Seasonal snow and wintertime carbon emissions in Arctic shrub tundra

Dutch, Victoria R. (2023) Seasonal snow and wintertime carbon emissions in Arctic shrub tundra. Doctoral thesis, Northumbria University.

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Abstract

The Arctic is at the forefront of global climate change, warming at a rate 4 times faster than the global average. Changes are particularly apparent during winter, which lasts for around 8 months of year. CO2 emissions at this time of year make a considerable contribution to the annual Arctic carbon budget, with enhanced soil CO2 losses due to winter warming exceeding growing season carbon uptake under future climatic conditions. However, high uncertainty surrounds estimates of winter CO2 fluxes across the Arctic region, which vary by a factor of three and a half, with considerable variation between measured and simulated fluxes.

Simulations of the Community Land Model (CLM5.0) were examined to address uncertainties impacting simulations of Arctic carbon fluxes; firstly of snow insulation and soil temperatures, and secondly rates of net ecosystem exchange of CO2. Improving simulated snow properties and the corresponding heat flux is important, as wintertime soil temperatures are an important control on subnivean soil respiration and hence impact Arctic winter carbon fluxes. The default model configuration was found to be inappropriate, with a change to the parameterisation of snow thermal conductivity required in order to correct for a cold soil temperature bias and better represent the insulation provided to the soil by the basal depth hoar not physically represented in CLM5.0.

Simulated CO2 emissions were then examined through sensitivity testing of the parameterisation of relationships between soil temperature, moisture and respiration as well as snow thermal conductivity. The default value of the minimum soil moisture threshold for decomposition prevented soil respiration for the majority of the winter, with no CO2 emission simulated between November and mid-May, in contrast to observations showing steady CO2 emission throughout the winter. Failure to simulate CO2 emissions for over half of the year has a considerable impact on the simulation of the annual Arctic carbon budget. Changes to the model parameterisation, most crucially a decrease in the minimum moisture required for soil decomposition, allowed emissions to occur throughout the winter.

Maintaining continuous wintertime eddy covariance measurements, typically used for model evaluation, in remote Arctic locations is very challenging. Consequently, as other measurement techniques would be advantageous, preliminary investigations with new low-cost CO2 sensors are presented and evaluated. Low-cost sensor measurements are used to examine spatial and temporal patterns in CO2 flux, and how these vary with changes in snow properties across the footprint of the eddy covariance tower. Measurements of CO2 flux using both new and established methods show similar magnitudes of CO2 release during March and April 2022, and asimilar relationship between CO2 and subnivean temperatures as previously shown across the Pan-Arctic. Future development of low-cost sensors should allow uncertainties to be reduced and an improvement in our understanding of the processes governing carbon fluxes from Arctic environments throughout the snow-covered season.

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