Description: Introduction We use high-resolution coupled ocean-atmosphere simulations to show that reasonable past melt rates of the Antarctic Ice Sheet can have led to shifts of the ITCZ through large-scale surface air temperature changes over the Southern Ocean. Through sensitivity experiments employing slightly negative to large positive meltwater fluxes we deduce that meridional shifts of the Hadley cell and therewith the ITCZ are, to a first order, a linear response to Southern Hemisphere high-latitude surface air temperature changes and Antarctic Ice Sheet melt rates. Methods The simulations were performed using the Community Earth System Model version1.2 (CESM1.2), a global climate model that includes interactive atmosphere (CAM4), ocean (POP2), land (CLM4.0; including carbon-nitrogen dynamics), and sea-ice (CICE4) components. For the atmosphere (running with a finite volume dynamical core) and land a horizontal resolution of 0.9° x 1.25° was used with the former having 26 vertical levels. The ocean and sea-ice components use a displaced dipole grid with a nominal horizontal resolution of 1° . The ocean grid has 60 levels. The AIS contribution to meltwater pulse 1A [Clark et al., 1996, doi:10.1029/96PA01419] is highly uncertain, but in the most thorough attempt thus far to quantify this flux, a mean value of 0.034 Sv was found, with maximum values up to 0.11 Sv for a period of 350 years [Golledge et al., 2014, doi:10.1038/ncomms6107]. The Holocene AIS variability suggested by Bakker et al. [2017, doi:10.1038/nature20582] is 0.048 Sv (1σ). Finally, the future response of the AIS if global warming is to continue in the next centuries again varies widely, ranging from ∼0.2-0.5 Sv for the year 2100 in, respectively the so-called Representative Concentration Pathway (RCP) scenario 4.5 and RCP8.5 [Meinshausen et al., 2011, doi:10.1007/s10584-011-0156-z], and ranging from ∼0.2-0.25 Sv for the year 2500 in again RCP4.5 and RCP8.5, respectively [deConto and Pollard , 2016, doi:10.1038/nature17145]. Taken together, credible past and future rates of AIS meltwater into the Southern Ocean seem to range from slightly negative values, i.e. periods of minor AIS growth [Bakker et al., 2017], to positive values of several tenths of Sverdrups. To assess the impact of such AIS meltwater rates we performed four 200 year long experiments in which different magnitudes of freshwater forcing (FWF), namely -48 mSv, 48 mSv, 100 mSv and 200 mSv, were continuously added to the surface of the Southern Ocean south of 60° S (referred to as PIm48, PI48, PI100 and PI200, respectively). The negative -48 mSv meltwater flux implies that freshwater is removed from the internally calculated freshwater flux that enters the ocean, comprising of precipitation, continental runoff and sea-ice melt. AIS freshwater input is not compensated for elsewhere. All experiments start from the same spinup pre-industrial state and in addition a 200 year long control simulation was performed in which no additional freshwater was added to the Southern Ocean (referred to as 'PI'). For the analyses averages over the last 20 years of each simulation are taken.

Description: Proxy-based indicators of past climate change show that current global climate models systematically underestimate Holocene-epoch climate variability on centennial to multi-millennial timescales, with the mismatch increasing for longer periods (Collins et al., 2002, doi:10.1175/1520-0442(2002)015〈1497:ACOTVO〉2.0.CO;2 ; Goosse et al., 2005, doi:10.1016/j.quascirev.2004.12.009 ; Zorita et al., 2010, doi:10.1007/s10584-010-9824-7 ; Lovejoy et al., 2013, doi:10.5194/esd-4-439-2013 ; Laepple and Huybers, 2014, doi:10.1073/pnas.1412077111 ). Proposed explanations for the discrepancy include ocean-atmosphere coupling that is too weak in models (Cane, 1998, doi:10.1126/science.282.5386.59 ), insufficient energy cascades from smaller to larger spatial and temporal scales (Ferrari & Wunsch, 2009, doi:10.1146/annurev.fluid.40.111406.102139 ) or that global climate models do not consider slow climate feedbacks related to the carbon cycle or interactions between ice sheets and climate (Lovejoy et al., 2013). Such interactions, however, are known to have strongly affected centennial- to orbital-scale climate variability during past glaciations (Broecker et al., 1992, doi:10.1007/BF00193540 ; Clark et al., 1999, doi:10.1126/science.286.5442.1104 ; Ganopolski & Rahmstorf, 2001, doi:10.1038/35051500 ; Liu et al., 2009, doi:10.1126/science.1171041 ), and are likely to be important in future climate change (Fogwill et al., 2015, doi:10.1002/2015EF000306 ; Green & Schmittner, 2015, doi:10.1175/JCLI-D-15-0110.1 ; Swingedouw et al., 2015, doi:10.1007/s00382-014-2270-x ). Here we show that fluctuations in Antarctic Ice Sheet discharge caused by relatively small changes in subsurface ocean temperature can amplify multi-centennial climate variability regionally and globally, suggesting that a dynamic Antarctic Ice Sheet may have driven climate fluctuations during the Holocene. We analysed high-temporal-resolution records of iceberg-rafted debris derived from the Antarctic Ice Sheet, and performed both high-spatial-resolution ice-sheet modelling of the Antarctic Ice Sheet and multi-millennial global climate model simulations. Ice-sheet responses to decadal-scale ocean forcing appear to be less important, possibly indicating that the future response of the Antarctic Ice Sheet will be governed more by long-term anthropogenic warming combined with multi-centennial natural variability than by annual or decadal climate oscillations.

Description: The Caspian Sea (CS) is the largest inland lake in the world. Large variations in sea level and surface area occurred in the past and are projected for the future. The potential impacts on regional and large-scale hydroclimate are not well understood. Here, we examine the impact of CS area on climate within its catchment and in the wider northern hemisphere. The Community Earth System Model (CESM1.2.2) is used to simulate the climate of four scenarios: (1) larger than present CS area, (2) current area, (3) smaller than present area, and (4) no-CS scenario. The results reveal large changes in the regional atmospheric water budget. Evaporation (E) over the sea increases with increasing area, while precipitation (P) increases over the south-west CS with increasing area. P-E over the CS catchment decreases as CS surface area increases, indicating a dominant negative lake-evaporation feedback. A larger CS area reduces summer surface air temperatures and increases winter temperatures. The impacts extend eastwards, where summer precipitation is enhanced over central Asia and the north-western Pacific region experiences warming with sea ice reduction in winter. Our results also indicate a weakening of the 500-hPa troughs over the northern Pacific with larger CS area. Lastly, we find a thermal response triggers a southward shift of the jet stream in the upper troposphere during summer. Our findings establish that changing CS area results in climate impacts of such scope that CS area variation should be considered for incorporation into climate model simulations, including palaeo and future scenarios.

Description: Several abrupt climatic events during the present interglacial have been associated with catastrophic freshwater forcing, such as the events at 9.2and 8.2 ka BP (Alley et al., 1997; Barber et al., 1999; Marshall et al. 2007; Fleitmann et al. 2008). Proxy evidence suggests that similar events may have occurred during the last interglacial (e.g., Beets & Beets 2003; Beets et al., 2006), suggesting that freshwater‐induced perturbations are an important mechanism for abrupt climate change in interglacial climates. In addition solar variability (Neff et al., 2001; Wang et al., 2005) and explosive volcanic eruptions (Crowley, 2000; Shindell et al., 2003; Jansen et al., 2007) can trigger centennial‐scale climate events during interglacials and may thus have been responsible for a part of interglacial climate variability. We investigate the sensitivity of the present and last interglacial climates to realistic perturbations resulting from freshwater, solar or volcanic forcings. We will compare the differences between the two interglacial periods, between different climate models and evaluate the resulting using proxy archives.

Description: Proxy-based indicators of past climate change show that current global climate models systematically underestimate Holocene-epoch climate variability on centennial to multi-millennial timescales, with the mismatch increasing for longer periods1,2,3,4,5. Proposed explanations for the discrepancy include ocean–atmosphere coupling that is too weak in models6, insufficient energy cascades from smaller to larger spatial and temporal scales7, or that global climate models do not consider slow climate feedbacks related to the carbon cycle or interactions between ice sheets and climate4. Such interactions, however, are known to have strongly affected centennial- to orbital-scale climate variability during past glaciations8,9,10,11, and are likely to be important in future climate change12,13,14. Here we show that fluctuations in Antarctic Ice Sheet discharge caused by relatively small changes in subsurface ocean temperature can amplify multi-centennial climate variability regionally and globally, suggesting that a dynamic Antarctic Ice Sheet may have driven climate fluctuations during the Holocene. We analysed high-temporal-resolution records of iceberg-rafted debris derived from the Antarctic Ice Sheet, and performed both high-spatial-resolution ice-sheet modelling of the Antarctic Ice Sheet and multi-millennial global climate model simulations. Ice-sheet responses to decadal-scale ocean forcing appear to be less important, possibly indicating that the future response of the Antarctic Ice Sheet will be governed more by long-term anthropogenic warming combined with multi-centennial natural variability than by annual or decadal climate oscillations.

Description: Climate change in Siberia is currently receiving a lot of attention because large permafrost-covered areas could provide a strong positive feedback to global warming through the release of carbon that has been sequestered there on glacial–interglacial timescales. Geological evidence and climate model experiments show that the Siberian region also played an exceptional role during glacial periods. The region that is currently known for its harsh cold climate did not experience major glaciations during the last ice age, including its severest stages around the Last Glacial Maximum (LGM). On the contrary, it is thought that glacial summer temperatures were comparable to the present day. However, evidence of glaciation has been found for several older glacial periods. We combine LGM experiments from the second and third phases of the Paleoclimate Modelling Intercomparison Project (PMIP2 and PMIP3) with sensitivity experiments using the Community Earth System Model (CESM). Together, these climate model experiments reveal that the intermodel spread in LGM summer temperatures in Siberia is much larger than in any other region of the globe and suggest that temperatures in Siberia are highly susceptible to changes in the imposed glacial boundary conditions, the included feedbacks and processes, and to the model physics of the different components of the climate model. We find that changes in the circumpolar atmospheric stationary wave pattern and associated northward heat transport drive strong local snow and vegetation feedbacks and that this combination explains the susceptibility of LGM summer temperatures in Siberia. This suggests that a small difference between two glacial periods in terms of climate, ice buildup or their respective evolution towards maximum glacial conditions can lead to strongly divergent summer temperatures in Siberia, allowing for the buildup of an ice sheet during some glacial periods, while during others, above-freezing summer temperatures preclude a multi-year snowpack from forming.

Description: The last extended time period when climate may have been warmer than today was during the Last Interglacial (LIG; ca. 129 to 120 thousand years ago). However, a global view of LIG precipitation is lacking. Here, seven new LIG climate models are compared to the first global database of proxies for LIG precipitation. In this way, models are assessed in their ability to capture important hydroclimatic processes during a different climate. The models can reproduce the proxy-based positive precipitation anomalies from the preindustrial period over much of the boreal continents. Over the Southern Hemisphere, proxy-model agreement is partial. In models, LIG boreal monsoons have 42% wider area than in the preindustrial and produce 55% more precipitation and 50% more extreme precipitation. Austral monsoons are weaker. The mechanisms behind these changes are consistent with stronger summer radiative forcing over boreal high latitudes and with the associated higher temperatures during the LIG.

Description: Paleoclimate proxy records from the North Atlantic region reveal substantially greater multicentennial temperature variability during the Last Glacial Maximum (LGM) compared to the current interglacial. As there was no obvious change in external forcing, causes for the increased variability remain unknown. Exploiting LGM simulations with a comprehensive coupled climate model along with high-resolution proxy records, we introduce an oscillatory mode of multicentennial variability, which is associated with moderate variations in the Atlantic meridional overturning circulation and depends on the large-scale salinity distribution. This self-sustained mode is amplified by sea-ice feedbacks and induces maximum surface temperature variability in the subpolar North Atlantic region. Characterized by a distinct climatic imprint and different dynamics, the multicentennial oscillation has to be distinguished from Dansgaard-Oeschger variability and emerges only under full LGM climate forcing. The potential of multicentennial modes of variability to emerge or disappear in response to changing climate forcing may have implications for future climate change.

Description: The Last Interglacial climatic optimum, ca. 128 ka, is the most recent climate interval significantly warmer than present, providing an analogue (albeit imperfect) for ongoing global warming and the effects of Greenland Ice Sheet (GIS) melting on climate over the coming millennium. While some climate models predict an Atlantic meridional overturning circulation (AMOC) strengthening in response to GIS melting, others simulate weakening, leading to cooling in Europe. Here, we present evidence from new proxy-based paleoclimate and ocean circulation reconstructions that show that the strongest warming in western Europe coincided with maximum GIS meltwater runoff and a weaker AMOC early in the Last Interglacial. By performing a series of climate model sensitivity experiments, including enhanced GIS melting, we were able to simulate this configuration of the Last Interglacial climate system and infer information on AMOC slowdown and related climate effects. These experiments suggest that GIS melt inhibited deep convection off the southern coast of Greenland, cooling local climate and reducing AMOC by ∼24% of its present strength. However, GIS melt did not perturb overturning in the Nordic Seas, leaving heat transport to, and thereby temperatures in, Europe unaffected.