Mid- to Late Holocene climate change: an overview

Holocene is the most recent interval of Earth history which lasted approximately 10 300 radiocarbon years, or 11 700 calibrated (calendar) years and extends to the present day. Despite a relatively short duration of the Holocene, significant changes in climate and landscape occurred during this interglacial epoch. Multiple studies of globally distributed high‐resolution climate proxy records indicate that a rapid warming developed at the transition from the Pleistocene to the Holocene. A subsequent slower rise of the mean global temperature reached its maximum (the Holocene climatic optimum) approximately 6–5.5 14 C kyr bp , or 6000–7000 cal year ago. The early Holocene warming was followed by approximately 0.7°C cooling through the Middle to Late Holocene, culminating in the coolest temperatures of the Holocene during the Little Ice Age, between 1350 and 1850 ce , when the lower summer insolation in the Northern Hemisphere due to orbital forcing coincided with minimum solar activity and several strong tropical volcanic eruptions.

Abstract. Large tropical (TROP) volcanic eruptions can influence North Atlantic climate by inducing a positive shift of the North Atlantic Oscillation (NAO), typically resulting in winter warming across northern Eurasia. However, these changes remain highly uncertain, as they may coincide with strong internal variability in Northern Hemisphere wintertime climate. In contrast, Northern Hemisphere extratropical (NHET) eruptions are proposed to have opposite impacts, but they have been comparatively less studied, and large uncertainties remain regarding the ability of climate models to capture volcanic responses. This study examines winter North Atlantic climate responses to TROP and NHET eruptions by comparing temperature and atmospheric circulation patterns from last millennium simulations with multiple proxy-based reconstructions. We find distinct differences in NAO-related climate changes in reconstructions, with TROP eruptions followed by a shift towards positive NAO and NHET eruptions associated with a negative NAO. In comparison, modelled responses exhibit a wide spread with strong dependence on the choice of volcanic forcing dataset. Notably, simulations using the latest volcanic forcing data show improved agreement with reconstructions, particularly for TROP eruptions. This model-proxy agreement provides a useful basis for investigating the mechanisms that drive positive NAO responses after TROP eruptions. However, the simulated impacts of NHET eruptions are less consistent and remain unclear. These results highlight the importance of improved volcanic forcing datasets, refined paleoclimate reconstructions, and robust statistical approaches to better constrain uncertainties in assessing volcanic impacts on North Atlantic climate.

Changes in the geometry of ocean basins have been influential in driving climate change throughout Earth’s history. Here we focus on the appearance of the Greenland-Scotland Ridge (GSR) and try to understand its impact on the ocean state, including global circulation, heat transport, T and S properties and ventilation timescales, which will be useful for interpreting paleoproxies. To this end, we use a coupled atmosphere-ocean-sea ice model with idealized geometry and consider two geometrical configurations. The reference configuration (noridge) comprises two wide strips of land set 90° apart extending from the North Pole to 40°S, separating the Northern Hemisphere ocean into a small and a large basin. In the ridge configuration a zonally symmetric oceanic ridge, that extends across the Atlantic-like basin at 60°N, mimicking the GSR, is added. In addition, we consider two climatic limits of noridge: a warm case where the northern high latitudes are seasonally ice-free and a cold case where a perennial sea ice cover is present. In both cases of noridge deep-water formation occurs at the North Pole in the Atlantic-like basin. When the ridge is introduced, the flow of warm Atlantic water to the high latitudes is hampered and the ocean heat transport across 70°N decreases by ~60% which causes cooling and freshening north of the ridge. Downwelling shifts south of the ridge, thereby altering the structure of the upper overturning cell dramatically. Despite these changes, the Northern Hemisphere surface climate response is surprisingly small for the warm climate case. This is because the subpolar gyre circulation continues to transport warm water across the ridge, keeping the northern North Atlantic relatively warm and ice-free. In the colder climate case, however, the presence of sea ice provides a strong non-linear feedback, which amplifies the cooling induced by the ridge, and causes sea ice to expand. Our results highlight the possible disconnect between changes in the localization of deep-water formation, the structure of the AMOC and the properties of water masses and changes in Northern Hemisphere climate. Implications for the interpretation of paleoproxy records from the North Atlantic region will be discussed.

Abstract. Compound wind and precipitation (CWP) extremes often cause severe impacts on human society and ecosystems, such as damage to crops and infrastructure. Spatially compounding events with multiple regions affected by CWP extremes in the same winter can impact the global economy and reinsurance industry; however, our understanding of these events is limited. While climate variability modes such as El Niño Southern Oscillation (ENSO) can influence the frequency of precipitation and wind extremes, their individual and combined effects on spatial co-occurrences of CWP extremes across the Northern Hemisphere have not been systematically examined. Here, by combining reanalysis data and climate model simulations, we investigate how two oceanic and two atmospheric variability modes – ENSO, the Atlantic Multidecadal Variability (AMV), the North Atlantic Oscillation (NAO), and the Pacific North American (PNA) – amplify the wintertime (December–February) frequency of daily CWP extremes and associated spatial co-occurrences across the Northern Hemisphere. We find many hotspot regions where concurrent variability mode anomalies significantly amplify wintertime CWP extreme event frequencies compared to single variability modes. By examining the relationships between frequencies of wintertime CWP extremes across regions, we identify dependencies enabling extreme spatially compounding events, that is winters with many regions experiencing CWP extremes. While ENSO is the most influential variability mode for such extreme spatially compounding events, the occurrence of these events increases further when multiple modes of variability are in anomalous phases. In particular, combinations of modes increase both the number of regions and the population exposed to daily CWP extremes in the same winter. For example, combined ENSO- and NAO+ nearly doubles the number of affected regions compared to neutral conditions on average. Our analysis highlights the importance of considering the interplay between variability modes to improve risk management and adapt to the impacts of spatially compounding CWP extremes.

North Atlantic climate evolution through the Plio-Pleistocene climate transitions

At present the El Nino system is one of the primary causes of inter-annual climatic variability, not only in tropical regions but also on a global level (Ropelewsky and Halpert 1987; Philander 1989; Kiladis and Diaz 1989; Glynn 1990). A considerable effort is dedicated to the understanding of the El Nino-Southern Oscillation (ENSO) system and its interactions with other kinds of climatic variations, at the decadal/interdecadal (e.g., the Pacific Decadal Oscillation) and centennial (Little Ice Age, Medieval Warm Period) scales (Anderson et al. 1992; Bradley and Jones 1992; Allan et al. 1997; Diaz and Markgraf 2000). Under societal and political pressure, several international research programs aim to decipher the relationships that link ENSO and the present global warming of the planet (Ropelewesky 1992; Cane et al. 1997; Timmermann et al. 1999). These scientific challenges and questions require that long series of climate variations, much longer than those available from instrumental data alone, be compiled and studied. A renewed interest for the climate evolution, at local, regional and global scales, during the last few centuries, is thus justified by the need to understand better the climatic particularities of the Little Ice Age (LIA) and the transition period to the modern situation (19th and 20th centuries) (e.g., Grove (1988), Bradley and Jones (1992, 1993), Allan and d’Arrigo (1999), Allan (2000), Verschuren (this volume), Scott and Lee-Thorp (this volume)). These studies should help to determine how much of the recent global warming results from anthropogenic activities as against a natural post-LIA evolution. Deciphering the relationships between the global state of the planet and the El Nino system (frequency and intensity of individual events, teleconnection links) requires us to document as precisely as possible former climate variations and ENSO

Researches on the effects of the El Nino Southern Oscillation (ENSO) over precipitation and temperature, such as drought, flood, and anomalous high or cold temperatures, have large importance because of the impacts on the environment, society, and economy. Some recent studies, focusing on the Northern Hemisphere, have indicated that the basic response of ENSO is dependent on the phase of the Madden-Julian Oscillation (MJO). The present work investigates the combined response of the phases of these two distinct phenomena, ENSO and MJO, over South America. Our goal is to explore the relative importance of the MJO to precipitation and temperature anomalies during each ENSO phase. A composite analysis with each combination of the phases of ENSO and MJO was performed to obtain the mean patterns of temperature and precipitation over South America for the months of November to March (austral summer) and May to September (austral winter). The results showed that the precipitation and temperature anomaly patterns observed during the ENSO phases, without the concurrent occurrence of the MJO, can be strengthened or weakened during events where ENSO and MJO occur simultaneously. Moreover, the effect on the anomaly patterns in these events depends on the MJO phase.

Three different reconstructed wind-stress fields which take into account variations of the North Atlantic Oscillation, one general circulation model wind-stress field, and three radiative forcings (volcanic activity, insolation changes and greenhouse gas changes) are used with the UVic Earth System Climate Model to simulate the surface air temperature, the sea-ice cover, and the Atlantic meridional overturning circulation (AMOC) since 1500, a period which includes the Little Ice Age (LIA). The simulated Northern Hemisphere surface air temperature, used for model validation, agrees well with several temperature reconstructions. The simulated sea-ice cover in each hemisphere responds quite differently to the forcings. In the Northern Hemisphere, the simulated sea-ice area and volume during the LIA are larger than the present-day area and volume. The wind-driven changes in sea-ice area are about twice as large as those due to thermodynamic (i.e., radiative) forcing. For the sea-ice volume, changes due to wind forcing and thermodynamics are of similar magnitude. Before 1850, the simulations suggest that volcanic activity was mainly responsible for the thermodynamically produced area and volume changes, while after 1900 the slow greenhouse gas increase was the main driver of the sea-ice changes. Changes in insolation have a small effect on the sea ice throughout the integration period. The export of the thicker sea ice during the LIA has no significant effect on the maximum strength of the AMOC. A more important process in altering the maximum strength of the AMOC and the sea-ice thickness is the wind-driven northward ocean heat transport. In the Southern Hemisphere, there are no visible long-term trends in the simulated sea-ice area or volume since 1500. The wind-driven changes are roughly four times larger than those due to radiative forcing. Prior to 1800, all the radiative forcings could have contributed to the thermodynamically driven changes in area and volume. In the 1800s the volcanic forcing was dominant, and during the first part of the 1900s both the insolation changes and the greenhouse gas forcing are responsible for thermodynamically produced changes. Finally, in the latter part of the 1900s the greenhouse gas forcing is the dominant factor in determining the sea-ice changes in the Southern Hemisphere.

High-resolution records of late-Holocene climate change and carbon accumulation in two north-west European ombrotrophic peat bogs

Two-way coupling of an ENSO model to the global climate model CLIMBER-3 [formula omitted

The impacts of North Atlantic Oscillation (NAO) and El Niño–Southern Oscillation (ENSO) on Great Lakes ice cover were investigated using lake ice observations for winters 1963–2010 and National Centers for Environmental Prediction reanalysis data. It is found that both NAO and ENSO have impacts on Great Lakes ice cover. The Great Lakes tend to have lower (higher) ice cover during the positive (negative) NAO. El Niño events are often associated with lower ice cover. The influence of La Niña on Great Lakes ice cover is intensity‐dependent: strong (weak ) La Niña events are often associated with lower (higher) ice cover. The interference of impacts of ENSO and NAO complicates the relationship between ice cover and either of them. The nonlinear effects of ENSO on Great Lakes ice cover are important in addition to NAO effects. The correlation coefficient between the quadratic Nino3.4 index and ice cover (−0.48) becomes significant at the 99% confidence level. The nonlinear response of Great Lakes ice cover to ENSO is mainly due to the phase shift of the teleconnection patterns during the opposite phases of ENSO. Multiple‐variable nonlinear regression models were developed for ice coverage. Using the quadratic Nino3.4 index instead of the index itself can significantly improve the prediction of Great Lakes ice cover (the correlation between the modeled and observed increases from 0.35 to 0.51). Including the interactive term NAO·Nino3.42 further improves the prediction skill (the correlation increases from 0.51 to 0.59). The analysis is also applied to individual lakes. The model for Lake Michigan has the highest prediction skill, while Lake Erie has the smallest skill.

Late-Holocene vegetation and climate change in Jeju Island, Korea and its implications for ENSO influences

The role of El Nino/Southern Oscillation (ENSO) and the mechanism through which ENSO influences the precipitation variability over northwest India and the adjoining (NWIA) region is well documented. In this study, the relative role of North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) and ENSO in modulating the Asian jet stream in the Northern Hemisphere winter and their relative impact on the precipitation variability over the region have been estimated through analysis of observed data. It is seen that interannual variations of NWIA precipitation are largely influenced by ENSO. An empirical orthogonal function (EOF) analysis has been carried out to understand dominant modes of interannual variability of zonal wind at 200 hPa of the Northern Hemisphere. The EOF-1 pattern in the tropical region is similar to that of an ENSO pattern, and the principal component (PC) time series corresponds to the ENSO time series. The EOF-2 spatial pattern resembles that of NAO/AO with correlation of PC time series with AO and NAO being 0.74 and 0.62, respectively. The precipitation anomaly time series over the region of interest has marginally higher correlation with the PC-2 time series as compared to that of PC-1. Regression analysis of precipitation and circulation parameters indicates a larger contribution of the second mode to variability of winds and precipitation over the NWIA. Moisture transport from the Arabian Sea during the active phase of NAO/AO and the presence of a cyclonic anomaly lead to higher precipitation over the NWIA region.

Using a coupled Earth climate model, freshwater forcing experiments are performed to study the Bjerknes compensation (BJC) between meridional atmosphere heat transport (AHT) and meridional ocean heat transport (OHT). Freshwater hosing in the North Atlantic weakens the Atlantic meridional overturning circulation (AMOC) and thus reduces the northward OHT in the Atlantic significantly, leading to a cooling (warming) in the surface layer in the Northern (Southern) Hemisphere. This results in an enhanced Hadley cell and northward AHT. Meanwhile, the OHT in the Indo-Pacific is increased in response to the Hadley cell change, partially offsetting the reduced OHT in the Atlantic. Two compensations occur here: compensation between the AHT and the Atlantic OHT, and that between the Indo-Pacific OHT and the Atlantic OHT. The AHT change undercompensates the OHT change by about 60% in the extratropics, while the former overcompensates the latter by about 30% in the tropics due to the Indo-Pacific change. The BJC can be understood from the viewpoint of large-scale circulation change. However, the intrinsic mechanism of BJC is related to the climate feedback of the Earth system. The authors’ coupled model experiments confirm that the occurrence of BJC is an intrinsic requirement of local energy balance, and local climate feedback determines the extent of BJC, consistent with previous theoretical results. Even during the transient period of climate change, the BJC is well established when the ocean heat storage is slowly varying and its change is much weaker than the net local heat flux change at the ocean surface. The BJC can be deduced from the local climate feedback. Under the freshwater forcing, the overcompensation in the tropics is mainly caused by the positive longwave feedback related to clouds, and the undercompensation in the extratropics is due to the negative longwave feedback related to surface temperature change. Different dominant feedbacks determine different BJC scenarios in different regions, which are in essence constrained by local energy balance.

This study examines associations of diurnal temperature range (DTR) changes in observations at the global, hemispheric, subcontinental, and grid box scales with five leading climate variability modes, including the Arctic Oscillation (AO), hemispheric Pacific–North America (PNA)–like mode, Pacific Decadal Oscillation, El Niño Southern Oscillation (ENSO), and Antarctic Oscillation (AAO) during the Northern Hemisphere winter season (Jan–Mar). Winter DTR variability in most subcontinental regions is significantly related to variations in either the AO, or hemispheric PNA‐like mode, or ENSO index in the Northern Hemisphere. In the Southern Hemisphere, the DTR variability appears closely coupled with variations in the ENSO and AAO. From 1951 to 2000, variations in the circulation patterns account for a significant fraction of the DTR increase at all scales although the strength of these associations varies geographically. After removing the linearly congruent component of leading climate variability modes from the total wintertime DTR trends in the observations, statistically significant residual trends in DTR are still found at the global, hemispheric, and most subcontinental regions. Ensemble mean multimodel averaged DTR trends to major anthropogenic and natural forcing are significantly smaller than not only observed total DTR trends but also residual trends at these large scales. The implication of changes in the leading climate variability modes on the detection of regional DTR trends is discussed. We find that the detection of the regional response to combined anthropogenic and natural forcing is robust to the exclusion of trends related to changes of the five modes considered here.