Lecture 2: Changing Earth

Types of Change (5)

• cyclic change

• stability and negative feedback

• episodic/catastrophic change

• chaos

• anthropogenic versus ‘natural’ change

Cyclic Change (5)

• Daily cycle: rotation of the Earth

• Seasonal cycle: orbit of Sun and axial tilt

• Croll-Milankovitch cycles

• Supercontinent cycle

• Non-seasonal biological cycles

Axial Tilt

• keeps seasonal difference of North and South Hemispheres

• changes heat budget with latitude

• change in weather patterns in terms of temperature

Solar Cycles

• sunspots ~11 cycle

• more sunspots = more solar radiation

• some believed that increase in solar flares caused global warming -> false

• Parker Solar Probe flew through Sun’s upper atmosphere 2021

Orbital Changes (Croll-Milankovitch Cycles)

• eccentricity -> shape of Earth’s orbit

• obliquity -> axial tilt with respect to orbital plane

• precession -> direction Earth’s axis of rotation is pointed

• glaciation follows these cycles

• orbit shape and axial tilt control seasons

  • distance + angle from sun

  • equinoxes and seasons

Climate History

• marine sediments and ice cores

• Antarctica 

  • trapped air bubbles show atmosphere composition

  • isotopes for carbon dating etc.

• marine sediment

  • isotopic fractionation

Isotopic Fractionation

• more Oxygen 16 = warmer

• H2O18 evaporates slightly easier than H2O16

• O18 is preferentially removed relative to O16 by precipitation

• therefore, when there is more evaporation than precipitation, i.e. in warmer temperatures, there ends up more O16 in the ocean

Croll-Milankovitch Cycles and Glacial-Interglacial Oscillations

• effects of solar radiation variations associated with Milankovitch cycles are small compared to observed climatic changes

• therefore, climatic system responds in complex ways

  • positive feedback (amplification) and damping (negative feedback) effects involving planetary albedo, greenhouse gases and ice sheets

• eccentricity has a much smaller effect on insolation than obliquity or precession

• over last ~Ma glacial-interglacial cyclicity is ~100ka

  • only appears to extend back to about 1Ma, before which ~40ka

Supercontinent Cycle

• climate, ocean currents, sea-level, mountain-building, environments, evolution, migrations

• 10s of millions of years timescale 

• tectonic shifts

• Pangea was 225 million years ago

• size of continents also impacts climate

• Nance and Murphy 2013: supercontinent cycle linked to evolutionary events

CO2 Sources and Sinks

• CO2 pumped into atmosphere through terrestrial and submarine volcanism

  • why does the atmosphere not have much higher CO2 concentration?

  • various sinks, e.g. silicate weathering, carbonate rocks, ocean

  • geoengineering proposal -> grind up basalt rocks to encourage silicate weathering

Silicate Weathering

• carbonic acid in rainwater reacts with silica compounds in rocks

• products of silicate weathering precipitate into oceans as carbonate rocks

• negative feedback loop → increased CO2 causes increased silicate weathering

• Himalayas caused some cooling?

Snowball Earth

• during Cryogenian (Sturtian) (ca.717-660 Ma) and Marinoan (ca.651-635 Ma) global glaciations covered the entire earth with ice, incl. low latitudes

• evidence that CO2 release from volcanic activity helped end at least Marinoan glaciation (Lan et al 2022)

Episodic/Catastrophic Change

• physical events

  • major earthquakes, major volcanic eruptions, tsunamis, impact events

• biological events

  • catastrophic behaviour of biological systems, pandemic, humans and mass extinctions

• larger events happen less frequently (frequency vs magnitude)

• for earthquakes, slip surface size determines strength

Catastrophic Change: Volcanic Eruptions

• supervolcano eruptions, e.g Yellowstone, Toba

  • 2.1Ma, 1.3Ma, 640ka, 74ka

• multiple smaller eruptions closer in time

• Hunga Tonga Hunga Ha’apai 2022 largest eruption since 1991 Pinatubo eruption

Catastrophic Change: Tsunamis

• movement of water caused by an eruption, earthquake, terrestrial or submarine landslide

• movement of whole column of water across oceans

Catastrophic Change: Impact Events

• Chicxulub crater, 66 million years old, 20 km deep, associated with Cretaceous Paleogene mass extinction event

  • global layer of iridium-rich clays from this time, indicating asteroid source

  • 300Gt of sulphur (short-term cooling)

  • 425Gt of CO2 (long-term warming)

  • 75% of all life extinct

Catastrophic Change: Biological Events

• 1918 flu pandemic killed 50-100 million worldwide

Chaos Theory: Butterfly Effect

• simple systems of equations can result in non-repeating behaviour that is very sensitive to initial conditions

• discovered by Edward Lorenz in 1961 based on observations of a weather model

Chaos Theory: Deterministic Chaos

• turbulence in fluid flows, meteorology, insect populations, geomorphological systems, hydrological systems, economic systems

• complex change patterns do not necessarily arise from complex causes

  • can arise from simple non-linear relationships

• some natural systems may be inherently ‘unpredictable’

  • implications for modelling, predicting, forecasting

Anthropogenic vs Natural Change

• diverse array of human-induced change

  • lithosphere, hydrosphere, atmosphere, biosphere

• in many cases more rapid change than ‘natural’ background rates

  • land use change, disruption of river systems

• humans now an order of magnitude more significant in several cases than natural processes

• can be direct or indirect