Human impacts prior to the Industrial Revolution are not well constrained. We investigate whether the decline in global atmospheric CO2 concentration by 7–10 ppm in the late 1500s and early 1600s ...which globally lowered surface air temperatures by 0.15∘C, were generated by natural forcing or were a result of the large-scale depopulation of the Americas after European arrival, subsequent land use change and secondary succession. We quantitatively review the evidence for (i) the pre-Columbian population size, (ii) their per capita land use, (iii) the post-1492 population loss, (iv) the resulting carbon uptake of the abandoned anthropogenic landscapes, and then compare these to potential natural drivers of global carbon declines of 7–10 ppm. From 119 published regional population estimates we calculate a pre-1492 CE population of 60.5 million (interquartile range, IQR 44.8–78.2 million), utilizing 1.04 ha land per capita (IQR 0.98–1.11). European epidemics removed 90% (IQR 87–92%) of the indigenous population over the next century. This resulted in secondary succession of 55.8 Mha (IQR 39.0–78.4 Mha) of abandoned land, sequestering 7.4 Pg C (IQR 4.9–10.8 Pg C), equivalent to a decline in atmospheric CO2 of 3.5 ppm (IQR 2.3–5.1 ppm CO2). Accounting for carbon cycle feedbacks plus LUC outside the Americas gives a total 5 ppm CO2 additional uptake into the land surface in the 1500s compared to the 1400s, 47–67% of the atmospheric CO2 decline. Furthermore, we show that the global carbon budget of the 1500s cannot be balanced until large-scale vegetation regeneration in the Americas is included. The Great Dying of the Indigenous Peoples of the Americas resulted in a human-driven global impact on the Earth System in the two centuries prior to the Industrial Revolution.
•Combines multiple methods estimating pre-Columbian population numbers.•Estimates European arrival in 1492 lead to 56 million deaths by 1600.•Large population reduction led to reforestation of 55.8 Mha and 7.4 Pg C uptake.•1610 atmospheric CO2 drop partly caused by indigenous depopulation of the Americas.•Humans contributed to Earth System changes before the Industrial Revolution.
The Humboldt coastal upwelling system in the eastern South Pacific ocean is one of the most productive marine ecosystems in the world. A weakening of the upwelling activity could lead to severe ...ecological impacts. As coastal upwelling in eastern boundary systems is mainly driven by wind stress, most studies so far have analysed wind patterns change through the 20th and 21st Centuries in order to understand and project the phenomenon under specific forcing scenarios. Mixed results have been reported, and analyses from General Circulation Models have suggested even contradictory trends of wind stress for the Humboldt system. In this study, we analyse the ocean upwelling directly in 13 models contributing to phase 5 of the Coupled Model Intercomparison Project (CMIP5) in both the historical simulations and an extreme climate change scenario (RCP8.5). The upwelling is represented by the upward ocean mass flux, a newly-included variable that represents the vertical water transport. Additionally, wind stress, ocean stratification, Ekman layer depth and thermocline depth were also analysed to explore their interactions with coastal upwelling throughout the period studied. The seasonal cycle of coastal upwelling differs between the Northern and Southern Humboldt areas. At lower latitudes, the upwelling season spans most of the autumn, winter and spring. However, in the Southern Humboldt area the upwelling season takes place in spring and the summertime with downwelling activity in winter. This persists throughout the Historical and RCP8.5 simulations. For both the Northern and Southern Humboldt areas an increasing wind stress is projected. However, different trends of upwelling intensity are observed away from the sea surface. Whereas wind stress will continue controlling the decadal variability of coastal upwelling on the whole ocean column analysed (surface to 300 m depth), an increasing disconnect with upwelling intensity is projected below 100 m depth throughout the 21st Century. This relates to an intensification of ocean stratification under global warming as shown by the sea water temperature profiles. Additionally, a divergence between the Ekman layer and thermocline depths is also evidenced. Given the interaction of upwelled nutrients and microscopic organisms essential for fish growth, a potential decline of coastal upwelling at depth could lead to unknown ecological and socio-economical effects.
Abstract
A key statistic describing climate change impacts is the ‘social cost of carbon dioxide’ (SCCO
2
), the projected cost to society of releasing an additional tonne of CO
2
. Cost-benefit ...integrated assessment models that estimate the SCCO
2
lack robust representations of climate feedbacks, economy feedbacks, and climate extremes. We compare the PAGE-ICE model with the decade older PAGE09 and find that PAGE-ICE yields SCCO
2
values about two times higher, because of its climate and economic updates. Climate feedbacks only account for a relatively minor increase compared to other updates. Extending PAGE-ICE with economy feedbacks demonstrates a manifold increase in the SCCO
2
resulting from an empirically derived estimate of partially persistent economic damages. Both the economy feedbacks and other increases since PAGE09 are almost entirely due to higher damages in the Global South. Including an estimate of interannual temperature variability increases the width of the SCCO
2
distribution, with particularly strong effects in the tails and a slight increase in the mean SCCO
2
. Our results highlight the large impacts of climate change if future adaptation does not exceed historical trends. Robust quantification of climate-economy feedbacks and climate extremes are demonstrated to be essential for estimating the SCCO
2
and its uncertainty.
Changes in inter-ocean gateways caused by tectonic processes have been long considered an important factor in climate evolution on geological timescales. Three major gateway changes that occurred ...during the Late Miocene and Pliocene epochs are the closing of the Central American Seaway (CAS) by the uplift of the Isthmus of Panama, the opening of the Bering Strait, and the closing of a deep channel between New Guinea and the Equator. This study compares the global climatic effects of these changes within the same climate model framework. We find that the closure of the CAS and the opening of the Bering Strait induce the strongest effects on the Atlantic meridional overturning circulation (AMOC). However, these effects potentially compensate, as the closure of the CAS and the opening of the Bering Strait cause similar AMOC changes of around 2 Sv (strengthening and weakening respectively). Previous simulations with an open CAS consistently simulated colder oceanic conditions in the Northern Hemisphere – contrasting with the evidence for warmer sea surface temperatures 10–3 million years ago. Here we argue that this cooling is overestimated because (a) the models typically simulated too strong an AMOC change not yet in equilibrium, (b) used a channel too deep and (c) lacked the compensating effect of the closed Bering Strait – a factor frequently ignored despite its potential influence on northern high latitudes and ice-sheet growth. Further, we discuss how these gateway changes affect various climatic variables from surface temperature and precipitation to ENSO characteristics.
•Opening of Bering Strait cooled the North Atlantic.•Overturning response to Central American Seaway (CAS) previously overestimated.•Opening of Bering Strait could compensate for CAS overturning changes.•New Guinea crossing Equator appears climatically less important.
The Atlantic meridional overturning circulation (AMOC) is a system of ocean currents that has an essential role in Earth's climate, redistributing heat and influencing the carbon cycle
. The AMOC has ...been shown to be weakening in recent years
; this decline may reflect decadal-scale variability in convection in the Labrador Sea, but short observational datasets preclude a longer-term perspective on the modern state and variability of Labrador Sea convection and the AMOC
. Here we provide several lines of palaeo-oceanographic evidence that Labrador Sea deep convection and the AMOC have been anomalously weak over the past 150 years or so (since the end of the Little Ice Age, LIA, approximately AD 1850) compared with the preceding 1,500 years. Our palaeoclimate reconstructions indicate that the transition occurred either as a predominantly abrupt shift towards the end of the LIA, or as a more gradual, continued decline over the past 150 years; this ambiguity probably arises from non-AMOC influences on the various proxies or from the different sensitivities of these proxies to individual components of the AMOC. We suggest that enhanced freshwater fluxes from the Arctic and Nordic seas towards the end of the LIA-sourced from melting glaciers and thickened sea ice that developed earlier in the LIA-weakened Labrador Sea convection and the AMOC. The lack of a subsequent recovery may have resulted from hysteresis or from twentieth-century melting of the Greenland Ice Sheet
. Our results suggest that recent decadal variability in Labrador Sea convection and the AMOC has occurred during an atypical, weak background state. Future work should aim to constrain the roles of internal climate variability and early anthropogenic forcing in the AMOC weakening described here.
The climate deterioration after the most recent African humid period (AHP) is a notable past example of desertification. Evidence points to a human population expansion in northern Africa prior to ...this, associated with the introduction of pastoralism. Here we consider the role, if any, of this population on the subsequent ecological collapse. Using a climate-vegetation model, we estimate the natural length of the most recent AHP. The model indicates that the system was most susceptible to collapse between 7 and 6 ka; at least 500 years before the observed collapse. This suggests that the inclusion of increasing elements of pastoralism was an effective adaptation to the regional environmental changes. Pastoralism also appears to have slowed the deterioration caused by orbitally-driven climate change. This supports the view that modern pastoralism is not only sustainable, but beneficial for the management of the world's dryland environments.
Foraminifera Mg/Ca paleothermometry forms the basis of a substantial portion of ocean temperature reconstruction over the last 5 Ma. Furthermore, coupled Mg/Ca–oxygen isotope (δ18O) measurements of ...benthic foraminifera can constrain eustatic sea level (ESL) independent of paleo-shoreline derived approaches. However, this technique suffers from uncertainty regarding the secular variation of the Mg/Ca seawater ratio (Mg/Casw) on timescales of millions of years. Here we present coupled seawater–test Mg/Ca–temperature laboratory calibrations of Globigerinoides ruber in order to test the widely held assumptions that (1) seawater–test Mg/Ca co-vary linearly, and (2) the Mg/Ca–temperature sensitivity remains constant with changing Mg/Casw. We find a nonlinear Mg/Catest–Mg/Casw relationship and a lowering of the Mg/Ca–temperature sensitivity at lower than modern Mg/Casw from 9.0% °C−1 at Mg/Casw = 5.2 mol mol−1 to 7.5±0.9%°C−1 at 3.4 mol mol−1. Using our calibrations to more accurately calculate the offset between Mg/Ca and biomarker-derived paleotemperatures for four sites, we derive a Pliocene Mg/Casw ratio of ∼4.3 mol mol−1. This Mg/Casw implies Pliocene ocean temperature 0.9–1.9 °C higher than previously reported and, by extension, ESL ∼30 m lower compared to when one assumes that Pliocene Mg/Casw is the same as at present. Correcting existing benthic foraminifera datasets for Mg/Casw indicates that deep water source composition must have changed through time, therefore seawater oxygen isotope reconstructions relative to present day cannot be used to directly reconstruct Pliocene ESL.
•G. ruber calibrations of Mg/Casw–Mg/Catest and Mg/Ca–temperature at Mg/Casw = 3.4.•The Mg/Catest–Mg/Casw–temperature relationship is nonlinear.•We reconstruct Mg/Casw for the last 5 Ma and show Pliocene Mg/Casw was 17% lower.•Pliocene surface and deep ocean temperature have been underestimated by 0.9–1.9 °C.•Pliocene sea level may have been overestimated based on coupled Mg/Ca–δ18O.
El Niño–Southern Oscillation (ENSO) is the strongest mode of interannual climate variability in the current climate, influencing ecosystems, agriculture, and weather systems across the globe, but ...future projections of ENSO frequency and amplitude remain highly uncertain. A comparison of changes in ENSO in a range of past and future climate simulations can provide insights into the sensitivity of ENSO to changes in the mean state, including changes in the seasonality of incoming solar radiation, global average temperatures, and spatial patterns of sea surface temperatures. As a comprehensive set of coupled model simulations is now available for both palaeoclimate time slices (the Last Glacial Maximum, mid-Holocene, and last interglacial) and idealised future warming scenarios (1 % per year CO2 increase, abrupt four-time CO2 increase), this allows a detailed evaluation of ENSO changes in this wide range of climates. Such a comparison can assist in constraining uncertainty in future projections, providing insights into model agreement and the sensitivity of ENSO to a range of factors. The majority of models simulate a consistent weakening of ENSO activity in the last interglacial and mid-Holocene experiments, and there is an ensemble mean reduction of variability in the western equatorial Pacific in the Last Glacial Maximum experiments. Changes in global temperature produce a weaker precipitation response to ENSO in the cold Last Glacial Maximum experiments and an enhanced precipitation response to ENSO in the warm increased CO2 experiments. No consistent relationship between changes in ENSO amplitude and annual cycle was identified across experiments.
The Pliocene warm interval has been difficult to explain. We reconstructed the latitudinal distribution of sea surface temperature around 4 million years ago, during the early Pliocene. Our ...reconstruction shows that the meridional temperature gradient between the equator and subtropics was greatly reduced, implying a vast poleward expansion of the ocean tropical warm pool. Corroborating evidence indicates that the Pacific temperature contrast between the equator and 32°N has evolved from ∼2°C 4 million years ago to ∼8°C today. The meridional warm pool expansion evidently had enormous impacts on the Pliocene climate, including a slowdown of the atmospheric Hadley circulation and El Niño-like conditions in the equatorial region. Ultimately, sustaining a climate state with weak tropical sea surface temperature gradients may require additional mechanisms of ocean heat uptake (such as enhanced ocean vertical mixing).
The mid-Pliocene Warm Period (mPWP, 3.3–3.0 Ma) was characterised by an atmospheric CO2 concentration exceeding 400 ppmv with minor changes in continental and orbital configurations. Simulations of ...this past climate state have improved with newer models but still show some substantial differences from proxy reconstructions. There is little information about atmospheric aerosol concentrations during the Pliocene, but previous work suggests that it could have been quite different from the modern period. Here we apply idealised aerosol scenario experiments to examine the importance of aerosol forcing on mPWP tropical precipitation and the possibility of aerosol uncertainty explaining the mismatch between reconstructions and simulations. The absence of industrial pollutants leads to further warming, especially in the Northern Hemisphere. The Intertropical Convergence Zone (ITCZ) becomes narrower and stronger and shifts northward after removal of anthropogenic aerosols. Though not affecting the location of monsoon domain boundary, removal of anthropogenic aerosol alters the amount of rainfall within the domain, increasing summer rain rate over eastern and southern Asia and western Africa. This work demonstrates that uncertainty in aerosol forcing could be the dominant driver in tropical precipitation changes during the mid-Pliocene: causing larger impacts than the changes in topography and greenhouse gases.
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