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Understanding the carbon cycle and carbon budgets

What is the carbon cycle?

The carbon cycle is a natural process that describes the movement of carbon through various components of the Earth's atmosphere, oceans, soil, and living organisms. It's a fundamental part of Earth's systems, regulating the distribution of carbon between different reservoirs and helping to maintain the planet's carbon balance.

The carbon cycle consists of several interconnected processes that continually exchange carbon dioxide and other carbon compounds between these reservoirs.

Key processes in the carbon cycle:

  1. Photosynthesis: Plants, algae, and some bacteria perform photosynthesis, a process where they use sunlight to convert carbon dioxide and water into glucose (a sugar) and oxygen. This glucose becomes a source of energy and carbon for these organisms.

  2. Respiration: All living organisms, including plants, animals, and microorganisms, undergo respiration. During respiration, organic matter, including glucose, is broken down to release energy, and carbon dioxide is produced as a by-product.

  3. Decomposition: When plants and animals die, their organic matter undergoes decomposition, often carried out by bacteria and fungi. This process releases carbon back into the soil and atmosphere as carbon dioxide or other carbon compounds.

  4. Fossil fuel formation: Over millions of years, the remains of plants and animals can become buried and undergo pressure and heat, transforming into fossil fuels like coal, oil, and natural gas. Burning these fuels releases carbon dioxide into the atmosphere.

  5. Ocean uptake and release: The oceans play a critical role in the carbon cycle. They absorb carbon dioxide from the atmosphere, helping to regulate atmospheric carbon levels. However, this process also leads to ocean acidification, which can impact marine ecosystems.

  6. Volcanic activity: Volcanic eruptions release carbon dioxide from Earth's interior into the atmosphere. While this is a natural process, it contributes a relatively small amount of carbon compared to human activities.

How do humans influence the carbon cycle?

The carbon cycle, in its natural state, maintains a balance between carbon sources and sinks, helping to regulate Earth's climate. However, human activities, particularly the burning of fossil fuels and deforestation, have significantly disrupted the natural carbon cycle.

The extraction and combustion of fossil fuels release large amounts of carbon dioxide into the atmosphere, contributing to the greenhouse effect and global warming. Deforestation reduces the planet's capacity to absorb carbon dioxide through photosynthesis. The Earth's climate is intricately connected with the carbon cycle, underscoring the importance of understanding and managing carbon emissions to mitigate the effects of climate change and work towards a more sustainable future.

What is the global carbon budget?

The global carbon budget refers to the total amount of carbon dioxide and other greenhouse gases that can be emitted into the atmosphere while still keeping global warming within a specific target, typically expressed as a maximum allowable temperature increase. The concept of the carbon budget is central to international efforts to address climate change and limit its impacts.

Key aspects of the global carbon budget

Emission targets

The global carbon budget sets a limit on the total cumulative emissions of greenhouse gases, primarily carbon dioxide, that can be released into the atmosphere. This target is informed by scientific research and aims to limit the increase in global average temperature to well below 2 degrees Celsius above pre-industrial levels, as set out in the Paris Agreement.

Carbon budget period

The carbon budget is usually expressed for a specific time frame, such as a decade or multiple decades. For example, a carbon budget might be set for the period between 2020 and 2030.

The UK carbon budget places a restriction on the total amount of greenhouse gases we can emit over a 5-year period. The UK is the first country to set legally binding carbon budgets.

Carbon sources and sinks

A carbon budget considers both the sources of greenhouse gas emissions and the sinks that absorb these gases. The balance between emissions and removals by sinks determines whether the budget is being adhered to.

  • Carbon sources: These are the processes and activities that release carbon dioxide into the atmosphere. The primary sources include the burning of fossil fuels for energy production, industrial processes, deforestation, and land-use changes. Additionally, certain natural processes like volcanic eruptions and wildfires contribute to carbon emissions.

  • Carbon sinks: These are the natural reservoirs that absorb and store carbon dioxide from the atmosphere. The main carbon sinks are forests, oceans, and soil. Plants absorb carbon dioxide during photosynthesis, storing carbon in their biomass and roots. The oceans also act as significant carbon sinks, with dissolved carbon dioxide being absorbed into the water. However, the capacity of these sinks to absorb carbon is being strained by increasing emissions.

Fair carbon allocation

Discussions around the global carbon budget also involve considerations of fairness, as different countries have contributed varying amounts to historical emissions. These discussions often centre on the principles of equity and the responsibility of developed nations to take a leading role in emissions reductions.

Emission pathways

To stay within the carbon budget, nations and industries need to follow emission reduction pathways, for example Net Zero. These pathways involve setting intermediate targets to progressively reduce emissions over time.

Carbon budget review and adjustment

The carbon budget is not fixed but is subject to scientific review and adjustment based on new data and research. This flexibility allows for adaptation as our understanding of the climate system and emission impacts evolves.

Why is the global carbon budget important?

The concept of the global carbon budget underscores the urgency of addressing climate change and highlights the finite nature of the Earth's capacity to absorb greenhouse gases. It guides policymaking, encourages emission reduction efforts, and emphasises the need for global cooperation to achieve climate goals.

Staying within the global carbon budget requires a rapid and substantial reduction in greenhouse gas emissions across all sectors of the economy. This entails transitioning to renewable energy sources, improving energy efficiency, reforestation, and adopting sustainable land-use practices. As the international community works together to limit global warming and its associated impacts, carbon accounting will continue to serve as a crucial tool to guide actions and decisions toward a more sustainable and resilient carbon budget.


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