Chapter 7 - The Changing Climate - Notes

Chapter 7: The Changing Climate

7.0 Introduction

• Global Warming:

  • The World Meteorological Organization (WMO) reported that 2024 is the hottest year on record, with warming temporarily hitting 1.5°C.
  • This is based on daily surface air temperature data.

• Nuisance Flooding:

  • Nuisance flooding is localized coastal flooding caused by extreme high tides coupled with higher sea levels.
  • Nuisance flooding events are increasing globally as sea levels rise, posing a major threat to low-lying coastal developments.
  • Example: Flood days projected to increase significantly by 2050 with a 0.3m to 0.6m sea level rise.

7.1 The Climate System

• Weather vs. Climate:

  • Weather: The state of the atmosphere at any given moment (e.g., sunshine, rain, heat waves).
  • Climate: The long-term average of weather and the average frequency of extreme weather events (e.g., average temperature, precipitation).
  • Quote: "Climate is what you expect, but weather is what you get."

• Weather and Climate Phenomena:

  • Weather: Cloudiness, rain shower, rainbow, sea breeze, tornado (minutes to hours); Night-and-day temperature difference (days); Hurricane, midlatitude cyclone (weeks).
  • Climate: Winter, hurricane season, drought (months); Asia monsoon (1 year); El Nino and La Nina (1 year or more); Younger Dryas (1,000 to 10,000 years); Quaternary glacial and interglacial cycles (10,000 to 1,000,000 years); Cenozoic cooling (millions of years).

• Climate Diagrams Example:

  • San Diego, CA: Average temperature of 17°C (64°F), average precipitation of 26 cm (10.3 in).
  • Tucson, AZ: Average temperature of 20°C (69°F), average precipitation of 29 cm (11.6 in).

• Components of the Climate System:

  • Climate results from interactions between the atmosphere, biosphere, lithosphere, and hydrosphere.
  • Energy and matter move through Earth’s systems, forming the climate system.

• Climate Change:

  • Occurs when the long-term average of any meteorological variable (e.g., temperature or precipitation) changes.
  • A single weather event typically does not change the long-term average, but enough extreme weather events can.

• Global Warming:

  • Since 1880, the average temperature of the lower atmosphere has increased by 1.1°C (2°F).

• Climate Forcing and Feedbacks:

  • Climate Forcing Factors: Operate outside of, and independent of, the climate system; cause climate change but are not affected by it (e.g., solar forcing, volcanic forcing).
  • Climate Feedback Factors: Arise within the climate system and are changed by it; enhance or diminish climate change already in motion (positive or negative).
  • Ice-Albedo Positive Feedback: Destabilizes the climate system, enhancing both warming and cooling trends.
    • $Albedo = \frac{\text{Reflected Radiation}}{\text{Incoming Radiation}}$
  • Cloud Negative Feedback: Initial warming increases evaporation and clouds, which increases Earth's albedo, decreasing absorbed light and thus decreasing Earth's temperature.

7.2 Climate Trends, Cycles, and Anomalies

• Paleoclimatology:

  • Paleoclimates are Earth's ancient climates, reconstructed by paleoclimatologists.
  • Example reconstructions of the temperature over Greenland indicate the Younger Dryas, Medieval Warm Period, and Little Ice Age.

• Paleoclimate Records: Temporal Resolution and Extent

  • Tree rings: Thousands of years back, annual resolution.
  • Corals: Tens of thousands of years back, annual resolution.
  • Glaciers: Hundreds of thousands of years back, annual resolution.
  • Cave deposits: Hundreds of thousands of years back, decades resolution.
  • Lake sediments: Hundreds of thousands of years back, decades resolution.
  • Marine sediments: Millions of years back, centuries resolution.

• Modes of Climate Change:

  • Long-term trends, repeating cycles, unpredictable anomalies caused by climate forcing factors and climate feedbacks.

• Long-Term Trends – Geologic Time Periods:

  • Holocene Epoch: Began 11,700 years ago, warm and stable interglacial climate.
  • Quaternary Period: Began 2.6 million years ago, alternating between cold glacial and warm interglacial periods.
  • Cenozoic Era: Began 66 million years ago, gradual cooling trend.

• Cenozoic Cooling Trend:

  • Began 66 million years ago due to a large asteroid impact in eastern Mexico, causing 75% of life to go extinct.
  • The building of the Himalayas and Tibetan Plateau exposed more rocks to the atmosphere, transferring carbon from the atmosphere to the lithosphere through weathering and erosion.

• Quaternary Period:

  • Began 2.6 million years ago with ice sheets growing in northern Europe and North America.
  • Climate cycled between glacial (cold, ~90,000 years) and interglacial (warm, ~10,000 years) periods 22 times.

• Milankovitch Cycles:

  • Changes in Earth–Sun orbital geometry cause Quaternary glacial–interglacial cycles due to orbital forcing.
  • Internal feedbacks (e.g., ice–albedo feedback) amplify these changes.
  • Eccentricity: Shape of Earth's orbit changes from circular to elliptical (~100,000 years).
  • Tilt: Angle of Earth's axial tilt changes between 21.5 and 24.5 degrees (~41,000 years). Greater tilt increases seasonality.
  • Precession: Earth wobbles on its axis (~24,000 years), changing the timing of the seasons.

• Holocene Epoch:

  • Began 11,700 years ago, Earth’s most recent interglacial (warm) period.

• Climate Anomalies:

  • Deviations from what is normal or expected, caused by changes in the Sun’s output, volcanic eruptions, or the ocean conveyor belt.

• Climate Anomalies & the Sun's Output

  • Sunspots are dark, cooler regions on the Sun's surface; sunspot activity peaks approximately every 11 years.
  • Increased sunspot numbers usually mean a slight increase in energy from the Sun reaching Earth.
  • Increased solar irradiance leads to warmer temperatures.
  • Solar irradiance has shown only small variations since 1880; it is not responsible for the recent warming.

• Climate Anomalies & Volcanic Eruptions:

  • Large volcanic eruptions cool Earth’s surface temperature for 1 or 2 years if ash and sulfur dioxide enter the stratosphere.

• Climate Anomalies & Ocean Conveyor Belt:

  • Younger Dryas: A cold period in the Northern Hemisphere between 12,900 and 11,600 years ago.
  • Caused by the ocean conveyor belt system slowing or shutting down due to melting of the Laurentide ice sheet.

• Ocean Conveyor Belt

  • Global system of surface and deep ocean currents that transfers heat toward the poles.
  • Driven by density and salinity differences.
  • The Gulf Stream's salty water does not sink at low latitudes because it is warm. As the Gulf Stream encounters cold air in the North Atlantic Ocean, it cools, becomes denser, sinks to the seafloor, and flows back south.

• Laurentide Ice Sheet: a large ice sheet (~2.5 miles thick) that covered much of North America during the most recent glacial period until about 12,000 years ago.

• Younger Dryas & Ocean Conveyor Belt:

  • Freshwater is more buoyant than saltwater. Therefore, if the Gulf Stream were freshened by a massive influx of freshwater, it would become more buoyant and would no longer sink in the North Atlantic Ocean.

7.3 Carbon and Climate

• Greenhouse Effect:

  • Earth's climate system is strongly influenced by greenhouse gases.
  • The greenhouse effect is a process that occurs when greenhouse gases in Earth's atmosphere trap the Sun's heat.

• Greenhouse Gases (GHG) & Habitable Planet:

  • Earth's average temperature is +15°C due to the greenhouse effect (+33°C).
  • Without GHG: -18 °C.
  • Mars: Average temperature of -50°C.
  • Venus: Average temperature of +420°C.

• Carbon Reservoirs:

  • 99.9% of Earth's carbon is stored in the lithosphere.
  • Atmosphere: 830.
  • Ocean: 38,000.
  • Plants: 450-650.
  • Soils: 1,500-2,400.
  • Earth's Crust/Sedimentary Rocks: 100,000,000.
  • Fossil Fuels: Gas: 380-1140; Oil: 170-260; Coal: 440-540.

• Carbon Cycle:

  • Carbon atoms continually move between the atmosphere, biosphere, hydrosphere, and lithosphere.
  • Involves processes like photosynthesis, respiration, decomposition, burning of fossil fuels, volcanic eruptions, and weathering.

• Earth Spheres Involved in Carbon Cycle:

  • Atmospheric CO2 is absorbed by the ocean.
  • Photosynthetic organisms absorb CO2 from the atmosphere.
  • Volcanoes erupt and emit CO2 into the atmosphere.
  • Chemical weathering and erosion of rocks remove CO2 from the atmosphere.
  • Fossil fuels are extracted and burned, releasing CO2 into the atmosphere.

• Short-Term Carbon Cycle:

  • Carbon transfers quickly among atmosphere, biosphere, and oceans.
  • Most carbon resides in the oceans.

• Long-Term Carbon Cycle:

  • Carbon transfers from atmosphere to the lithosphere through burial and preservation of photosynthetic organisms (fossils), weathering, and erosion.

• Carbon Leaving the Lithosphere and returning to the Atmosphere:

  • Volcanic activities (similar amount as weathering).
  • Burning of fossil fuels.

• Human Activity and Carbon Cycling:

  • Since the Industrial Revolution, burning fossil fuels has accelerated the transfer of carbon from the lithosphere to the atmosphere.
  • Deforestation also contributes significantly.

7.4 Climate at the Crossroads

• CO2 Concentrations Since 1000 CE:

  • Between 1000 and 1800 CE, CO2 hovered near 280 ppm.
  • By 1850, CO2 began rising quickly.
  • Today, CO2 is at 408 ppm and rising about 2.5 ppm per year (Keeling Curve).

• Anthropogenic Greenhouse Gas Emissions:

  • Human activities have caused atmospheric CO2 concentrations to rise (428 ppm nowadays).
  • Current rate of increase is 2.5 ppm per year.

• Greenhouse Gas Sources:

  • Anthropogenic:
    • Carbon dioxide (CO2): Burning fossil fuels, deforestation, cement production.
    • Methane (CH4): Livestock, fossil-fuel mining, agricultural fields, burning vegetation.
    • Nitrous oxide (N2O): Fertilizers, wastewater, fossil fuels, industrial activity, vehicles.
    • Halogenated gases (CFCs, HFCs): Industrially manufactured.
  • Natural:
    • Carbon dioxide (CO2): Volcanic activity, vegetation decomposition.
    • Methane (CH4): Bacterial processes in wetlands, mammals, and termite mounds.
    • Nitrous oxide (N2O): Soil bacterial processes.
    • Halogenated gases (CFCs, HFCs): None.

• Anthropogenic Greenhouse Effect:

  • Human emissions of CO2 and other greenhouse gases are creating an anthropogenic greenhouse effect.

• The Anthropogenic Greenhouse Effect and Arctic Amplification:

  • The Arctic is warming over twice as fast as anywhere else.
  • This is due to arctic amplification: the tendency of high-latitudes to warm faster than the rest of the planet due to the ice–albedo positive feedback.

• Comparing Today with the Last 800,000 Years:

  • Earth’s average atmospheric temperature is higher now than at any time during the instrumental era (18th century to now).

• The Eemian:

  • The Eemian was the most recent interglacial warm period (between 130,000 and 115,000 years ago).
  • CO2 was 300 ppm (compared to today’s 428 ppm).
  • 2°C (3.6°F) warmer than today.
  • Global sea level was as much as 9 m (30 ft) higher due to melting ice in Greenland and Antarctica.

• Causes of Warming Trend:

  • Temperature warming since 1880 is caused by anthropogenic GHG (especially CO2).
  • Natural climate forcing factors cannot explain the current warming trend.

• Earth’s Changing Physical Systems: Positives and Negatives:

  • Positives: Improved agricultural outputs, new Arctic economy (shipping, fishing, tourism, petroleum and gas exploration).
  • Negatives: Sea-level rise, extreme weather, extinctions, changes in wind and precipitation patterns.

• Signs of Climate Change:
  Sea level increase; Ocean temperature increase; Ocean pH decrease; Extreme rainfall increase; Snowpack decrease; Groundwater loss increase; Wildfires increase; Spatial ranges of plants and animals shift.

• Climate Change Projections:

  • The Intergovernmental Panel on Climate Change (IPCC) provides evidence-based statements about the state of the world’s present and future climate.

• General Circulation Model (GCM):

  • Models long-term climate projections.
  • Climate change projections vary depending on assumptions about future CO2 emissions.

• Future Projections:

  • By 2100, CO2 will be between 550 and 900 ppm.
  • Global temperature will be between 1.8°C and 4.2°C warmer.

• Climate Change Risks in the US:

  • Climate risks to the US will continue to grow without deeper cuts in global net emissions.
  • People born in North America in 2020 will experience more climate hazards during their lifetime than people born in 1965.

• An Ice-Free World:

  • Occurred when CO2 reached 1,000 ppm about 55 million years ago.

7.5 Geographic Perspectives: Fixing Climate: The 2-Degree Limit

• Paris Agreement:

  • Ratified December 2015 with 197 countries signing on.
  • Primary goal: Limit warming by 2100 to 2°C above pre-industrial temperatures to avoid positive feedbacks and uncontrolled warming.

• Key Points of the Paris Agreement:

  • Keep warming well below 2 degrees Celsius.
  • Aim for greenhouse gases emissions to peak "as soon as possible".
  • From 2050: rapid reductions to achieve a balance between emissions from human activity and the amount that can be captured by "sinks".
  • Developed countries must continue to "take the lead" in the reduction of greenhouse gases.
  • Rich countries must provide 100 billion dollars from 2020, as a "floor".

• The Carbon Budget:

  • To stay below 2°C, the world cannot emit more than 1 trillion tons (1,000 PgC) in total.
  • We have already emitted 550 PgC since the Industrial Revolution, leading to 1.1 °C of warming.
  • We therefore have about 450 PgC left to emit (current emission rate: ~10 PgC per year).
  • By 2100, the world must become carbon neutral.

• How to Reduce Carbon:

  • Develop a green economy based on carbon-free energy sources (solar, wind, hydroelectricity, geothermal).
  • Improve energy efficiency in cars, buildings, and coal power plants.
  • Implement carbon capture and storage (CCS) technologies.

• Actions to reduce carbon:

  • Double car fuel efficiency from 30 to 60 mpg.
  • Halve the number of miles traveled by cars each year.
  • Reduce energy use in all buildings by 25%.
  • Improve coal power plant efficiency from 40% to 60%.
  • Increase photovoltaic solar power capacity by 700 times.
  • Increase wind power capacity by a factor of about 40.
  • Reverse deforestation in the tropics.

• Other Measures:

  • Carbon taxes on CO2 emissions.
  • Cap-and-trade systems.
  • Individual actions to reduce one’s carbon footprint.

• Carbon Footprint Reduction:

  • Reduce, reuse, recycle.
  • Use less water.
  • Using solar and wind energy decreases reliance on fossil fuels.
  • Diet: eat less meat.
  • Flying less decreases fuel use.
  • Political engagement can be an effective way to manage greenhouse gas emissions.

• Geoengineering:

  • May be necessary; involves deliberate, global-scale modification of Earth’s environments.
  • Climate engineering approaches include CO2 removal and solar radiation management.

• Geoengineering Methods:

  • CO2 removal: Afforestation, CO2 capture from air and storage, CO2 capture from fossil fuels and storage, ocean iron fertilization.
  • Solar radiation management: Reflective aerosols, cloud seeding, space mirrors.