Chapter 23 - Lecture

Chapter 23: Global Ecology

23.1 The El Niño Southern Oscillation (ENSO)

• Definition: ENSO is a large-scale atmospheric and oceanic phenomenon that influences ecological systems globally.

• Impact:

  • Warmer currents appear off Peru, typically during the Christmas season (El Niño relates to "Christ child").

  • The Southern Oscillation refers to atmospheric pressure fluctuations across the Pacific Ocean.

Historical Context

• Walker (1924): Identified a link between Pacific barometric pressure and monsoon rainfall, establishing the concept of the Southern Oscillation, which is linked to droughts in various regions.

• Bjerknes's Contribution: Connected El Niño's temperature changes to atmospheric circulation, leading to storm formation over the eastern Pacific.

Climate Variability with ENSO Phases

• El Niño Phase:

  • Characterized by warmer sea surface temperatures in the eastern tropical Pacific and lower pressure.

  • Results in increased storms and precipitation in North and South America.

• La Niña Phase:

  • Cooler sea surface temperatures and higher pressure in the eastern Pacific, causing droughts in North America and increased rainfall elsewhere.

Effects on Marine Populations

• Under normal conditions, upwelling supports nutrient-rich waters along the South American coast.

• During El Niño: Upwelling is reduced, compromising nutrient availability and leading to declines in fish populations.

Influence on Terrestrial Ecosystems

• Impact on the Great Salt Lake: The strong El Niño of 1982-83 resulted in high precipitation and significant ecological changes in the lake, demonstrating the interconnectedness of climate phenomena and ecosystems.

• Effect on Australia:

  • El Niño leads to droughts, while La Niña creates favorable conditions for plant establishment and diverse animal populations.

  • Red kangaroos adapt breeding patterns based on rainfall, showcasing the influence of climate shifts on their life cycles.

23.2 Human Activity and the Global Nitrogen Cycle

• Nitrogen Fixation: Human activities have increased nitrogen cycling significantly, with sources such as agriculture and industry contributing to 135-145 Tg/yr, which exceeds natural levels.

23.3 Changes in Land Cover

• Tropical Deforestation:

  • Critical for biodiversity; tropical forests contain half of Earth’s species. Brazil experiences the highest deforestation rates.

  • Historical data shows a significant loss of forest area, adversely affecting global biodiversity and ecosystem health.

Edge Effects of Deforestation

• Loss of forest and the creation of edges expose fragments to harsher environmental conditions, impacting species diversity and survival.

23.4 Human Influence on Atmospheric Composition

• Ice cores have revealed atmospheric CO2 records over the last 800,000 years, showing strong correlations between CO2 levels and global temperature patterns.

Recent Trends in CO2 Emissions

• Fossil Fuel Impact: Burning fossil fuels contributes substantially to atmospheric CO2 levels, with several historical events disrupting the pattern of increases.

• Ozone Layer Depletion: Discovery of significant ozone depletion led to international efforts, including the Montreal Protocol, showing human ability to manage atmospheric issues.