Soil Temperature & Global Warming

Soil & Global Warming

Surface Carbon Water Biota Climate

Estimated Surface Temperatures :

On a sunny day - 30C, arable land can be 5-10C warmer than pasture 32-36 and arable 40-45C. Concrete can get 20–25°C hotter than the air temperature on a sunny day, so the concrete surface could reach around 50–55°C. Tarmac can become 25–30°C hotter than the air temperature, so the tarmac could reach 55–60°C

The widespread use of surfaces like concrete and tarmac in urban areas contributes significantly to the urban heat island effect, where cities become much warmer than surrounding rural areas. These artificial surfaces absorb and retain far more heat than natural landscapes like forests, grasslands, or pastures, exacerbating temperature rises. This also contributes to global warming because:

Increased energy use: Higher temperatures in cities mean more demand for cooling, which can increase greenhouse gas emissions if powered by fossil fuels.
Reduced cooling from vegetation: Replacing natural landscapes with buildings and pavement eliminates the cooling effects of plants (through processes like transpiration and shading).
Less reflection: Urban surfaces generally have a lower albedo than natural environments, so they absorb more solar radiation, further warming the surface and the air above.

The relationship between land use changes (trees to grass & grass to arable) and global warming is not easy to calculate. All soils are different, climate changes all the time and there all different plants. Most studies involving soil temperatures and global warming are quite recent, as few people were interested although yet their significance may be profound.

"Rising rangelands and reducing forest areas have indirectly increased surface temperature" (Barati et al 2023).

Land-Use & Land-cover changes

“The results indicate that the Land-Use & Land-cover (LULC) changes have contributed to warming over this region by 0.06°C per decade. Comparison of the change in temperature trend with the change in LULC indicates warming due to LULC changes because of the reduction of area under open forest and subsequent increase of the area under agricultural land. The study highlights the impact of land-use change to be more significant and the utility of satellite data for periodic LULC studies in climate change research." (Nayak & Mandel 2012)

Topsoil Temperatures

Soil temperatures are determined by a number of components, such as specific heat capacity, thermal conductivity, and thermal diffusivity, which are affected by basic soil properties that include bulk density, texture, and water content. Thermal conductivity and diffusivity depend a lot on soil wetness and other soil properties. The flow of water and heat is an interactive process, where temperature gradients affect the moisture potential and both liquid and vapour movement in soil. Soil temperatures are being studied much more to help guide cropping techniques.

Big Data

There is rapid increase in available big data on species distribution and soil temperature, particularly a new database SoilTemp database. We can ask questions we didn’t dare think about a few years ago. “Mean annual topsoil temperature was on average 3.6 ± 2.3°C higher than gridded air temperature in cold and/or dry biomes, namely tundra, boreal forests, temperate grasslands and subtropical deserts. In contrast, offsets were slightly negative in warm and wet biomes (tropical savannas, temperate forests and tropical rainforests) where soils were, on average, 0.7 ± 2.7°C cooler than gridded air temperature. Soils in the temperate seasonal forest biome were on average 0.8°C (±2.2°C) cooler than air temperature within 1-km2 grid cells of forested habitats, and 1.0°C (±4.0°C) warmer than the air within 1-km2 grid cells of non-forested habitats, resulting in a biome-wide average of 0.5°C. Similar patterns were observed in other biomes" (Lembrechts & Lenoir 2019)

“Results indicate that, across 60 contrasting sites in four countries in Europe, land use intensification (from intensive annual crop growing - High, extensive rotation including legumes or ley - medium, and permanent grassland - low) consistently reduces the biomass of all components of the soil food web, including the key functional groups fungi and bacteria, protozoa, nematodes, earthworms, enchytraeids, mites, and collembolans. Land use intensification equally reduced the biomass of most feeding groups in the soil food web. However, the biomass of the groups that are part of the root energy channel was reduced more than that of the organisms of the fungal and bacterial (detritus) energy channel together" (Vries et al 2013)

Global average temperatures are around 1.1C warmer now than in pre-industrial times. However the land surface has warmed by approximately 1.7C. Why is this?

Most research global change in temperatures relies on estimates of air temperature around 2m above the ground, so do not reflect conditions below vegetation canopies and near the ground surface, which can be up to 10 degrees different. Now we have global maps of soil temperature, that define the what most terrestrial species do better than these air temperatures. Ecological patterns and processes often relate more directly to below-canopy soil temperature rather than to well-ventilated air temperature inside a weather station. “Soil temperature, macroclimate and land-use change will interact to define the future climate as experienced by organisms, and high-resolution soil temperature data are needed to tackle current and future challenges." (Lembrechts et al 2021)

Land Use

As the CEH said: “climate and land use changes that affect plants and below ground ecology will play an important part in controlling the release of soil carbon stores in the form of greenhouse gas carbon dioxide.”

The IPCC said in their 6th Assessment Report 2021 Chapter 2 (2.2.7) under the heading ‘land use and land cover’ that: ”The AR5 assessed that land use change very likely increased the Earth’s albedo with a radiative forcing of –0.15 (± 0.10) W m–2.” This means that changes in land use (like deforestation to agriculture) have very likely increased the reflectivity of Earth’s surface (its albedo). This change has had a cooling effect on the climate, reducing the amount of energy absorbed by Earth's surface, with an estimated impact of –0.15 W m–2 on the planet's radiative balance. The uncertainty range indicates that this cooling effect could be somewhat smaller or larger.

Land use changes also affect surface temperatures through non- radiative processes, and particularly through the hydrological cycle. These processes are less well known and are difficult to quantify but tend to offset the impact of albedo changes. The changes to soil structures can make more profound impacts to global temperature.

Challenges & Developments

The complexities of land use change and global warming include:..

1. Local vs. Global Effects:

While land use changes, such as deforestation or conversion of forests to agriculture, do increase the surface albedo (reflectivity), the overall climate impact is more complex than just a direct cooling effect:

Local Cooling but Global Warming Effects:
- The increase in albedo from deforestation can lead to local cooling, especially in high latitudes where snow cover is more significant.
- However, deforestation also reduces the capacity of land to sequester carbon, which leads to an increase in atmospheric CO₂, causing a global warming effect.
- This results in a trade-off: local cooling due to increased albedo, but global warming from higher CO₂ levels. Studies indicate that the warming from the additional CO₂ may offset or even exceed the cooling from albedo changes.

2. Non-radiative Effects of Land Use Changes:

Some challenges to the conclusion highlight the importance of non-radiative effects:
- Evapotranspiration: Vegetation, especially forests, cools the surface through the process of evapotranspiration (the combined process of water evaporating from the surface and from plant leaves). When forests are cleared, this cooling effect is reduced, leading to local warming.
- Surface Roughness: Forests also affect atmospheric circulation by changing surface roughness, which can influence local wind patterns and weather systems.
These non-radiative processes may counterbalance or amplify the radiative cooling effect from increased albedo, and the net climate effect varies depending on location and type of land cover change.

3. Albedo Changes Depend on Geographic Location:

The cooling effect from albedo change is strongest in high latitudes, where forests are replaced with snow-covered land. In tropical or mid-latitude regions, where snow is not a factor, the albedo effect may be smaller, and deforestation may contribute more to warming.
Recent studies have shown that in the tropics, deforestation leads to net warming, as the increase in albedo is relatively small compared to the warming effects of reduced carbon sequestration and changes in moisture dynamics.

4. Revised Estimates in AR6 (2021):

In the 6th Assessment Report (AR6, 2021), the IPCC revisited and refined their estimates of the radiative forcing from land use change. The estimate for net radiative forcing from land use change is slightly different from AR5, reflecting a deeper understanding of both the albedo changes and other effects.
The total radiative forcing from land use changes in AR6 is assessed at –0.05 W m–2 (with uncertainty ranging from –0.25 to +0.10 W m–2). This represents a reduced cooling effect compared to the AR5 estimate, suggesting that earlier assessments might have overestimated the cooling from albedo changes.

5. Emerging Evidence on Land Use and Biophysical Feedbacks:

A growing body of research emphasizes that biophysical feedbacks, such as changes in cloud formation, regional precipitation patterns, and atmospheric circulation, must be considered when evaluating the impact of land use changes on the climate.
These biophysical effects may interact with albedo changes in ways that make the overall impact less straightforward.

6. Climate Models and Uncertainty:

Climate models have become more sophisticated since AR5, incorporating more complex interactions between land, atmosphere, and vegetation. These models sometimes show different results depending on the region, season, and type of land cover change.
The uncertainties highlighted in AR5 (± 0.10 W m–2) reflected the challenge of accurately modeling all these interactions. The refinement in AR6’s estimate suggests that scientists are more cautious about attributing a simple cooling effect to land use changes.

Summary of Challenges:

The radiative cooling effect due to increased albedo from land use changes has been refined, with some challenges highlighting the complexity of the carbon cycle, non-radiative effects (like evapotranspiration), and biophysical feedbacks.
Newer research and the IPCC’s AR6 suggest that the overall impact of land use changes is less cooling than previously estimated, and in some cases, might even lead to net warming when taking into account other factors like CO₂ emissions and reduced moisture cycling.

Wet & Warmth

Wet soils will take more energy to warm in the spring and stay warmer in the winter than dry soils.

The climate in the hot desert is characterised by high temperatures, up over 40C in the daytime but very cold at night often freezing. In tropical rainforests, the daily temperature is about 28C and never goes below 20C and rarely above 35C. In temperate forests The average daily temperatures range between -30°C and 30°C. In grasslands, temperatures peak at around 35c just before the wet season and lowest (around 15c) just after it. It would be interesting to find many more studies measuring the difference between grasslands and arable land in tropical an temperate areas.

One of the last questions I asked Prof Tony Allen who helped me write a Masters degree in Regenerative farming, was whether we could calculate the following: “What is the impact of regenerating a desert like the Sahara back to Savanah in terms of global cooling?” We’ve seen what trees can do to the water cycle and keeping soil cool, but can we calculate what difference grass and shrub could make? See desertification. It may not only help the people trying to live there, but also the planet. This Emeritus professor of geography, famous for coining the phrase 'virtual water' at Kings College London put me on to Walter Jehne 'Cooling the Climate'.

With global warming almost by definition, the soil is bound to warm up. But where will that happen most? Can we put a lid on it - with better animal and plant management? The main impacts of increased soil temperature, will affect all soil processes like microbial communities, nutrient cycling processes, and plant growth. There will be changes in soil moisture, due to changes precipitation patterns. Drier conditions will lead to soil degradation, increased erosion, and reduced water availability for plants. Conversely, in some regions, increased rainfall intensity can lead to soil erosion and the loss of topsoil. So shouldn’t we be studying the soil much more closely?

Bare Soil

Roger Savory asks on FB in 2023 “Why are there 10s of thousands of acres across America looking like this with ploughed up bare soil in the middle of summer. ..there is 84% evaporation from bare soil compared with 10% when covered. “ “One study in Northern Carolina found “Bare soil had a 0.43°C, or 1.9%, greater mean daily temperature and a 5.23°C greater overall range than the covered soil."

This is just one study, where the bare soil was in the middle of grass. Imagine if it were in the middle of an acre of bare soil, what would be the temperature difference? And what about different soils like sand, loam and clay, what are their differences? What about different seasons and different continents? We need 10’s of thousands of these soil temperature measurements across the world. We could do it now with the advent of AI and data collection. By doing this we would have a direct measurement of where global warming is happening most.

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