Earth Sciences 240A - Lecture 32 - Climate I

Introduction

Climate vs. Weather

It’s just a matter of time/scale

21 thousand years ago: Glacial period

Enormous ice sheets; sea level down ~110 m

100 mya: No ice anywhere

Why the variations?

Climate System

‘Forcing’

Factors that cause change

‘Response’

Result of forcing factors

Some Forcing Factors:

Tectonic Processes

Dictated by Earth’s internal heat engine

Very slow to change climate

Ocean currents partly directed by placement of continent blocks

Earth Orbital Changes “Milankovitch Factors”

Circular to oval orbit about Sun: 100,000 year period

Earth’s axis tilt; 21.5o to 24.5o: 41,000 year period

Earth’s axis wobble (‘precession’): 26,000 year period

Combine to produce cyclical solar radiation variation

Sun Energy Change

Long-term

Sun’s radiation has increased since ‘birth’

Short-term

‘Sun spots’: 11 year period

Climate Change Responses

Importance of time of forcing vs. time of response

Full on – full off: never in nature

“Rate of response is fastest when ‘system’ is far from equilibrium”

“Each component of climate responds at a different rate”

Feedback

Case: global cooling

Positive feedback

Example: spread of snow/ice strengthens glacial growth

Negative feedback

Example: increase of ice sheets means decreased weathering, which means more CO2 left in greenhouse

Earth’s Surface Heat

Sun Radiation

Latitude variation

Albedo : % of incoming radiation that is reflected rather than absorbed by a surface

Snow/vegetation

Climate Data Archives

Historical Data

Quality of harvests

Distribution of pollen

Tree rings

Pre-Historical Data

Sediment and Ice Cores

Oxygen Isotopes

Oxygen Isotopes

16O (99.8%) and 18O (17O too rare)

Stronger bonds - heavier isotopes

Fractionation

Evaporation

Temperature dependency

Geothermometer

Climate Models

”Models put numbers on ideas”

Types

Physical

Most simulate today’s climate

control case’

Geochemical

Follow material masses through climate systems

Geochemical tracers

Past Glaciations

Terminology confusing, so use:

Icehouse

Time of continental ice somewhere

May be glacial/interglacial periods

Greenhouse

Time of no continental ice

 

Next: Climate II

26 November

Climate III

28 November

End of course

 

Review of previous exam

1 December