The response of the terrestrial biosphere to climate change is still largely unknown and represents a key uncertainty in climate change predictions. High latitude regions, including Arctic and boreal ecosystems, constitute a key component of the earth system due to significant soil carbon stocks. High latitude regions are net sources of greenhouse gases, such as methane (CH4) and nitrous oxide (N2O), but there is significant disagreement among flux estimates with further uncertainty due to a rapidly changing environment. Climate change effects are particularly strong during the non-growing season, altering the timing of spring snowmelt, fall freeze-up, and increasing winter temperatures. The changes have significant implications for biogeochemical cycles and ecosystem function across high latitude regions. Despite growing evidence of the importance of non-growing season greenhouse gas emissions, few measurements have been made in pristine Arctic and boreal ecosystems. Non-growing season CH4 emissions can account for 10-100% of annual CH4 flux, while next to nothing is known about emissions of N2O during this period. Process-based models miss non-growing season emissions of CH4, underestimating them by 67% and annual emissions by 25%. I will use complementary observations (WP1), modelling (WP2), and experiments (WP3) to quantify the annual magnitude of CH4 and N2O flux, identify controls on non-growing season flux, and assess why existing models of CH4 flux fail outside of the growing season. Are environmental conditions so different that existing model parameters fail, or is non-growing season biogeochemistry fundamentally different? The overall impact is to shift the paradigm from “nothing happens outside of the growing season” to “capturing non-growing season processes is key to understanding ecosystem dynamics.” Ultimately, results will provide novel insights into greenhouse gas budgets and transform our understanding of fundamental earth system dynamics.

Climate change is primarily driven by atmospheric greenhouse gas levels, making it crucial to identify the Earth system processes that regulate these gases in order to better predict and mitigate climate impacts. While the ocean has long been considered the primary driver of atmospheric CO2 fluctuations, oceanic processes alone cannot fully explain the magnitude of the variations or the observed carbon isotopic signatures. In this context, the vast ancient carbon reservoirs stored beneath Arctic ice sheets, which reconfigured in sync with glacial-interglacial climate cycles, likely represent an additional carbon source released during past ice sheet retreats. The overarching aim of ARCTIVITY is to evaluate the reactivity of organic carbon remobilized by Arctic ice sheet retreat, focusing on its impact on the carbon cycle and paleoclimate change. ARCTIVITY will utilize multiple advanced techniques and methodologies to investigate different aspects of the reactivity of glacially-derived organic carbon, archived in marine sediments. These assessments include: (1) thermal reactivity – assessing carbon reactivity in response to thermal decomposition by applying RPO-14C on Northeast Greenland shelf sediments; (2) bioavailability – determining carbon reactivity through its accrssibility to microbial assimilation by leveraging CSRA of IPLs in Northeast Greenland shelf sediments; (3) weathering intensity – evaluating carbon reactivity in response to environmental exposure using sedimentary δ187Re analysis in Beaufort Sea sediments. These analyses will generate new insights into the linkage between Arctic ice sheet retreat and the global carbon cycle, as well as its potential impact on past climate change.

Photosymbioses between heterotrophic hosts and phototrophic algal endosymbionts have repeatedly and independently evolved across the tree of life. The immense productivity of photosymbioses affects global nutrient cycles, yet global warming is disrupting these nutrient-exchange symbioses, destabilizing the ecosystems they support. Heat stress erodes the metabolic controls underlying the establishment and maintenance of mutualistic carbon recycling in these symbioses. A link between endosymbiont nutrient release and their intracellular maintenance could thus explain the evolutionary establishment as well as the ecological collapse of photosymbioses in the Anthropocene. Harnessing recent advances in cell isolation and culture techniques, PhagoPhoRe seeks to understand the processes underlying the establishment, maintenance, and breakdown of the cnidarian-algal symbiosis. Specifically, I will investigate the idea that the release of photosynthates enables algal endosymbionts to evade phagosomal digestion and autophagic immune responses of the host by mimicking ongoing digestion processes. In three complementary work packages that combine state-of-the-art single-cell transcriptomics, proteomics, and isotopic imaging techniques, I will: 1) Characterize the role of host phagosome maturation processes in the intracellular establishment and maintenance of algal endosymbionts. 2) Investigate the postulated link between algal nutrient release and host phagosome arrest and immune suppression. 3) Examine the drivers of co-evolution towards obligate metabolic interactions in photosymbioses. Deciphering the interplay between symbiotic nutrient cycling and host phagosome maturation will be key to understanding the establishment and subsequent breakdown of photosymbioses. These processes are unlikely restricted to photosymbioses and could reveal fundamental mechanisms enabling the establishment of endosymbioses and, as such, eukaryotic life itself.

The Greenland and Antarctic ice sheets (GrIS and AIS, respectively) and the Atlantic Meridional Overturning Circulation (AMOC) are prominent examples of tipping elements in the Earth system that have the potential to respond nonlinearly to small changes in forcing. Tipping elements can thus give rise to climate surprises, i.e., low-probability, high-impact events that may be triggered earlier than expected. Simulating such climate surprises and their impacts, on the relevant multi-centennial timescales and beyond, is particularly challenging. Today, the right methods are not available, resulting in deep uncertainty in future projections. Here I aim to develop a novel, probabilistic methodology to robustly forecast climate surprises such as ice-sheet and AMOC collapse on long timescales. This requires simultaneous advances beyond the state of the art on two fronts. First, a new generation Fast Earth System Model (FESM) will leverage the latest advances in our understanding of key processes to represent the GrIS, AIS and AMOC realistically, in a coupled framework and on long timescales. Critically, this will be the first comprehensive model fast enough to run the large ensembles of simulations needed to quantify the uncertainty associated with deeply uncertain processes. Second, a highly novel and generalized probabilistic approach will be developed, to constrain the FESM to be consistent with output from the latest generation of Earth System Models. FORCLIMA will generate probabilistic estimates of climate surprises for the medium-term future (centuries to millennia) with much higher confidence than we have today, and inform about interactions between key tipping elements in the climate system. This project will therefore greatly advance the state of the art in coupled climate – ice-sheet modeling, and lead to an unparalleled understanding of the long-term impacts of climate change on the Earth system.