Temperature History of Earth Notes

Climate Change: Ancient & Modern

  • Earth's climate has continuously changed since its birth 4.5 BYBP (billion years before present).
  • Direct temperature measurements are available since approximately 1850.
  • Prior to 1850, temperature proxies are used.
    • T proxy = A measurable quantity that stands in for temperature.
    • Examples of T proxies:
      • Ocean sediments:
        • Measure chemicals and organism shells.
        • Records extend to tens of millions of years ago.
      • Ice cores:
        • Measure water isotopes.
        • Records extend to 800,000 years ago.
      • Other Proxies:
        • Tree rings
        • Boreholes

Ancient Temperature Record

  • The rock record provides evidence of climate over the past 700+ million years.
    • Earth was generally warm, but with occasional glaciations.
    • Snowball Earth events occurred at approximately 700 and 300 MYBP (million years before present).
    • P = Paleocene-Eocene Thermal Maximum (PETM), ~55 MYBP
      • Rapid warming due to greenhouse gases (GHGs).
      • Considered a good analog for present conditions.
    • Eocene Climate Optimum ~50 MYBP
      • 15 °C warmer than present.
      • Tropical plants & animals (e.g., alligators) found in polar regions.
      • No polar ice sheets.
    • Slow cooling occurred after this time.
    • All natural changes
    • Consideration: If temperatures were much warmer 50 MYBP, why worry about warming now?
  • Over past 70M years (Fig 2.11)

Temperatures in the Past 5M Years

  • Ice sheets appear in the Northern Hemisphere.
  • Oscillations occur between glacial periods (ice ages) and interglacial periods (warm).
  • The glacial/interglacial cycle initially took ~ 41,000 years (until ~ 1 MYBP) and then ~ 100,000 years, with larger temperature oscillations after 1 MYBP.
  • General cooling until ~ 1 MYBP
  • At ~ 3 MYBP

Temperatures in Past 400,000 Years

  • Triggered by variations in Earth’s orbit, which alter insolation (sunlight amount).
  • Temperature is correlated with CO2CO_2 (yellow line) and other greenhouse gases (GHGs).
  • Glacial periods (ice ages) characteristics:
    • ~ 6-8 °C colder than interglacial periods.
    • Large expansion of NH ice sheets (covering much of North America & N Europe).
    • Sea level ~ 100 m lower than during interglacial periods.
  • We are currently in an interglacial period.
  • Current interglacial expected to last for 1000s of years (even without our GHG emissions).
  • Strong glacial/interglacial variations with 100,000-yr period.
  • Interglacial periods are roughly similar to today’s climate.

Temperatures in Past 11,000 Years (Holocene)

  • Four general periods - what is happening to T in each? Why?
  • Is current rate of warming faster than warming at end of last glacial period?
  • Warming starts ~ 1900.
  • Current climate is warmer than any time in past 11,000 years.

Temperatures from 1850 to present

  • 2024 anomaly is 1.6 ºC relative to 1850-1900 reference.
  • Graph of air T above land & ocean clearly shows Earth is warming …2024 anomaly? …Rate of warming?
  • Warming is spatially heterogeneous
    • Two main spatial differences?

Weather and Climate Extremes

  • Human-induced climate change has increased weather and climate extremes.
  • Hot extremes/heat waves are more frequent & more intense
    • Occurring throughout the globe
    • e.g., summer 2021 heatwave in northwest North America
      • Set record highs throughout NW
      • Killed > 1000 people
      • CC made it ~150 times more likely to occur
  • Precipitation (precip) changes
    • Increased frequency and intensity of heavy precip events
    • Increased frequency and intensity of droughts
  • What parts of globe are seeing more extreme heat?
  • How confident is the IPCC that humans are contributing to this change?

Consistency and Conclusion

  • Are the changes described today and in the last class self-consistent?
  • Based on this evidence, the latest IPCC assessment report (AR6, 2021) concludes that:
    • “It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred. ”
    • “The scale of recent changes across the climate system as a whole and the present state of many aspects of the climate system are unprecedented over many centuries to many thousands of years. ”
    • “Human-induced climate change is already affecting many weather and climate extremes in every region across the globe. ”