Substantial decrease in CO2 emissions from Chinese inland waters due to global change

Carbon dioxide (CO2) evasion from inland waters is an important component of the global carbon cycle. However, it remains unknown how global change affects CO2 emissions over longer time scales. Here, we present seasonal and annual fluxes of CO2 emissions from streams, rivers, lakes, and reservoirs throughout China and quantify their changes over the past three decades. We found that the CO2 emissions declined from 13831 Tg C yr(-1) in the 1980s to 98 +/- 19 Tg C yr(-1) in the 2010s. Our results suggest that this unexpected decrease was driven by a combination of environmental alterations, including massive conversion of free-flowing rivers to reservoirs and widespread implementation of reforestation programs. Meanwhile, we found increasing CO2 emissions from the Tibetan Plateau inland waters, likely attributable to increased terrestrial deliveries of organic carbon and expanded surface area due to climate change. We suggest that the CO2 emissions from Chinese inland waters have greatly offset the terrestrial carbon sink and are therefore a key component of China's carbon budget. Inland waters emit greenhouse gases, but robust estimations are hampered by a dearth of spatio-temporally resolved measurements. Here the authors present annual fluxes of CO2 from Chinese inland waters over the past several decades, showing that emission fluxes have significantly declined since the 80s.

Spatiotemporal Drivers of CO2 Dynamics and Evasion Fluxes from Mountain Streams

Asa Lovisa Viktoria Horgby

Inland waters are key components of the global carbon cycle, emitting a substantial amount of carbon dioxide (CO2) to the atmosphere. Climate change and global warming affect mountain catchments significantly, leading to glacier retreat and changing precipitation patterns. What this means for the streams in the mountains, and for CO2 sources, dynamics and fluxes from those streams, is poorly understood. Generally, mountain streams have low CO2 concentrations, largely due to low organic carbon content in catchments above the tree line and poorly developed soil horizons. However, it is yet unclear from where the streamwater CO2 is coming, what drives the CO2 dynamics, and the magnitudes of the CO2 evasion fluxes from the stream surfaces to the atmosphere. This PhD thesis aims to fill this knowledge gap. By using a combination of field sampling, sensor monitoring stations, and analyses of stable isotopes, I identify potential drivers of CO2 concentrations and fluxes from mountain streams across different spatial and temporal scales. Moreover, in combination with recent insights of gas exchange from mountain streams, this thesis comprises new quantifications of CO2 evasion fluxes from Swiss mountain streams, as well as mountain streams worldwide. Within the frame of this thesis, two major field studies were carried out. The first field study consisted of repeated seasonal sampling campaigns every 50 meter across an Alpine stream network. We found large seasonal and spatial variations in streamwater CO2 concentrations, and identified soil respiration as the major source of streamwater CO2. With spring snowmelt, and increased catchment connectivity, more respiratory CO2 could be transported to the stream, however due to high gas transfer velocities, the CO2 was rapidly evaded downstream. The second major field study consisted of high-frequency (10 minutes) monitoring of streamwater CO2 in 12 streams located in 4 Alpine catchments. This enabled us to further explore the role of snowmelt for CO2 dynamics in mountain streams. We identified different responses in CO2 concentration and CO2 evasion fluxes to increasing runoff across different temporal scales. On annual time scales, increasing runoff led to lower streamwater CO2 concentration. However, during the onset of snowmelt we found increasing CO2 concentrations with higher runoff, leading to higher CO2 evasion fluxes. To better contextualize our results and estimate CO2 evasion fluxes from mountain streams, we combined insights gained from the two field studies with recent insights of gas exchange in turbulent streams. We developed a simple CO2 prediction model, and modelled the CO2 evasion fluxes from all small mountain streams in Switzerland, as well as all mountain streams worldwide. We estimate that mountain streams worldwide emit 167 ± 1.5 Tg C yr-1. This estimate is unexpectedly high given the small total surface area of small mountain streams. This thesis brings new light on the CO2 dynamics in mountain streams, not only by highlighting the role of spatiotemporal catchment dynamics and in particular the important role of snowmelt for facilitating transport of catchment-derived CO2 to the streams, but also by providing one of the first global estimates of the quantity of CO2 that is evaded from mountain streams every year.

EPFL2019

The effects of land use and climate change on the carbon cycle of Europe over the past 500 years

Jed Oliver Kaplan, Kristen Marie Krumhardt

The long residence time of carbon in forests and soils means that both the current state and future behavior of the terrestrial biosphere are influenced by past variability in climate and anthropogenic land use. Over the last half-millennium, European terrestrial ecosystems were affected by the cool temperatures of the Little Ice Age, rising CO2 concentrations, and human induced deforestation and land abandonment. To quantify the importance of these processes, we performed a series of simulations with the LPJ dynamic vegetation model driven by reconstructed climate, land use, and CO2 concentrations. Although land use change was the major control on the carbon inventory of Europe over the last 500years, the current state of the terrestrial biosphere is largely controlled by land use change during the past century. Between 1500 and 2000, climate variability led to temporary sequestration events of up to 3Pg, whereas increasing atmospheric CO2 concentrations during the 20th century led to an increase in carbon storage of up to 15Pg. Anthropogenic land use caused between 25Pg of carbon emissions and 5Pg of uptake over the same time period, depending on the historical and spatial pattern of past land use and the timing of the reversal from deforestation to afforestation during the last two centuries. None of the currently existing anthropogenic land use change datasets adequately capture the timing of the forest transition in most European countries as recorded in historical observations. Despite considerable uncertainty, our scenarios indicate that with limited management, extant European forests have the potential to absorb between 5 and 12Pg of carbon at the present day.

2012

Spatiotemporal Drivers of CO2 Dynamics and Evasion Fluxes from Mountain Streams

Asa Lovisa Viktoria Horgby

EPFL2019