Volcanic forcing, a major natural source of climate variability, represents a challenge for current climate modeling because of the unpredictability and specificity of individual eruptions, and ...because of the complexity of processes linking the eruption to the climate response. Volcanic forcing is largely underrepresented in available future climate projections, which is a critical problem. The study by Man Mei Chim and Colleagues (Chim et al., 2023, https://doi.org/10.1029/2023GL103743) tackles this known unknown and reveals how climatically relevant volcanic activity may be stronger than currently thought in a future warmer climate, enhancing uncertainty of climate projections. The study exemplifies the profound implications of inaccuracies within simplified climate scenarios and motivates new research on volcanically forced climate variability. It also arouses some thoughts on climate uncertainty communication.
Plain Language Summary
Volcanic eruptions are a critical part of the earth system, capable of causing large climatic fluctuations over periods from years to decades. Available scenario experiments typically neglect volcanic forcing, which may cause systematic errors in future climate projections. Man Mei Chim and Colleagues (Chim et al., 2023, https://doi.org/10.1029/2023GL103743) use a series of statistical and mathematical models to comprehensively simulate how volcanic eruptions may affect climate during the 21st century. The results indicate that a more realistic representation of volcanic eruptions yields slightly less future warming and larger uncertainty than standard projections. In other words, we are less confident of future climate changes once we account for the (unpredictable) volcanic forcing. By considering future volcano‐climate interactions in their whole complexity and revealing a substantial role for small‐to‐moderate eruptions, the study places a new important piece in the volcano‐climate puzzle. It will motivate new coordinated research to improve the simulated representation of volcano‐climate interactions not only in the ambit of future climate scenarios but in paleoclimatic investigations and idealized volcanic experiments well. The research also reminds us of limitations inherent in climate model experiments and should foster improved communication of natural climate variability and associated uncertainties.
Key Points
The study by Man Mei Chim and Colleagues shows that volcanic forcing can strongly affect future climate projections
The study reaffirms the importance of natural variability for future climates, stimulating research on volcano‐climate interactions
Volcanic forcing remains unpredictable, which rises the question of how to communicate irreducible climate projection uncertainties
Oceanic heat transport variations, carried by the northward-flowing Atlantic Water, strongly influence Arctic sea-ice distribution, ocean-atmosphere exchanges, and pan-Arctic temperatures. ...Palaeoceanographic reconstructions from marine sediments near Fram Strait have documented a dramatic increase in Atlantic Water temperatures over the 20th century, unprecedented in the last millennium. Here we present results from Earth system model simulations that reproduce and explain the reconstructed exceptional Atlantic Water warming in Fram Strait in the 20th century in the context of natural variability during the last millennium. The associated increase in ocean heat transfer to the Arctic can be traced back to changes in the ocean circulation in the subpolar North Atlantic. An interplay between a weakening overturning circulation and a strengthening subpolar gyre as a consequence of 20th-century global warming is identified as the driving mechanism for the pronounced warming along the Atlantic Water path toward the Arctic. Simulations covering the late Holocene provide a reference frame that allows us to conclude that the changes during the last century are unprecedented in the last 1150 years and that they cannot be explained by internal variability or natural forcing alone.
Decadal and bi-decadal climate responses to tropical strong volcanic eruptions (SVEs) are inspected in an ensemble simulation covering the last millennium based on the Max Planck Institute—Earth ...system model. An unprecedentedly large collection of pre-industrial SVEs (up to 45) producing a peak annual-average top-of-atmosphere radiative perturbation larger than −1.5 Wm
−2
is investigated by composite analysis. Post-eruption oceanic and atmospheric anomalies coherently describe a fluctuation in the coupled ocean–atmosphere system with an average length of 20–25 years. The study provides a new physically consistent theoretical framework to interpret decadal Northern Hemisphere (NH) regional winter climates variability during the last millennium. The fluctuation particularly involves interactions between the Atlantic meridional overturning circulation and the North Atlantic gyre circulation closely linked to the state of the winter North Atlantic Oscillation. It is characterized by major distinctive details. Among them, the most prominent are: (a) a strong signal amplification in the Arctic region which allows for a sustained strengthened teleconnection between the North Pacific and the North Atlantic during the first post-eruption decade and which entails important implications from oceanic heat transport and from post-eruption sea ice dynamics, and (b) an anomalous surface winter warming emerging over the Scandinavian/Western Russian region around 10–12 years after a major eruption. The simulated long-term climate response to SVEs depends, to some extent, on background conditions. Consequently, ensemble simulations spanning different phases of background multidecadal and longer climate variability are necessary to constrain the range of possible post-eruption decadal evolution of NH regional winter climates.
Tidal measurements from the Italian city of Venice, available since 1872 and constituting the longest sea-level record in the Mediterranean area, indicate that local flooding statistics have ...dramatically worsened during the last decades. Individual flooding episodes are associated with adverse meteorological conditions, and their increased frequency is mainly attributed to the rise of the average local Relative Sea Level (RSL). However, the role of interannual-to-multidecadal modes of average RSL variability in shaping the evolution of Venice flooding is highly significant and can cause sharp increases in the flood frequency episodes. Here, we use local tidal measurements in Venice covering 1872–2020 to deeply inspect the contribution and predictability of the different components characterizing the observed average RSL variability, including a long-term trend and four quasi-periodic modes. Our results demonstrate that the observed increase in flooding frequency is not only due to the average RSL rise but also due to a progressive widening of tidal anomalies around the average RSL, revealed by opposite trends in mean tidal maxima and minima. Moreover, interannual and decadal periodicities are not negligible in modulating the timing of annual mean RSL and flood frequency extremes. This study demonstrates that the last decades experienced an unprecedented sharp increase in sea level, which significantly affected the decadal predictability of RSL with statistical methods. Our work contributes to a deeper understanding of the sources of uncertainty in decadal sea-level variability and predictability in the Venice lagoon.
This work provides evidence of the influence of large volcanic eruptions on Sahel rainfall relying on PMIP4/past1000 multi‐model simulations, covering the last millennium. A classification of ...volcanic eruptions in the last millennium according to the meridional symmetry of the associated radiative forcing reveals different mechanisms of the West African Monsoon response at inter‐annual timescale. In all cases, these simulated changes result in Sahel drying up to 2 years after an eruption. Besides, we add evidence of a role of varying volcanic activity across the past millennium in the Sahel precipitation variability at multi‐decadal to secular timescales.
Plain Language Summary
Relying on climate simulations of the past millennium, this work shows that the largest volcanic eruptions documented induce different mechanisms in the West African Monsoon response depending on whether the eruption occurs at extra‐tropical or tropical latitudes. In both cases, such volcanic impacts can induce droughts the two following rainy seasons in the West African Sahel region. Moreover, we show first evidence of an influence of the frequency of volcanic eruptions on the Sahel precipitation regime over decades to centuries across the past millennium.
Key Points
Climate model simulations of the past millennium show Sahel drought in response to large volcanic eruptions up to the following 2 years
The mechanisms leading to the Sahel drying are different if it responds to extra‐tropical or tropical eruptions
The increasing frequency of eruptions throughout the past millennium modulates Sahel precipitation variability on multi‐decadal timescales
We present an assessment of the probabilistic and climatological consistency of the CMIP5/PMIP3 ensemble simulations for the last millennium relative to proxy-based reconstructions under the paradigm ...of a statistically indistinguishable ensemble. We evaluate whether simulations and reconstructions are compatible realizations of the unknown past climate evolution. A lack of consistency is diagnosed in surface air temperature data for the Pacific, European and North Atlantic regions. On the other hand, indications are found that temperature signals partially agree in the western tropical Pacific, the subtropical North Pacific and the South Atlantic. Deviations from consistency may change between sub-periods, and they may include pronounced opposite biases in different sub-periods. These distributional inconsistencies originate mainly from differences in multi-centennial to millennial trends. Since the data uncertainties are only weakly constrained, the frequently too wide ensemble distributions prevent the formal rejection of consistency of the simulation ensemble. The presented multi-model ensemble consistency assessment gives results very similar to a previously discussed single-model ensemble suggesting that structural and parametric uncertainties do not exceed forcing and internal variability uncertainties.
We assess the responses of North Atlantic, North Pacific, and tropical Indian Ocean Sea Surface Temperatures (SSTs) to natural forcing and their linkage to simulated global surface temperature (GST) ...variability in the MPI-Earth System Model simulation ensemble for the last millennium. In the simulations, North Atlantic and tropical Indian Ocean SSTs show a strong sensitivity to external forcing and a strong connection to GST. The leading mode of extra-tropical North Pacific SSTs is, on the other hand, rather resilient to natural external perturbations. Strong tropical volcanic eruptions and, to a lesser extent, variability in solar activity emerge as potentially relevant sources for multidecadal SST modes’ phase modulations, possibly through induced changes in the atmospheric teleconnection between North Atlantic and North Pacific that can persist over decadal and multidecadal timescales. Linkages among low-frequency regional modes of SST variability, and among them and GST, can remarkably vary over the integration time. No coherent or constant phasing is found between North Pacific and North Atlantic SST modes over time and among the ensemble members. Based on our assessments of how multidecadal transitions in simulated North Atlantic SSTs compare to reconstructions and of how they contribute characterizing simulated multidecadal regional climate anomalies, past regional climate multidecadal fluctuations seem to be reproducible as simulated ensemble-mean responses only for temporal intervals dominated by major external forcings.
Using spectral and statistical analyses of discharges and basin average precipitation rates acquired over the Po River since the early 1800s, we investigate the impact of variations in solar activity ...on hydrological decadal patterns over northern Italy. Wet and dry periods appear to alternate in accordance with polarized sunspot cycles. Intriguingly, a solar signature on Po River discharges is detected to be highly significant since the late 1800s, before the onset of sunspots hyperactivity established by the middle 1900s. In particular, observed hydrological patterns over northern Italy are significantly correlated, under periods of quiet sunspot activity, with parameters characterizing the Sun's orbital motion, specifically with the time derivative of the solar angular momentum (τ) which is thought to modulate the strength of the solar wind and sunspot dynamics under weak sunspot activity. The North Atlantic Oscillation (NAO) is detected as potential link between the Sun and Po River discharges, since it is significantly correlated with both solar activity and the decadal variability in the north Italian climate. In particular, positive (negative) NAO anomalies, which are associated with comparatively lower (higher) Po River discharges, are assessed to alternatively correlate at decadal timescales either with τ or with the Earth's geomagnetic activity (GA), which closely follows sunspot activity. This changing correlation seems to be regulated by the strength of sunspot activity: under periods of quiet sunspot activity, a weakening of the GA‐NAO connection and a reinforcement of the τ‐NAO connection is observed. In this sense, the strength of solar activity apparently modulates the connection between the NAO and Po River discharges.
Sea‐level rise is one of the most critical consequences of global warming, with potentially vast impacts on coastal environments and societies. Sea‐level changes are spatially and temporally ...heterogeneous on multiannual‐to‐multidecadal timescales. Here, we demonstrate that the observed rate of winter sea‐level rise in the Italian city of Venice contains significant multidecadal fluctuations, including interdecadal periods of near‐zero trend. Previous literature established a connection between the local sea‐level trend in Venice and over the broad subpolar and eastern North Atlantic. We demonstrate that for multidecadal variations in sea‐level trend such connection holds only since the mid‐20th Century. Such multidecadal sea‐level fluctuations relate to North Atlantic sea‐surface temperature changes described by the Atlantic multidecadal variability, or AMV. The link is explained by combined effect of AMV‐linked steric variations in the North Atlantic propagating in the Mediterranean Sea, and large‐scale atmospheric circulation anomalies over the North Atlantic with a local effect on sea level in Venice. We discuss the implications of such variability for near‐term predictability of winter sea‐level changes in Venice. Combining available sea‐level projections for Venice with a scenario of imminent AMV cooling yields a slowdown in the rate of sea‐level rise in Venice, with the possibility of mean values remaining even roughly constant in the next two decades as AMV effects contrast the expected long‐term sea‐level rise. Acknowledging, understanding, and communicating this multidecadal variability in local sea‐level rise is crucial for management and protection of this world‐class historical site.
Plain Language Summary
Environmental and socioeconomic impacts of sea‐level rise are one of the major concerns of global warming. Here, we consider the case of the Italian city of Venice, one of the iconic locations for the potentially dramatic effects of sea‐level rise. We show that the sea‐level evolution in Venice during the past ∼150 years contains strong multidecadal fluctuations, so that periods of more than two decades when there is little or no trend occurred even in the recent past. We link these fluctuations with sea‐level and climatic variations in the North Atlantic. In particular, we focus on the phenomenon known as Atlantic multidecadal variability, or AMV, which describes the alternation over multidecadal periods of warm and cold phases of the North Atlantic surface. Our results indicate that warm AMV phases are linked to faster sea‐level rise in Venice and vice versa. Accordingly, we build sea‐level rise scenarios for Venice until 2035 by considering an imminent AMV cooling as suggested by recent studies. The scenarios yield a temporary slowdown of sea‐level rise as the AMV contrasts the effects of global warming. This sea‐level variability can strongly impact on the management of protective measures against flooding currently operative in Venice.
Key Points
The historical rate of sea‐level rise in Venice contains large multidecadal fluctuations and interdecadal periods of near‐zero trend
Multidecadal sea‐level trend variations in Venice follow those in the subpolar North Atlantic since the mid‐20th Century
Atlantic multidecadal variability paves the way for the exploration of decadal predictability of sea‐level rise in Venice
We assess internally-generated climate variability expressed by a multi-model ensemble of unperturbed climate simulations. We focus on basin-scale annual-average sea surface temperatures (SSTs) from ...twenty multicentennial pre-industrial control simulations contributing to the fifth phase of the Coupled Model Intercomparison Project. Ensemble spatial patterns of regional modes of variability and ensemble (cross-)wavelet-based phase-frequency diagrams of corresponding paired indices summarize the ensemble characteristics of inter-basin and regional-to-global SST interactions on a broad range of timescales. Results reveal that tropical and North Pacific SSTs are a source of simulated interannual global SST variability. The North Atlantic-average SST fluctuates in rough co-phase with the global-average SST on multidecadal timescales, which makes it difficult to discern the Atlantic Multidecadal Variability (AMV) signal from the global signal. The two leading modes of tropical and North Pacific SST variability converge towards co-phase in the multi-model ensemble, indicating that the Pacific Decadal Oscillation (PDO) results from a combination of tropical and extra-tropical processes. No robust inter- or multi-decadal inter-basin SST interaction arises from our ensemble analysis between the Pacific and Atlantic oceans, though specific phase-locked fluctuations occur between Pacific and Atlantic modes of SST variability in individual simulations and/or periods within individual simulations. The multidecadal modulation of PDO by the AMV identified in observations appears to be a recurrent but not typical feature of ensemble-simulated internal variability. Understanding the mechanism(s) and circumstances favoring such inter-basin SST phasing and related uncertainties in their simulated representation could help constraining uncertainty in decadal climate predictions.
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