Trees, soil, and climate: What is the connection?
Abstract
The role of land
Land plays an important role in climate, biodiversity, and people's lives. Land absorbs carbon dioxide, a greenhouse gas affecting Earth's temperature. Plants use carbon dioxide, sunlight, and other nutrients to grow and produce oxygen. Carbon is stored in plants and soils, but can be lost when land is cleared, such as by deforestation and degradation. When carbon dioxide is released by burning fossil fuels or by deforestation, some of it goes into the ocean, some of it is absorbed by plants as they grow, and some stays in the atmosphere. The carbon dioxide that stays in the atmosphere traps heat, warming our planet. Cows and sheep also produce methane, a potent greenhouse gas, as they digest their food. Light-colored and snow-covered land surfaces reflect sunlight, but darker surfaces like trees absorb sunlight, making the air above them hotter. Plants affect how energy flows across the land surface, influencing local temperature, winds, and rainfall.
Over the last 150 years, changes in land cover, such as cutting down trees to plant crops and grow livestock, have led to a release of greenhouse gases, contributing to global warming. Those changes have also produced more light-colored surfaces on Earth, which reflect more sunlight, and have caused some cooling [1]. However, the total global warming from increases in greenhouse gases is larger than the cooling from changing land cover. So, overall, Earth has warmed over the last 150 years by 1.15°C.
Land provides food and recycles and stores water. Farmers use land to grow crops, like wheat, maize, fruit, and vegetables. Land is also used for livestock, like cattle, pork, and poultry—both for the animals to live on and to grow their feed. Today, about 12%-14% of Earth’s land is used to grow crops, around 37% is used for livestock, and 30% is covered by forests (both natural and managed) [2]. Forests can provide timber for building, paper, and energy production. Some forests are untouched and provide homes for a wide variety of plants and animals, which means they have high biodiversity. Ecosystems with high biodiversity are more resilient to future climate change.
Land is also impacted by changes in climate. Temperature over land has risen considerably faster than temperature over the ocean. Rainfall has also changed, with some parts of the world becoming drier and more at risk of drought and wildfires. Other areas are having more powerful floods and storms. These changes in weather patterns are affecting when, where, and how well plants grow, and whether animals thrive.
How do we know?
Scientists have several ways of understanding the role of land in climate. We can take measurements of land cover from the ground, from airplanes, and from space. For example, satellites can tell us where forests and crops are by taking very precise pictures from space. There is also an instrument on the International Space Station that uses lasers to measure how tall trees are (Figure 1A). Some of the tallest trees in the world are redwood trees in California, USA (Figure 1B). From the ground, we can measure the size of individual trees in a forest, and we can also measure the uptake and emissions of greenhouse gases from plants and animals. We can also get data from timber producers and farmers. We use all these types of data to work out how much carbon is stored, gained, or lost. By taking these measurements repeatedly, we can see how things change over time, and under different environmental conditions.
Scientists also use computer models to understand land and climate. These models use mathematical equations to represent various processes that happen on Earth, such as the way a tree grows, how much carbon it absorbs, how much sunlight the land reflects, and more. With these models, scientists can ask questions about what has happened in the past and what might happen in the future. For example, we can use a computer model to understand what would happen to the climate if we planted many more trees. This way, we can test what might happen before we try it in the real world.
How land can help address climate change and improve sustainability
The way we manage land can help limit climate change. Reducing deforestation and improving land management can reduce carbon dioxide emissions, while planting trees can remove carbon dioxide from the atmosphere (although it can take several decades for new trees to grow). People may consider changing their diets to eat less meat, or they may try to waste less food, because these actions will reduce greenhouse gas emissions. Good land management can also free up land for forests and other natural ecosystems, increasing biodiversity and food security at the same time. Restoring natural ecosystems, like wetlands, can also help slow down natural river flow and prevent flooding. Increasing how much carbon is stored in soils can help farmers grow more food on the same amount of land.
Land products can replace goods and services that produce a lot of greenhouse gases. For example, we can burn wood or crops (called bioenergy) as an energy source instead of fossil fuels like oil and coal; or we could use timber instead of concrete and steel in buildings. While these steps could reduce greenhouse gases, we would need to use large areas of land to produce this bioenergy. This might mean there is less land available to grow crops for food, which could lead to challenges for food security and sustainability [2]. While land can help limit climate change, it cannot stop it all: Greenhouse gas emissions from burning fossil fuels would need to be reduced to near zero to stop global warming.
Using computer models, scientists can look at how different choices people make will change the land in the future. There are many choices we can make now that can improve life on land and limit climate change. For example, we could have a future in which the Earth has grown back much of the forest areas that have been cut down. Improved farming technology could help ensure that food is available to everyone around the world, while still increasing forest areas.
Looking at the same area of North America shown in Figure 1A, Figure 2 shows the results of a computer model predicting how the world would look in the year 2100 if tree cover increased and greenhouse gas emissions were reduced. The green colors show where there are more trees—partly because more trees are planted in this scenario and partly because higher levels of CO2 allow trees to grow a bit better. We hope that having this knowledge of our possible options for the future will help society make better plans to tackle climate change and produce enough food.
Land is vital for our lives, and we know how to use it responsibly. Over many centuries, some communities and nations have learned how to use land sustainably. However, land is vulnerable to changes in climate as well as changes made by people. In recent decades, our use of the land has grown rapidly affecting climate, natural ecosystems, and biodiversity. We know that land management can help reduce greenhouse gas emissions and limit global warming. Our scientific knowledge of the problem can help us understand these issues and plan for a sustainable future.
References
[1] Jia, G., Shevliakova, E., Artaxo, P., De Noblet-Ducoudré, N., Houghton, R., House, J., et al. 2019. “Land–climate interactions”, in Climate Change and Land: an IPCC Special Report on Climate Change, Desertification, Land Degradation, Sustainable Land Management, Food Security, and Greenhouse Gas Fluxes in Terrestrial Ecosystems, eds. P. R. Shukla, J. Skea, E. Calvo Buendia, V. Masson-Delmotte, H.-O. Pörtner, D. C. Roberts, et al.
[2] IPCC. 2019. “Summary for policymakers”, in Climate Change and Land: An IPCC Special Report on Climate Change, Desertification, Land Degradation, Sustainable land Management, Food Security, and Greenhouse Gas Fluxes in Terrestrial Ecosystems, eds. P. R. Shukla, J. Skea, E. Calvo Buendia, V. Masson-Delmotte, H.-O. Pörtner, D. C. Roberts, et al.
[3] O'Neill, B. C., Tebaldi, C., van Vuuren, D. P., Eyring, V., Friedlingstein, P., Hurtt, G., et al. 2016. The scenario model intercomparison project (ScenarioMIP) for CMIP6. Geosci. Model Dev. 9:3461–82. doi: 10.5194/gmd-9-3461-2016
