Lecture 11 – The Human Dimension of Climate Change

Q: What is the Holocene Epoch?

The current interglacial epoch, spanning the last 11,700 years.

Q: How far back do ice core records extend?

Over 800,000 years.

Q: What proxy data from ice cores reveal past temperatures?

Oxygen (δ¹⁸O) and hydrogen (δ²H) isotope ratios.

Q: How stable was Holocene climate (excluding the last 200 years)?

Very stable — natural temperature variations remained within ±1°C.

Q: List the five lines of evidence for 20th century warming.

(1) Mountain glacier retreat (avg. 1.5 km since 1800s); (2) decreased N. Hemisphere snow cover (1978–2003); (3) Arctic sea ice decreased by 40% since 1970s; (4) longer growing seasons (e.g., Alaska +2 weeks in 50 years); (5) sea level rise of 17 cm.

Q: What are the three causes of sea level rise?

Thermal expansion of warming ocean water, melting mountain glaciers, and melting of Greenland/Antarctic ice sheets.

Q: How much has mean annual global temperature risen in the last 100 years?

Over 1°C, supported by both instrumental measurements and proxy data.

Q: How does air become trapped in ice cores?

Snow compacts and sinters into ice, sealing air in bubbles. The trapped air is younger than the surrounding ice but older than today's atmosphere.

Q: What is sintering?

The compaction and bonding of snow grains into solid ice, which seals air in closed bubbles.

Q: What are the three zones of air behavior from surface to depth in an ice sheet?

Surface: air moves freely. ~15 m: air diffuses slowly. ~50 m: air is fully sealed in bubbles.

Q: What is Greenland's deposition rate and its ice–gas age offset?

0.5 m/year; air bubbles are centuries younger than surrounding ice.

Q: What is Antarctica's deposition rate and its ice–gas age offset?

0.05 m/year (10× lower than Greenland); air bubbles are ~2,000 years younger than surrounding ice.

Q: What was pre-industrial CO₂ and what is the current level?

Pre-industrial: 280 ppm. Current: ~425 ppm (50% above pre-industrial; over 70% of the increase since 1970).

Q: When did atmospheric CO₂ start rising, and since when have instruments confirmed the trend?

Started rising ~1800 AD; instrumental confirmation since 1958.

Q: What is the MRT of CO₂ in the atmosphere?

Approximately 10 years.

Q: What was pre-industrial CH₄?

750 ppb; has increased since the Industrial Revolution and accelerated in the last 50 years.

Q: What are the three anthropogenic sources of the CO₂ increase?

Deforestation, biomass burning (tropical rainforests), and fossil fuel combustion (peat, coal, oil, natural gas).

Q: What is the potential natural source of CO₂ increase?

Volcanic emissions.

Q: What are the δ¹³C values for pre-industrial atmospheric CO₂, fossil fuels/biomass, and volcanic CO₂?

Pre-industrial atmosphere: −6.5‰. Fossil fuels and biomass: −27‰. Volcanic CO₂: −6‰.

Q: How does δ¹³C evidence identify the CO₂ increase as anthropogenic rather than volcanic?

Fossil fuels/biomass (−27‰) are much lighter than volcanic CO₂ (−6‰). The observed decrease in atmospheric δ¹³C from −6.5‰ points to fossil fuel/biomass sources, not volcanic.