Greenhouse effect & Global warming - Lecture 6

Why is it warm inside a greenhouse ?
Two reasons :
(1) Greenhouse is made of glass;
a selective absorber of radiation,
its absorptivity is very different for different wavelengths : a(l)
Specifically,
a ( visible light) ~ 0
a ( infrared light) ~ 1
Most sunlight passes through glass;
absorbed by objects inside greenhouse ;
Most IR radiation emitted by objects inside is absorbed by glass ;
warms up glass & evenutally air inside

(2) The glass enclosure prevents warm air from leaving the room.

Another example caused by the same process :
T inside a car in a hot summer day can become dangerously high.

ATMOSPHERIC GREENHOUSE EFFECT
selective absorbers in the atmosphere :
CO2 (carbone dioxide),
H2O (water vapor) ,
CH4 (methane), and
O3(ozone).
called Greenhouse gases
* Note: T of such gases < T of the earth's surface
* Such gases would absorb some of the radiation from the earth's warm surface;
would emit radiation at a lower temperature
both upward to space & downward to surface ;
* effectively cut down IR radiation to space ;

Due to radiative process alone, surface & atmospere eventually settle to
a state of no net gain or loss of radiation .
called Radiative Equilibrium
with a temperature , Radiative equilibrium temperature, T(re)
Summary :
T(re) with greenhouse gases >
T(re) without greenhouse gases
this process moderated the climate ;
makes it more favorable for life forms .

Issue of Global Warming :
* Significant increase of temperature due to anthropogenic increase of greenhouse gases .
* Such increase could take place in a short time.
* Impacts of such increase of T could be serious .

Possible consequences of anticipated global warming :
* agricultural impacts : hydrological cycle,
* changes in energy consumption level & patterns
* sea-level rises,
* changes in drought/flood, severe weather frequency & distribution
* impacts on ecological systems : health problems .

Natural variability vs Anthropogenic global warming
* Earth's temperature did fluctuate a lot in geological past, ( +10F, -10F )
e.g. Much warmer 130 million years ago (dinosaur bones found at high latitude in many continents)
* Also sometimes much colder many times before;
e.g. ice ages (e.g. 20,000 years ago)
* But such rate of change is very small
e.g. 5 F in 20,000 yrs means 0.025 F in a century

* Relatively stable climate in the past 10,000 years;
but regional climate seems to have fluctuated considerably more
Evidences : Ancient rivers flowed across Sahara Desert
In ~1000 AD, Northern Europe and N Atlantic were much warmer :
Vikings settled in a much warmer Greenland
1550 to 1850 : Little Ice Age

* The past 100 years : Average global temperature seems to have risen by ~ 1F since 1880
Lots of claims & counter-claims recently

Nature of Controversy :
(1) Large uncertainty in data
(2) Uncertainty in how representative are the data
(3) Uncertainty in how to separate the impact of human activity from "natural variation"
(4) Uncertainty in quantifying the greenhouse gas effect
(5) Uncertainty in what happens to the released C02 (carbon cycle)
(6) Uncertainty in additional factors, e.g. impacts of volcanic eruption
(7) Impacts on different parts of the world could be very different.

Factors influencing climate :
(1) shift in the earth's orbit around the sun ( very long time scale)
(2) variation in the sun's radiation itself (different time scales)
(3) amounts of greenhouse gases ( quite short time scale)
(4) volcanic eruption (episodic )

An easily established fact :
* Large increase of CO2 in the atm due to fossil burning since ~ 1880 :
60 million tons of C02 released daily ;
equivalent to 6 tons per person per year;
* US alone emits ~ 22% ;
(5% population; 20 tons per person per year)
* Industrial countries emit ~ 50%;
All other countries together emit ~ 50% ;
* Industrialization & population increase in every country :
Expect increasingly quicker pace in increase of CO2

Increase of other greenhouse gases :
CH4 : rice farming, landfills
N2O : fertilizers

a useful resource ENVIRONLINK

"Official assessment "
(1995 IPCC report (Intergovernmental Panel of Climate Change),1996, Cambridge U. Press)
IPCC general conclusions :
the balance of evidence suggests that there is a discernible human influence on global climate change;
T increase by 1.4 to 6.3 F by 2100

Policy issues :
* What is a sound policy in light of considerable uncertainty ?
* A problem of cost-effectiveness.
* How much reduction of greenhouse gases is necessary & feasible ?
* Optimal time to begin reduction ?
* How ?
* How to adapt to climate change & consequences ?

Exhibition

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