Fossil fuels on the carbon cycle

What is the carbon cycle

• The carbon cycle is the natural process by which carbon moves between the atmosphere, oceans, land, and living organisms, involving processes like photosynthesis, respiration, decomposition, and combustion

How much of the extra carbon from human activities is absorbed by land plants and oceans

• About 55% is absorbed; the remaining 45% stays in the atmosphere

How long may some of the extra atmospheric CO₂ remain in the atmosphere

• Up to many thousands of years, with around 20% potentially remaining long-term

How does excess carbon in the atmosphere affect the planet

• It warms the planet and can enhance plant growth on landHow does excess carbon in the ocean affect marine life

How does excess carbon in the ocean affect marine life

• It makes the water more acidic, putting marine life in danger

Why is the persistence of CO₂ in the atmosphere significant

• CO₂ is the most important gas for controlling Earth’s temperature

What percentage of Earth’s greenhouse effect is caused by CO₂, water vapor, and clouds

• CO₂: ~20%, water vapor: ~50%, clouds: ~25%

What contributes to the remaining portion of the greenhouse effect

• Small particles (aerosols) and minor greenhouse gases like methane

How do water vapor concentrations in the air depend on temperature

• Warmer temperatures increase evaporation and humidity; cooler temperatures cause condensation and precipitation

How does rising CO₂ affect water vapor in the atmosphere

• Higher CO₂ raises air temperatures, which increases evaporation, adding more water vapor and amplifying the greenhouse effect

Why is CO₂ considered the “gas that sets the temperature”

• Because it controls the amount of water vapor in the atmosphere, determining the size of the greenhouse effect

How much anthropogenic CO₂ is absorbed by ocean surfaces

• Approximately 30%

What happens to CO₂ when it dissolves in seawater

• It forms carbonic acid, causing ocean acidification and impacting aquatic ecosystems

How does ocean acidification affect marine life

• Corals, mussels, and some plankton have deformed or eaten-away shells, disrupting the food chain

How is the ocean’s ability to absorb CO₂ changing

• Warm waters are less effective than cool waters, slowing carbon uptake and leaving more CO₂ in the atmosphere

What is the atmospheric lifetime of CO₂

• Approximately 300–1,000 years

How has ocean pH changed since pre-industrial times

• Pre-industrial pH was 8.2; in 2022 it is 8.1—a 30% increase in acidity

How do warmer oceans affect phytoplankton abundance

• Warmer oceans reduce phytoplankton abundance because they grow better in cool, nutrient-rich waters

How could reduced phytoplankton affect the carbon cycle

• It limits the ocean’s ability to take carbon from the atmosphere through the fast carbon cycle

Can increased CO₂ fertilize phytoplankton and ocean plants

• Some species that directly take up CO₂, like certain phytoplankton and sea grasses, may grow more

Does increased CO₂ benefit all ocean plant and phytoplankton species

• No, most species are not helped by higher CO₂ levels

How much of human-emitted CO₂ has been absorbed by land plants

• Approximately 25%

How has plant carbon uptake changed since 1960

• Plants have generally absorbed more CO₂, though only some of this increase is directly due to fossil fuel emissions

What is carbon fertilization

• Increased plant growth due to higher levels of CO₂ available for photosynthesis

How much might plant growth increase if atmospheric CO₂ doubled

• Models predict a 12–76% increase, assuming no other limiting factors like water shortages

What factors besides CO₂ are essential for plant growth

• Water, sunlight, and nutrients (especially nitrogen)

Is there a limit to how much CO₂ plants can absorb

• Yes, the limit varies by region and depends on availability of water, nutrients, and other growth factors

How do higher CO₂ and temperatures affect plants

• They can extend the growing season and increase growth, but warmer temperatures can also stress plants

How does a longer, warmer growing season affect plants

• Plants need more water; water shortages can slow growth, especially in summer in the Northern Hemisphere

Why are dry, water-stressed plants more vulnerable

• They are more susceptible to fires, which release carbon from plants and soil into the atmosphere

How does warming affect tropical forests

• Reduced water slows tree growth, lowers carbon uptake, or causes trees to die and release stored carbon

What happens when soil warms, particularly permafrost

• Organic matter decays faster, releasing carbon as methane and CO₂ into the atmosphere

How much carbon is stored in Northern Hemisphere permafrost

• Approximately 1,672 billion tons (Petagrams) of organic carbon

What could happen if 10% of permafrost thaws

• It could release enough carbon to raise global temperatures by ~0.7°C (1.3°F) by 2100