Carbon storage and changes in periglacial ice environments

Periglacial regions, characterised by consistently low temperatures and permafrost (permanently frozen soil), play a significant role in the global carbon cycle. These areas store vast amounts of carbon, and with rising global temperatures, their dynamics are shifting in ways that could impact the climate further.

Carbon storage in permafrost

Periglacial soils hold approximately 300–400 gigatonnes (Gt) of carbon as dead organic matter (DOM), compared to about 600–700 Gt in the atmosphere. This represents around 20–30% of the world's soil carbon storage. The cold conditions slow down the breakdown of organic material, allowing significant carbon accumulation over time.

Impacts of warming on periglacial environments

Productivity and decomposition changes:

• Rising temperatures could boost net primary productivity (the rate at which plants produce organic matter), provided nutrient availability isn't a limiting factor.
• Currently, low temperatures restrict nutrient release from DOM.
• Warmer conditions may speed up the decay of DOM, releasing more nutrients into the soil and potentially increasing plant growth.

Greenhouse gas emissions:

• Periglacial areas, especially in regions like eastern Siberia and northern Canada, are major sources of methane, releasing around 15–25 megatonnes (Mt) annually.
• Warming could amplify these emissions as permafrost thaws.

Feedback mechanisms:

• Reduced snow cover lowers surface reflectivity, leading to greater absorption of solar radiation and further warming.
• Thawing permafrost releases methane, a potent greenhouse gas, which could elevate air temperatures even more.

Carbon dynamics in oceans and the impact of climate change

Oceans are a critical component of the Earth's carbon cycle, holding far more carbon than the atmosphere. However, increasing atmospheric carbon due to human activity is altering ocean chemistry and ecosystems in significant ways.

Carbon storage in oceans

Oceans contain approximately 36,000–38,000 petagrams (Pg, where 1 Pg = 1 billion tonnes) of carbon, roughly 40–45 times more than the pre-industrial atmospheric content of about 560 Pg. While atmospheric carbon has risen by around 1.5% since pre-industrial times, the ocean's vast storage capacity makes it a key regulator of global carbon levels.

Effects of climate change on ocean carbon

Ocean acidification:

• Increased atmospheric carbon dioxide (CO₂) dissolves into seawater, forming carbonic acid.
• This reacts with carbonate ions to produce bicarbonate, reducing carbonate availability.
• Since 1750, oceanic pH has decreased by roughly 0.08, representing a 25% increase in acidity.
• Lower carbonate levels affect shell-building organisms like crabs and corals, resulting in thinner, weaker shells that hinder their survival.

Phytoplankton decline:

• Warmer ocean temperatures may reduce phytoplankton populations, which thrive in cooler, nutrient-rich waters.
• Since phytoplankton absorb CO₂ during photosynthesis, their decline could reduce oceanic carbon uptake.

Carbon storage in the terrestrial biosphere and human influences

The terrestrial biosphere, encompassing plants, soils, and ecosystems on land, is another vital carbon store. Human activities have both enhanced and disrupted its capacity to sequester carbon, with significant implications for the global carbon cycle.

Carbon absorption by terrestrial plants

Land-based plants have absorbed around 15–20% of the CO₂ emitted by human activities, acting as a natural buffer against climate change. A doubling of atmospheric CO₂ can substantially increase plant growth, assuming other factors like water or nutrients are not limiting.

Human impacts on biosphere carbon

Agricultural effects:

• Farming practices have mixed outcomes on carbon storage.
• Abandoned farmland can revert to forest, sequestering carbon.
• The use of fire to clear land for agriculture releases large amounts of CO₂ into the atmosphere.

Forest dynamics:

• Historically, many forests acted as carbon sinks, converting CO₂ into biomass through photosynthesis.
• Forest soils also store substantial carbon.
• However, deforestation reverses this process, releasing carbon from both biomass and soils back into the atmosphere.

Wildfire prevention:

• By suppressing natural wildfires, humans prevent carbon release in the short term, allowing it to accumulate in vegetation.
• However, this can increase the risk of larger, more intense fires in the future.

Feedback mechanisms and interactions in carbon storage systems

The storage of carbon in ice, oceans, and the biosphere is interconnected, with changes in one system often influencing the others through feedback loops. Understanding these interactions is crucial to predicting future climate trends.

Key interactions and feedbacks in carbon systems

Positive feedback loops:

• As periglacial areas warm, thawing permafrost releases methane, which enhances greenhouse warming and accelerates further thawing.
• Warmer oceans may hold less CO₂ due to decreased solubility of gases in warmer water, potentially increasing atmospheric CO₂ levels and amplifying global warming.

Negative feedback mechanisms:

• Increased plant growth from higher CO₂ levels can sequester more carbon.
• However, deforestation and land-use changes counteract this by releasing stored carbon.

Complex interactions:

• The interplay between these systems can create complex outcomes.
• For instance, reduced oceanic phytoplankton due to warming could increase atmospheric CO₂, which might then enhance terrestrial plant growth if conditions allow.

Visualising carbon flux trends