The world’s soils are the largest active carbon reservoir on Earth’s land surface – recording at least three more time than in forests and plants.
According to the last Global carbon budget, the emission of an additional 120 billion tonnes of carbon into the atmosphere should be enough to push global warming beyond the 1.5°C limit. That’s a very small number compared to valued 2,000 to 4,000 billion tonnes of carbon stored in soils on a global scale.
The consequence of such a large carbon stock is that if even a small fraction were to be released into the atmosphere due to climate change, it could result in a significant positive climate feedback, causing even more warming.
Global climate models allow scientists to project what might happen to soil carbon in the future as the Earth warms. However, with many factors influencing how soils gain and lose carbon, it’s a complicated picture.
In a new study published in Biogeosciences, we test the ability of the latest models to replicate what we see today. This gives us an idea of how confident we can be in their future predictions and highlights areas where the models could be improved.
Our results show that models are increasingly better at simulating the amount of carbon entering soils, but still need improvement to reproduce the amount lost when soil microbes break down carbon and release it into the atmosphere.
New advances in future modeling will be essential to successfully project how soil carbon will be affected by warming and whether this will help – or hinder – global efforts to reduce carbon emissions.
Soil carbon in the face of climate change
The carbon stored in soils is a key part of the Earth’s carbon cycle – the natural cycle of carbon between the atmosphere, land and ocean.
The carbon cycle is important in determining future climate change because it is sensitive to increasing atmospheric CO2 concentrations and the warming it causes.
Without climate change, carbon uptake by soils would generally be in long-term equilibrium with carbon losses, keeping the overall amount of carbon stored in soil stable.
Globally, soil carbon fluctuations are large – about five times bigger than current man-made CO2 emissions each year. They are also should increase as the climate warms and therefore determining the overall impact on future soil carbon is a major challenge.
The main source of carbon to the soil is the “litter” of vegetation, where fallen leaves and debris end up as carbon in the soil. In the event of an increase in atmospheric CO2 levels, plants and trees are often more “productive” because there is more CO2 available for photosynthesis. This results in more carbon being absorbed from the atmosphere to the land surface and can potentially lead to more carbon being added to the soil.
However, to counteract this effect, there are additional factors that must also be taken into account. The first is that rising temperatures due to global warming allow microbes that live in the soil to decompose carbon at a faster rate. This can cause more soil carbon to be released into the atmosphere, thereby reducing stored carbon. A good example of this is frozen permafrost ground, which is known to be particularly sensitive to climate change and pose a great threat to future climate change.
These changes in the carbon cycle that affect the amount of CO2 in the atmosphere are known as carbon cycle feedbacks, which can speed up or slow down climate change. The future fate of soil carbon therefore plays an important role in quantifying future warming.
To predict the future fate of soil carbon, climatologists use Earth System Models (ESM). These are climate models capable of simulating both climate and carbon cycle processes and present the best tool for making future projections of climate change.
ESMs simulate the carbon cycle and associated feedbacks; however, modeling soil carbon and related processes presents many challenges.
Carbon input from plant litter and output from microbial decomposition are expected to increase as a result of climate change. However, these factors have opposite effects on soil carbon change. The main problem with soil carbon modeling is that we don’t know how much incoming and outgoing carbon will change as a result of global warming and the balance between them is crucial in predicting the net effect on climate.
An additional complication in understanding the future evolution of soil carbon is that soils around the world differ significantly, affecting how carbon is stored in them.
For example, we refer to some of the carbon as being in a “slow soil carbon pool”, which means that the soil may have properties that make it more resistant to decay, which is more common in soils. colder high latitudes of the northern hemisphere. Similarly, we have ‘fast soil carbon pools’, where carbon in the soil is short-lived and is rapidly broken down in the atmosphere, which is more common in warmer tropical regions.
The balance between these types of carbon pools plays an important role in the behavior of soil carbon on a global scale and understanding how this balance will change over time presents a significant challenge when modeling the evolution of the soil. soil carbon.
Our new study assesses how soil carbon is modeled in the latest generation of ESMs in the sixth – and most recent – iteration of the Coupled Model Intercomparison Project (CMIP6). We compare our results with observational datasets and also with the previous generation of models (CMIP5).
CMIP is a globally coordinated climate modeling effort that powers the Intergovernmental Panel on Climate Change assessment process (IPCC). There are many ESMs that have been developed by different modeling centers around the world and are constantly being updated and improved. The CMIP defines consistent experiments on climate change to allow comparison between each of the ESMs and between different generations.
In our study, we compare model output data from CMIP6 and CMIP5 and observational data drawn from the real world, such as satellite-derived estimates. From this, we can see where improvements have been made to CMIP6 over CMIP5, and where limitations remain when it comes to soil carbon modeling.
We consider soil carbon globally and the factors that influence carbon uptake and loss. For uptake, we focus on the “overhead” input of terrestrial vegetation productivity. For losses, we focus on “underground” production, looking at how long carbon survives in the soil before it is broken down by microbes and released into the atmosphere.
We have identified improvements in the simulation of surface processes in CMIP6 models compared to CMIP5. This is largely due to the inclusion of nutrient limitations on plant growth in the CMIP6 models. For plants to be able to photosynthesize, they need CO2, light and water, but also enough nutrients like nitrogen and phosphorus. Therefore, if any of these requirements are limited in the future, this will limit the potential increase in productivity and carbon input to the soil as CO2 increases.
However, the same consistent improvements are not observed in the simulation of underground processes. As shown in the maps below, this mainly translates to a general underestimation of soil carbon in high northern latitudes in CMIP6 models (top) compared to observations (bottom). On the maps, the darker green shading shows higher levels of soil carbon.
Soil carbon varies greatly around the world, both in terms of soil type and amount of carbon stored. For example, in tropical regions where the Earth experiences much warmer temperatures, there are generally smaller amounts of carbon stocks in the soil. On the other hand, it tends to be larger soil carbon stocks in high northern latitude regions where the Earth experiences much colder temperatures.
However, this northern hemisphere high latitude soil carbon is not replicated in the majority of CMIP6 climate models.
Focus on underground processes
The results of our study suggest that much of the remaining uncertainty in soil carbon modeling is related to the simulation of subsurface processes.
This highlights the need to focus more on improving the simulation of these processes in the next generation of ESM for CMIP7.
Improvements would help reduce the uncertainty of projected carbon emissions from global soils under climate change and increase confidence in the carbon budgets associated with different levels of global warming.
Due to the significant amounts of carbon stored in soils globally, understanding and quantifying the potential release of carbon from soils is essential if the temperature limits of the Paris Agreement are to be met.
This research was supported by the European Research Council projects, Emergent Constraints on Climate-Land feedbacks in the Earth System (ECCLES) and Climate-Carbon Interactions in the Current Century (4C).
Varney, R. et al. (2022) Evaluation of Soil Carbon Simulation in Earth System Models CMIP6, Biogeosciences, doi:10.5194/bg-19-4671-2022.
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