CLIMATE CHANGE : SCIENCE AND SOLUTIONS | BRIEFING 7 The carbon cycle: Better understanding carbon-climate feedbacks and reducing future risks In brief Carbon is everywhere on Earth. It is within all living Study of the carbon cycle is critical to narrow gaps in our organisms, in soils, in the ocean and in the atmosphere. understanding of the Earth system and more accurately It is also stored in rocks and fossils. It constantly flows project the effect of CO2 emissions on global temperatures. between these reservoirs, which are interlinked through While the fundamental processes driving flows of carbon a variety of processes. This is the carbon cycle. More than are largely known, uncertainties remain, particularly in the half of the carbon dioxide (CO2) emitted to the atmosphere magnitude of change. Understanding carbon-climate feedback by human activity has been absorbed by natural carbon processes – the effect that a changing climate has on the reservoirs (or ‘sinks’), which have therefore considerably carbon cycle which in turn further changes the climate – is dampened climate change. However, as we continue into particularly important, as these are among the largest sources an era of unprecedented CO2 emissions, we are disrupting of uncertainty in climate change projections. Risks associated the carbon cycle, with potentially severe consequences for with large carbon-climate feedbacks are minimised by our climate. reducing CO2 emissions as much and as quickly as possible. INSIGHTS • Nature’s carbon cycle has alleviated climate change • Enhancement of natural carbon sinks by human and its impacts by absorbing more than half of the intervention will be essential to achieve net zero carbon that human activities have emitted, but such emissions. This includes sustainable afforestation, a large uptake is not guaranteed to continue. reforestation, agricultural soil management, and peatland restoration. • The future of carbon sinks will depend on the level of CO2 in the atmosphere and how fast it rises or • Research to improve understanding of the carbon cycle decreases, on the impacts of climate change, and should include: continuous observational monitoring in the potentially on direct human intervention. Safeguarding atmosphere, on land, and in the ocean, by both in situ and the sinks, in particular forests, is essential to maintain satellite data; better understanding of potential instabilities their functions. in carbon sinks, as well as the development of models that more fully represent the carbon cycle’s complexity. • Carbon-climate feedbacks are expected to amplify climate change and its impacts, with the greatest • The risks associated with carbon cycle uncertainties will and most uncertain effects in a high carbon be minimised by increasing momentum towards halving emissions future. global emissions by 2030 and continuing with deep emissions cuts afterwards. CLIMATE CHANGE : SCIENCE AND SOLUTIONS THE CARBON CYCLE 1 1. Climate change and the carbon cycle 1.1 Why the carbon cycle matters 1.2 The natural carbon cycle For 800,000 years The processes of the carbon cycle are The Earth has an active natural carbon fundamentally tied to the climate of our cycle, inhaling and exhaling carbon like a until the Industrial planet. Carbon is constantly cycling between living organism. Before humans released Revolution (starting atmospheric, oceanic, and terrestrial reservoirs. CO2 at large scale from burning fossil around 1750), ice Before humans perturbed the environment, fuels, deforestation, and other use of land core data show the carbon emitted by natural processes (like (termed ‘the anthropogenic perturbation’), decay of organic matter) was approximately the atmospheric reservoir stored around CO2 concentration in the atmosphere equal to the carbon absorbed (for instance, 2200 billion tonnes of CO2 (2200 GtCO2). by vegetation growth), leading to a relatively Each year, around 400 GtCO is removed remained 2 stable atmospheric CO concentration since from the atmosphere by trees and other approximately 2 the end of the last glaciation ten thousand vegetation on land through photosynthesis, around 300 parts years ago. This helped ensure that surface and a similar amount is returned to it by plant per million, with air temperature was also relatively stable. and soil respiration. In the ocean, around 330 CO2 concentration GtCO2 each year is exchanged between the However, the carbon cycle can move out of fluctuating over atmosphere and surface waters, with more CO2 its fragile equilibrium with the global climate. glacial-interglacial absorbed in the cold waters of high-latitudes, Cold and warm periods in Earth’s history have and some CO out-gassed in the tropical cycles. CO 2 2 shown that natural perturbations, such as oceans where carbon-rich deep waters come concentration glaciations triggered by changes to Earth’s to the surface via upwelling of ocean waters1. reached 415 ppm axial tilt and orbit around the sun, can disrupt in 2020 and the carbon cycle, altering the level of CO2 in For 800,000 years until the Industrial continues to rise. the atmosphere and other reservoirs, with Revolution (starting around 1750), ice core data implications for the Earth’s climate. Such natural shows CO2 concentration in the atmosphere fluctuations reveal the intricate functioning remained below approximately 300 parts of the carbon cycle and its influence on the per million (ppm), with CO2 concentration climate system. fluctuating over glacial-interglacial cycles. In the centuries leading up to the Industrial Anthropogenic, or human-generated, Revolution, CO2 concentration was around 270 emissions have rapidly increased the – 280ppm2. While preindustrial humans emitted concentration of CO in the atmosphere to 2 some CO2 from fires and land clearance, such levels unprecedented in the last three million emissions were very small; it was not until years. Some of these emissions are absorbed the Industrial Revolution that anthropogenic by the natural carbon cycle, but it has been emissions began to significantly perturb the disrupted beyond natural fluctuations. It is carbon cycle. therefore essential to better understand the cycle in order to project future climate change. 2 CLIMATE CHANGE : SCIENCE AND SOLUTIONS THE CARBON CYCLE FIGURE 1 CO2 concentrations in the atmosphere have fluctuated over the last 800,000 years as the Earth moves between 1 glacial and interglacial periods, but human activity since 1750 is increasing CO2 concentrations at unprecedented rates (A). As of 2020, Earth’s atmospheric CO2 concentration is around 415 ppm and rising by around 2.5 ppm each year. This growth rate has accelerated in recent decades11 (B). (A) 450 2020 400 2000 350 1980 1960 300 concentration (ppm) 2 250 CO 200 150 800,000 700,000 600,000 500,000 400,000 300,000 200,000 100,000 0 Years before present (B) 420 400 380 360 concentration (ppm) 2 CO 340 320 300 1960 1980 2000 2020 Year CLIMATE CHANGE : SCIENCE AND SOLUTIONS THE CARBON CYCLE 3 1.3 The carbon cycle since 1750: the Total fossil plus land-use change emissions were partitioned among the atmosphere (19 GtCO ), Without the ‘anthropogenic perturbation’ 2 Since 1750, anthropogenic emissions of CO land (13 GtCO ), and ocean (9 GtCO )2. sequestration of 2 2 2 have started to disturb the global carbon cycle, carbon by the rising from negligible levels of anthropogenic Carbon sinks have prevented much higher levels of climate change and more severe land and ocean, CO2 before the Industrial Revolution, to around impacts. Without the sequestration of carbon the level of CO2 in 42 GtCO2 in 2019 from burning of fossil fuels 2 by the land and ocean, the level of CO in the the atmosphere and land-use change . This increase in CO2 2 would now be emissions is the main driving cause of climate atmosphere would now be around 600 ppm, change. With methane, nitrous oxide and other with an associated warming about double that around 600 ppm, 2 greenhouse gases (GHGs) included, annual currently observed . with an associated emissions have risen to around 59 billion warming about 4 The land and ocean respond to rising CO2 in tonnes of CO2 equivalent (GtCO2e) (see panel the atmosphere. Land has helped to alleviate double that on methane and nitrogen cycles). currently observed. climate change because elevated atmospheric CO increases photosynthesis, which in In the past decade, CO2 emissions originated 2 primarily from fossil fuel combustion, averaging turn tends to encourage more plant growth (especially of trees). More plant growth leads to 34 GtCO2 per year during 2010 – 2019 (see figure 2). During the same period, land- more biomass, resulting in more carbon storage in living plants. When plants die, some of this use change emitted 16 GtCO2, mainly from additional biomass becomes soil organic matter, deforestation, and reabsorbed 10 GtCO2, mainly from the regrowth of abandoned agricultural increasing soil carbon storage. This is a primary reason why the land sink has helped dampen land, leading to net emissions of 6 GtCO2. anthropogenic climate change. However, this BOX 1 process has limits. The land sink will saturate when it becomes limited by other factors, such The methane and nitrogen cycles as the availability of water or nutrients, meaning Stabilising the global climate does not depend it may sequester proportionally less of the future CO2 humans emit. Impacts of climate change, only on CO2 emissions but other greenhouse gases (GHG) which make up more than such as forest droughts, fires and permafrost 25% of total anthropogenic GHG emissions, thaw, may even reverse the sink. particularly methane and nitrous oxide4. Total In the ocean, rising atmospheric CO2 pushes GHG emissions, including CO2, methane, additional CO into the ocean, which leads to nitrous oxide and other gases reached an 2 4 a proportional increase in the amount of CO2 estimated 59 GtCO2e in 2019 , of which around dissolved in the ocean surface. Most of this 10 GtCO2e comprised methane and 3 GtCO2e CO reacts with carbonate ions in seawater to nitrous oxide.