Alkalinity responses to climate warming destabilise the Earth's thermostat

The weathering of carbonate and silicate rocks is a crucial process in Earth's carbon cycle, consuming atmospheric CO2 and increasing ocean alkalinity. While silicate weathering acts as a long-term CO2 sink, carbonate weathering's impact is more nuanced, acting as a CO2 sink only beyond the calcium carbonate compensation time (~10 ka). Carbonate weathering, due to its faster kinetics, is more responsive to short-term environmental changes like acid rain and shifts in soil CO2. Understanding the short- and long-term impacts of anthropogenic, climatic, and geologic factors on global alkalinity fluxes is essential. Previous research has established terrestrial river discharge and watershed lithology as dominant controls, along with acidity influenced by atmospheric and soil CO2. The role of temperature remains debated, with some studies suggesting optimal weathering in temperate climates. However, a precise quantification of these controls at various scales remains challenging due to the scarcity of spatially and temporally comprehensive datasets.

Existing literature highlights the significant roles of river discharge, watershed lithology, and acidity in controlling global alkalinity fluxes. The influence of temperature has been a subject of ongoing debate, with some studies indicating that temperate climates are optimal for weathering. However, quantifying these controls, especially soil CO2 content, across regional and global scales, remains a significant challenge due to limited data availability. Regional studies have demonstrated that physical erosion can enhance weathering, particularly at low-to-moderate erosion rates. However, a global-scale relationship between physical erosion and alkalinity flux has not been established. This study addresses the gap by combining riverine alkalinity measurements with erosion rates from various climate zones to reveal the impact of erosion on global alkalinity generation.

This research compiled data from 233 sampling locations across six continents, incorporating both alkalinity and 10Be-derived erosion rate measurements. To account for variations in runoff and alkalinity concentration, runoff-normalized alkalinity (alkalinity/runoff) was employed. A generalized linear model (GLM) was used to analyze the relationships between runoff-normalized alkalinity and predictor variables including erosion rate, areal carbonate proportion, MAT, catchment area, and soil regolith thickness. The GLM allowed for the establishment of linear relationships, even if the actual relationships were non-linear, facilitating interpretation. The model was then used to assess the impact of future climate scenarios (represented by SSP1-2.6 and SSP5-8.5) on riverine alkalinity flux. Data sources included the OCTOPUS database for 10Be erosion rates, various databases and publications for alkalinity data, and multiple sources for catchment characteristics such as lithology, temperature, precipitation, and soil properties. The global hydrological model HydroPy was utilized to simulate runoff and river discharge data for the climate change impact assessment.

The study's primary finding is that erosion rate is a first-order control on riverine alkalinity at the global level. An 'efficient erosion rate regime' (approximately 10-1000 mm ka-1) was identified, where optimal alkalinity concentrations were observed. Outside this regime, alkalinity was low. Within the efficient regime, areal carbonate proportion and MAT significantly impacted alkalinity. Areal carbonate proportion showed a positive relationship with alkalinity, with the highest concentrations found in carbonate-rich catchments. MAT exhibited a non-linear relationship, with peak alkalinity observed in temperate climates (5-15°C) due to higher soil-rock CO2 content and favorable carbonate solubility. Cold climates (<5°C) limited weathering due to low acid availability and slow reaction rates, while warmer climates (>15°C), particularly in areas with low carbonate proportion, exhibited lower alkalinity. At high erosion rates (>100 mm ka-1), decreased acid availability and turbulent river flow (leading to CO2 degassing) contributed to lower alkalinity. The GLM developed incorporated erosion rate, areal carbonate proportion, MAT, catchment area, and soil regolith thickness as significant predictors of runoff-normalized alkalinity. The model indicated that normalized alkalinity increased with catchment area and soil regolith thickness, although this trend may be nonlinear beyond a certain thickness. A sensitivity analysis was conducted to evaluate the influence of MAT on alkalinity across different climatic zones, showing a strong impact within the 0-20°C range. The model was then used to project the impact of climate change on alkalinity flux under two Shared Socioeconomic Pathways (SSP1-2.6 and SSP5-8.5). Under SSP1-2.6 (low emissions), a decrease in mid-latitude alkalinity flux was projected due to aridification, while SSP5-8.5 (high emissions) suggested an increase in alkalinity flux. This resulted in projections of CO2 sequestration reduction under low emissions and increased CO2 sink under high emissions.

The findings address the research question by demonstrating the importance of erosion rate, along with carbonate proportion and MAT, in controlling global riverine alkalinity. The non-linear response of alkalinity to MAT highlights the complexity of the system, with both increased acid availability at moderate temperatures and decreased carbonate solubility at higher temperatures influencing the outcome. The projections of alkalinity flux changes under different climate scenarios highlight the potential for significant feedback mechanisms between climate and the carbon cycle. The observed decrease in alkalinity at high erosion rates suggests that morphology and associated turbulent flow play a crucial role in CO2 degassing and subsequent alkalinity reduction. The study’s relevance to the field is significant as it provides a more comprehensive understanding of the factors regulating alkalinity generation and the potential impacts of climate change on the Earth’s carbon cycle.

This study presents a comprehensive analysis of the factors influencing global riverine alkalinity and its sensitivity to climate warming. The development of a global model incorporating erosion rate, carbonate proportion, and MAT provides valuable insights into the complex interactions between geology, climate, and the carbon cycle. The projection of altered alkalinity fluxes under various emissions scenarios highlights the potential for substantial feedback effects on atmospheric CO2. Future research should focus on improving data availability for high-latitude regions and refining the model to incorporate more detailed hydrological and biogeochemical processes. Further investigation into the role of non-carbonate weathering reactions would enhance model accuracy.

The study's scope is limited by the availability of consistent erosion rate and alkalinity data globally. While the dataset covers a wide range of conditions, potential biases may exist due to uneven spatial distribution and the exclusion of certain extreme values. The use of long-term averaged 10Be-derived erosion rates might not fully capture short-term variations in erosion. The model assumes a constant discharge for the climate change impact assessment, which might not accurately reflect future hydrological changes. Further, focusing solely on the mid-latitudes for the climate change analysis limits the generalizability of the findings to other regions. Future research should address these limitations by expanding data collection and refining model complexity.