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
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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
