Climate Change: Mechanisms, Evidence, and Biological Impacts

Climate Change: Overview

Introduction to Climate Change

Climate change refers to significant, long-term changes in the global climate, particularly temperature and precipitation patterns. It is driven by both natural processes and human activities, with profound impacts on ecosystems and natural resources.

• Key Questions: How does climate change work? What causes it? How do we monitor it? What are its impacts?

Evidence for Climate Change

Global Temperature Trends

Multiple lines of evidence show a consistent rise in global average surface temperature since the late 19th century.

• Instrumental Records: Data from NOAA and other agencies show a marked increase in temperature, especially since the 1970s.

• 2023: The warmest year on record globally (1850–2023).

• Convergence of Evidence: Multiple independent datasets confirm warming trends.

Natural Climate Variation

Earth's Climate Cycles

Earth's climate has naturally fluctuated over geological time due to several factors.

• Orbital Changes: Variations in Earth's orbit (eccentricity, tilt, precession) affect climate cycles (Milankovitch cycles).

• Solar Activity: Changes in solar output can influence global temperatures.

• Volcanic Activity: Eruptions can inject aerosols into the atmosphere, temporarily cooling the planet.

• Meteor Impacts: Large impacts can alter climate by blocking sunlight.

The Anthropocene Epoch

Human Influence on the Environment

The term Anthropocene describes the current geological epoch, characterized by significant human impact on Earth's systems.

• Dominance of Human Activities: Industrialization, deforestation, and urbanization have altered key environmental processes.

The Greenhouse Effect

Natural Greenhouse Effect

The greenhouse effect is the process by which certain gases in Earth's atmosphere trap heat, maintaining temperatures suitable for life.

• Energy Balance: About 30% of the sun's energy is reflected back into space; the rest is absorbed by Earth's surface and atmosphere.

• Key Percentages:

  • 51% absorbed by Earth

  • 20% scattered/reflected by clouds

  • 19% absorbed by atmosphere/clouds

  • 6% scattered by atmosphere

  • 4% reflected by surface

Human Intensification of the Greenhouse Effect

Human activities have increased the concentration of greenhouse gases, intensifying the greenhouse effect and leading to global warming.

• Less heat escapes into space due to higher greenhouse gas concentrations.

Greenhouse Gases

Major Greenhouse Gases

Several gases contribute to the greenhouse effect by absorbing and emitting infrared radiation.

• Carbon dioxide (CO2)

• Methane (CH4)

• Nitrous oxide (N2O)

• Ozone (O3)

• Chlorofluorocarbons (CFCs)

• Water vapor (H2O)

CO2 Trends and Sources

CO2 is the most significant greenhouse gas emitted by human activities.

• Current Atmospheric Level: ~426 ppm (parts per million)

• Pre-industrial Level: ~280 ppm

• Sources:

  • Transportation

  • Electricity generation

  • Industry

  • Residential/commercial

CO2 Emissions by Source (US EPA)

Source	Percentage
Transportation	29%
Electricity	25%
Industry	23%
Residential/Commercial	12%
Other	8%

Methane (CH4)

• Accounts for 12% of human-contributed greenhouse gases.

• 20% more effective than CO2 at trapping heat.

• Sources:

  • Natural gas and petroleum systems

  • Enteric fermentation (livestock)

  • Landfills

  • Coal mining

  • Manure management

Methane Emissions by Source (US EPA)

Source	Percentage
Natural Gas/Petroleum	29%
Enteric Fermentation	25%
Landfills	18%
Coal Mining	10%
Manure Management	9%
Other	9%

Nitrous Oxide and Fluorinated Gases

• Nitrous oxide: 6% of human-contributed greenhouse gases; 265–310 times more potent than CO2.

• Fluorinated gases: 3%; extremely high global warming potential (GWP).

Water Vapor and Feedback Loops

Role of Water Vapor

Water vapor is the most abundant greenhouse gas and plays a key role in climate feedbacks.

• Positive Feedback: As Earth warms, evaporation increases, raising water vapor levels and further enhancing warming.

• Arctic Sea Ice Melting: Another positive feedback loop, as less ice means less sunlight is reflected, increasing warming.

Carbon Cycle

Pre-industrial Carbon Cycle

The carbon cycle describes the movement of carbon among Earth's atmosphere, biosphere, oceans, and crust.

• Reservoirs:

  • Atmosphere: 420 GtC

  • Terrestrial biosphere: 2160 GtC

  • Ocean surface: 900 GtC

  • Ocean deep: 36,400 GtC

  • Earth's crust: 90,000,000 GtC

• Fluxes:

  • Atmosphere to ocean: 0.6 GtC/yr

  • Terrestrial biosphere to atmosphere: 0.6 GtC/yr

  • Rivers: 0.8 GtC/yr

Pre-industrial Carbon Cycle Table

Reservoir	Amount (GtC)
Atmosphere	420
Terrestrial Biosphere	2160
Ocean Surface	900
Ocean Deep	36,400
Earth's Crust	90,000,000

Anthropogenic Perturbation (1980–1999)

• Atmospheric CO2 increased to 750 GtC (+3.3 GtC/yr)

• Earth's crust: 4000 GtC (+5.9 GtC/yr)

• Land use change and deforestation: Major contributors to increased atmospheric CO2

Determining Past Climate Patterns

Methods of Climate Reconstruction

• Dendrochronology: Tree rings reveal environmental conditions, droughts, and fire intervals.

• Ice Core Analysis: Air bubbles trapped in ice contain ancient atmospheric gases, including CO2.

• "Fossil air" in glacier ice: Used to reconstruct past atmospheric composition.

Impacts of Climate Change

Changes Beyond Temperature

• Precipitation Patterns: U.S. receives 6% more rain than 100 years ago, but changes are not evenly distributed.

• Extreme Weather Events: Increased frequency and severity of hurricanes, droughts, floods, and fires.

Phenology

Phenology is the study of the timing of biological events, such as migrations, bloom dates, and frost dates, which are shifting due to climate change.

• Examples:

  • Striped bass biting when dogflower trees bloom

  • Black-capped chickadees begin mating songs

  • Canada geese arrival

  • Hawthorne trees flowering

Impacts on Wildlife

• Range Shifts: Species move to new areas as habitats change.

• Extinction Risk: Species with narrow niches may die out.

• Hybridization: May occur as ranges overlap.

• Disease Rates: May increase or decrease.

• Food and Habitat Availability: Altered by climate change.

Impacts on Fish

• Growth and Development: Influenced by temperature.

• Fewer Coldwater Species: As waters warm.

• Higher Incidence of Disease and Invasives.

• Changes to Forage Base: Phytoplankton and zooplankton affected.

• Physical Changes: Turnover cycles and ice cover duration change.

Impacts on Forests

• Warmer Temperatures: Affect species distribution.

• Drier or Wetter Conditions: Influence forest health.

• Range Shifts: Some species may not be able to migrate.

• Increased Insects and Disease: More prevalent in warmer climates.

• Loss of Carbon Sequestration: Reduces forests' ability to absorb CO2.

Deforestation

Role in Climate Change

• Trees absorb CO2 and store it as biomass.

• Burning or clearing forests releases CO2 into the atmosphere.

• Since 1990, 420 million hectares of forest have been lost.

Mitigation and Adaptation

Strategies to Address Climate Change

• Mitigation: Reducing greenhouse gas emissions (e.g., renewable energy, energy efficiency).

• Adaptation: Adjusting to changes (e.g., infrastructure, conservation).

Climate Change Perceptions and Local Impacts

Public Perceptions

• Climate Anxiety: Varies from very high to none among individuals.

• Local Impacts: Many U.S. adults report climate change affecting their communities.

Key Equations and Concepts

Greenhouse Effect Equation

• Radiative Forcing: $\Delta F = \alpha \ln \left( \frac{C}{C_0} \right)$ Where $\Delta F$ is the change in radiative forcing, $\alpha$ is a constant, $C$ is the current CO2 concentration, and $C_0$ is the reference concentration.

Global Warming Potential (GWP)

• Definition: GWP is a measure of how much heat a greenhouse gas traps in the atmosphere over a specific time period compared to CO2.

Summary Table: Major Greenhouse Gases

Gas	Contribution (%)	Relative Potency	Main Sources
CO2	82	1 (reference)	Fossil fuels, deforestation
CH4	12	20x CO2	Agriculture, landfills, fossil fuels
N2O	6	265–310x CO2	Agriculture, industry
Fluorinated gases	3	1000s–10,000s x CO2	Industrial processes

Additional info:

• Thermohaline circulation (Great Ocean Current) is a key factor in global climate regulation.

• Water transparency and ice cover duration are indicators of ecosystem health and climate change impacts.