Chapter 8: Emissions scenarios and future climate predictions
Summary of IPCC conclusions
Warming of the climate system is unequivocal
Humans are very likely in control of the climate today
IPCC conclusions (2007)
Most of the warming since 1950 very likely due to observed increase in GHG
concentration (90% probability)
Past climate change important
Future climate change of concern
Given our favorite equation
S(1-α)/4 = σ T4
can estimate the evolution of T if we have estimates of n, S, and α.
Need predictions of n and α for climate models
What are the factors that control emissions?
P. Population
Each person consumes goods and services, which leads to emissions, so as population
increases, so does emissions
A.Affluence
As people get wealthier, they consume more goods and services, which leads to increased
emissions
T.Technology
Efficiency with which energy is consumed, technology used to generate the energy
Technology = energy intensity x carbon intensity
Energy intensity = J/$ GDP
Carbon intensity = CO2 emitted/J
Energy intensity is the “conservation” term. Buy an energy efficient light bulb,
hybrid car, add insulation to your attic, etc., you produce the same output, but use less
energy
Carbon intensity is the “generation technology” term, and represents how you
generate energy.
Wind, solar, hydro, nuclear have zero; natural gas is the best fossil fuel, followed
by oil, the worst is coal
That’s why gas is often referred to as “green” and why there’s such an emphasis
on “clean coal”
IPAT = population x affluence x technology
IPAT = CO2 emitted
Population x wealth/person = total wealth of society = GDP
= total goods and services produced/consumed by a society
Technology = GHG emitted/$ = energy intensity x carbon intensity
Renewables + nuclear are best; of the fossil fuels, natural gas is best, followed by oil;
coal is the worst
Energy required: PxA ; Energy intensity = # of Joules required
Carbon intensity * energy required = amount of CO2 emitted
Technology term also called “greenhouse gas intensity” - policy debates
Recent historical data population: GDP, EI, CI
Population + affluence is going up, EI and CI are going down
Population has been rising for millennia
EI and CI have both declined because
1) efficiency has improved because of price pressure
2) CI has improved as we move away from coal to nat. gas
Rise in P and A faster than fall in EI and CI
What about the future?
For each of these terms, the IPCC has made predictions of “plausible” futures.
They are not predictions, but “internally consistent storylines”:
Terms are linked: Population is linked to affluence; technology is linked to
economic growth
Predicting the future
A2 > A1 > B2 > B1
“A” scenarios are high growth (BAU)
“B” scenarios are “sustainable”: limited growth
“1” scenarios are “convergent”: narrowing gap between rich and poor
“2” scenarios are “non-convergent”
Feed emission calculations into carbon cycle model
Carbon model is required to figure out how much CO2 stays in the atmosphere.
Recall time scales for removal of carbon: about 50% removed within one year, about
80% removed after a few centuries, all removed after a few tens of thousands of years
CO2 projections = economic + demographic model + carbon cycle model
Carbon cycle model estimates how much the ocean and land take up
A2 > A1 > B2 > B1
Can calculate total radiative forcing: [4-8]W/m2
C02 responsible for 80% of the forcing
To calculate future temperatures, input C02 estimates above into climate models
How much warming for each scenario? IPCC FIGURE
Temp rise vs time (RHS = the year 2100)
Recall: warming = 2-4°C, midpoint of about 3°C including fast feedback
Committed warming of 0.5°C
1) uncertainties in the model, 2) uncertainties in the future emissions
How do we compare with reality?
We are at or above the “worst-case” scenario
China and other developing countries (BRIC) are driving enormous increases in CO2
emissions due to coal: populations, wealth, reliance on dirty technology
Long-term future warming:
Emissions in the long run
Carbon cycle slowly removes CO2 from the atm/bios/ocean system
Fossil fuel use finite: note peak around 2100
Global surface temperatures in the long run
Note peak around 2150
Actions we take will determine the climate for the next thousand generations
Predictability of the climate
Weather vs. Climate
Requirements of a weather forecast: must predict the exact state of the atmosphere over
the time frame
If your prediction of rain is off by one day, it’s a blown forecast. If the prediction is off
by 50 km, it’s a blown forecast
The number one determinant of the weather tomorrow is the weather today.
By “weather today” I mean the exact state of today’s atmosphere
But there are errors in our specification of today’s state, and these errors grow, so that
after about 1 week we can no longer accurately describe the exact state of the weather.
For a climate forecast, we don’t care what the exact weather is on June 15, 2080
What we care is that the model gets the right statistics (distribution of temperatures,
number of rain storms, etc.) for the summer of the 2080s.
You don’t need to know the exact state of the atmosphere today for a climate prediction.
Question: is Aug. warmer or cooler than Jan. in CLL?
You just made a climate prediction months in advance, even though you cannot predict
the weather 2 weeks in advance.
For climate predictions, there is large year-to-year variability
SHOW thermometer record
We can predict warming in 100 years for the same reason we can predict that July is
hotter than January.