Carbon cycle feedbacks play a key role in past climate variations and may also have a significant impact on future climate. However, it was not until the early 2000s that general circulation models (GCMs) of the climate started to include carbon cycle feedbacks on climate change. The results of the first two climate-carbon GCMs were compared in a paper published in Tellus. Following those first model runs, many other such 'Earth System Models (ESMs)' have been developed and compared as part of the Coupled Climate-Carbon Cycle Model Intercomparison project (C4MIP). Modelled changes in global land and ocean carbon storage have been routinely compared using a simple formalism that assumes a linear dependence of global land and ocean carbon storage on both atmospheric CO2 increase and global warming. This j3-y formulation has been very helpful to identify the zeroth-order reasons for differences between ESM projections, but it is unable to reproduce the saturating time-evolution of global land and ocean carbon storage as simulated by the C4MIP models. In this paper, we summarise work to date on modelling the coupled climate-carbon cycle system, and suggest an update to the j3-y formulation to include a representation of sink saturation as well as the timescale dependencies of land and ocean carbon uptake. Our new approach adds half-saturation constants, x, for quasi-equilibrium land and ocean carbon increases with CO2, and land and ocean timescales, z, that approximate how quickly this quasi-equilibrium is approached. This new 'j3-y-x-z' formulation reproduces the C4MIP results with much greater accuracy than the existing j3-y formulation, without requiring any additional ESM runs to be analysed.
Ocean stratification plays a crucial role in regulating climate and marine ecosystems. This study investigates global ocean stratification changes over a 26-year period (1992-2017) from the ECCOv4r4 ocean state estimate. We quantify stratification trends using the depth anomaly of the ocean's center of mass (COM), defined as the excess depth of the COM relative to a fully mixed reference state. The global depth anomaly has increased at a steady rate of 0.66 cm per decade, corresponding to a significant 1% stratification increase per decade. The global mean squared buoyancy frequency has increased at a similar rate, albeit with larger interannual variability, making it a less robust metric. The primary driver of stratification increase is the upper ocean warming, with a nearly perfect correlation between the global depth anomaly and the ocean heat content. Depth anomalies are mapped spatially for the upper 2000 m water column, showing large spatial heterogeneity. Most of the stratification increase occurs in Indo-Pacific tropical regions and along western boundary currents, with local rates more than five times higher. Salinity changes provide a secondary contribution, dominant only in polar regions. In contrast, stratification has weakened in most of the North Atlantic sector during the same period.
Many important questions in Earth system science require an integrated understanding on the duration of processes such as mass flows, particularly for components of the Earth system that are interconnected and are changing rapidly due to human activity. This type of questions require an integrated and general approach to study timescales of Earth system processes. A number of publications in Tellus have been fundamental to establish the basis for such a general approach. In this contribution, I present a review on how Tellus helped to define the basis to study timescales of Earth system processes, establishing the definitions of timescale metrics such as age, transit-, and turnover time. In addition, I present an overview of an expanded theory based on recent progress in hydrology and carbon cycle science where new methods to study timescales for time-dependent systems out of equilibrium have been recently developed. The new theory facilitates the study of timescale problems in which rates of mass flows are changing fast and interacting nonlinearly with other system components. I discuss the application of the theory to studying the time required to remove anthropogenic carbon dioxide from the interconnected atmosphere-land-ocean system, the time that anthropogenic reactive nitrogen spends in the natural environment, and the time that methane spends in the atmosphere. Overall, the new theory helps to clarify concepts and expands the range of scientific questions to problems on the anthropogenic perturbation of timescales of Earth system processes.
We describe the coordination of ORCESTRA, a field campaign designed to advance understanding of how mesoscale convective processes influence Earth's climate. ORCESTRA harmonized the execution of eight sub-campaigns that spanned the tropical North Atlantic from 10 August to 30 September 2024. The ORCESTRA sub-campaigns performed measurements from three research aircraft, a research vessel, two ground stations, and EarthCARE, a new Earth Explorer Satellite. These platforms performed thousands of atmospheric soundings, supported a rich constellation of active and passive remote sensing, conducted a wide variety of in situ measurements, and deployed autonomous sensors in the air and sea. Together with its measurements, ORCESTRA is being accompanied by extensive numerical experimentation to advance the development of a new generation of more physical climate models.
Despite the urgency to mitigate climate change, global greenhouse gas concentrations are still increasing, and even global emissions are still growing. Furthermore, the global sinks-both natural and technical-are not improving rapidly enough. Therefore, discussion and investigations to utilize solar radiation modification (SRM) have resurfaced. Stratospheric Aerosol Injection (SAI) is one of the most studied methods, particularly using sulfate aerosols. However, sulfate SAI includes several risks, such as deposition of acidic aerosol particles on the Earth's surface, increasing acidification and ozone depletion, and changing weather patterns. Here we propose an alternative approach, namely Organic Stratospheric Aerosol Injection (OSAI). The OSAI is initiated by an injection of isoprene or monoterpenes, which, according to our conceptual investigations and global model simulations, seem to be plausible candidates for this purpose. The organic SAI is likely to avoid some of the problems inherent to sulfate SAI, even though further research is needed in terms of its potential influences on stratospheric heating and the ozone layer. While SRM is still preferably avoided, our results suggest that organic aerosols may be an alternative to sulfate for further consideration if it is deemed a viable and necessary option.
