Lecture 12 – The Human Dimension of Climate Change, Orbital Variations

Q: What are the two main sources of anthropogenic CO₂ and their timelines?

Deforestation/biomass burning (~1×10¹⁵ g C/yr, dominant 1800–1900) and fossil fuel combustion (~10×10¹⁵ g C/yr, dominant since 1970s).

Q: What is the fate of emitted anthropogenic carbon?

~55% stays in atmosphere, ~15–20% taken up by biosphere, ~25–30% absorbed by shallow ocean. Biosphere uptake is limited by insufficient nutrient supply.

Q: What are the pre-industrial and current levels of CH₄?

Pre-industrial: ~750 ppb; current: ~1930 ppb — a 250% increase.

Q: What are the pre-industrial and current levels of N₂O?

Pre-industrial: 270 ppb; current: 340 ppb — a 26% increase.

Q: What is 100-year global warming potential (GWP)?

A measure of how much heat a gas traps over 100 years relative to CO₂ (assigned factor of 1). CH₄ = 30×, N₂O = 280×, CFCs = 10,000×.

Q: What are CFCs and why were they banned?

Entirely anthropogenic gases (coolants/solvents/fire retardants) with MRT of ~100 years. Banned by the Montreal Protocol; their role is now decreasing steadily.

Q: What is each greenhouse gas's percentage contribution to the anthropogenic greenhouse effect?

CO₂: 74%, CH₄: 17%, N₂O: 6%, CFCs: 3%.

Q: What causes global dimming (the cooling effect)?

Sulfate aerosols (SO₂) in the lower atmosphere and particles from forest fires reflect solar radiation, partially offsetting warming.

Q: How much radiative forcing does the enhanced anthropogenic greenhouse effect add?

+2.7 W/m², which is a 1.8–2% addition to the natural greenhouse effect (+31°C).

Q: What happens if atmospheric CO₂ is doubled?

Direct warming of +1.25°C, plus feedbacks: more water vapour (+2.5°C) and reduced albedo from ice melt (+0.6°C).

Q: What are the positive feedback mechanisms for warming?

Water vapour feedback (2× the CO₂ effect) and albedo feedback from ice/snow melt (0.5× CO₂ effect). Together they approximately double the initial warming.

Q: What is the negative feedback mechanism for warming?

Increased cloud cover causes more backscattering of solar radiation, partially offsetting warming. Cloud effects are hard to predict from geological records.

Q: How does recent warming compare to the rest of the Holocene?

Mean annual global temp has risen >1°C in the last 100 years (especially since 1980). The extent and rate of change are unprecedented compared to the last 2,000 years and the Holocene.

Q: What does the δ¹³C trend reveal about the source of rising CO₂?

Fossil fuels and biomass have δ¹³C of −27‰ vs. atmospheric CO₂ at −6.5‰. If they are the source, atmospheric δ¹³C should decrease (~0.02‰/yr), which is exactly what's observed.

Q: What three orbital variables control natural climate change?

Obliquity (axial tilt), eccentricity (orbit shape), and precession.

Q: What is obliquity and what is its periodicity?

Earth's axial tilt, currently 23.5°. It varies between 22.2° and 23.5° with a ~41,000-year cycle. More tilt = more seasonality, especially at polar regions (10–15% more summer radiation).

Q: What is orbital eccentricity?

A measure of how elliptical Earth's orbit is (e = 0 is circular, e > 0 is elliptical). Earth's distance to the Sun varies ~3% from the mean of 155.5 million km.

Q: What are perihelion and aphelion?

Perihelion (Jan 3): closest to Sun (~153M km). Aphelion (July 4): furthest from Sun (~158M km). The radiation effect is small (~0.2%) compared to seasonal variations from tilt.