MEVE 011: Unit 03 - Natural Causes of Climate Change

 UNIT 3: NATURAL CAUSES OF CLIMATE CHANGE

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3.1 Introduction

While human activities are a major cause of modern climate change, the Earth's climate has always changed due to natural factors. This unit focuses on these natural causes, such as variations in Earth’s position and movement, volcanic eruptions, changes in ocean currents, and natural emissions of greenhouse gases. Understanding these natural drivers helps us differentiate between natural and human-induced changes in the Earth’s climate system.

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3.2 Objectives

The objectives of this unit are to:

• Understand how Earth's movement in space affects its climate
• Explore the role of volcanic eruptions and meteor impacts
• Learn about oceanic patterns like El Niño, La Niña, and Arctic Oscillation
• Examine tectonic plate movements and their influence on climate
• Understand natural greenhouse gas emissions from wetlands and soils

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3.3 Earth’s Tilt, Rotation and Orbital Changes

The Earth’s movement around the Sun influences how much solar energy different parts of the planet receive over time, leading to long-term changes in climate, such as ice ages. These are known as Milankovitch cycles, and they involve:

3.3.1 Earth’s Obliquity (Tilt)

Earth’s axis is tilted at an angle of about 23.5 degrees, but this tilt changes slowly over a cycle of about 41,000 years. A greater tilt means more extreme seasons, while a smaller tilt leads to milder seasons.

3.3.2 Earth’s Eccentricity

This refers to the shape of Earth's orbit around the Sun, which changes from more circular to more elliptical over a period of about 100,000 years. When the orbit is more elliptical, Earth receives varying amounts of solar energy, contributing to glacial and interglacial periods.

3.3.3 Earth’s Precession

Earth wobbles like a spinning top in a cycle that lasts about 26,000 years. This changes the timing of seasons and can affect the contrast between them, influencing climate patterns over long periods.

3.3.4 Sunspot Activity

Sunspots are dark spots on the Sun’s surface that represent areas of high solar activity. Increased sunspots often correlate with more solar radiation reaching Earth, which can cause slight warming. Periods of low sunspot activity, such as the Maunder Minimum, have been associated with cooler climates.

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3.4 Meteors and Volcanic Eruptions

3.4.1 Meteors

Large meteor impacts can drastically alter the climate by ejecting dust and particles into the atmosphere. These particles block sunlight and cause a temporary cooling of the Earth’s surface. One famous example is the meteor impact linked to the extinction of dinosaurs.

3.4.2 Volcanic Eruptions

Volcanoes release ash, dust, and sulfur dioxide into the atmosphere. These aerosols reflect sunlight and can cool the Earth temporarily. For example, the eruption of Mount Pinatubo in 1991 caused a global temperature drop of about 0.5°C for several years.

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3.5 Changes in Ocean Currents

Ocean currents help distribute heat around the globe. Changes in these currents can lead to significant climate shifts. For instance, the disruption of the Gulf Stream could cause major changes in European and North American climates.

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3.6 El Niño, La Niña Cycle and Arctic Oscillation (AO)

These are natural climate variability patterns caused by interactions between the ocean and atmosphere.

3.6.1 El Niño

An El Niño event occurs when warmer-than-normal water accumulates in the eastern Pacific Ocean. It leads to dry conditions in India and Australia and flooding in South America, and it disrupts global weather patterns.

3.6.2 La Niña

La Niña is the opposite of El Niño—cooler-than-normal Pacific Ocean temperatures. It often brings heavier rainfall to Asia and cooler, wetter conditions in North America.

3.6.3 Arctic Oscillation (AO)

AO refers to changes in the air pressure patterns over the Arctic. A strong AO traps cold air in the Arctic, while a weak AO allows cold air to move southward, affecting winter weather in Europe, Asia, and North America.

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3.7 Tectonic Plate Movements

The Earth’s surface is divided into tectonic plates that slowly move. Over millions of years, these movements shift the positions of continents and oceans. Such shifts affect ocean currents, mountain formation, and even atmospheric circulation, all of which influence climate over geological time scales.

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3.8 Greenhouse Gas Emissions

While human activities have increased greenhouse gas levels, natural sources also contribute:

3.8.1 Emissions from Wetlands

Wetlands are a natural source of methane (CH₄), produced by bacteria in waterlogged soils. Methane is a powerful greenhouse gas that contributes to global warming.

3.8.2 Emissions from Soils

Soils, especially in tropical forests and grasslands, release carbon dioxide (CO₂) and nitrous oxide (N₂O) through decomposition and microbial activity. These emissions vary with temperature, moisture, and vegetation cover.

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3.9 Let Us Sum Up

This unit explained the natural causes of climate change that operate over different timescales. These include orbital variations (obliquity, eccentricity, precession), sunspot cycles, volcanic eruptions, meteor impacts, tectonic movements, and natural greenhouse gas emissions. Oceanic phenomena like El Niño, La Niña, and the Arctic Oscillation also contribute to short-term climate variability. Understanding these factors is crucial to separating natural climate variability from human-caused climate change.