Lecture 21 – Causes of Long-Term Climate Change (p.1)

Q: What is the Faint Young Sun Paradox?

Early in Earth’s history, the Sun was 25–30% weaker than today — yet the geological record shows Earth was not frozen, contradicting climate models that predict a frozen planet prior to 2 billion years ago at today’s CO₂ levels.

Q: What geological evidence contradicts a frozen early Earth?

Sedimentary rocks older than 3.5 billion years were deposited in oceans (liquid water, not ice), with the oldest sedimentary rock (3.9 Ga) from the Isua Formation in Greenland, and abundant evidence for life in oceans older than 3.5 Ga.

Q: What do carbon isotope data reveal about early life?

Organic material is always ~25‰ lower in δ¹³C than associated bicarbonate/carbonate values, providing evidence that primitive life was capable of photosynthesis as early as 3.45 billion years ago.

Q: What are stromatolites, and when were they abundant?

Layered sedimentary structures formed by the action of cyanobacteria and algal mats growing symbiotically with biofilms; abundant between 2.5 and 3.5 billion years ago.

Q: Were there major glaciations in Earth’s early history?

Few signs of major glaciations prior to 1.0 Ga, with one exception: the Huronian Glaciation from 2,450 to 2,100 million years ago.

Q: What are the proposed solutions to the Faint Young Sun Paradox?

Higher concentrations of greenhouse gases (CO₂, CH₄ from early volcanism), less continental landmass (lower albedo, more heat absorption), a different atmospheric gas mixture (mainly reducing compounds like CH₄), and darker volcanic rock surfaces.

Q: What is the “thermostat-like” process concept?

A self-regulating mechanism that warms Earth during cold periods and cools it during warm periods, keeping climate within conditions suitable for life over geological time.

Q: What is the hypothesis for how Earth’s temperature is regulated over geological time?

Redistribution of carbon between the lithosphere and atmosphere regulates atmospheric CO₂ and CH₄ concentrations, acting as a thermostat.

Q: What are the main greenhouse gases, and what is the total greenhouse effect?

H₂O vapour, CO₂ (0.003%), CH₄ (<0.001%), and N₂O (<0.001%); together they produce a greenhouse effect of 31°C, raising Earth’s average surface temperature to 15°C.

Q: How much solar radiation does Earth absorb?

242 W/m².

Q: Why is the atmosphere especially susceptible to changes in carbon content?

Because it is one of the smallest carbon reservoirs, so even small transfers of carbon in or out cause relatively large changes in atmospheric CO₂.

Q: What is the global carbon cycle paradox?

Although atmospheric CO₂ significantly influences climate and the atmosphere is a small, easily changed reservoir, Earth’s climate appears remarkably stable over the last billion years.

Q: How quickly would atmospheric CO₂ run out if volcanism were shut off?

In ~4,000 years with all other fluxes unchanged; ~25,000 years including near-surface carbon (continental and oceanic); ~300,000 years including deep ocean carbon.

Q: Why is volcanism NOT the thermostat-like process regulating atmospheric CO₂?

It acts on relatively short timescales, involves relatively small fluxes, is driven from Earth’s interior by plate tectonics, and critically does not react to external atmospheric changes.

Q: What is the difference between carbonate weathering and silicate weathering in terms of atmospheric CO₂?

Carbonate weathering operates on timescales less than 1 million years but has no net impact on atmospheric CO₂ (equal exchange). Silicate weathering removes CO₂ from the atmosphere on timescales of millions to tens of millions of years.

Q: Why does only silicate weathering remove CO₂ from the atmosphere?

Because the carbon from silicate weathering sinks to the ocean floor as solid carbonate and remains sequestered for many millions of years until subduction, rather than being returned to the atmosphere.

Q: What happens when the weathering flux exceeds the volcanism flux?

Atmospheric CO₂ decreases (fast, small reservoir) and lithospheric carbon increases (slow, large reservoir).

Q: What happens when the volcanism flux exceeds the weathering flux?

Atmospheric CO₂ increases (fast, small reservoir) and lithospheric carbon decreases (slow, large reservoir).

Q: How could smaller continents have helped counteract the Faint Young Sun?

Smaller continents meant less silicate weathering, so less CO₂ was removed from the atmosphere, allowing higher atmospheric CO₂ concentrations to compensate for weaker solar output.

Q: What regulates volcanic CO₂ emissions?

Plate tectonic activity — which is driven from Earth’s interior and does not exhibit thermostat-like behaviour in response to surface climate changes.