AS

Lecture 13: Climate Change

Weather

  • Atmospheric conditions at a certain place and time with reference to temperature, pressure, humidity, wind, and other key parameters (meteorological elements)

Climate

  • The average weather or the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years

    • temperature, precipitation and wind

    • 30-year average (World Meteorological Organization)

  • Climate Change

    • Change in the state of the climate that can be identified by changes in the mean and/or the variability of its properties, and that persists for an extended period, typically decades or longer

The Earth’s Changing Climate Over Time

  • Oxygen Explosion (2.5 Ga - 500 Ma)

    • The evolution of cyanobacteria, which produced oxygen as a bi-product of photosynthesis, meant that O2 levels dramatically increased

  • Paleocene - Eocene Thermal Maximum (55 Ma)

    • Records show a massive warming of between 5-8° in just 20,000 years, possibly due to sudden release of carbon into the atmosphere (i.e. methane from the ocean bed or from within ice structures called clathrates)

Natural Causes of Climate Change

  • Plate tectonics and volcanism

    • Plate tectonics refer to the movement of Earth's continents and ocean floors. Over millions of years, these slow movements can change the shape and position of continents, which alters ocean currents and wind patterns, influencing global climate.

    • Volcanism (volcanic activity) can inject large amounts of ash and gases like sulfur dioxide into the atmosphere. These particles can block sunlight and cause short-term cooling. On longer time scales, the release of carbon dioxide (a greenhouse gas) can contribute to warming.

  • Solar output and variations

    • The Sun's energy output is not constant. Solar cycles, typically lasting about 11 years, include variations in sunspot activity that slightly increase or decrease solar radiation reaching Earth.

  • Ocean variability

    • The oceans store and transport vast amounts of heat. Natural ocean cycles, such as El Niño and La Niña, cause shifts in sea surface temperatures and weather patterns across the globe.

    • Thermohaline circulation (also called the global ocean conveyor belt) distributes heat around the planet. Changes in this circulation can lead to regional or even global climate shifts over decades or centuries.

  • Orbital variations

    • Milankovitch Cycles

      • 3 cycles related to the Earth’s orbit, tilt, and wobbling that contribute to cyclical (predictable) climate changes

      • Eccentricity

        • The shape of the Earth’s orbit around the sun is not constant. Sometimes, it’s more elliptical, sometimes it’s less. The Earth’s orbit fluctuates from 0-5% ellipticity every ~100,000 years

        • These changes in ellipticity alters the Earth’s distance from the sun

          • closer => takes less time for sun rays to reach Earth

          • farther => takes longer time for sun rays to reach Earth

      • Axial Tilt

        • The inclination of the Earth’s axis. Currently, the Earth is tilted ~23.5° from its axis

          • This accounts for the Earth’s seasons

        • However, this angle of tilt fluctuates from 21.5 - 24.5° every ~41,000

          • If the Earth’s axis is tilted less from the sun, the solar radiation received by Earth is more even between summer & winter months

          • If the tilt is larger, the difference between winter & summer is more pronounced

          • Tilt angle also affects the difference between the amount of radiation received by equator vs poles

        • “One hypothesis for Earth’s reaction to a smaller degree of axial tilt is that it would promote the growth of ice sheets. This response would be due to a warmer winter, in which warmer air would be able to hold more moisture, and subsequently produce a greater amount of snowfall. In addition, summer temperatures would be cooler, resulting in less melting of the winter’s accumulation. At present, axial tilt is in the middle of its range.”

      • Precession

        • This is the wobbling of the Earth as it spins on its axis

          • Sometimes the Earth’s “north” points to Polaris (current north star)

          • Sometimes it points to Vega at a cycle of ~23,000 years due to this wobbling

            • Northern Hemisphere winter and summer solstices ~ coincide with the aphelion and perihelion, respectively => greater seasonal contrasts

          • This top-like wobble, or precession, has a periodicity of 23,000 years

Greenhouse Gases Effect

  • Sunlight reaches Earth

  • Some radiation are reflected back into space

  • Some are absorbed as heat

    • Some of this is re-radiated as heat

      • Instead of going straight out to space, this heat is absorbed by GHGs in the atmosphere

      • The GHGs re-emit this heat in all directions => warming the Earth

  • Natural GHG Emissions

    • Volcanism and CO2

      • If global volcanism slows, as would be the case when supercontinents stabilize, less atmospheric CO2 would trigger global cooling. Increased volcanism puts more CO2 in the atmosphere and results in more greenhouse warming.

  • Anthropogenic GHG Emissions

    • Human-caused greenhouse gas emissions that began to rise significantly around the start of the Industrial Revolution (mid-1700s to early 1800s) and have continued increasing since.

      • Carbon dioxide (CO₂): From burning coal, oil, and natural gas for energy and industrial processes.

      • Methane (CH₄): From agriculture (especially livestock), fossil fuel extraction, and landfills.

      • Nitrous oxide (N₂O): From agriculture (fertilizers), waste, and some industrial activities.

      • Fluorinated gases: Synthetic gases from refrigerants and industrial applications (only present since the 20th century).