control our climate?
What is the role of our atmosphere in Earth’s radiation budget?
Does today
today’s
s climate differ from what we have experienced before?
Are we responsible for greenhouse warming and climate change?
According to the IPCC report – what can we say about southern
Africa?
Are there any observed rainfall and temperature changes over
southern Africa?
How do these changes compare to future projections?
What does the future hold for southern Africa?
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The atmosphere is remarkable thin
i comparison
in
i
with
ith the
th radius
di off
the Earth, which is 6 x 106 m or
6000 km. The depth of the
atmosphere is estimated at 100km.
100km
This is a fraction of 0.02 of the
radius of the Earth.
Half (50%) of the mass of the
atmosphere lies below 6 km above
mean sea level ((MSL)) which is
less than 0.001 times the Earth’s
radius.
About 99% of the mass of the
atmosphere lies within the lowest
30 km above MSL.
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CONTROL FORCE 1: TEMPERATURE GRADIENTS → PRESSURE GRADIENTS
WARM EQUATOR + COLD POLES + NO ROTATION
HADLEY CELL
COLD
HIGHER PRESSURES + DRY
HADLEY CELL
60oN
30oN
WARM + LOWER SURFACE PRESSURES + WET
0o
30oS
60oS
HADLEY CELL
HIGHER PRESSURES + DRY
COLD
HADLEY CELL
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CONTROL FORCE 2: ROTATION OF THE EARTH
WARM EQUATOR + COLD POLES + ROTATION = 24 HOURS PER REVOLUTION
POLAR CELL
POLAR CELL
HIGHER PRESSURES + DRY
FERREL CELL
HADLEY CELL
LOWER PRESSURES + WET
FERREL CELL
60oN
30oN
HIGHER PRESSURES + DRY
LOWER PRESSURES + WET
HADLEY CELL
0o
HIGHER PRESSURES + DRY
LOWER PRESSURES + WET
FERREL CELL
30oS
60oS
HIGHER PRESSURES + DRY
POLAR CELL
HADLEY CELL
HADLEY CELL
FERREL CELL
POLAR CELL
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Earth surface temperatures
are determined by:
Monthly average surface temperature (C) – NCEP fields
Incoming solar irradiance
Outgoing heat
Surface characteristics
Incoming solar irradiance are
linked to the Sun
Sun’ss
declination angle and
follows a seasonal cycle.
With surface characteristics
seasonal surface
temperature patterns
develop, and the Intertropical Conversion Zone
(ITCZ) or heat equator is
formed.
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Rainfall is formed when lower
level moisture is available
and is pushed upwards in
the atmosphere.
Monthly average relative humidity (%) – NCEP fields
Ai will
Air
ill rise
i as a resultlt of:f
• Surface heat convection
• Mountain forcing
• Less dense air that is
pushed over denser cold
air
Risingg moist air cools
adiabatically, which will
eventually lead to droplet
formation and rain.
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During summer in the midlatitudes (±30oS), on average,
southern Africa emits more heat
towards the atmosphere than
the surrounding oceans, which
l d to
leads
t a temperature
t
t gradient
di t
between the continent and
surrounding oceans.
How will this affect
atmospheric flow over the
southern Africa?
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South Africa with typical summer weather
conditions
Average rainfall totals for January
(1960-2002)
Summer rainfall over South Africa is predominantly caused by moist tropical air
that propagates southwards towards the central and eastern parts of the country.
The flow is possible because of a continental low or trough that forms as a result
off a hi
higher
h continental
i
l hheat flflux – relative
l i to the
h surrounding
di oceans.
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South Africa with typical winter weather
conditions
Average rainfall totals for July
(1960-2002)
Winter rainfall over South Africa is predominantly the result of moist air
convection against the slope of heavy cold air masses (cold fronts) that
propagates from west to east over the southern continent and ocean. Cold fronts
are followed by higher pressures that ridge in to the south of the continent
continent, and
with counterclockwise rotation, allow for moisture convection over the east.
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METEOSAT images
25-30 September 2007
Every 6 hours
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Carbon Dioxide
Greenhouse Gasses: Nitrogen
Water Vapour
Solar constant
1300 W.m-2
Albedo
430 W.m-2
870 W.m-2
30o
S
Dry with
subsidence
L H
With no atmosphere and
30% reflection Earth’s
average surface temperature
would have been -18°C
18 C
30o
S
Dry with
subsidence
L H
With an atmosphere
t
h andd
30% reflection, greenhouse
gasses allow for an average
surface temperature of
+15°C
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Historic relationship between Atmospheric Temperatures and Carbon Dioxide (CO2)
~370 ppmv in 2000
Lower Atmospheric Temperature (blue) and CO2 Concentrations (red)
250
200
150
Carbon Dioxid
C
de (ppmv)
300
It is thought that the onset of glaciation and the subsequent interglacial periods have been brought on by
changes in the Earth's orbit around the Sun, also known as the Milankovich cycles.
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NASA / GISS : Global lower atmospheric temperature change: blue = colder , red = warmer
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Globally observed surface temperatures over the past 146 years
Currently highest in past 1000 years
Warmest 12 years: 1998,2005,2003,2002,2004,2006,
2001,1997,1995,1999,1990,2000
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WHAT IS AN ATMOSPHERIC MODEL?
A computer programme of highly complex flow dynamic equations and expressions that
explain surface and atmospheric physics that simulate the flow of the atmosphere
INITIAL FIELD (data assimilation) / PROGNOSTIC FIELD (u,v,w,T,q)
AT
TMOSPHERE
MODEL GRID
RADIATION
BUDGET
ATMOSPHERIC
CHEMISTRY
EARTH SURF
FACE
RAINFALL
ESTIMATION
Z
X
Y
BOUNDARY FORCING
At each time-step (5-30 min) the full set of atmospheric equations (momentum,
continuity, thermodynamic, moisture) as well as all physical parameterization schemes
are solved for each model grid point
point.
This is done on the world’s strongest supercomputers
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Future CO2 scenarios
A2 scenario envisions population growth to 15 billion by the year 2100 and rather slow
economic and technological development.
B2 scenario envisions slower population growth (10.4 billion by 2100) with a more rapidly
evolving economy and more emphasis on environmental protection.
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Globally observed surface temperatures over the period 1900 to 1990
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Estimated change between 2000 and 2099 (100 years) – A2 scenario
Model projected surface air temperature change
South
North
South Africa
Model projected rainfall change
South
North
South Africa
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What can we expect?
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Can the models tell us anything about South Africa?
IPCC GCMs:
Ruosteenoja
classification
Absolute temperature
and percentage
precipitation
change relative to 19611990 (baseline period)
for SRES emissions
scenario ( A2 ,B2, A1fI,
B1)
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S
Southern
Africa reggion, P-change vss. T-change
((2079-2098) – (19779-1998)
A1B scenario, 21 m
A
models
Can the models tell us anything about South Africa?
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Can the models tell us anything about South Africa?
Southern Africa region, P-change vs. T-change
(2079-2098) – (1979-1998)
A1B scenario, 21 models
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But temperatures are already rising – can we see anything in what we observe?
Observed trends in annual
rainfall totals
1960-2002
No spatially coherent
statistically significant trends
could be identified
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Are there trends in monthly rainfall?
Observed trends in
April (left) and May
(right) rainfall totals
1960-2002
960 00
R a in fa ll to ta ls ( m m ) fo r M a y a re a : 7 2 s ta tio n s
100
80
60
40
20
120
100
80
60
40
20
Year
Year
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2000
1998
1996
1994
1992
1990
1988
1986
1984
1982
1980
1978
1976
1974
1972
1970
1968
1966
1964
1962
1960
2 00 0
1 99 8
1 99 6
1 99 4
1 99 2
1 99 0
1 98 8
1 98 6
1 98 4
1 98 2
1 98 0
1 97 8
1 97 6
1 97 4
1 97 2
1 97 0
1 96 8
1 96 6
0
1 96 4
0
1 96 2
Are we perhaps
experiencing a shift in
season?
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1 96 0
– the summer rainfall
season might
g gget
shorter.
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R a in fa ll totals (m m ) for A p ril a r ea : 2 16 s tatio ns
Spatially coherent and
statistically significant
trends indicate that the
late summer is getting
dryer over the summer
rainfall
i f ll region.
i
Are there trends in monthly rainfall?
Year
Year
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20 0 0
19 9 8
19 9 6
19 9 4
19 9 2
19 9 0
19 8 8
19 8 6
19 8 4
19 8 2
19 8 0
19 7 8
19 7 6
0
19 7 4
2000
1998
1996
1994
1992
1990
1988
1986
1984
1982
1980
1978
1976
1974
1972
1970
1968
1966
1964
1962
0
10
19 7 2
20
20
19 7 0
40
30
19 6 8
60
40
19 6 6
80
50
19 6 4
100
60
19 6 2
120
1960
Are we pperhaps
p
experiencing a shift in
season?
R a inn fa ll to ta ls ( m m ) fo r J u ly a r e a : 2 0 2 s ta tio n s
– the winter rainfall
season appear to get
longer.
140
19 6 0
Spatially coherent and
statistically
t ti ti ll significant
i ifi t
trends indicate that the
late winter is getting
wetter over the winter
rainfall region.
R a in fall to ta ls (m m ) fo r S e p te m b er a r e a : 1 9 2 s ta tio ns
Observed trends in July
(left) and September
(right) rainfall totals
1960-2002
How do what we observed compare to the latest model projections?
ANNUAL
DEC, JAN, FEB
JUN, JUL, AUG
4 0 ºC
4.0
C
3.5 ºC
3.0 ºC
2.5 ºC
Temperature changes over South Africa from the MMD-A1B simulation. Annual mean, December, January, February
and June-July-August temperature change between 1980 to 1999 and 2080 to 2099, averaged over 21 models.
+ 15%
+ 10%
- 5%
- 10%
- 15%
- 20%
- 25%
Precipitation changes over South Africa from the MMD-A1B simulation. Annual mean, December, January, February
and June-July-August % precipitation change between 1980 to 1999 and 2080 to 2099
2099, averaged over 21 models
models.
8-13
5-7
3-4
1-2
0
Number of models out of 21 that project increases in precipitation
. Temperature and precipitation changes over Africa from the MMD-A1B
MMD A1B simulation. Top row:
Annual mean, DJF and JJA temperature change between 1980 to 1999 and 2080 to 2099,
averaged over 21 models. Middle row: same as top, but for fractional change in precipitation.
Bottom row: number of models out of 21 that project increases in precipitation
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What does the future holds for summer and winter rainfall
over southern Africa?
Trends: Dry
Models: Dry
Trends: Annual – none
Late summer – drier
Summer rain season shorter?
Models: Annual - none
Trends: Annual - none
Late winter – wetter
Winter rain season longer?
Models: Dry
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