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Our decadal climate prediction system, which is based on the Max-Planck-Institute Earth System Model, is initialized from a coupled assimilation run that utilizes nudging to selected state parameters from reanalyses. We apply full-field nudging in the atmosphere and either full-field or anomaly nudging in the ocean. Full fields from two different ocean reanalyses are considered. This comparison of initialization strategies focuses on the North Atlantic Subpolar Gyre (SPG) region, where the transition from anomaly to full-field nudging reveals large differences in prediction skill for sea surface temperature and ocean heat content (OHC). We show that nudging of temperature and salinity in the ocean modifies OHC and also induces changes in mass and heat transports associated with the ocean flow. In the SPG region, the assimilated OHC signal resembles well OHC from observations, regardless of using full fields or anomalies. The resulting ocean transport, on the other hand, reveals considerable differences between full-field and anomaly nudging. In all assimilation runs, ocean heat transport together with net heat exchange at the surface does not correspond to OHC tendencies, the SPG heat budget is not closed. Discrepancies in the budget in the cases of full-field nudging exceed those in the case of anomaly nudging by a factor of 2-3. The nudging-induced changes in ocean transport continue to be present in the free running hindcasts for up to 5 years, a clear expression of memory in our coupled system. In hindcast mode, on annual to inter-annual scales, ocean heat transport is the dominant driver of SPG OHC. Thus, we ascribe a significant reduction in OHC prediction skill when using full-field instead of anomaly initialization to an initialization shock resulting from the poor initialization of the ocean flow. [ABSTRACT FROM AUTHOR]

Attempts to predict changes in Atlantic Meridional Overturning Circulation (AMOC) have yielded little success to date. Here, we demonstrate predictability for monthly mean AMOC strength at 26.5°N for up to 4 years in advance. This AMOC predictive skill arises predominantly from the basin-wide upper-mid-ocean geostrophic transport, which in turn can be predicted because we have skill in predicting the upper-ocean zonal density difference. Ensemble forecasts initialized between January 2008 and January 2011 indicate a stable AMOC at 26.5°N unfit at least 2014, despite a brief wind-induced weakening in 2010. Because AMOC influences many aspects of climate, our results establish AMOC as an important potential carrier of climate predictability. [ABSTRACT FROM AUTHOR]

The vigor of Atlantic meridional overturning circulation (Moo is thought to be vulnerable to global warming, but its short-term temporal variability is unknown so changes inferred from sparse observations on the decadal time scale of recent climate change are uncertain. We combine continuous measurements of the MOC (beginning in 2004) using the purposefully designed transatlantic Rapid Climate Change array of moored instruments deployed along 26.5°N, with time series of Gulf Stream transport and surface-layer Ekman transport to quantify its intra-annual variability. The year-long average overturning is 18.7 ± 5.6 sverdrups (Sv) (range: 4.0 to 34.9 Sv, where 1 Sv = a flow of ocean water of 106 cubic meters per second). Interannual changes in the overturning can be monitored with a resolution of 1.5 Sv. [ABSTRACT FROM AUTHOR]

The Atlantic meridional overturning circulation (MOC), which provides one-quarter of the global meridional heat transport, is composed of a number of separate flow components. How changes in the strength of each of those components may affect that of the others has been unclear because of a lack of adequate data. We continuously observed the MOC at 26.5°N for 1 year using end-point measurements of density, bottom pressure, and ocean currents; cable measurements across the Straits of Florida; and wind stress. The different transport components largely compensate for each other, thus confirming the validity of our monitoring approach. The MOC varied over the period of observation by ±5.7 × 106 cubic meters per second, with density-inferred and wind-driven transports contributing equally to it. We find evidence for depth-independent compensation for the wind-driven surface flow. [ABSTRACT FROM AUTHOR]

When the Marinoan snowball Earth deglaciated in response to high atmospheric carbon dioxide (CO 2 ) concentrations, the planet warmed rapidly. It is commonly hypothesized that the ensuing supergreenhouse climate then declined slowly over hundreds of thousands of years through continental weathering. However, how the ocean affected atmospheric CO 2 in the snowball Earth aftermath has never been quantified. Here we show that the ocean's carbon cycle drives the supergreenhouse climate evolution via a set of different mechanisms, triggering scenarios ranging from a rapid decline to an intensification of the supergreenhouse climate. We further identify the rapid formation of carbonate sediments from pre-existing ocean alkalinity as a possible explanation for the enigmatic origin of Marinoan cap dolostones. This work demonstrates that a moderate and relatively short-lived supergreenhouse climate following the Marinoan snowball Earth is a plausible scenario that is in accordance with geological data, challenging the previous hypothesis., (© 2024. The Author(s).)

I review the history of ideas that have led to the establishment of the RAPID monitoring system for the Atlantic Meridional Overturning Circulation (AMOC) at 26.5° N. This history is closely connected to important events in my personal career. Starting from early and largely unsuccessful attempts at formulating a dynamically consistent force balance for the AMOC, I made theoretical progress by separately predicting the density at the eastern and western boundaries and by invoking exact geostrophic balance throughout, including the western boundary current. A remarkable confluence of individuals and ideas then enabled the establishment of the RAPID array, at its core based on monitoring boundary densities and on geostrophy. The RAPID results, such as the surprisingly large sub-seasonal variability, have encouraged AMOC monitoring approaches at other latitudes. I finish by pointing at two theoretical concepts-first, acknowledging the difference between convective mixing and sinking and, second, considering the advective rather than wave propagation of density perturbations in the deep western boundary current-that, together with continued observations and newly available global coupled simulations at very high resolution, should substantially improve our understanding of the causes of AMOC variability. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.

The late Nobel laureate Elinor Ostrom envisioned a polycentric approach to mitigating climate change rather than a centralised solution. Debating about global efforts to solve climate-change problems has yet not led to an effective global treaty. Ostrom argued that instead of focusing only on global efforts, it is better to encourage polycentric efforts to reduce the risks associated with the emission of greenhouse gases. Many problems conceptualised as 'global problems' are the cumulative results of actions taken by individuals, families, small groups, private firms, and local, regional, and national governments. Ostrom and colleagues pointed to many examples of successfully managing a common good through interaction within a community. Energy-saving actions undertaken by individuals, families and actors at a small-scale pay off and, when multiplied, may reduce emissions globally. The incentive to achieve an individual net gain may trigger human investment decisions. Here we provide experimental support for Ostrom's basic ideas using methods of experimental economics. By subdividing experimental populations in subgroups that approach sub-goals of mitigating simulated dangerous climate change combined with incentives, the 'global' solution is achieved by combined subgroup contributions exceeding the 'global' threshold for averting simulated dangerous climate change. Incentives from refunded saved energy motivate reaching sub-goals, as Ostrom suggested. By contrast, coercing free-riding subgroups through sanctioning at a cost fails, because sanctioning also hits fair individuals who then reduce their contributions. However, the power of polycentricity with numerous successful units can help mitigate climate change., Competing Interests: Competing interestsThe authors declare no competing interests., (© The Author(s) 2022.)

Separating how model-to-model differences in the forced response (U MD ) and internal variability (U IV ) contribute to the uncertainty in climate projections is important, but challenging. Reducing U MD increases confidence in projections, while U IV characterises the range of possible futures that might occur purely by chance. Separating these uncertainties is limited in traditional multi-model ensembles because most models have only a small number of realisations; furthermore, some models are not independent. Here, we use six largely independent single model initial-condition large ensembles to separate the contributions of U MD and U IV in projecting 21st-century changes of temperature, precipitation, and their temporal variability under strong forcing (RCP8.5). We provide a method that produces similar results using traditional multi-model archives. While U MD is larger than U IV for both temperature and precipitation changes, U IV is larger than U MD for the changes in temporal variability of both temperature and precipitation, between 20° and 80° latitude in both hemispheres. Over large regions and for all variables considered here except temporal temperature variability, models agree on the sign of the forced response whereas they disagree widely on the magnitude. Our separation method can readily be extended to other climate variables.

A new release of the Max Planck Institute for Meteorology Earth System Model version 1.2 (MPI-ESM1.2) is presented. The development focused on correcting errors in and improving the physical processes representation, as well as improving the computational performance, versatility, and overall user friendliness. In addition to new radiation and aerosol parameterizations of the atmosphere, several relatively large, but partly compensating, coding errors in the model's cloud, convection, and turbulence parameterizations were corrected. The representation of land processes was refined by introducing a multilayer soil hydrology scheme, extending the land biogeochemistry to include the nitrogen cycle, replacing the soil and litter decomposition model and improving the representation of wildfires. The ocean biogeochemistry now represents cyanobacteria prognostically in order to capture the response of nitrogen fixation to changing climate conditions and further includes improved detritus settling and numerous other refinements. As something new, in addition to limiting drift and minimizing certain biases, the instrumental record warming was explicitly taken into account during the tuning process. To this end, a very high climate sensitivity of around 7 K caused by low-level clouds in the tropics as found in an intermediate model version was addressed, as it was not deemed possible to match observed warming otherwise. As a result, the model has a climate sensitivity to a doubling of CO 2 over preindustrial conditions of 2.77 K, maintaining the previously identified highly nonlinear global mean response to increasing CO 2 forcing, which nonetheless can be represented by a simple two-layer model., (©2019. The Authors.)

Most present-generation climate models simulate an increase in global-mean surface temperature (GMST) since 1998, whereas observations suggest a warming hiatus. It is unclear to what extent this mismatch is caused by incorrect model forcing, by incorrect model response to forcing or by random factors. Here we analyse simulations and observations of GMST from 1900 to 2012, and show that the distribution of simulated 15-year trends shows no systematic bias against the observations. Using a multiple regression approach that is physically motivated by surface energy balance, we isolate the impact of radiative forcing, climate feedback and ocean heat uptake on GMST--with the regression residual interpreted as internal variability--and assess all possible 15- and 62-year trends. The differences between simulated and observed trends are dominated by random internal variability over the shorter timescale and by variations in the radiative forcings used to drive models over the longer timescale. For either trend length, spread in simulated climate feedback leaves no traceable imprint on GMST trends or, consequently, on the difference between simulations and observations. The claim that climate models systematically overestimate the response to radiative forcing from increasing greenhouse gas concentrations therefore seems to be unfounded.