Natural causes of climate change

How natural forcing has driven climate change in the geological past, including: plate tectonics, including volcanic activity and continental drift Milankovitch cycles solar output the role of natural atmospheric greenhouse gases.

What is the Holocene and has its climate been completely stable?

• The current interglacial period began 11,700 years ago at the end of the last glacial period.

• Climate has not been continuous.

• Examples include:

  • 6,000 years ago: Global temperatures were higher than today.

  • Medieval Warm Period (1100–1300): A short-lived warm phase.

  • Little Ice Age (1550–1850): Global temperatures fell, sending European winters into a deep freeze.

What are the key time scales and characteristics of the Quaternary period?

• Spans the last 2.6 million years.

• Characterized by cyclic climate shifts of long cold glacials (~100,000 years) interrupted by shorter interglacials (10,000–15,000 years).

• Last Glacial Maximum (Devensian): Peaked around 20,000 years ago, covering ~1/3 of the Earth's surface in snow/ice.

Name the natural causes of climate change

• Plate tectonics

• Milankovitch cycles

• Solar output

• Natural atmospheric greenhouse gases

What are Milankovitch cycles

• Shifts in the Earth's orbit and axis

• These forces affect the amount of solar radiation reaching the planet's surface

• Milankovitch cycles were responsible for the last ice age, around 20,000 years ago

What are the three Milankovitch cycles

• Obliquity

• Eccentricity

• Precession

Obliquity

• Aka tilt

• Over 40,000 years the earth tilts on its axis from 22 to 24.5 degrees

How does obliquity influence climate

• 22 degrees is not enough to melt ice sheets in the summer, as it means the seasonal temperature differences are reduced (cooler summers, warmer winters)

• 22 degrees means snow and ice accumulated during winter do not melt in the summer, allowing glaciers and ice sheets to expand

• This creates a positive feedback loop, increasing the reflection of incoming solar radiation and further lowering temperatures

How does eccentricity influence climate

• Aka stretch - The orbit of Earth varies from near circular to elliptical

• With maximum eccentricity, the earth receives roughly 30% more solar energy when it is closest to the Sun than when it is farthest away

• Ice corresponds to periods of maximum eccentricity

Precession

• Aka wobble

• The earth gyrates on its axis like a spinning top

• Therefore, the point in the earth's orbit when the planet is closest to the sun (perihelion) changes over time

• This shift which occurs over 22,000 years affects the intensity of the seasons

How does precession influence climate

• If perihelion occurs during the northern hemisphere's winter, winters will be warmer and summers cooler

• Snow and ice accumulating in winter will not melt completely in the following summer, so that ice and snow cover expands, triggering a glacial period

What are sunspots

• Sunspots occur when intense magnetic activity on the Sun's surface creates cooler, darker areas

How do sunspots influence climate

• There is a positive correlation between the number of sunspots and solar energy output

• It is on an 11-year cycle but the impact is not enough to greatly affect global climate significantly

• On longer timescales, solar output is more variable

• The number of sunspots declined to almost 0 for several decades which led to the Little Ice Age

How has plate tectonics (including continental drift) driven climate change?

• Driven by plate tectonics and sea-floor spreading, the global distribution and configuration of the continents have changed dramatically over geological time

• 250 million years ago the continents formed a single huge land mass - Pangaea

• Since then, the continents have drifted apart to their present-day position

• Tectonic changes on this scale have a direct impact on global climate

• in part they explain the periodic extreme shifts of the Earth's climate between greenhouse and icehouse conditions

• For example, as a larger continental area occupies higher latitudes, the land area with permanent ice cover expands

• This in turn increases global albedo and positive feedback, which forces global cooling.

How do volcanic eruptions influence climate

• SO2 and volcanic ash is released from volcanic eruptions into the stratosphere

• This is converted to sulphuric acid which forms sulphate aerosols. These reflect solar radiation and lower temperatures, but only on the short term (eg. A drop of 1.3 C over 3 years as a result of Mt Pinatubo in 1991)

• Large or prolonged volcanic eruptions release CO2 into the atmosphere, strengthening the greenhouse effect and increasing global temperatures

Ocean circulation

• What it is:

  • The movement of water across the globe through surface currents and deep-water (thermohaline) circulation.

• How it influences climate:

  • Heat Transfer: Ocean currents act like a conveyor belt, redistributing heat from tropical regions to higher latitudes.

  • Changes in past climate: Shifts in ocean circulation (often triggered by plate movements opening or closing oceanic gateways, or massive freshwater influxes from melting ice sheets) can disrupt global heat distribution, pushing the Earth into glacial or interglacial periods.

  • The Atlantic Meridional Overturning Circulation (AMOC): Changes in salinity and temperature can stall or redirect major currents, causing rapid regional climate shifts.

Natural greenhouse gases

• What it is:

  • The presence of naturally occurring gases in the atmosphere—such as carbon dioxide, methane, water vapour and nitrous oxide—that trap outgoing longwave terrestrial radiation.

• How it influences climate:

  • The Natural Greenhouse Effect: Without these gases, Earth's average surface temperature would be about -18°C instead of a habitable +15°C.

  • Geological Past Variations: Natural events alter greenhouse gas levels over millions of years. For instance, intense volcanic activity releases massive amounts of CO2 into the atmosphere, enhancing the greenhouse effect and creating warm "Greenhouse Earth" periods. Conversely, weathering of rocks and biological drawdown can pull CO2 out of the atmosphere, cooling the planet into "Icehouse Earth" conditions.

Natural greenhouse PSD

• The PETM (56 Million Years Ago) — Average CO2 levels reached 1,000 ppm and temperatures were 10 degrees higher during the Palaeocene–Eocene Thermal Maximum, driven by large quantities of carbon entering the atmosphere, potentially from volcanic activity.

Ocean circulation PSD

• Opening of the Drake Passage (34 Million Years Ago) — A deep-water connection developed between South America and Antarctica, establishing the Antarctic Circumpolar Current (ACC).

• This reduced the exchange of warmer lower-latitude waters with Antarctica, contributing to major ice sheet growth