Download Global Warming: The Scientific Basis for Anthropogenic Climate

Global Warming:
g
The Scientific Basis for Anthropogenic
Climate Change
g
The context: 6.7 billion people – 1 planet. Is there a future?
• “Th
“The global
l b l average nett effect
ff t off human
h
activities
ti iti since
i
1750 has been one of warming, with a radiative forcing
of 1.6 Wm-2” (IPCC 2007).
• Mean g
global CO2 concentrations are now at 386ppm
pp
(2008 mean) which is 30% greater than at anytime in the
past 800,000 years, while the rate of increase is 200
times faster than at any time over the same period.
period
• “Warming
Warming of the climate system is unequivocal
unequivocal, as is
now evident from observations of increases in global
average air temperature, widespread melting of snow
and
d iice, and
d rising
i i global
l b l average sea llevel”
l” (IPCC
2007).
“If humanity wishes to preserve a planet similar to that
on which civilization developed and to which life on
Earth is adapted, palaeoclimate evidence and ongoing
climate change suggest that CO2 will need to be
reduced from its current 386ppm to at most
350ppm
350ppm….
If the present overshoot of this target CO2
is not brief, there is a possibility of seeding irreversible
catastrophic
catast
op c events”.
e e ts ((Hansen
a se et a
al. p
p1,, 2008).
008)
“Human p
population
p
g
growth is a root cause of the stress
that humanity is placing upon the global environment
and upon the other species sharing our planet’s
resources A deliberate policy of population growth (as
resources.
promoted by Australia) is inconsistent with
preservation of climate and nature” ((Hansen 2009).
p
)
Recent Climate Change
Global average air temperature
• 100-year linear trend of temperature increase 0.74 [0.56 to
0.92] oC for 1906-2005.
• Average ocean temperature increased to depths of at least
3000 m – ocean has absorbed 80% of heat added
> seawater expansion - Sea Level Rise.
• Annual average Arctic sea ice extent shrunk by 2.7 % per
decade, decreases in summer 7.4 %.
• The maximum area covered by seasonally frozen ground has
decreased byy about 7%
% in the Northern Hemisphere
p
since
1900, in spring of up to 15%.
(Source: IPCC 2007)
Global warming – the scientific basis for
human induced climate change
• Evidence of human induced climate
change – how does it work?
• Climate forcing by greenhouse gases and
landuse change
change.
• Our future climate.
Images of climate change:
Glacier and Ice cap retreat
Rh
Rhone
Glacier,
Gl i Switzerland
S it l d
1859
2004
Arctic Sea Ice Loss
(Source: National Snow and Ice Data Centre, USA)
Evidence of Past Climate Change
(Source Petit et. al. 1999)
•
Ice cores provide excellent geologic archives of climate variability and
changes in atmospheric trace gas concentrations, i.e. CO2, CH4.
•
This record (eg Vostock) shows that the rapid increase in greenhouse gases
(CO2) monitored over the last 50 yrs and the corresponding rate of
temperature change has not been exceeded for at least the last 425ka yrs.
CO2
CH4
The atmospheric concentration of CO2 and CH4 in 2009
exceeds by far the natural range of the last 650,000 years
The Hockey Stick
Curve
The Mann temperature curve
identified for the first time the
recent rapid increase
in air temperature.
Evidence of Anthropogenic
Forced Climate?
Te
emperatu
ure anom
maly (°C)
Are we starting to cool?
(Reference 1961-1990)
YEAR
(Source: Climate Research Centre, UEA, Norwich, UK)
ENSO forcing
g of recent global
g
temperatures
p
El Niño events correlate with
warmer years, while La Nina
correlate with cooler years.
y
Volcanic eruptions increase
global albedo and result
in a cooling IF the
eruption is of sufficient size.
(Source: Hansen et al 2009)
Solar forcing
• Th
The currentt solar
l cycle
l h
has llasted
t d about
b t 2 years llonger th
than
usual. A possible cause for slowing of the warming trend?
• If it doesn’t recover, the associated negative forcing relative to
the mean solar irradiance is equivalent to about 7 years of CO2
increase at current rates – so no new Ice Age!!
Spatial trends in global warming
IPCC (2007).
Global Mean Temperature Trends
(IPCC 2007)
FAQ 3.1, Figure 1
Recent Change in Australian Climate
Ob
Observed
d change
h
iin maximum
i
air
i ttemperature
t
(C/decade) 1970-2008
18
Ob
Observed
d change
h
annuall ttotal
t l rainfall
i f ll ((mm/decade)
/d
d ) 1970
1970-2008
2008
The Basic Cause of Global Warming:
Modification of the Global Energy Balance
• Under ideal conditions the global energy balance is in equilibrium,
i.e. 342 Wm-2 incoming solar radiation is balanced by 235 Wm-2 of
outgoing longwave and 107 Wm-2 of reflected solar radiation.
• Disruption of this system causes global warming or cooling.
Source: Kiehl and Trenberth (1997).
Modification of the Radiation Budget
g
and Energy Balance:
The Cause of Global Warming?
• Change in atmospheric composition
(greenhouse gas emissions).
• Change in land surface type (land
clearing, reduced snow & ice cover).
• Change in cloud cover
cover.
Atmospheric absorption of radiation
1
0
1
N 2O
Absorptivitty
A
0
1
O 2 and O 3
0
1
CO2
0
1
Gre
eenhouse
e Gases
CH 4
H 20
0
1
Atmosphere
0
0.1
0.2 0.3 0.4
0.6 0.8 1
1.5
2
3
4 5 6 8 10
S hort wave
Long wave
20 30
‘W
W indow
indow’
Wavelength (  m)
Figure 2.12 Absorption of short- and long-wave radiation by consitituents of the atmosphere and by the
atmosphere
h as a whole
h l (after
( f Fleagle
Fl l & Businger
B i
1980)
1980).
Atmospheric absorption of radiation
1
0
1
N 2O
Abbsorptivity
0
1
Gre
eenhouse
e Gases
CH 4
O 2 and O 3
0
1
CO2
0
1
H 20
0
1
Atmosphere
0
0.1
0.2 0.3 0.4
0.6 0.8 1
1.5
2
3
4 5 6 8 10
S hort wave
Long wave
20 30
‘W indow’
Wavelength (  m)
Fi
Figure
2 12 Absorption
2.12
Ab
ti off shorth t andd long-wave
l
radiation
di ti by
b consitituents
itit t off the
th atmosphere
t
h
andd by
b the
th
atmosphere as a whole (after Fleagle & Businger 1980).
Radiative Forcing and our Understanding
(IPCC 2007)
Anthropogenic factors
• Human activities that affect climate through:
Variable
changed
Scale of effect
Sources of
change
Atmospheric composition Local-global
Local global
Release of aerosols and
trace gases
Surface properties;
energy budgets
b d t
Regional
Deforestation;
d
desertification;
tifi ti
urbanization
Wind regime
Local-regional
Deforestation;
urbanization
Hydrological cycle
components
p
Local-regional
Deforestation;
desertification;; irrigation;
g
;
urbanization
100 Year Global Warming Potentials
for selected greenhouse gases and atmospheric lifetimes
Greenhouse Gas
Global Warming Potential
Lifetime
• CO2 (carbon dioxide)
1
variable
• C
CH4 ((methane)
et a e)
 21
12.2 y
yrs
s
• N2O ((nitrous oxide))
 206
120 y
yrs
• HFC (hydrofluorocarbons)
 140 - 11700
1.5 - 264 yrs
• CFC (chlorofluorocarbons)
 12000 – 16000
• PFC (perfluorocarbons)
 6500 - 9200
• SF6 (sulfur hexafluoride)
 23000
3200 - 50000yrs
3200 yrs
Modification of atmospheric composition
• Enhanced warming by greenhouse gas emissions since 1765AD is
estimated at 1.6 W m-2 (0.6 – 2.4 W m-2 ).
• Human emissions of CO2 are believed to contribute to 60% of this
warming CH4 20%,
warming,
20% CFCs 12% and NO 6%
6%.
• The warming has been attributed to increased absorption of L↑.
• CFCs have also lead to a dramatic depletion of stratospheric O3 resulting
in modification of stratospheric temperature.
• Aerosols may cause either a warming or cooling or both, i.e. black
carbon absorbs solar radiation thereby changing the vertical temperature
structure
t t
where
h
minerals
i
l such
h as C
Ca may cause a cooling.
li
• Water soluble inorganic species (SO2 (sulphur dioxide), NO3 (Nitrate) and
NH4 (ammonium)) may backscatter the solar beam (direct forcing) and/or
affect cloud micro-physical processes (increase cloud lifetime).
Longest (most reliable) measured
record of CO2, Mauna Loa
Loa, Hawaii
CO2 Mauna Loa
400
2008: 386 ppm
CO2 ppm
C
380
360
340
Exceeding 400ppm is now believed to be
a possible trigger for dangerous climate
change (2013?)
320
300
20
030
20
020
20
010
20
000
19
990
19
980
19
970
19
960
19
950
Year
Atmospheric concentrations
i CO2, CH4, N2O
in
• Atmospheric concentrations of
carbon
b di
dioxide,
id methane
h
and
d
nitrous oxide over the last 10,000
years (large panels) and since
1750 (inset panels).
• Measurements are shown from ice
cores (symbols with different
colours for different studies) and
atmospheric samples (red lines).
• The corresponding radiative
forcings are shown on the right
h d axes off th
hand
the llarge panels
l
GHG Emissions by Source
(IPCC 2007)
Change in Land Cover
• Occurs at a range of scales – urban heat islands
to Amazon deforestation.
• Australian land cover change: clearing of 1.2
million km2 (~13%) of the continent since
European settlement.
• Global land cover change: 25 -30%
30% of GHG
emission each year are from deforestation.
Example of land clearing in western
Queensland
34
Modeling the impact of landcover
change in Australia.
• Uncoupled CSIRO Mark 3 atmospheric GCM (T63);
• Two sets of model simulations ((ensemble of 10 each)) for
the period 1949-2003, (a) pre-European and (b) Modern
Day Australian Vegetation;
• The mapping of Australian LCC was derived from the
structural and floristic vegetation maps of Australian
vegetation;
• Analysed the annual averages and seasonal means for
the 1951-2003 period averaged over all model ensembles
for temperature and rainfall.
Results: Difference in the ensemble annual climate averages
(1951-2003) between pre-European and modern day conditions
Temp (oC)
Soil Moisture (% Change)
Rainfall (% Change)
Surface Wind Speed (m/sec)
Results: Seasonal Climate
DJF (Summer) Temp (oC)
Summer Rainfall (% Change)
JJA (Winter) Temp (oC)
Winter Rainfall (% Change)
37
Land cover change and climate
• Modification of the surface energy balance. Reduced
evaporation/evapotranspiration and lowered soil
moisture meaning more net radiation is converted into
sensible heat.
• Changes in surface roughness. Removal of trees
p
– higher
g
evaporation
p
rates and
increases wind speeds
moisture divergence.
• Changes in seasonal land cover and vegetation types
influence exchanges of heat, moisture and momentum
between the atmosphere and surface.
• Changes in surface temperature influence synoptic scale
pressure field patterns leading to change in regional
circulation patterns.
Where are we heading: IPCC 4th Assessment
• On average 0.6°C globally over the last 100yrs.
• 1920 to mid
id 1940
1940s considerable
id bl warming
i off 0
0.4
4 °C.
°C
• Mid 1940s to early 1970s temperature oscillations with a range of
0.4 °C with the northern hemisphere cooling slightly and the
southern hemisphere remaining fairly constant.
• Mid 1970s to present has witnessed a pronounced warming of 0.5
°C. In a few regions temperature has increased > 1 °C
• Antarctic peninsula has experienced a warming of >2.5 °C in the
last 50 yrs
• Mid-troposphere temperatures above the Antarctic have risen
between 0.5 °C to 0.75 °C in the last 30 yrs
Future scenarios
• Modeled p
predictions of future climate run a wide
range of scenarios, e.g. Scenario A1F1
Assumes a fossil fuel intensive society where
fossil fuels are used to promote economic
development. It is the most CO2 heavy scenario
and assumes of tripling of CO2 level by 2100
(1134
(1134ppm)
) with
ith coall b
being
i used
d extensively
t
i l tto
drive economic development in developing
economies coupled with lower population growth
(worse case scenario – perhaps?).
IPCC – 4th Assessment Predictions
Global GHG Emissions
(IPCC 2007)
Global surface warming
But how good are IPCC predictions?
• The IPCC is heavily influenced by politics.
• Leads to dumbing down of predictions to the lowest denominator.
• Climate change scenarios used by the IPCC are more than 10
years old.
• Incorporate scientific findings up to 2005/06
2005/06.
• IPCC predictions do no consider recent rapid growth in GHG
emissions from China and India
India.
• They assume a certain amount of spontaneous decarbonisation
without direct policy intervention
intervention.
• In seeking global consensus I believe they may be highly
conservative
ti iin th
their
i predictions
di ti
off ffuture
t
climate.
li t
Temperature
Predicted v.
Observed
Temperature
predictions – good
Sea Level
Sea level – well?
(Source: Pielke 2008)
Sea level
• S
Sea level
l
l rise
i iis predicted
di t d b
by th
the IPCC(2007) tto range ffrom
18 to 59cm by 2100. A further 10 to 20cm may occur due to
melting of ice sheets.
• However further ice sheet contributions cannot be
discounted or at present quantified.
• 20,000 yrs BP when temperatures were 4 to 7°C less than
present sea level was 120m below present.
• In the Pliocene 3 million years ago temperatures were 2 to
g
than
3°C warmer and sea level was 25 to 35 m higher
present.
• Sea level would therefore appear to respond to a change in
temperature of 1°C on the order of 10 to 30m !!
Future Sea Level
• Prediction of future sea level is difficult because we do not have
adequate physical models to model all associated physical
processes.
• Rahmstorf (2007) developed a semi-empirical model based on
the simple relationship between temperature change and sea
level change.
Predicted Sea Level Change
Rahmstorf (2007)
(1.4 to 5.8°C warming)
IPCC (2007)
The Future for the Gold Coast?
Assessment of Target CO2
Phenomenon
Target CO2 (ppm)
1. Arctic Sea Ice
300-325
2 Ice Sheets/Sea Level
2.
300 350
300-350
3. Shifting Climatic Zones
300-350
300
350
4. Alpine
p
Water Supplies
pp
300-350
5. Avoid Ocean Acidification
300-350
 Initial Target CO2 = 350* ppm
*assumes
assumes CH4, O3, Black Soot decrease
Reference: Hansen et al. Target Atmospheric CO2, Open Atmos. Sci., 2008
Questions