Description: Our planet is in crisis! The latest report of the United Nations Intergovernmental Panel on Climate Change (IPCC AR6) confirms that human influence is causing widespread, rapid, and intensifying changes in our weather and climate that are affecting every region on Earth in multiple ways. With every additional ton of carbon we emit, the frequency and intensity of storms, floods, droughts, and fires become greater and the effects on the environment and on human health and civilization become more severe. As geoscientists and journal editors, most of us have been accustomed to being on the leading edge of human knowledge and understanding of climate change, where we deal in objectivity, uncertainty, and debate, but now we find ourselves at the core of this climate crisis. It is no longer scientific discoveries at stake, but also humanity itself. This is an uncomfortable place for many of us. We are trained to be dispassionate observers and cautious thinkers, yet the alarming rate of recent climate change impels us to turn our attention directly toward mitigating this impending crisis. We are making a plea for collective action: we must make the switch to a green economy, put a just and effective price on carbon now, and consider a portfolio of other equitable public investments in climate solutions. These actions will ensure that the true costs and risks of burning fossil fuels are accounted for and global carbon emissions are rapidly reduced. Rich countries must lead the way in making drastic cuts to carbon emissions and in helping low- and middle-income countries to develop sustainably. We are running out of time. For decades, American Geophysical Union (AGU) journals have been at the forefront of documenting human-caused climate change and warning of a worsening climate crisis. Over 2,000 publications from AGU journals are cited in the new IPCC AR6 report. But we too can do more than just document and scientifically explain the ongoing crisis—our profession must help lead the way to solutions. Finding solutions and adapting to change have become not only necessary, but essential in ensuring safe, sustainable, and healthy human communities in the future. The geosciences have an essential role to play in these efforts by pivoting toward more cross-sector, solution-based science. To help lead this vision, the AGU is adding a new publication forum for community science in partnership with associations outside the geosciences. This forum will enhance interactions among AGU's existing, more disciplinary journals and give local communities a voice in leading solutions to global challenges. We are scientists, but we also have families and loved ones alongside our fellow citizens on this planet. The time to bridge the divide between scientist and citizen, head and heart, is now. The lead-up to the 2021 UN Climate Change Conference, COP26, being held in Glasgow in November, is our “last best chance” to urge world leaders to come together and commit to keeping climate change and its devastating impacts in check.

Description: The Atlantic meridional overturning circulation (AMOC) makes the strongest oceanic contribution to the meridional redistribution of heat. Here, an observation-based, forty-eight-month-long time series of the vertical structure and strength of the AMOC at 26.5°N is presented. From April 2004 to April 2008 the AMOC had a mean strength of 18.7 ±2.1 Sv with fluctuations of 4.8 Sv rms. The best guess of the peak-to-peak amplitude of the AMOC seasonal cycle is 6.7 Sv, with a maximum strength in autumn and a minimum in spring. While seasonality in the AMOC was commonly thought to be dominated by the northward Ekman transport, this study reveals that fluctuations of the geostrophic mid-ocean and Gulf Stream transports of 2.2 Sv and 1.7 Sv rms, respectively, are substantially larger than those of the Ekman component (1.2 Sv rms). A simple model based on linear dynamics suggests that the seasonal cycle is dominated by wind stress curl forcing at the eastern boundary of the Atlantic. Seasonal geostrophic AMOC anomalies might represent an important and previously underestimated component of meridional transport and storage of heat in the subtropical North Atlantic. There is evidence that the seasonal cycle observed here is representative of much longer intervals. Previously, hydrographic snapshot estimates between 1957 and 2004 had suggested a long-term decline of the AMOC by 8 Sv. This study suggests that aliasing of seasonal AMOC anomalies might have accounted for a large part of the inferred slowdown.

Description: The Agulhas Current system has been analyzed in a nested high-resolution ocean model and compared to observations. The model shows good performance in the western boundary current structure and the transports off the South African coast. This includes the simulation of the northward-flowing Agulhas Undercurrent. It is demonstrated that fluctuations of the Agulhas Current and Undercurrent around 50–70 days are due to Natal pulses and Mozambique eddies propagating downstream. A sensitivity experiment that excludes those upstream perturbations significantly reduces the variability as well as the mean transport of the undercurrent. Although the model simulates undercurrents in the Mozambique Channel and east of Madagascar, there is no direct connection between those and the Agulhas Undercurrent. Virtual float releases demonstrate that topography is effectively blocking the flow toward the north.

Description: Continuous estimates of the oceanic meridional heat transport in the Atlantic are derived from the Rapid Climate Change–Meridional Overturning Circulation (MOC) and Heatflux Array (RAPID–MOCHA) observing system deployed along 26.5°N, for the period from April 2004 to October 2007. The basinwide meridional heat transport (MHT) is derived by combining temperature transports (relative to a common reference) from 1) the Gulf Stream in the Straits of Florida; 2) the western boundary region offshore of Abaco, Bahamas; 3) the Ekman layer [derived from Quick Scatterometer (QuikSCAT) wind stresses]; and 4) the interior ocean monitored by “endpoint” dynamic height moorings. The interior eddy heat transport arising from spatial covariance of the velocity and temperature fields is estimated independently from repeat hydrographic and expendable bathythermograph (XBT) sections and can also be approximated by the array. The results for the 3.5 yr of data thus far available show a mean MHT of 1.33 ± 0.40 PW for 10-day-averaged estimates, on which time scale a basinwide mass balance can be reasonably assumed. The associated MOC strength and variability is 18.5 ± 4.9 Sv (1 Sv ≡ 106 m3 s−1). The continuous heat transport estimates range from a minimum of 0.2 to a maximum of 2.5 PW, with approximately half of the variance caused by Ekman transport changes and half caused by changes in the geostrophic circulation. The data suggest a seasonal cycle of the MHT with a maximum in summer (July–September) and minimum in late winter (March–April), with an annual range of 0.6 PW. A breakdown of the MHT into “overturning” and “gyre” components shows that the overturning component carries 88% of the total heat transport. The overall uncertainty of the annual mean MHT for the 3.5-yr record is 0.14 PW or about 10% of the mean value.

Description: The Atlantic Meridional Overturning Circulation (MOC) carries up to one quarter of the global northward heat transport in the Subtropical North Atlantic. A system monitoring the strength of the MOC volume transport has been operating since April 2004. The core of this system is an array of moored sensors measuring density, bottom pressure and ocean currents. A strategy to mitigate risks of possible partial failures of the array is presented, relying on backup and complementary measurements. The MOC is decomposed into five components, making use of the continuous moored observations, and of cable measurements across the Straits of Florida, and wind stress data. The components compensate for each other, indicating that the system is working reliably. The year-long average strength of the MOC is 18.7±5.6 Sv, with wind-driven and density-inferred transports contributing equally to the variability. Numerical simulations suggest that the surprisingly fast density changes at the western boundary are partially linked to westward propagating planetary waves