Historic Climate Change

• Earth’s climate has varied over geologic time, largely due to variations in earth’s orbit around the sun

  • Varies in eccentricity (~100,000 years) bringing it closer to and further from the sun at different times

    • More eccentric = further from sun

  • Varies in Obliquity (~40000 years) exposing northern latitudes to higher insolation at different times

  • Leads to predictable variation in Earth’s climate called Milankovitch cycles

Earth’s Historical Climate

• Scientists have measured and estimated earth’s historical temperature and CO₂ levels using 3 main pieces of evidence

  • Foraminifera shells in ocean sediments - different species have different temperature tolerance

  • Air bubbles in ice cores that contain ancient atmospheric gas (CO₂ levels)

  • 16O vs. 18O isotope concentrations in ancient ice (higher 18O = higher temperature)

• Global ice ages, folowed by warmer periods occur roughly every 100,000 years

• CO₂ levels are strongly correlated with temperature but casuality isn’t fully understood

Effects of Climate Change

• Rising Temperature - habitat/sepcies loss, drought, soil dessiccation, heat waves, increasesd precipitation in some regions

• Rising Sea level - due to glacial, polar ice melt + thermal expansion

• Melting of Permafrost - permanetly frozen tundra soils that begin to thaw and release methane & CO₂ from anaerobic decomposition

Impact on Coastal Communities 

• Property Loss, damage, potential relocation: Coastal communities, especially poorer ones that can’t build up may need to relocate inland

  • Seawalls or other barriers can be built higher, but this just delays eventual flooding

• Loss of barrier islands: islands that buffer coastal communities/ecosystems from wind & waves may be lost as sea level rises

Impact on Atmospheric Currents

• Widening & weakening of hadley cell: as temp. diff. between equator and poles decreases, air ascending and expanding form equator travels further before sinking

  • This shifts subtropical zones (dry, desert biomes) toward the poles and expands the tropics

  • Regions between 30^o and 60^o may experience drier climate as cool, dry, descending air from hadley cell shifts north & south

• Weakened, destabilized Jet Stream: as arctic warms faster than other areas of earth, temperature difference between equator & poles weakens

  • Because temperature & pressure differs between polar & subtropical regions is what drives polar jet stream, less differerence between them means weaker, wobblier jet stream

    • Leads to extreme cold spells in eastern US & dry spells in western US

Impact on Marine Ecosystems

• Altered range of marine ecosystems: some new marine habitats will be formed by rising sea level flooding coastline

  • Some areas of ocean will become too deep to receive sunlight & photic zone will shift up, further from ocean floor

• Altered ranges for organisms: warm water holds less O₂, so many fish populations have declined, or migrated to cooler waters

Impact on Ocean Circulation

• Suppression of thermohaline circulation: global ocean current that redistributes heat from the equator, salt, and nutrients by mixing ocean waters could slow or stop altogether 

  • Ice melt from Greenland → especially cold, fresh water buildup in north atlantic 

  • Freshwater is less dense than salt, preventing it from sinking

  • The cold north atlantic slows warmer Gulf Stream waters, cooling Europe & slowing global thermohaline circulation

Unequal Global Warming

• Polar regions of earth are warming faster than other regions (polar amplification)

  • Especially the arctic (N pole) becasue there is more land & less water to absorb heat

    • Melting sea ice = more exposed ocean water, which absorbs more sunlight than ice & snow, leading to more ice melting (positive feedback loop)

  • Distribution of tropical heat to poles by thermohaline circulation also warms poles

• Melting of Permafrost - permanently frozen tundra soils that begin to thaw & release methane & CO₂ from anaerobic decomposition

  • Air pollution adds soot & other PM to atmosphere, distributed to poles by atmospheric circulation

    • Darker, soot/PM covered ice absorbs even more heat due to lower albedo

Impact on Polar Ecosystems

• Arctic sea ice loss = habitat

  • Seals use it for resting and find holes for breathing

  • Algae grow on ice, forming base of arctic food web

  • Polar bears use ice for hunting seals at breathing holes