(land:methane:sources)=
# Methane Sources
One of the first things we can ask ourselves is why are the bottom-up
emissions suggesting such a large source of methane from 'other natural'
sources?
Much of this is that we are measuring
directly high fluxes of methane related to hotspots of methane
emissions, and then there may be something wrong with how we are
extrapolating these, or they may be missed by satellites. Every day
another 10-15 studies are published that discuss a methane hotspot or
some environment that is releasing far more methane than we thought.
For example, Gabby Kleber in the department has been measuring methane
coming out from springs formed from retreating glaciers and found that
they are releasing a lot of methane -- much more than that released by
permafrost.
```{figure} figures/07_landcarbon.4.1.png
---
name: fig:land:methane:permafrost
figclass: margin-caption
width: 80%
---
```
The other large issue is that we don't necessarily understand the impact
of our actions on methane production, and sometimes we do something to
mitigate CO2 release but it has the unintended consequence of
releasing more methane. There is currently a large project in the Fens
studying greenhouse gas emissions from peat as a function of changes in
the water table.
The basic premise is that peat, which formed during glacial retreat in
flooded fields and valleys, is being exposed and oxidised to CO2.
If we reflood the fields, or raise the water table then in theory the rate
of oxidation will slow down dramatically. But what will happen to
methane?
```{figure} figures/07_landcarbon.4.2.png
---
name: fig:land:methane:peat
figclass: margin-caption
width: 80%
---
````
To constrain the depth dependent rate of methane production we need to
use Fick's second law. Fick's first law is useful when concentrations
are in steady state, and is useful to understand static flux
measurements. Fick's second law explores what happens to the
concentration as a function of time and as a function of depth. This
allows us to understand transient diffusion (as opposed to steady state
diffusion) and allows us to constrain the depth-dependent rate of
methane production and consumption.
To derive Fick's second law (which is done in detail in the lecture), we
take the derivative of Fick's first law with respect to depth.
```{math}
\pdv{C}{t} = \pdv{}{x} \left( -D \pdv{C}{x} \right)
```
Here if we assume the diffusion coefficient ($D$) is not a function of
depth, and we define depth in the positive direction going down in the
soil, then this becomes
```{math}
\pdv{C}{t} = D \pdv{^2C}{x^2}
```
To this we can add chemical reactions ($R_i$) that make or destroy methane.
```{math}
\pdv{C}{t} = D \pdv{^2C}{x^2} + \sum^i R_i
```
When we do this we find that in many studies, in soil-saturated
wetlands, the rate of methane flux into a static chamber can be higher
than the rate of methane production in the soil, and in places 10x
higher (Shoemaker and Schrag, 2010).
```{figure} figures/07_landcarbon.4.3.png
---
name: fig:land:methane:production
figclass: margin-caption
width: 80%
---
Column-integrating rates of methane production, oxidation and respiration for each month. Values are monthly integrated and shown in units of μmol cm-2 month-1.
```
This has been suggested to be due to the rapid production and
consumption in water-logged soils at the surface, where aerobic bacteria
can break down organic matter into small compounds that can fuel
methanogenesis.
All together these two stories are part of a wide and growing body of
literature that suggests that local variability in methane production
and consumption, hot spots associated with methane release due to
climate change, and a lack of understanding of where and how methane is
being oxidised lead to great uncertainties in the overall methane
budget.
Moving away from methane, we can consider other ways that the land
surface impacts climate. Back in Air Pollution we discussed the role of
NOx in generating ozone. Another class of compounds that also generates
ozone are volatile organic compounds (these also make up a lot of the
particulate matter that is so harmful to human health). Because of the
nature of photolysis of these compounds and their ability to make ozone,
it has long been recognised that at low VOC concentrations, ozone
production is strongly dependent on the concentration of VOC, while at
low NOx concentrations, it is strongly dependent on the concentration of
NOx. This is shown in the figure below as the 'VOC' and 'NOx' limited
parts of ozone production, with the lines of equal ozone production
called 'isopleths'.
```{figure} figures/07_landcarbon.4.4.png
---
name: fig:land:methane:isopleths
figclass: margin-caption
width: 80%
---
```
Based on emissions in the USA, it was thought that the country was in
the VOC-limited part of this graph. So there was great effort in the
1990's and 2000's to reduce VOC emissions, with little effort to reduce
NOx as it was thought not to have as much of an impact. But there was no
decrease in ozone.
Plants emit many volatile organic compounds, for example isoprene (\>50%
of biogenic VOC emissions). Which react quickly to form formaldehyde.
Satellite maps of formaldehyde concentrations over the US show that the
US has massive amounts of biogenic VOC emissions. When this is taken
into account, the country is in the NOx limited regime and to reduce
ozone should focus on NOx emission reductions. Indeed, the background
biogenic VOC concentration is so high that focusing on NOx reductions
makes sense anyway.
```{figure} figures/07_landcarbon.4.5.png
---
name: fig:land:methane:VOCs
figclass: margin-caption
width: 80%
---
```
```{figure} figures/07_landcarbon.4.6.png
---
name: fig:land:methane:isoprene
figclass: margin-caption
width: 80%
---
```
The final thing to note is that the land surface plays a large role in
amplifying heat waves and climate change. This is shown in the schematic
below. When soil moisture is high, there is a large latent heat flux
from the land to the atmosphere, and this causes clouds, which lead to
precipitation, which keep the soil moisture high. When soil moisture
declines, then the land emits more of the sensible heat flux, which
produces a deeper, drier atmosphere, which enhances the overall dryness
and exacerbates the heat wave. These dry periods also greatly impact the
storage of carbon in the soil, as oxygen gets into the soil and produces
CO2.
```{figure} figures/07_landcarbon.4.7.png
---
name: fig:land:methane:wet-dry
figclass: margin-caption
width: 80%
---
schematic of the net raditation budget at the land surface. **a)** In areas with high soil moisture, the latent heat flux by evapotranspirtation and transpiration dominates, enhancing cloud formation and a tendency for cooling. **b)** By contrast, if the soil moisture deficit is high, the dry soils raise the sensible heat flux, producing deeper, warmer, drier, low-level atmosphere.
This process inhibits convection and cloud formation and creates a positive feedback loop.
```
In conclusion, below is the figure from the IPCC which allows you to
consider the role that various types of land surface play in storage of
carbon, production of methane or other trace gases, and release of these to the atmosphere.
```{figure} figures/07_landcarbon.4.8.png
---
name: fig:land:methane:production
figclass: margin-caption
width: 80%
---
The structure and fucntioning of managed and unmanaged excosystems that affect local, regional, and global climate.
Land surface characteristics such as albedo and emissivity determine the amount of solar and long-wave radiation absorbed by land and reflected or emitted to the atmosphere.
Surface roughness influences turbulent exchanges of momentum, energy, water and biogeochemical tracers.
Land ecosystems modulate the atmospheric composition through emissions and removals of many GHGs and precursors of SLCFs, including biogenic volatile organic compounds (BVOCs) and mineral dust.
Atmospheric aerosols formed from these precursors affect regional climate by altering the amounts of precipitation and radiation reaching land surfaces through their role in clouds physics.
```