Abrupt CO2 release to the atmosphere under glacial and early interglacial climate conditions

Shifting Antarctic Circumpolar Current south of Africa over the past 1.9 million years

The Antarctic Circumpolar Current (ACC) dominates the transfer of heat, salt, and tracers around the Southern Ocean (SO), driving the upwelling of carbon-rich deep waters around Antarctica. Paleoclimate reconstructions reveal marked variability in SO circulation; however, few records exist coupling quantitative reconstructions of ACC flow with tracers of SO upwelling spanning multiple Pleistocene glacial cycles. Here, we reconstruct near-bottom flow speed variability in the SO south of Africa, revealing systematic glacial-interglacial variations in the strength and/or proximity of ACC jets. These are superimposed by warmer-than-present "super-interglacials," whereby extreme slowdown in the midlatitude ACC (41°S) is opposed by faster flow at higher latitudes (>54°S), implying poleward strengthening of the ACC. Coupled with reconstructions of the subsurface-deep stable carbon isotope gradient, we show that the reorganization of ACC coincides with the upwelling of isotopically light deep waters around Antarctica, likely contributing to the interglacial rise in atmospheric carbon dioxide (CO2) levels.

Asynchronicity of deglacial permafrost thawing controlled by millennial-scale climate variability

Permafrost is a potentially important source of deglacial carbon release alongside deep-sea carbon outgassing. However, limited proxies have restricted our understanding in circumarctic regions and the last deglaciation. Tibetan Plateau (TP), the Earth's largest low-latitude and alpine permafrost region, remains underexplored. Using speleothem growth phases, we reconstruct TP permafrost thawing history over the last 500,000 years, standardizing chronology to investigate Northern Hemisphere permafrost thawing patterns. We find that, unlike circumarctic permafrost, TP permafrost generally initiates thawing at the onset of deglaciations, coinciding with Weak Monsoon Intervals and sluggish Atlantic Meridional Overturning Circulation (AMOC) during Terminal Stadials. Modeling elaborates that the associated Asian monsoon weakening induces anomalous TP warming through local cloud-precipitation-soil moisture feedback. This, combined with high-latitude cooling, results in asynchronous boreal permafrost thawing. During the last deglaciation, however, anomalous AMOC variability delayed TP and advanced circumarctic permafrost thawing. Our results indicate that permafrost carbon release, influenced by millennial-scale AMOC variability, may have been a non-trivial contributor to deglacial CO2 rise.

Sustained North Atlantic warming drove anomalously intense MIS 11c interglacial

The Marine Isotope Stage (MIS) 11c interglacial and its preceding glacial termination represent an enigmatically intense climate response to relatively weak insolation forcing. So far, a lack of radiometric age control has confounded a detailed assessment of the insolation-climate relationship during this period. Here, we present 230Th-dated speleothem proxy data from northern Italy and compare them with palaeoclimate records from the North Atlantic region. We find that interglacial conditions started in subtropical to middle latitudes at 423.1 ± 1.3 thousand years (kyr) before present, during a first weak insolation maximum, whereas northern high latitudes remained glaciated (sea level ~ 40 m below present). Some 14.5 ± 2.8 kyr after this early subtropical onset, peak interglacial conditions were reached globally, with sea level 6-13 m above present, despite weak insolation forcing. We attribute this remarkably intense climate response to an exceptionally long (~15 kyr) episode of intense poleward heat flux transport prior to the MIS 11c optimum.

Elevated [CO2] and temperature augment gas exchange and shift the fitness landscape in a montane forb

Climate change is simultaneously increasing carbon dioxide concentrations ([CO2]) and temperature. These factors could interact to influence plant physiology and performance. Alternatively, increased [CO2] may offset costs associated with elevated temperatures. Furthermore, the interaction between elevated temperature and [CO2] may differentially affect populations from along an elevational gradient and disrupt local adaptation. We conducted a multifactorial growth chamber experiment to examine the interactive effects of temperature and [CO2] on fitness and ecophysiology of diverse accessions of Boechera stricta (Brassicaceae) sourced from a broad elevational gradient in Colorado. We tested whether increased [CO2] would enhance photosynthesis across accessions, and whether warmer conditions would depress the fitness of high-elevation accessions owing to steep reductions in temperature with increasing elevation in this system. Elevational clines in [CO2] are not as evident, making it challenging to predict how locally adapted ecotypes will respond to elevated [CO2]. This experiment revealed that elevated [CO2] increased photosynthesis and intrinsic water use efficiency across all accessions. However, these instantaneous responses to treatments did not translate to changes in fitness. Instead, increased temperatures reduced the probability of reproduction for all accessions. Elevated [CO2] and increased temperatures interacted to shift the adaptive landscape, favoring lower elevation accessions for the probability of survival and fecundity. Our results suggest that elevated temperatures and [CO2] associated with climate change could have severe negative consequences, especially for high-elevation populations.

Southern Ocean drives multidecadal atmospheric CO2 rise during Heinrich Stadials

The last glacial period was punctuated by cold intervals in the North Atlantic region that culminated in extensive iceberg discharge events. These cold intervals, known as Heinrich Stadials, are associated with abrupt climate shifts worldwide. Here, we present CO2 measurements from the West Antarctic Ice Sheet Divide ice core across Heinrich Stadials 2 to 5 at decadal-scale resolution. Our results reveal multi-decadal-scale jumps in atmospheric CO2 concentrations within each Heinrich Stadial. The largest magnitude of change (14.0 ± 0.8 ppm within 55 ± 10 y) occurred during Heinrich Stadial 4. Abrupt rises in atmospheric CO2 are concurrent with jumps in atmospheric CH4 and abrupt changes in the water isotopologs in multiple Antarctic ice cores, the latter of which suggest rapid warming of both Antarctica and Southern Ocean vapor source regions. The synchroneity of these rapid shifts points to wind-driven upwelling of relatively warm, carbon-rich waters in the Southern Ocean, likely linked to a poleward intensification of the Southern Hemisphere westerly winds. Using an isotope-enabled atmospheric circulation model, we show that observed changes in Antarctic water isotopologs can be explained by abrupt and widespread Southern Ocean warming. Our work presents evidence for a multi-decadal- to century-scale response of the Southern Ocean to changes in atmospheric circulation, demonstrating the potential for dynamic changes in Southern Ocean biogeochemistry and circulation on human timescales. Furthermore, it suggests that anthropogenic CO2 uptake in the Southern Ocean may weaken with poleward strengthening westerlies today and into the future.

Reconciling ice core CO2 and land-use change following New World-Old World contact

Ice core records of carbon dioxide (CO2) throughout the last 2000 years provide context for the unprecedented anthropogenic rise in atmospheric CO2 and insights into global carbon cycle dynamics. Yet the atmospheric history of CO2 remains uncertain in some time intervals. Here we present measurements of CO2 and methane (CH4) in the Skytrain ice core from 1450 to 1700 CE. Results suggest a sudden decrease in CO2 around 1610 CE in one widely used record may be an artefact of a small number of anomalously low values. Our analysis supports a more gradual decrease in CO2 of 0.5 ppm per decade from 1516 to 1670 CE, with an inferred land carbon sink of 2.6 PgC per decade. This corroborates modelled scenarios of large-scale reorganisation of land use in the Americas following New World-Old World contact, whereas a rapid decrease in CO2 at 1610 CE is incompatible with even the most extreme land-use change scenarios.

Community context and pCO2 impact the transcriptome of the "helper" bacterium Alteromonas in co-culture with picocyanobacteria

Many microbial photoautotrophs depend on heterotrophic bacteria for accomplishing essential functions. Environmental changes, however, could alter or eliminate such interactions. We investigated the effects of changing pCO2 on gene transcription in co-cultures of 3 strains of picocyanobacteria (Synechococcus strains CC9311 and WH8102 and Prochlorococcus strain MIT9312) paired with the 'helper' bacterium Alteromonas macleodii EZ55. Co-culture with cyanobacteria resulted in a much higher number of up- and down-regulated genes in EZ55 than pCO2 by itself. Pathway analysis revealed significantly different transcription of genes involved in carbohydrate metabolism, stress response, and chemotaxis, with different patterns of up- or down-regulation in co-culture with different cyanobacterial strains. Gene transcription patterns of organic and inorganic nutrient transporter and catabolism genes in EZ55 suggested resources available in the culture media were altered under elevated (800 ppm) pCO2 conditions. Altogether, changing transcription patterns were consistent with the possibility that the composition of cyanobacterial excretions changed under the two pCO2 regimes, causing extensive ecophysiological changes in both members of the co-cultures. Additionally, significant downregulation of oxidative stress genes in MIT9312/EZ55 cocultures at 800 ppm pCO2 were consistent with a link between the predicted reduced availability of photorespiratory byproducts (i.e., glycolate/2PG) under this condition and observed reductions in internal oxidative stress loads for EZ55, providing a possible explanation for the previously observed lack of "help" provided by EZ55 to MIT9312 under elevated pCO2. If similar broad alterations in microbial ecophysiology occur in the ocean as atmospheric pCO2 increases, they could lead to substantially altered ecosystem functioning and community composition.

Insolation triggered abrupt weakening of Atlantic circulation at the end of interglacials

Abrupt cooling is observed at the end of interglacials in many paleoclimate records, but the mechanism responsible remains unclear. Using model simulations, we demonstrate that there exists a threshold in the level of astronomically induced insolation below which abrupt changes at the end of interglacials of the past 800,000 years occur. When decreasing insolation reaches the critical value, it triggers a strong, abrupt weakening of the Atlantic meridional overturning circulation and a cooler mean climate state accompanied by high-amplitude variations lasting for several thousand years. The mechanism involves sea ice feedbacks in the Nordic and Labrador Seas. The ubiquity of this threshold suggests its fundamental role in terminating the warm climate conditions at the end of interglacials.

[A review of the state of the climate crisis in the midst of the COVID-19 pandemic]

The atmospheric concentration of well-mixed greenhouse gases has drastically increased since 1850. The prime cause for this increase is anthropogenic activity, particularly the burning of fossil fuels. As a consequence of the changing atmospheric composition, we observe a net positive radiative forcing, which manifests in global warming. The global mean surface temperature has increased since the preindustrial by about 1.0 °C. Under the assumption of continued greenhouse gas emissions, climate models project a temperature increase between 3.7 °C and 4.8 °C until 2100 (compared to the 1850-1900 mean). The assessment reports of the Intergovernmental Panel on Climate Change detail the catastrophic consequences of global warming of such extent for both ecosystems and mankind. As a consequence, the Paris Agreement aims to limit global warming to below 2 °C, ideally 1.5 °C, when compared to the preindustrial. To achieve this goal, fast and ambitious emission controls are required, reaching net zero emission by 2050 at the latest. Examining the global greenhouse gas emissions of recent decades, it becomes obvious how far away we are at present from reaching this goal. Also, the currently determined national contributions for emission reduction do not suffice to meet the 1.5 °C target. Thus, it is of uttermost importance to raise the global ambition in climate protection. The 1.5 °C target can still be reached, however, the time to set the required measures will expire within this decade.