Project Funding: 2017 - 2020

NRA: 2016 NASA: Carbon Cycle Science   

Funded by NASA

Abstract:
An important question for the climate research community pertains to how the ocean’s solubility and biological pumps will be perturbed by anthropogenic climate change. This will have important consequences not only for the coupled carbon-climate system, but also for ocean acidification and ecosystems. However, natural variability in the climate system will complicate the task of detecting anthropogenic changes in the carbon pumps, as measurements collected through the global carbon observing system represent a mixture of anthropogenic signals and natural background climate variability. One promising approach to unraveling the natural and forced trends is through the use of Earth system models (ESMs). Large (≥30 member) ensemble experiments run with ESMs provide a powerful means to identify both the forced (signal) and natural variability (noise) components of the Earth system evolution. By ensemble here, we refer to a suite of simulations with the same model with identical boundary conditions, but different initial conditions, so that modes of natural variability are randomized amongst the ensemble members for the duration of the simulation. From a modeling perspective, emergence of an anthropogenic signal occurs if/when the magnitude of the anthropogenic trend is sufficiently larger than the natural climate variability. Preliminary results with a large ensemble of simulations with GFDL’s ESM2M indicate that anthropogenic changes in the solubility carbon pump emerge significantly earlier than the biological carbon pump during the course of the 21st century. However, the timing and spatial structures of emergence of anthropogenic signals in the ocean carbon pump, in particular the biological carbon pump, are likely to be highly dependent on the particular ESM evaluated. ESMs diverge widely in their representation of the amplitude of the response of the biological pump. Therefore we propose to consider systematically the emergence characteristics of the solubility and biological carbon pumps for four Earth system models. Each of these ESMs have large ensemble experiments, allowing for the clean separation of anthropogenic and natural forcings, providing a means of evaluating both model uncertainty and random uncertainty in state-of-the-art ESM components of the global carbon cycle. Particular emphasis will be placed on identifying mechanistic controls on the separation between the solubility and carbon pump emergence timescales, and how these vary among models and compare to observations. Additionally, we will use a multi-model suite of ESMs to evaluate climate-carbon feedbacks on the ocean carbon pumps, but consider both seasonal and mechanistic dependence of the climate-carbon feedback parameter. Finally, we will address model biases in the representation of natural variability through a novel suite of data-override experiments with GFDL-ESM2M that utilize remotely sensed products to constrain simulated recent and future decadal variability. The relevance to NASA’s Carbon Cycle Science synthesis goals will be derived from (1) the proposal leading to improved understanding, through model-to-model and model-to-measurement comparisons of the causes and impact of rising CO2 on ocean ecology, namely impacts and feedbacks on the ocean’s biological pump, and (2) the project helping to set priorities for optimization of the remote sensing contributions to optimization of the observing system, so as to better capture both the natural variability of the carbon cycle and its anthropogenic transients. Central to the proposed work will be a framework for quantifying uncertainty and contributing to quantification of errors in trend detection with remote sensing.

Publications:

Schwalm, C. R., Huntzinger, D. N., Michalak, A. M., Schaefer, K., Fisher, J. B., Fang, Y., Wei, Y. 2020. Modeling suggests fossil fuel emissions have been driving increased land carbon uptake since the turn of the 20th Century. Scientific Reports. 10(1). DOI: 10.1038/s41598-020-66103-9

More details may be found in the following project profile(s):

• NASA Ocean Biology & Biogeochemistry (OBB) Project Profile