9.5-9.9

9.5

Historical Climate Change

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

  • Varies in obliquity (~40,000 yrs.) exposing northern latitudes to higher insolation at different times

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

    • More eccentric = further from the sun

Earth’s Historical Climate

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

  1. Foraminifera shells in ocean sediments - different species have different temperature tolerance

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

  3. 16O vs 18O isotope concentrations in ancient ice (increased 18O = increased temperature)

CO2 levels are strongly correlated with temperature, but causality isn’t fully understood

Effects of Climate Change

Rising temperature - habitat/species loss, drought, soil, desiccation, heat waves, increased precipitation in some regions

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

Melting of permafrost - permanently frozen tundra soils that begin to thaw and release methane and 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 and waves may be lost as sea level rises

Impact on Atmospheric Currents

Widening and weakening of Hadley cell: as temperature differences between equator and poles decreases, air ascending and expanding from equator travels further before sinking

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

  • Regions between 30 and 60 degrees may experience drier climate as cool, dry, descending air from Hadley cell shifts north and south

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

  • Because temperature and pressure differences between polar and subtropical regions is what drives the polar jet stream, less difference between them means weaker, wobblier jet stream

    • Leads to extreme cold spells in eastern US and 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 and photic zone will shift up, further from ocean floor

Altered ranges for organisms: warm water hols less O2, so many fish populations have declined, or migrated to cooler waters (MSY = maximum sustainable yield)

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

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

Unequal Global Warming

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

  • Especially the arctic (N pole) because there is more land and less water

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

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