Climate Change

and its consequences

Bill Menke

October 4, 2005

1. The factors that influence global temperature

2. The role of greenhouse gasses

3. Anthropogenic emissions of greenhouse gasses

4. Natural Variability and Modeling efforts

5. Hazards of increased temperatures

Summary

Sunlight – mostly visible light

Maximum = 1370 W/m2

Average* = 1370 / 4 = 343 W/m2

Aboput 30% reflected as visible light

The rest converted absorbed by earthEarth’s surface gets hot

and re-radiated energy as Invisible Infrared radiation343 W/m2

103 W/m2

240 W/m2

*average: surface area of earth is 4r2, but only a disk of diameter r2 faces the sun, hence the average is one fourth of the maximum

1370 W/m2

One toaster per square meter

that’s a lot of heat …

1370 Watts

1 square meter

Infrared energy radiated by a hot object depends

strongly

upon it temperature

1 square meter

temperature, T

E = c T4

With T in Kelvin andc = 5.6 x 10-8 W/m2K4

So how hot is the earth’s surface

1 square meter

temperature, T

240 = 5.6 x 10-8 T4

240 Wattsdownward

Balanced by 240 Wattsupward

T = ( 240 / 5.6 x 10-8 )1/4

= 255 K = -17 deg C = 1 deg F

Reality Check ! 255 K is too cold

We must be doing something wrong ...

New style of summer clothing

We forgot the Atmosphere

Layer just thick enough to absorb allInfrared radiation that shines upon it

Energy balance:IR shining into a layerMust be balanced by

energy radiated by layer

Model with two layer atmosphereTop of Atmosphere

upward radiation must balance solar input

cT04 = 240 W/m2

T0 = 255K

Top surface240 up

Bottom surface240 down

Sun240 down

Total energy leaving upper atmosphere2 x 240 W/m2

Must equal energy received from lower atmosphere

T0 = 255K

cT14 = 2 x 240 W/m2

T1 = 304K

Lower atmosphere must be hotter to balanceheat shining down from upper atmosphere

240 up

240 down

480 up

Suppose you had three layers …

T0 = 255K

cT24 = (480 + 480 – 240) W/m2

T1 = 304K

Lower atmosphere must be hotter to balanceheat shining down from upper atmosphere

T2 = 337K

480 up

480 down

720 up

240 down

The more layers, each just opaque enough to absorb IR radiation, the hotter the lower

atmosphere is

How many layers in the Earth’ atmosphere?

Somewhere between 2 and 3, probably closer to 2(well, this is a very simplified model …)

What controls the IR opacity of the atmosphere

The concentration of greenhouse gasses:

Water vaporCarbon dioxide

MethaneHalocarbonsNitrous oxide

How do you compare greenhouseGas concentrations ?

The effect on the IR opacity is what’s important

Not their actual concentration in the atmosphere

Standard practice – represent as an equivalent change in solar radiation,

“radiative forcing”

Greenhouse gasses have been increasing due to anthropogenic causese.g. burningfossil fuels

Can future rise in atmosphericCO2 be predicted?

Sources of atmospheric CO2:burning fossil fuelsdeforestation

Sinks of atmospheric CO2:absorption into oceanforest growth

Spatial pattern of oceans absorption and emmission of CO2 is very variable

But remember … water vapor is the major greenhouse gas

That’s a problem for models of global warming

concentration of water vapor very variable

they are also temperature dependenthotter climate – more water vapor

but water vapor creates clouds, which increase the earth’s albedo

The earth is getting hotter

Is the increase in radiative forcing causing this global warming n?

And if so

Will temperatures continue to increase as we continue to add

greenhouse gasses to the atmosphere?

Lesson from the Ice Age: global temperature has been very variable, soNatural variation needs to be seriously considered …

Models of climate necessary to sort out causes

But

How do you build confidence that models are correctly predicting the climate system?

Traditional well-controlled experiments are impractical …

One approach … model the past

An attempt to modelLast 150 yearTemperature variationsRequires bothNatural andAnthropogenicForcing

Note big increase inAnthropogenic forcingSince 1950

EXEMPLARY IMPACT : SEA LEVEL RISE

Light grey: parts of MississippiDelta that are less than 5 metersbelow sea leavel.

Why Sea Level Rise?

• Melting of Glaciers, especially in Greenland and West Antarctica

– Huge effect: 7 meters if you melt Greenland alone

• Thermal expansion of ocean waters as you heat them

– Huge effect: 1 meter per deg C, if you warmed the entire world ocean down to the sea floor

A very large percentage ofpeople live at low elevations

List of Physical ImpactsTemperature

hotter peak temperaturesfewer really cold dayshotter nights

Weathermore droughtsmore intense wind in hurricaneswarmer winters

Precipitationmore intense rain stormsmore evaporation, less runoff in rivers