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Global Warming and Global Warming and

Sea Level RiseSea Level Rise

Is it warm in here or is it just us (humans)?

The greenhouse effectThe greenhouse effectWhat is it?� a natural process � works sort of like an actual greenhouse

(oversimplifying here …)� sun is the heat engine for both� Earth’s surface acts like the plants and

soil in the greenhouse� certain gases in Earth’s atmosphere act

like the glass of the greenhouse walls

Is it good or bad?� It is decidedly good - beneficial,

nay, essential, to life on Earth!� keeps Earth ~ 33oC (59oF) warmer

than it otherwise would be � 15oC instead of –18oC (Brrr!!!)

Venus Venus -- too much of a too much of a

good thinggood thingThe Venusian atmosphere is comprised almost entirely of CO2, an important greenhouse gas.

Venus receives even more solar energy than Earth.

So what would you expect Venus’s surface to be like?

HOT! ~ 430oC (can melt lead)“runaway greenhouse” effect� CO2 builds up in the atmosphere� atmosphere warms� no life can develop/survive� no hydrologic cycle (no liquid H2O)� no hydrosphere or biosphere to

take CO2 out of atmosphere

a lesson for Earthlings?

Incoming!Incoming!The Sun emits radiant energy at wavelengths between 100-10,000 nm of the electromagnetic spectrum� reaches Earth’s atmosphere

as 1630 W/m2 of energy� vast majority is between

wavelengths 200-2500 nm� 200-400 nm – ultraviolet (UV)� 400-700 nm – visible� 700+ nm – infrared (IR)

� peak wavelength is 500nm (yellow-orange visible light)

� relatively little infrared

~ 45% never reaches Earth’s surface� 6% is reflected by atmosphere� 20% is reflected by clouds� 3% is absorbed by clouds� 16% is absorbed by atmosphere

� UV and visible wavelengths – ozone (O3)� IR wavelengths – water (H2O)

& carbon dioxide (CO2)

~ 55% reaches surface (yellow curve)� 4% is reflected (mostly by ice)� 51% is absorbed (by dark land & water)

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Outgoing!Outgoing!This energy is emitted from Earth’s surface to Earth’s atmosphere and to space.Of the original 51%� 6% escapes directly to space� 45% is absorbed by gases in

the atmosphere

At night, Earth’s surface emits the 51% of incoming energy it absorbed from the Sun during the day,but at long wavelengths, in the infrared portion of the electromagnetic spectrum� peak wavelength is 10

�m

(= 10,000 nm) IR� no UV, very little visible light

emitted

The greenhouse gasesThe greenhouse gasesThese are trace gases in the atmosphere� CO2

≈0.035% of total

atmospheric gases� H2O

≈0.1-2.0% of total

(highly variable)

but, together with clouds, they absorb the 45% of outgoing longwave radiation plus 19% of incoming shortwave radiation.Greenhouse gases (ghg’s) occur as molecules of two or more different atoms (note that N2 and O2 are not ghg’s)Which ghg contributes most to the greenhouse effect?� choices (major contributors):

� methane (CH4)� nitrous oxide (N2O)� carbon dioxide (CO2)� water vapor (H2O)� ozone (O3)� chlorofluorocarbons (CFC’s)

approximate contributions� H2O

≈60-70%

� CO2

≈20-25%

� O3

≈5-7%

� others

≈3-8%

So why all the fuss about CO2? Shouldn’t we be more worried about water vapor?� answer: residence time (RT)

� RTH2O

≈1 week

� RTCO2

≈1 century (100 yrs)

� RTCH4

≈1 decade (10 yrs)

Global CO2 riseGlobal CO2 rise

Ice core data from Antarctica� actual atmospheric gas

bubbles trapped in ice� showed CO2 rose drastically

over last 1000 years� sharp increase at start of the

industrial revolution (~1800)

The famous “Keeling curve” from Mauna Loa, Hawaii� far from big city influence

� oscillations are seasonal� spring/summer – vegetation

takes up CO2

� fall/winter – vegetation releases CO2

Keeling curve

Global temperature riseGlobal temperature riseThe famous “hockey stick” curve of Mann et al., 1999

instrumental data since 1860� general increase over time� 1998 was the warmest year on

record until 2005

natural forcings (volcanic and solar) do not account for observed warming after 1970

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Global sea level riseGlobal sea level rise

Sea level rise from warming occurs via two processes:� thermal expansion

� water expands when heated

� accounts for ~2/3 of total rise

� melting glaciers� only glacial ice (land-

based) contributes� melting/thinning sea

ice does not contribute

� accounts for ~1/3 of total rise

Since 1993 global sea level has risen 34 mm (1.33 inches)

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Sea level pastSea level pastSea level has risen more than 120 m since the peak of the last ice age about 18000 years ago. However, only 2-4 m of this increase has occurred in the last 6000 years. From 3000 years ago to the 19th century the long term change was roughly 0.5 meters at a rate of 0.1 to 0.2 mm/yr.

Since 1850, sea level has risen again at 1 to 2 mm/yr.

Since 1992 satellite altimetry from TOPEX/Poseidon suggests a rate of about 3 mm/yr.

This higher value may indicate accelerating sea level change from global warming.

Sea level futureSea level futureDifferent climate models predict different amounts of sea level change in the next century (~0.2-0.6 m)There is a great deal of uncertainty as to how much melting will occur� It is possible that increased snowfall

will compensate for glacial melting in a global warming scenario

� It is equally possible that positive feedbacks, such as basal melting leading to higher glacial flow rates, will speed up the glacial loss.

Melting or thinning sea ice does not add to sea level because it is already floating in the ocean.The biggest concerns lie with melting of the ice sheets on Greenland and Antarctica� The Greenland ice sheet would

contribute about 7 m of sea level if it melted completely.

� This is possible, but it would take several millenia.

� The Antarctic ice sheets would contribute about 60 m of sea level of they melted!

� But this is highly unlikely to ever occur.

Sea level in actionSea level in action