Land and Vegetation
Land is where we live
We depend on the land surface for almost all of our food and water, work and recreational needs. About one fifth of the Earth’s surface area is habitable land, and today we use half of this habitable land for human food production.
In the early 1700s, before the industrial revolution, half of the ice-free land area was in a wild state, such as uninhabited forests and grasslands, and only 5% was actively used by humans1. By 2000 this situation had reversed, just over half was in direct use by humans and only one quarter remained wild. Overall, human activities have affected more than 70% of the ice-free land area2. Clearly, the land surface and vegetation are crucial for our well-being and are where we are particularly vulnerable to the effects of environmental change. But these regions are on the front line of climate change, with temperature increase over land since 1850 roughly twice as large as the global average increase2.
Land interacts with physical climate
Properties of the land environment affect the flow of energy through the climate system. The land absorbs energy from the Sun (solar radiation) and from the atmosphere (thermal radiation) and maintains an overall balance by returning that energy to the atmosphere. How much solar radiation is absorbed rather than reflected varies greatly between land cover types and conditions. For example, tropical forest and grass absorb around 90% and 75% of solar radiation arriving at the surface, respectively, whereas fresh snow may absorb only 10%.
The energy returned from land to the atmosphere can take several forms, such as thermal radiation, heat, and water vapour. This vapour comes from the evaporation of water stored on the land in soil, as snow and ice, and on surfaces, and is an important contribution to the land component of the global water cycle.
Land environments with no water, or where water is less accessible, tend to be warmer because they are less able to lose absorbed solar energy by evaporation. Land cover strongly affects this ability to access water, with bare ground only able to evaporate from the top few centimetres of soil, whereas deep-rooted trees may transpire (i.e., evaporate through their leaves and stems) using water from many metres below the surface.
Similarly, taller vegetation helps to stir the air above the surface, leading to a greater ability to lose energy as heat and evaporation. Water evaporated to the atmosphere will fall from the atmosphere again somewhere as rain or snow, which can lead to strong relationships between vegetation cover and climate. In tropical forests, these relationships can be further influenced by the ability of trees to emit molecules (volatile organic compounds) that can affect the formation of clouds and rainfall.
Land is a carbon reservoir
Land is a store of carbon and an important component of the global carbon cycle. Vegetation combines carbon dioxide (CO2) from the air, water from the soil, and energy from solar radiation to store carbon on land as biomass, a process known as photosynthesis or primary production. Some of this vegetation biomass is consumed by other living things, such as animals and soil microbes, to form their own biomass, a process known as secondary production. Both primary and secondary producers also respire CO2 back to the atmosphere as they use the energy stored in their biomass, contributing to a natural cycle of carbon between land and atmosphere.
Globally, land exchanges approximately 120 billion tonnes of carbon with the atmosphere each year. Over thousands of years, a small imbalance between the incoming photosynthesis and the outgoing respiration has led to a gradual accumulation of carbon reservoirs on the land, with around 500 billion and 4 trillion tonnes of carbon stored in vegetation and soils, respectively3.
This imbalance in carbon exchange affects the amount of CO2 that is in the atmosphere and is itself affected by climate conditions such as temperature, precipitation, and radiation. Currently, climate is changing because our activities are releasing greenhouse gases, such as CO2 and methane (CH4), into the atmosphere. Much of this release is from the burning of fossil fuels, which is carbon that has been stored on the land for millions of years. This climate change has also perturbed the natural carbon cycle, such that land is currently a net sink of CO2 (i.e. the land absorbs CO2) from the atmosphere, and around 1/3 of current fossil fuel emissions are taken up by the global land through vegetation3. The land is also a notable contributor to global CH4 emissions through vegetated ecosystems, such as wetlands.
Land use change could affect future climate change
Fossil fuel burning is not the only source of greenhouse gas emissions from land, a significant fraction of emissions is the result of how we use the land and how that use changes over time. This includes things like the management of farmland, deforestation and wildfire. Recent agriculture, forestry and other land use activities accounted for around 13% of CO2, 44% of methane (CH4), and 81% of nitrous oxide (N2O) emissions globally, amounting to 23% of total net man-made greenhouse gas emissions2. As well as affecting the land carbon reservoir by releasing stored carbon and nitrogen as greenhouse gases, these land use changes can also alter how the land interacts with physical climate through the exchange of energy, water, trace gases and aerosols with the atmosphere.
Land could help combat future climate change
The best way to avoid further climate change is for us to reduce our greenhouse gas emissions. We can use the land to help with this replacing some of our energy generation from the burning of fossil fuels, in which carbon was accumulated slowly over hundreds of millions of years, with the burning of bioenergy crops, in which carbon was accumulated rapidly over just a few years. We can reduce our land use greenhouse gas emissions by improving our management of farming, forestry and fires, and by reducing the amount of land that is converted from high- to low-carbon storing ecosystems. Despite these land use emissions, uptake of CO2 by natural ecosystems means that the land is currently a net sink of carbon from the atmosphere. We have the potential to slow the accumulation of CO2 in the atmosphere by enhancing this land sink through the protection and expansion of carbon rich ecosystems like forests and peatlands.
References
- Ellis et al (2010) https://doi.org/10.1111/j.1466-8238.2010.00540.x
- IPCC SR Land (2019)
- Friedlingstein et al (2020) https://doi.org/10.5194/essd-12-3269-2020
- Saunois et al (2020) https://doi.org/10.5194/essd-12-1561-2020
See also: https://ncas.ac.uk/the-future-of-our-forests-climate-change-and-trees
