Large-scale emergence of regional changes in year-to-year temperature variability by the end of the 21st century

Global warming is expected to not only impact mean temperatures but also temperature variability, substantially altering climate extremes. Here we show that human-caused changes in internal year-to-year temperature variability are expected to emerge from the unforced range by the end of the 21st century across climate model initial-condition large ensembles forced with a strong global warming scenario. Different simulated changes in globally averaged regional temperature variability between models can be explained by a trade-off between strong increases in variability on tropical land and substantial decreases in high latitudes, both shown by most models. This latitudinal pattern of temperature variability change is consistent with loss of sea ice in high latitudes and changes in vegetation cover in the tropics. Instrumental records are broadly in line with this emerging pattern, but have data gaps in key regions. Paleoclimate proxy reconstructions support the simulated magnitude and distribution of temperature variability. Our findings strengthen the need for urgent mitigation to avoid unprecedented changes in temperature variability.

A proxy-model comparison for mid-Pliocene warm period hydroclimate in the Southwestern US

Hydroclimate proxy reconstructions and paleoclimate models of the mid-Pliocene warm period provide insight into how, under a moderate greenhouse warming scenario, Earth-system feedbacks may impact regional hydroclimate. However, in the Southwestern United States there is discord between these two types of information: proxy data have been interpreted to indicate much wetter conditions, while the most recent generation of mid-Pliocene warm period climate models simulates drying. We use a water and energy balance framework to directly compare paleoclimate model output to a refined compilation of proxy records of the presence and areal extent of mid-Pliocene lakes. Within this framework, we quantify uncertainties in the proxy system model parameters and in the interpretation of available proxy records. We find that despite these significant uncertainties, most paleoclimate models simulate a regional balance between precipitation and evaporative demand that could not have sustained the extent of recorded lakes from this time. Moreover, the extensive lakes included as boundary conditions in mid-Pliocene warm period climate models are inconsistent with the regional climate simulated by those same models. This study identifies and quantifies the remaining unknowns in our picture of regional mid-Pliocene warm period hydroclimate, with implications for analyses of climate dynamics during this time.

The long-standing dilemma of European summer temperatures at the Mid-Holocene and other considerations on learning from the past for the future using a regional climate model

The past as an analogue for the future is one of the main motivations to use climate models for paleoclimate applications. Assessing possible model limitations in simulating past climate changes can lead to an improved understanding and representation of the response of the climate system to changes in the forcing, setting the basis for more reliable information for the future. In this study, a Regional Climate Model (RCM) is used for the investigation of the Mid-Holocene (MH, 6000 years ago) European climate, aiming to contribute to end the long-standing debate on the reconstruction of MH summer temperatures for the region, and gaining more insights on the development of appropriate methods for the production of future climate projections. Two Physically Perturbed Ensembles (PPEs) are first built by perturbing model physics and parameter values, consistently over two periods characterized by different forcing (i.e. the MH and Pre-Industrial (PI)). The goal is to uncover possible processes associated with the considered changes, that could deliver a response in MH summer temperatures closer to evidence from continental-scale proxy reconstructions. None of the investigated changes in model configuration produces remarkable differences with respect to the mean model behaviour. This indicates a limited sensitivity of the model to changes in the climate forcing, in terms of its structural uncertainty. Additional sensitivity tests are further conducted for the MH, by perturbing the model initial soil moisture conditions at the beginning of spring. A strong spatial dependency of summer near surface temperatures on the soil moisture available in spring is evinced from these experiments, with particularly remarkable differences evident over the Balkans and the areas north of the Black Sea. This emphasizes the role of soil-atmosphere interactions as one of the possible drivers of the differences in proxy-based summer temperature evident between Northern and Southern Europe. A deficiency of the considered land scheme of COSMO-CLM in properly retaining spring soil moisture, evinced from the performed tests and further confirmed by the evidence of present-day studies, suggests that the consideration of more sophisticated schemes may help bridging the gap between models and proxy-reconstructions. Finally, the distribution of the PPEs with changes in model configuration is analyzed for different variables (T2, PREC, TCLC). In almost all of the considered cases the results show that what is optimal for one period, in terms of a model configuration, is not the best for another characterized by different radiative forcing. These results raise a concern about the usefulness of automatic and objective calibration methods for RCMs, suggesting that a preferable approach is the production of small PPEs that target a set of model configurations, properly representing climate phenomena characteristic of the target region and that will be likely to contain the best model answer under different forcing.

Multi—Proxy Reconstructions of Climate Change and Human Impacts Over the Past 7000 Years From an Archive of Continental Shelf Sediments off Eastern Hainan Island, China

Abrupt climatic events and the history of human activities on Hainan Island are poorly understood, due to the lack of high-resolution records. We present high-resolution multiproxy records from the coastal shelf off eastern Hainan Island in China to investigate abrupt climate change and regional human–environment interaction over the last 7,000 years. A prominent climatic anomaly occurred during 5,400–4,900 cal yr BP. This abrupt monsoon failure has been detected in various paleoclimatic records from monsoonal regions. Anomalous summer monsoon intensity during 5,400–4,900 cal yr BP is probably driven by solar variability, ENSO activity and ice-rafting events in the North Atlantic. Over the past 1,500 years, with the growing population and progress in production technology, human activity has increasingly become the dominant factor controlling the natural environment of Hainan Island.

Identifying exposure biases in early instrumental data

<p>Long observational records of land surface air temperature are vital to our understanding of climate variability and change, as well as for testing predictions of climatic trends. However, of the relatively few long observational records which exist, many contain inhomogeneities or biases resulting from changing instrumentation, station location/surroundings and/or observing practises. One of the most significant issues is the exposure bias. Prior to the widespread adoption of louvered Stevenson-type screens in the late-19<sup>th</sup> century, various (often insufficient) approaches were used to shield thermometers. Each approach exposed the thermometer to differing levels of solar radiation, thus introducing inhomogeneities into individual station records and biases across regions, if similar approaches were used. Poorly shielded thermometers, for example, tended to read higher during the summer half year than those in Stevenson-type screens. Despite a number of studies documenting the presence of the exposure bias in early instrumental data, relatively few corrections have been applied or incorporated into global temperature datasets. This is largely due to the pervasive nature of the bias and a lack of observational metadata impeding bias identification or estimation of the appropriate correction.</p><p>In this work we explore a range of datasets to identify the potential for exposure bias in early instrumental data. We analyse historical data, corrections applied to homogenized datasets, as well as the small number of parallel measurements from differentially-shielded thermometers, in order to better define the characteristics of the exposure bias. These characteristics are then used to identify potential instances of exposure bias in early instrumental temperature records. We consider differences in seasonal anomalies, which is a key feature of many exposure biases, as well as their geographical variation (focussing mostly, but not solely, on Europe). We analyse how these behave at stations where it is known that exposure bias has already been adjusted for (though perhaps not completely) versus those that have not been. We also make comparisons with proxy reconstructions of temperature as an independent reference that is not susceptible to the same biases as the early instrumental data.</p><p>This work forms part of the NERC-funded GloSAT project which is developing a global surface air temperature dataset starting in 1781. The ultimate aim of the work reported here is to refine the error associated with these biases, in order to improve the representation of the exposure bias in error models used for gridded instrumental temperature datasets.</p>

The Atlantic Meridional Overturning Circulation (AMOC) simulated during interglacials and its impact on climate

<p>The Atlantic Meridional Overturning Circulation (AMOC) is a key mechanism of poleward heat transport and an important part of the global climate system. How it responded to past changes inforcing, such as experienced during Quaternary interglacials, is an intriguing and open question. Previous modelling studies suggest an enhanced AMOC in the mid-Holocene compared to the pre-industrial period. In previous simulations from the Palaeoclimate Modelling Intercomparison Project (PMIP), this arose from feedbacks between sea ice and AMOC changes, which also depended on resolution. Here I present aninitial analysis of the recently available PMIP4 simulations. This shows the overall strength of the AMOC does not markedly change between the mid-Holocene and piControl experiments (at least looking at the maximum of the mean meridional mass overturning streamfunction below 500m at 30<sup>o</sup>N and 50<sup>o</sup>N). This is not inconsistent with the proxy reconstructions using sortable silt and Pa/Th for the mid-Holocene. Here we analyse changes in the spatial structure of the meridional overturning circulation, along with their fingerprints on the surface temperature (computed through regression). We then estimate the percentage of the simulated surface temperature changes between the mid-Holocene and pre-industrial period that can be explained by AMOC. Furthermore, the analysis for the changes in the AMOC spatial structure has been extended to see if the same patterns of change hold for the last interglacial. The simulations will be compared to existing proxy reconstructions, as well as new palaeoceanographic reconstructions.</p>

Comparing temperature trends and variability over the Holocene in climate models of low and high complexity

<div> <p>Modeled and observed temperature trends over the Holocene disagree. Proxy reconstructions suggest global cooling during the late Holocene. Model simulations, on the other hand, show a warming trend for the entire Holocene, a contradiction known as the Holocene temperature conundrum.  </p> </div><div> <p>A recent study by Bader et. al. (2020) introduced a new approach to the question by proposing the coexistence of a cooling and warming climate mode. While the warming mode is proposed to be related to changes in greenhouse gas concentrations, the physical process behind the cooling mode might be a change in the seasonal cycle of Arctic sea-ice. It’s unclear to what extent this process is responsible for the observed climate response. Depending on their strength and location these modes have strong implications for proxy data interpretation and location selection when calculating global mean temperatures.   </p> </div><div> <p>Here, we investigate if similar modes and temperature trends can be found in models of different complexity. Therefore, we use a 2D Energy Balance Model (EBM), with solar, volcanic, ice-sheet and greenhouse gas forcing, for transient simulations of the Holocene climate. We analyze these Holocene climate simulations in terms of global and regional temperature trends, modes and variability patterns. We conduct sensitivity tests to examine the influence of the forcings on those trends and modes. In particular, we are interested in the influence of volcanic eruptions on the Holocene climate. Furthermore, we compare our model results with temperature reconstructions and simulations from Earth System Models.    </p> </div><div> <p>Altogether, we comprehensively analyze Holocene climate as simulated by a conceptual EBM, a state-of-the-art Earth System Model and proxy reconstructions. The results provide insight into whether models of different complexity produce similar modes and trends and whether these occur due to climate forcing rather than internal processes of the earth system. Finally, we will provide a better understanding of Holocene cooling and warming and the interpretation of differences between Holocene temperature proxy reconstructions and climate model simulations.    </p> </div><div> <p> </p> </div><div> <p>References:  <br>Bader, J., Jungclaus, J., Krivova, N. et al. Global temperature modes shed light on the Holocene temperature conundrum. Nat Commun 11, 4726 (2020). https://doi.org/10.1038/s41467-020-18478-6 </p> </div>

Critical transitions in Earth system dynamics

<p>It has been argued that several components of the Earth system may destabilise in response to gradually changing forcing such as rising atmospheric greenhouse gas concentrations and temperatures. Key examples of potentially unstable parts of the Earth system include the polar ice sheets and sea ice cover, the Atlantic Meridional Overturning Circulation, as well as tropical rainforests and monsoon systems. There are reasons to believe that the leading dynamical modes of these subsystems may essentially mimic bifurcations in low-order random dynamical systems. The stability loss on the way to critical transitions associated with such bifurcations typically leaves characteristic imprints in the statistics of time series encoding the dynamics of the system in question, which can hence serve as a proxy to assess the stability of the system. Here, we present recent advances in detecting stability loss along these lines and investigate proxy reconstructions and observations of several of the Earth system components that have been proposed to be at risk of destabilisation. We discuss the control parameters relevant for the different Earth system components and report on the posterior distributions of the critical thresholds, beyond which stability would be lost. </p>