A recent temperature reconstruction of global annual temperature shows Early Holocene warmth followed by a cooling trend through the Middle to Late Holocene [Marcott SA, et al., 2013, Science 339(6124):1198-1201. This global cooling is puzzling because it is opposite from the expected and simulated global warming trend due to the retreating ice sheets and rising atmospheric greenhouse gases. Our critical reexamination of this contradiction between the reconstructed cooling and the simulated warming points to potentially significant biases in both the seasonality of the proxy reconstruction and the climate sensitivity of current climate models.global temperature | Holocene temperature | model-data inconsistency I n the latest reconstruction of the global surface temperature throughout the Holocene (1) (hereafter M13), the most striking feature is a pronounced cooling trend of ∼0.5°C following the Holocene Thermal Maximum (HTM) (∼10-6 ka) toward the late Holocene, with the Neoglacial cooling culminating in the Little Ice Age (LIA; ∼1,800 common era) (Fig. 1, blue). Numerous previous reconstructions have shown cooling trends in the Holocene, but most of these studies attribute the cooling trend to regional and/or seasonal climate changes (2-6). The distinct feature of the M13 reconstruction is that it arguably infers the cooling trend in the global mean and annual mean temperature. This inferred global annual cooling in the Holocene is puzzling: With no direct net contribution from the orbital insolation, the global annual mean radiative forcing in the Holocene should be dominated by the retreating ice sheets and rising atmospheric greenhouse gases (GHGs), with both favoring a globally averaged warming. Therefore, how can the global annual temperature exhibit a cooling trend in response to global warming forcing? This inconsistency between the reconstructed cooling and the inferred warming forced by GHGs and ice sheet poses the so-called Holocene temperature conundrum and will be the subject of this study. Here, we study the global annual temperature trend in the Holocene and its physical mechanism by comparing the temperature reconstruction with three different transient climate model simulations. Our analysis shows a robust warming trend in current climate models, opposite from the cooling in the M13 reconstruction. This model-data discrepancy suggests potentially significant biases in both the reconstructions and current climate models, and calls for a major reexamination of global climate evolution in the Holocene. Model ExperimentsWe analyzed transient climate simulations in three coupled ocean-atmosphere models [Community Climate System Model 3 (CCSM3) (7), Fast Met Office/UK Universities Simulator (FAMOUS) (8), and Loch-Vecode-Ecbilt-Clio-Agism Model (LOVECLIM) (9); Methods] that are subject to realistic climate forcings of orbitally driven insolation variations, GHGs, continental ice sheets, and the associated meltwater fluxes. The three models all simulate a robust annual mean warming (∼0.5°C) throughout ...

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Glaciers in the tropical Andes have been retreating for the past several decades, leading to a temporary increase in dry season water supply downstream. Projected future glacier shrinkage, however, will lead to a long-term reduction in dry season river discharge from glacierized catchments. This glacier retreat is closely related to the observed increase in high-elevation, surface air temperature in the region. Future projections using a simple freezing level height-equilibrium-line altitude scaling approach suggest that glaciers in the inner tropics, such as Antizana in Ecuador, may be most vulnerable to future warming while glaciers in the more arid outer tropics, such as Zongo in Bolivia, may persist, albeit in a smaller size, throughout the 21st century regardless of emission scenario. Nonetheless many uncertainties persist, most notably problems with accurate snowfall measurements in the glacier accumulation zone, uncertainties in establishing accurate thickness measurements on glaciers, unknown future changes associated with local-scale circulation and cloud cover affecting glacier energy balance, the role of aerosols and in particular black carbon deposition on Andean glaciers, and the role of groundwater and aquifers interacting with glacier meltwater.The reduction in water supply for export-oriented agriculture, mining, hydropower production and human consumption are the most commonly discussed concerns associated with glacier retreat, but many other aspects including glacial hazards, tourism and recreation, and ecosystem integrity are also affected by glacier retreat. Social and political problems surrounding water allocation for subsistence farming have led to conflicts due to lack of adequate water governance. Local water management practices in many regions reflect cultural belief systems, perceptions and spiritual values and glacier retreat in some places is seen as a threat to these local livelihoods.Comprehensive adaptation strategies, if they are to be successful, therefore need to consider science, policy, culture and practice, and involve local populations. Planning needs to be based not only on future scenarios derived from physically-based numerical models, but must also consider societal needs, economic agendas, political conflicts, socioeconomic inequality and cultural values. This review elaborates on the need for adaptation as well as the challenges and constraints many adaptation projects are faced with, and lays out future directions where opportunities exist to develop successful, culturally acceptable and sustainable adaptation strategies.

Abstract. In this manuscript we have attempted to consolidate the common signal in previously defined proxy reconstructions of the El Niño-Southern Oscillation into one individual proxy titled the Unified ENSO Proxy (UEP). While correlating well with the majority of input reconstructions, the UEP provides better representation of observed indices of ENSO, discrete ENSO events and documented historical chronologies of ENSO than any of these input ENSO reconstructions. Further to this, the UEP also provides a means to reconstruct the PDO/IPO multi-decadal variability of the Pacific Ocean as the low-pass filtered UEP displays multidecadal variability that is consistent with the 20th century variability of the PDO and IPO. The UEP is then used to describe changes in ENSO variability which have occurred since 1650 focusing on changes in ENSOs variance, multiyear ENSO events, PDO-like multi-decadal variability and the effects of volcanic and solar forcing on ENSO. We find that multi-year El Niño events similar to the 1990-1995 event have occurred several times over the last 3 1/2 centuries. Consistent with earlier studies we find that volcanic forcing can induce a statistically significant change in the mean state of ENSO in the year of the eruption and a doubling of the probability of an El Niño (La Niña) event occurring in the year of (three years after) the eruption.

We compare several statistical routines that may be used to calculate d 18 O sw and SSS from paired coral Sr/Ca and d 18 O measurements. Typically, the d 18 O coral -SST relationship is estimated by linear regression of coral d 18 O vs. SST. If this method is applied, evidence should be given that at a particular site SST and SSS do not co-vary. In the tropical oceans, SST and d 18 O sw (SSS) often co-vary, and this will bias the estimate of the regression slope of d 18 O coral -SST. Using a stochastic model, we show that covariance leads to a bias in the coefficients of the univariate regression equations. As the slope of the d 18 O coral -SST relationship has known, we propose to insert this value for c 1 in the regression models. This requires that the constants of the regression equations are removed. To omit the constants, we propose to center the regression equations (i.e., to remove the mean values from the variables). The statistical error propagation is calculated to assess our ability to resolve past variations in d 18 O sw (SSS). At Tahiti, we find that the combined analytical uncertainties of coral d 18 O and Sr/Ca equal the amplitude of the seasonal cycle of d 18 O sw (SSS). Therefore, we cannot resolve the seasonal cycle of SSS at Tahiti. At Timor, the error of reconstructed d 18 O sw (SSS) is lower than the magnitude of seasonal variations of d 18 O sw (SSS), and the seasonal cycle of d 18 O sw (SSS) can be resolved.

[1] To study the response of the global carbon cycle to a weakening of the Atlantic Meridional Overturning Circulation (AMOC), a series of freshwater perturbation experiments is conducted both under preindustrial and glacial conditions using the earth system model of intermediate complexity LOVECLIM. A shutdown of the AMOC leads to substantial cooling of the North Atlantic, a weak warming of the Southern Hemisphere, intensification of the northeasterly trade winds, and a southward shift of the Intertropical Convergence Zone (ITCZ). Trade wind anomalies change upwelling in the tropical oceans and hence marine productivity. Furthermore, hydrological changes associated with a southward displacement of the ITCZ lead to a reduction of terrestrial carbon stocks mainly in northern Africa and northern South America in agreement with paleoproxy data. In the freshwater perturbation experiments the ocean acts as a sink of CO 2 , primarily through increased solubility. The net atmospheric CO 2 anomaly induced by a shutdown of the AMOC amounts to about +15 ppmv and À10 ppmv for preindustrial and glacial conditions, respectively. This background state dependence can be explained by the fact that the glacial climate is drier and the terrestrial vegetation therefore releases a smaller amount of carbon to the atmosphere. This study demonstrates that the net CO 2 response to large-scale ocean circulation changes has significant contributions both from the terrestrial and marine carbon cycle.

Climate sensitivity reconstructed from eight glacial cycles exhibits state dependence and confirms global warming projections.

The termination of the last ice age (Termination 1; T1) is crucial for our understanding of global climate change and for the validation of climate models. There are still a number of open questions regarding for example the exact timing and the mechanisms involved in the initiation of deglaciation and the subsequent interhemispheric pattern of the warming. Our study is based on a well-dated and high-resolution alkenone-based sea surface temperature (SST) record from the SE-Pacific off southern Chile (Ocean Drilling Project Site 1233) showing that deglacial warming at the northern margin of the Antarctic Circumpolar Current system (ACC) began shortly after 19,000 years BP (19 kyr BP). The timing is largely consistent with Antarctic ice-core records but the initial warming in the SE-Pacific is more abrupt suggesting a direct and immediate response to the slowdown of the Atlantic thermohaline circulation through the bipolar seesaw mechanism. This response requires a rapid transfer of the Atlantic signal to the SE-Pacific without involving the thermal inertia of the Southern Ocean that may contribute to the substantially more gradual deglacial temperature rise seen in Antarctic ice-cores. A very plausible mechanism for this rapid transfer is a seesawinduced change of the coupled ocean-atmosphere system of the ACC and the southern westerly wind belt. In addition, modelling results suggest that insolation changes and the deglacial CO 2 rise induced a substantial SST increase at our site location but with a gradual warming structure. The similarity of the two-step rise in our proxy SSTs and CO 2 over T1 strongly demands for a forcing mechanism influencing both, temperature and CO 2 . As SSTs at our coring site are particularly sensitive to latitudinal shifts of the ACC/southern westerly wind belt system, we conclude that such latitudinal shifts may substantially affect the upwelling of deepwater masses in the Southern Ocean and thus the release of CO 2 to the atmosphere as suggested by the conceptual model of [Toggweiler, J.R., Rusell, J.L., Carson, S.R., 2006. Midlatitude westerlies, atmospheric CO 2 , and climate change during ice ages. Paleoceanography 21.