Water, carbon, climate and life on Earth

Why Water and Carbon Matter for Life on Earth

Water and carbon are fundamental to all living systems:

Water:

• All organisms depend on water for chemical reactions, transporting nutrients, photosynthesis and cell structure.
• The hydrosphere and atmosphere store and circulate water through evaporation, condensation and precipitation, sustaining ecosystems on land and in oceans.
• Water is also a powerful climate regulator: oceans absorb heat and move it around the planet via currents, while water vapour is a natural greenhouse gas that keeps Earth warm enough for life.

Carbon:

• Carbon forms the backbone of organic molecules, including carbohydrates, proteins and DNA.
• Plants absorb carbon dioxide through photosynthesis to build biomass; animals gain carbon by eating plants or other animals.
• Respiration and decomposition return carbon to the atmosphere and soil, where it can be reused.
• Carbon dioxide in the atmosphere helps trap heat, maintaining temperatures that allow liquid water to exist.

Together, these cycles keep Earth habitable. Both are dynamic systems with stores and flows that change over time.

How the Water and Carbon Cycles Interact in the Atmosphere

Although often taught separately, the cycles are strongly linked — especially in the atmosphere.

1. Carbon dissolves in water

• Atmospheric CO₂ is soluble in water droplets.
• When carbon dioxide dissolves in rainwater, weak carbonic acid forms.
• This acid contributes to the chemical weathering of rocks, transferring carbon into rivers and eventually into the oceans.

2. Water vapour and CO₂ act as greenhouse gases

• Water vapour is the most abundant greenhouse gas.
• Carbon dioxide reinforces the greenhouse effect by trapping long-wave radiation.
• Together, they maintain Earth’s temperature within a range that supports life.

3. Evaporation and precipitation move carbon

• As water evaporates from oceans, CO₂ is released back to the atmosphere.
• Rainwater removes CO₂ from the air and carries dissolved carbon to the ground or ocean surface.

Key idea:

Changes in one cycle influence the other. For example, warmer temperatures caused by increased CO₂ levels increase evaporation, adding more water vapour to the atmosphere, which then further strengthens the warming effect.

Feedback Loops Linking the Water and Carbon Cycles

Feedback is a response within a system that can amplify or reduce change.

Positive feedback (amplifies change)

These processes make warming stronger.

Melting permafrost (carbon cycle feedback)

• Rising temperatures melt permafrost in Arctic regions.
• Frozen soils contain huge amounts of organic carbon.
• When they thaw, microbes decompose the material, releasing CO₂ and methane.
• These gases warm the atmosphere further, causing more melting.

Result: The Arctic can switch from a carbon sink to a carbon source.

Sea-ice feedback (water cycle feedback)

• Ice reflects sunlight due to its high albedo.
• As ice melts, darker ocean water absorbs more heat.
• This increases temperatures and melts even more ice.
• More exposed water also means more evaporation and more water vapour, another greenhouse gas.

Result: Faster warming, especially at high latitudes.

Negative feedback (reduces change)

Some responses help stabilise the climate.

Weathering and carbon removal

• Carbon dioxide dissolves in rainwater, forming weak carbonic acid.
• The acid reacts with carbonate rocks in a process called carbonation weathering.
• The dissolved carbon is transported by rivers to the oceans.
• Marine organisms build shells from carbonate.
• When they die, shells can be buried as sediments, locking carbon away for thousands to millions of years.

Result: Over long timescales, this reduces atmospheric CO₂.

Vegetation growth

Higher CO₂ can increase rates of photosynthesis.
Plants absorb more carbon and store it in biomass and soils.
This helps to counteract rising CO₂ — although only if forests remain healthy and widespread.

How Feedback Influences Climate and Life on Earth

• Positive feedbacks, like melting ice and thawing permafrost, accelerate climate change, risking faster sea-level rise, extreme weather and ecosystem loss.
• Negative feedbacks, such as weathering or increased plant growth, stabilise the carbon cycle, but act too slowly to offset rapid human-driven emissions.
• As climate shifts, freshwater supplies, ocean circulation and global temperatures change, affecting agriculture, biodiversity, health, and human settlements.

Most important point:

Life depends on a stable carbon and water budget. When warming increases faster than stabilising processes can act, the systems that support life come under pressure.

Summary

• Water and carbon cycles store, transfer and recycle essential elements that support all ecosystems.
• In the atmosphere, carbon dioxide and water vapour regulate global temperatures through the greenhouse effect.
• Feedback loops connect both cycles — some amplify warming, others reduce it.
• Human activities are tipping the balance toward stronger positive feedback, threatening long-term climate stability.