Global Warming
Prof. Dr. Carsten Vogt
Bochum University of Applied Sciences
Summer term 2013
Global warming: Some facts
• Average global surface temperature:
– Depends on two things:
– Amount of incoming solar radiation
– Amount of reflected energy (reflection by surface and
clouds)
• Depends on concentration of certain gases in the atmosphere
• Some gases let incoming short wave radiation pass through
• But absorb the reflected long wave radiation (wave length is
altered by reflection – remember your physics courses!)
– So called natural greenhouse effect
Global warming: Some facts
• main effect: global average surface temperature of
15°C
• Without greenhouse gases: -18°C!
• Simple positive relation: The higher the
concentration of greenhouse gases, the higher the
average temperature!
• Most important gases: water vapour, carbon dioxide
(CO2), methane (CH4)
Global warming: Some facts
• Main problem: concentration of CO2 and CH4
increases due to human activities
– CO2: burning of fossil fuels
– CH4: emerges in agriculture (ranching, rice cultivation)
•  so called anthropogenic greenhouse effect
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How strong?
Measure: radiative forcing
Natural greenhouse effect: 324 W/m2
Anthropogenic effect: 2.7 W/m2
• 60% due to CO2!
Global warming: Some facts
Global warming: Keeling curve
CO2 concentration over past 1000 years
Global temperature (°C) since 1850
(deviation from long run average)
Climate Sensitivity
• Definition: temperature increase resulting from a
doubling of CO2 concentration
• Can be directly measured (in experiment): 1,2°C
• Problem: Different additional effects in the field
•  several feedback loops
• Positive feedbacks:
– e.g. water vapour
Climate Sensitivity. Positive feedbacks
Atmospheric content
of water vapour
increases
+
+
T
Temp.
increase
Climate Sensitivity: Feedbacks
• Another positive feedback: melting of arctic ice
• Reduced ‚albedo‘ (ability to reflect sunlight)
• Leads to further warming
• Example for a negative feedback loop:
– Increased temperature leads to more clouds
– Clouds can have a cooling effect (more sunlight is
reflected)
Climate Sensitivity
• One important difficulty in climatology: quantifying
these feedbacks
• Taking feedbacks into account, climatologists end up
with a measure of CS of 2°C up to 4,5°C (best guess
at 3°C)
• Note: if we have a meaure for CS AND if we know
development of future CO2-emissions we can predict
future temperature increase!
Future temperature
• Depends on several variables like, e.g.
–
–
–
–
Level of economic activity (economic growth)
Population growth
energy policy
Technical progress
• impossible to predict the future, but:
• We can calculate scenarios (this is much different
from a forecast!)
Future temperature increase
• Economists have developed 40 plausible scenarios
for IPCC (Intergovernmental Panel on Climate
Change) (SRES scenarios)
• Most optimistic scenario
– Low population growth
– High speed of technical progress
– Fast substitution of fossil fuel based energy generation to
renewables
– CO2 concentration increases up to 540 ppm
– Leading to a temperature increase of 1,1°C
Future temperature increase (until 2100)
• Most pessimistic scenario (high economic growth,
high population growth, low speed of energy
substitution…)
• CO2 increases to 970 ppm
• Temperature increases by 6,4°C
What do we know about Global Warming for
sure ?
• Atmospheric concentration of CO2 has increased from 280
ppm (1850) up to 379 ppm (2005)
• Without doubt due to anthropogenic causes (burning of
fossils)
• Climate sensitivity lies in a range from 2,0 to 4,5 °C
• Global warming since 1850: +0,8°C
Global Warming: Risks
• Melt down of glaciers worldwide
• E.g. in the Alpes, glaciers have lost half of their original
volume
• May cause water stress in different areas of the world
• Melting of arctic ice
– Loss of habitat for animal species and Inuit
– Positive feedback loop: less reflection of sunlight in the polar region
stimulates the greenhouse effect
• Melting of permafrost
– Loss of infrastructure (buildings, streets, plants …)
Global Warming: Risks
• Melting of inland iceshelfs (Greenland, Antarctica)
• Greenland: Meltdown of iceshelf already expected
for a global temperature increase by 2°C
• Caution: Melting process will last several hundreds
years
• However: Complete meltdown lets the sealevel rise
by 7m!
– Problems for low lying coastal areas
– Many big cities are located at coastal lines
Global Warming: Risks
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Antarctica: contains 57 m sea level rise !
Temperatures in Antarctica very low
Global warming will not directly affect the iceshelf
Indirect: Warmer incoming water lets shelfs melt
down
• This process has accelerated during the past years
Global Warming: Risks
• Sea level rise: Even small changes in global temperature can
cause huge sea level rise or fall:
• E.g. 120000 years ago (last warm period), temperatures were
only 1°C higher
• Sea level was 2 to 6 meters higher than today
• 20.000 years ago (last ice age): Temperature was 4 to 7°C
below nowadays average
• Sea level was 120 m lower than today
Global Warming: Risks
• Break down (partial or total) of thermohaline circulation
• Atlantic ocean: water transport from southern areas to
northern areas
• Cold water sinks down in the Labrador sea, creating a gigantic
flow of water and heat (15 mio cubicmeters of water every
second, 2000 times energy generated in all europan power
plants)
• Warming of northern sea may cause to break down this flow
– Warm water is less dense than cold water
– In addition: melting of Greenland ice contributes (sweet water flow in
the ocean, reducing salt concentration in the sea water and thus the
density)
Global Warming: Risks
• Total breakdown of northern atlantic circulation: decrease of
temperatures in northern and western Europe by several
degrees
• Most models predict at worst a 50% weakening of
thermohaline circulation
• Higher probability of extreme weather events
– Droughts and heating waves (Europe 2003: 20.000 to 30.000 lives lost
in addition)
– More destructive hurricanes (e.g. 2005: estimated loss of 160 Billion
US-Dollars and 3900 lives lost)
– More intense rainfalls and floodings
Global Warming: Benefits
• Less lives lost due to cold weather in winter
• Increased crop yields in northern areas (Canada, northern
parts of US, northern Europe)
• New sea routes available in arctic sea (northwest and northeastern passage)  decrease in transportation costs
• Increased availability of natural resources, e.g. in the arctic
sea
Global Warming: Summary
• Clearly, costs of global warming more than outweigh benefits
• Central question: What should we do against global warming?
– How much action is needed ?
– How strongly should CO2-emissions be decreased?
– And: how fast?
–  question of OPTIMAL CLIMATE POLICY