Climate Change and its consequences Bill Menke October 4, 2005.

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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/m 2 Average* = 1370 / 4 = 343 W/m 2 Aboput 30% reflected as visible light The rest converted absorbed by earth Earth’s surface gets hot and re-radiated energy as Invisible Infrared radiation 343 W/m2 103 W/m 2 240 W/m 2 *average: surface area of earth is 4  r 2, but only a disk of diameter  r 2 faces the sun, hence the average is one fourth of the maximum

1370 W/m 2 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 T 4 With T in Kelvin and c = 5.6 x 10 -8 W/m 2 K 4

So how hot is the earth’s surface 1 square meter temperature, T 240 = 5.6 x 10 -8 T 4 240 Watts downward Balanced by 240 Watts upward 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 all Infrared radiation that shines upon it

Energy balance: IR shining into a layer Must be balanced by energy radiated by layer

Model with two layer atmosphere Top of Atmosphere upward radiation must balance solar input cT 0 4 = 240 W/m 2 T 0 = 255K Top surface 240 up Bottom surface 240 down Sun 240 down

Total energy leaving upper atmosphere 2 x 240 W/m2 Must equal energy received from lower atmosphere T 0 = 255K cT 1 4 = 2 x 240 W/m 2 T 1 = 304K Lower atmosphere must be hotter to balance heat shining down from upper atmosphere 240 up 240 down 480 up

Suppose you had three layers … T 0 = 255K cT 2 4 = (480 + 480 – 240) W/m 2 T 1 = 304K Lower atmosphere must be hotter to balance heat shining down from upper atmosphere T 2 = 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 vapor Carbon dioxide Methane Halocarbons Nitrous oxide

How do you compare greenhouse Gas 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 causes e.g. burning fossil fuels

Can future rise in atmospheric CO 2 be predicted? Sources of atmospheric CO 2 : burning fossil fuels deforestation Sinks of atmospheric CO 2 : absorption into ocean forest growth

Spatial pattern of oceans absorption and emmission of CO 2 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 dependent hotter 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, so Natural 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 model Last 150 year Temperature variations Requires both Natural and Anthropogenic Forcing Note big increase in Anthropogenic forcing Since 1950

EXEMPLARY IMPACT : SEA LEVEL RISE Light grey: parts of Mississippi Delta that are less than 5 meters below 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 of people live at low elevations

List of Physical Impacts Temperature hotter peak temperatures fewer really cold days hotter nights Weather more droughts more intense wind in hurricanes warmer winters Precipitation more intense rain storms more evaporation, less runoff in rivers

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