Alan M. Haywood

Species-specific responses of Late Quaternary megafauna to climate and humans

Despite decades of research, the roles of climate and humans in driving the dramatic extinctions of large-bodied mammals during the Late Quaternary period remain contentious. Here we use ancient DNA, species distribution models and the human fossil record to elucidate how climate and humans shaped the demographic history of woolly rhinoceros, woolly mammoth, wild horse, reindeer, bison and musk ox. We show that climate has been a major driver of population change over the past 50,000 years. However, each species responds differently to the effects of climatic shifts, habitat redistribution and human encroachment. Although climate change alone can explain the extinction of some species, such as Eurasian musk ox and woolly rhinoceros, a combination of climatic and anthropogenic effects appears to be responsible for the extinction of others, including Eurasian steppe bison and wild horse. We find no genetic signature or any distinctive range dynamics distinguishing extinct from surviving species, emphasizing the challenges associated with predicting future responses of extant mammals to climate and human-mediated habitat change.

The Anthropocene: a new epoch of geological time?

Anthropogenic changes to the Earth’s climate, land, oceans and biosphere are now so great and so rapid that the concept of a new geological epoch defined by the action of humans, the Anthropocene, is widely and seriously debated. Questions of the scale, magnitude and significance of this environmental change, particularly in the context of the Earth’s geological history, provide the basis for this Theme Issue. The Anthropocene, on current evidence, seems to show global change consistent with the suggestion that an epoch-scale boundary has been crossed within the last two centuries.

Modelling Pliocene warmth: contribution of atmosphere, oceans and cryosphere

The relative role of the atmosphere, oceans and cryosphere in contributing towards middle Pliocene warmth (ca 3 Ma BP) is investigated using the HadCM3 coupled ocean-atmosphere general circulation model. The model was initialised with boundary conditions from the USGS PRISM2 data set and a Pliocene atmospheric CO2 level of 400 ppmv and run for 300 simulated years. The simulation resulted in a global surface temperature warming of 3degreesC compared to present-day. In contrast to earlier modelling experiments for the Pliocene, surface temperatures warmed in most areas including the tropics (1-5degreesC). Compared with present-day, the model predicts a general pattern of ocean warming (1-5degreesC) in both hemispheres to a depth of 2000 m, below which no significant differences are noted. Sea ice coverage is massively reduced (up to 90%). The flow of the Gulf Stream/North Atlantic Drift is up to 100 mm s(-1) greater in the Pliocene case. Analysis of the model-predicted meridional streamfunction suggests a global pattern of reduced outflow of Antarctic bottom water (AABW; up to 5 Sv), a shallower depth for North Atlantic deep water formation and weaker thermohaline circulation (3 Sv). The decrease in AABW occurs mainly in the Pacific rather than Atlantic Ocean. Model diagnostics for heat transports indicate that neither the oceans nor the atmosphere are transporting significantly more heat in the Pliocene scenario. Rather, these results indicate that the major contributing mechanism to global Pliocene warmth was the reduced extent of high-latitude terrestrial ice sheets (50% reduction on Greenland, 33% reduction on Antarctica) and sea ice cover resulting in a strong ice-albedo feedback. These results highlight the need for further studies designed to improve our knowledge regarding Pliocene terrestrial ice configurations.

Deep-time Perspectives on Climate Change: Marrying the Signal from Computer Models and Biological Proxies

This book unites climate modelling, palaeoceanography and palaeontology to address fundamental events in the climate history of Earth over the past 600 million years. Understanding the ‘tipping points’ that have led to rapid changes in the Earths climate is vitally important with the realization that humans modify global climate. In an effort to better understand past and future climate change, general circulation models have become the forerunners of attempts to simulate future climate. Although extraordinarily sophisticated, they remain imperfect tools that require ‘grounding’ in geological data. In this, the study of past major climate transitions like the Palaeozoic icehouse worlds and the extreme greenhouse of the Cretaceous are invaluable. Both the mechanisms that forced changes in the Earths climate as well as the proxies that track these changes are discussed. The central message of the book is that general circulation models tested with geological data in an iterative ‘ground truth’ process provide the best estimates of the Earths ancient climate.

Late Pliocene Greenland glaciation controlled by a decline in atmospheric CO2 levels

It is thought that the Northern Hemisphere experienced only ephemeral glaciations from the Late Eocene to the Early Pliocene epochs (about 38 to 4 million years ago), and that the onset of extensive glaciations did not occur until about 3 million years ago. Several hypotheses have been proposed to explain this increase in Northern Hemisphere glaciation during the Late Pliocene. Here we use a fully coupled atmosphere–ocean general circulation model and an ice-sheet model to assess the impact of the proposed driving mechanisms for glaciation and the influence of orbital variations on the development of the Greenland ice sheet in particular. We find that Greenland glaciation is mainly controlled by a decrease in atmospheric carbon dioxide during the Late Pliocene. By contrast, our model results suggest that climatic shifts associated with the tectonically driven closure of the Panama seaway, with the termination of a permanent El Niño state or with tectonic uplift are not large enough to contribute significantly to the growth of the Greenland ice sheet; moreover, we find that none of these processes acted as a priming mechanism for glacial inception triggered by variations in the Earth’s orbit.

Global Scale palaeoclimate reconstruction of the Middle Pliocene climate using the UKMO GCM: Initial Results

Abstract This paper outlines the results of a new climate modelling study for the middle Pliocene (ca. 3 Ma). The work was carried out using the UK Meteorological Office GCM (UKMO, Version 3.0) which is a grid point model using a grid of 2.5° in latitude by 3.75° in longitude. The model utilised the new PRISM2 2°×2° enhanced data set of boundary conditions for the middle Pliocene supplied by the U.S. Geological Surveys Pliocene Interpretations and Synoptic Mapping Group (PRISM). By comparison with the present, the model simulation predicted a 1.9°C annual mean warming over the globe. Warming was at its greatest in high latitudes and resulted in a reduced equator-to-pole temperature gradient of 6°C and a reduction in the general circulation of the atmosphere. Annual mean values for total precipitation (mm/day) increased by 6% with a minor increase in the high cloud component. In low and equatorial regions, temperature decreases by a maximum of 8°C (e.g. over East Africa), whilst precipitation increases, associated with a broadening of the Hadley Cell, promoted greater evaporation and further cooling. Comparison of our results with previous modelling studies, which have examined the character of the middle Pliocene climate, reveal a broad pattern of agreement between the models, but with significant differences observed in detail. These differences are caused by: (a) the greater spatial resolution and different physical parameterisations used in the UKMO GCM compared to other models and (b) variations in the PRISM2 data set compared to the original 8°×10°, or the PRISM1 2°×2° data set used in earlier modelling studies. Comparison of the simulated palaeoclimate to available geological data shows a broad accordance. However, disparities do exist when examining the apparent strengthening of upwelling suggested by some geological data, against the simulated reduction in strength of the general circulation of the atmosphere predicted by the UKMO GCM. Furthermore, the well-documented uncertainty of the geological record concerning the extent of the East Antarctic ice sheet during the middle Pliocene makes the overall accuracy of the imposed PRISM2 boundary conditions, and hence the predictions of the UKMO GCM, difficult to truly assess.

A permanent El Niño–like state during the Pliocene?

The Pliocene may have been characterized by permanent El Nino–like conditions. Initial modeling studies suggest that this may have contributed to Pliocene warmth. The termination of this state may have influenced Northern Hemisphere glaciation (NHG). We use the Hadley Centre Coupled Model version 3 to examine the role of the oceans and ocean structure on Pliocene warmth. A permanent El Nino–like state is not predicted. Annual mean sea surface temperatures in the eastern equatorial Pacific at Ocean Drilling Program Sites 847 and 851 increase by 1.71°C and 1.15°C, respectively. However, El Nino Southern Oscillation events are clearly expressed by the model. Sensitivity tests indicate that a prescribed permanent El Nino–like condition increases annual global mean surface temperatures by a maximum of 0.6°C. If the Pliocene was characterized by such a condition, it is questionable that it provided a major contribution to global warmth and therefore unlikely that the termination of this state contributed significantly to the onset of NHG.

Regional warming: Pliocene (3 Ma) paleoclimate of Europe and the Mediterranean

The paleoclimate of the middle Pliocene (ca. 3 Ma) was generally warmer than present, particularly at middle to high latitudes. It has been suggested that this period may represent an analogue for future climate change. Mechanisms that have been proposed to account for this warming are enhanced thermohaline circulation and/or greater concentrations of CO 2 in the atmosphere. We present new climate model simulations, supported by geological data that suggest that in the European and Mediterranean region the climate was warmer (by 5 °C), wetter (by 400–1000 mm/yr), and less seasonal than present. Modeling results suggest that an intensification of the Icelandic low-pressure system and the Azores high-pressure system occurred during the middle Pliocene as a direct result of higher annual sea-surface temperatures and reduced ice cover in the Northern Hemisphere. This change increased the surface pressure gradient over the region and strengthened annual westerly wind velocity by 4 m/s −1 . Associated increases in wind stress (by 20 N/m −2 ) over the North Atlantic Ocean may have enhanced the flow of surface currents such as the Gulf Stream and North Atlantic Current, which in turn would have sustained the higher sea-surface temperatures. These changes to the regional climate system would have driven greater atmospheric and oceanic transport of heat from equatorial regions to the North Atlantic Ocean, particularly during winter. Such conditions are proposed as a likely cause of the warming indicated for Europe and the Mediterranean ca. 3 Ma.

Sea Surface Temperature of the mid-Piacenzian Ocean: A Data-Model Comparison

The mid-Piacenzian climate represents the most geologically recent interval of long-term average warmth relative to the last million years, and shares similarities with the climate projected for the end of the 21st century. As such, it represents a natural experiment from which we can gain insight into potential climate change impacts, enabling more informed policy decisions for mitigation and adaptation. Here, we present the first systematic comparison of Pliocene sea surface temperature (SST) between an ensemble of eight climate model simulations produced as part of PlioMIP (Pliocene Model Intercomparison Project) with the PRISM (Pliocene Research, Interpretation and Synoptic Mapping) Project mean annual SST field. Our results highlight key regional and dynamic situations where there is discord between the palaeoenvironmental reconstruction and the climate model simulations. These differences have led to improved strategies for both experimental design and temporal refinement of the palaeoenvironmental reconstruction.

CO2-driven ocean circulation changes as an amplifier of Paleocene-Eocene thermal maximum hydrate destabilization

Changes in ocean circulation have been proposed as a trigger mechanism for the large coupled climate and carbon cycle perturbations at the Paleocene-Eocene Thermal Maximum (PETM, ca. 55 Ma). An abrupt warming of oceanic intermediate waters could have initiated the thermal destabilization of sediment-hosted methane gas hydrates and potentially triggered sediment slumps and slides. In an ensemble of fully coupled atmosphere-ocean general circulation model (AOGCM) simulations of the late Paleocene and early Eocene, we identify such a circulation-driven enhanced intermediate-water warming. Critically, we find an approximate twofold amplification of Atlantic intermediate-water warming when CO2 levels are doubled from 2x to 4x preindustrial CO2 compared to when they are doubled from 1x to 2x. This warming is largely focused on the equatorial and South Atlantic and is driven by a significant reduction in deep-water formation from the Southern Ocean. This scenario is consistent with altered PETM circulation patterns inferred from benthic carbon isotope data and the intensity of deep-sea carbonate dissolution in the South Atlantic. The linkage between intermediate-water warming and gas hydrate destabilization could provide an important feedback in the establishment of peak PETM warmth.