Unit 3: Changing Climate

1. Definition of Climate Change

Climate Change refers to a statistically significant, long-term change in the average state of the climate or its variability (e.g., changes in average temperature, rainfall, or wind patterns) that persists for an extended period, typically decades or longer.

It's important to distinguish this from climate variability, which refers to natural fluctuations (like a single cold year) around the long-term average.

2. Natural Causes of Climate Change

The Earth's climate has always been changing, long before humans. Natural drivers of climate change include:

• Milankovitch Cycles: Slow, predictable changes in Earth's orbit, tilt, and wobble over thousands of years. These cycles alter the amount and distribution of sunlight reaching the Earth, and are the main driver of ice ages.
• Solar Variations: Changes in the Sun's energy output. The sun has natural cycles (like the 11-year sunspot cycle) that slightly alter the energy Earth receives.
• Volcanic Eruptions: Large eruptions inject massive amounts of sulfur dioxide (SO₂) into the stratosphere. This SO₂ forms aerosols (tiny particles) that reflect sunlight, leading to a temporary global cooling effect for 1-3 years.

3. Detection of Climate Change

Detecting climate change involves finding a long-term "signal" of change within the "noise" of natural weather variability. Scientists do this by:

• Analyzing Instrumental Records: Using data from thermometers, weather stations, and satellites collected over the last ~150 years to track changes in global temperature, sea level, etc.
• Using Proxies: For pre-instrumental times, scientists use "proxies" to reconstruct past climates. These include:
  • Ice Cores: Bubbles trapped in ice sheets contain samples of the ancient atmosphere (like CO₂ levels).
  • Tree Rings: The width of tree rings indicates past conditions (a good year = wide ring).
  • Sediment Cores: Fossils and chemicals in lake/ocean mud reveal past temperatures.

4. CO₂, Trace Gases, and Climate Change

While Carbon Dioxide (CO₂) from fossil fuel burning is the main driver of current climate change, other "trace gases" (present in small amounts) also play a powerful role. These are all Greenhouse Gases (GHGs).

• Methane (CH₄):
  • Sources: Agriculture (livestock), natural gas leaks, landfills, rice paddies.
  • Impact: On a per-molecule basis, it's much more potent (a stronger GHG) than CO₂, but it has a shorter lifetime in the atmosphere (~12 years).
• Nitrous Oxide (N₂O):
  • Sources: Agricultural fertilizers, industrial processes, biomass burning.
  • Impact: Very potent and has a long atmospheric lifetime (~114 years). Also destroys stratospheric ozone.

5. Greenhouse Effect and Global Warming

The Greenhouse Effect (Natural)

The greenhouse effect is a natural and essential process that keeps the Earth warm enough to support life.

1. Solar radiation (mostly visible light) passes through the atmosphere and warms the Earth's surface.
2. The Earth's surface radiates this energy back as infrared radiation (heat).
3. Greenhouse gases (like H₂O, CO₂) in the atmosphere are "transparent" to visible light but "opaque" to infrared. They absorb this outgoing heat and re-radiate it in all directions, including back towards the surface.

This trapping of heat keeps the global average temperature at ~15°C instead of a frigid -18°C.

[Image of the greenhouse effect mechanism]

Global Warming (Enhanced)

Global Warming is the enhancement of the natural greenhouse effect caused by human activities. By burning fossil fuels, we are adding extra greenhouse gases (especially CO₂) to the atmosphere, thickening this "thermal blanket" and trapping more heat, leading to a rise in global average temperatures.

6. Manifestations of Global Warming

"Manifestations" are the observable consequences and symptoms of a warming planet.

• Sea Level Rise: This is a major consequence caused by two factors:
  1. Thermal Expansion: As ocean water warms, it expands, taking up more volume.
  2. Melting of Land Ice: Glaciers and ice sheets (on Greenland and Antarctica) are melting, adding water that was previously stored on land into the ocean. (Note: melting *sea ice* does not raise sea level).
• Melting of Glaciers: Glaciers worldwide are retreating at an unprecedented rate. This threatens the water supply for millions of people who depend on glacial meltwater in the dry season.
• Variation in Monsoon Patterns: A warmer atmosphere can hold more water vapor. This can lead to more intense rainfall events during monsoon seasons, increasing flood risk. The changes in land-sea temperature contrast can also make monsoon timing and duration more erratic.

7. Weather Extremes

Global warming doesn't just raise the average temperature; it "loads the dice," making extreme weather events more frequent and/or more intense.

• Heat Waves: Become more frequent, last longer, and reach higher temperatures.
• Cyclones, Hurricanes, and Tornadoes: Warmer oceans provide more "fuel" for tropical cyclones (hurricanes/typhoons), potentially making them stronger (higher wind speeds, more rainfall). The link to tornadoes is less certain but an active area of research.
• Droughts & Floods: Changes in precipitation patterns lead to "weather whiplash." Some areas experience more intense, prolonged droughts, while others see an increase in heavy downpours, leading to flooding.

8. EL Nino / LA Nino

This is the El Niño-Southern Oscillation (ENSO), a natural and powerful climate cycle centered in the tropical Pacific Ocean. It is not caused by global warming, but it interacts with it.

[Image of El Nino vs La Nina ocean currents]

• Neutral Phase: Normal conditions. Trade winds blow east-to-west, "piling up" warm water in the western Pacific (near Indonesia).
• EL Niño Phase: The trade winds weaken. The warm water sloshes back across the Pacific towards South America. This shifts global weather patterns, typically causing droughts in Australia/Indonesia and heavy rains in Peru. El Niño years are often associated with a temporary *spike* in global average temperatures.
• LA Niña Phase: The trade winds become *stronger* than normal, pushing even more warm water to the west. This causes opposite weather patterns and can lead to a temporary *dip* in global average temperatures.

Climate change may be affecting the frequency and intensity of these natural El Niño and La Niña events.