The carbon cycle is closely linked to the climate system and is influenced by the growing human population and associated demands for resources, especially fossil fuel energy and land.
The rate of change in atmospheric Carbon dioxide (CO2) reflects the balance between carbon emissions from human activities and the dynamics of a number of terrestrial and ocean processes that remove or emit CO2.
The long-term evolution of this balance will largely determine the speed and magnitude of human-included climate change and the mitigation requirements to stabilize atmospheric CO2 concentrations at any given level.
Carbon emissions from fossil fuel combustion and cement production in 2008 were 8.7 Gt C. 41% higher than in 1990 (Kyoto Protocol base year), following the average of the most carbon-intense scenarios of the Intergovernmental Panel on Climate Change (IPCC). According to a report by the REN21 thinktank, fossil fuels (coal, oil and gas) which are the main cause of global warming accounted for 80.2% of final energy consumption in 2019.
Renewable energy grew three times faster than fossil fuels and nuclear over a five-year period, but accounted for less than one-third of the increase in total final energy demand.
The largest advances have been in the power sector (in both capacity and generation), while significantly less progress has occurred in heating, cooling and transport.
Renewables continued to meet low shares of final energy demand in the buildings, industry and transport end-use sectors, where support policy remained crucial to spurring uptake, yet was lacking. Emissions from land use change are on average 1.5 Gt C per year. They are largely determined by deforestation in tropical regions resulting from domestic policies, economic development, and global commodity prices which are often interlinked in complex ways.
Combined emissions for fossil fuels and land use change increased by over 3% per year since 2000, up from 1.9% over the period 1959-1999. The growth of these emissions is driven by population, per capita Gross Domestic Product (GDP), and carbon intensity of GDP.
The increase in the growth of population and per capita income weigh equally in driving emissions upward, the latter becoming more important in recent years. Carbon dioxide is responsible for more than 60% of the 2.6 Watts/m2 warming that has resulted from the increase in human-induced long-lived greenhouse gases.
The relative importance of carbon dioxide in climate change will further rise as we continue to burn larger amounts of fossil fuels. Carbon has a uniquely long residence time.
As estimated 20-35% of today’s emissions will remain in the atmosphere for several centuries into the future. There is no consensus about the level of global temperature increase defining “dangerous anthropogenic interference of the climate systems”.
However, growing evidence show that keeping global warming below two degrees Celsius above pre-industrial levels could avoid the worst impacts of climate change.
To keep below this two-degree limit with 50% probability, only an additional 500 billion tonnes of carbon can be emitted into the atmosphere.
This would bring the total anthropogenic cumulative emission allowance close to one trillion tonnes (including the 500 billion tonnes already emitted over the past 200 or so years).
This budget approach can help policymakers to explore how the remaining carbon emissions for a given global temperature target can be partitioned among countries and citizens around the world.
Unless urgent emission reductions are implemented, 500 billion tonnes of carbon will be emitted within the next 30 years. Climate change and land use change can destabilize large carbon reservoirs and reduce the efficiency of natural CO2 sink in the ocean and in the land ecosystem leading to an acceleration in the accumulation of atmospheric CO2.
Changing atmospheric CO2 concentrations affect ocean carbon sinks leading to ocean acidification and widespread changes in marine biota thus affecting the capacity of oceans to store carbon.
Other vulnerable reservoirs include carbon in frozen soils and northern peat, tropical peat, forests vulnerable to deforestation, drought and wildfires, and methane hydrates in permafrost and continental shelves.
There are indications now that the efficiency of CO2 uptake by natural sinks may have already declined over the last 50 years. Natural sinks remove on average 55% of every tonne of anthropogenic CO2 emitted to the atmosphere, down from 60% fifty years ago.
Urban areas contributed 71% of global energy-related CO2 emissions in 2006. World urbanisation – 49.6% in 2007 – is expected to reach 70% by 2050.
Almost all of this increase will come from further urbanization of developing countries, providing a challenge and an opportunity to manage carbon emissions.
Cities import large amounts of energy and products which result in carbon emissions at the point of production far away from the cities themselves (e.g. in coal power plants that generate electricity or industries producing cement or goods for cities).
For the first time, developing countries are now emitting more fossil fuels CO2 emissions (55%) than developed countries. Per capita emissions, however, continue to be led by developed countries by an ample margin. The world’s energy demand is expected to rise by 50% by 2030, and unless major changes are implemented rapidly, 80% of that increase will depend on fossil fuels (oil, gas, and coal).