Polar chimneys

The potential of carbon sequestration by marine based plants such as mangroves, seagrass and algae, often referred to as blue carbon, and the importance of better understanding it, has clearly been recognised (Mcleod et al. 2011). The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (2019) concluded blue carbon can play an important role in both climate regulation and adaptation. The term algae groups together several kinds of marine photosynthetic organisms. These are often subdivided into very small microalgae like phytoplankton, and larger macroalgae like kelp and seaweed. Although there is still large uncertainty about the total amount of carbon sequestered by these marine organisms, a recent estimate by Duarte et al. (2022) indicated that all macroalgae took in as much CO2 as the Amazon rainforest.

Methane is a highly potent greenhouse gas and its reduction is given ever greater priority in international emission reduction policies (see, for example, https://www.globalmethanepledge.org/). There are several suggested ways to remove atmospheric methane (see Nisbet Jones et al. 2021; and Ming et al. 2022; see in this report also iron salt aerosols and zeolites). One of the main issues with methane removal is that atmospheric methane concentrations are very low. This means that very large volumes of air, and related energy demands, are required, making the use of ventilators like those used for DACCS (see direct air capture) more complicated (Nisbet-Jones et al. 2021).

Black Carbon (BC), also known as soot, is produced through the incomplete combustion of fossil fuels and biomaterials. Apart from its negative health impacts, BC also has significant climate effects because it generally has a lower albedo than its surroundings, which increases the amount of radiation absorbed both when BC is present in the atmosphere and when it is deposited on land (Stjern et al. 2017). Due to the large albedo differences, the effects of BC are especially significant in areas that are normally covered in snow or ice (Hadley and Kirchstetter 2012; Sand et al. 2016; Kang et al. 2020).

Clouds play an important role in the Earth’s energy system. The effects of clouds are complex and diverse, often having simultaneous cooling and warming effects. Mixed-phase clouds (MPC) are clouds that contain water vapor, ice particles, and supercooled water droplets. MPCs are still poorly understood and 'notoriously difficult to represent in numerical weather prediction and climate models' (Korolev et al. 2017).

Up to 50% of the world’s glaciers are predicted to disappear this century, with many more at risk if emission reduction targets are not met (Rounce et al. 2023). Furthermore, the melting of mountain glaciers impacts the rates and seasonality of meltwater abundance and scarcity.

Large areas of the Northern and Arctic regions consist of permafrost, almost permanently frozen soil. As global temperatures rise, these permafrost areas are thawing at an ever faster rate. This thawing leads to massive amounts of GHGs being released into the atmosphere, either as CO2 or CH4 in generally dry or wet areas. Because methane is a very potent GHG, the thawing of the permafrost is considered a major tipping point in the climatic system. Permafrost preservation is of the utmost importance because Arctic terrestrial regions alone hold up to 1500 Pg C (Schuur et al. 2015), and although there is large uncertainty about the total amount of emissions from permafrost (Miner et al. 2022), especially when it comes to nonlinear abrupt thawing (Turetsky et al. 2020), significant amounts of carbon release is to be expected as the Northern regions warm.

The deep waters in the Baltic are severely deoxygenated. Although the causes of the current state are complex, this is mainly a result of increased eutrophication from sewage and agricultural runoff from surrounding lands, which leads to extreme bioproductivity (Rolff et al. 2022). Some species manage to survive in the upper water layers, but many organisms living on the seafloor are severely impacted by the hypoxia, thereby influencing the health of a wide network of ecosystems and biochemical processes. There are attempts to reduce nutrient runoff into the Baltic (see for example: https://helcom.fi/baltic-sea-action-plan/). However, some argue these will be insufficient and argue for engineering solutions to the issue.

The pH of water is lowered when it takes up atmospheric carbon. Given that the Earth’s oceans serve as a major carbon sink, there is increasing interest in the possibility to artificially increase the alkalinity of water to restore pH to previous levels, and/or increase carbon uptake potential.