Modality of the Tropical Rain Belt Across Models and Simulated Climates

The tropical rain belt varies between unimodal and bimodal meridional precipitation distributions, both regionally and on seasonal to geological time scales. Here we show that this variation is largely driven by equatorial precipitation inhibition, and quantify it using an equatorial modality index (EMI) that varies continuously between 1 and 2 for purely unimodal and bimodal distributions. We show that tropical modality is a fundamental characteristic of tropical climate, which we define as annual-mean EMI. We examine large-scale aspects of tropical modality across 73 climate models from phases 5 and 6 of the Coupled Model Intercomparison Project, 45 paleo simulations (;300 million years ago to present), and observations. We find increased tropical modality to be strongly related to increased width of the tropical rain belt, wider and weaker meridional overturning circulation, colder equatorial cold tongues, and more severe double intertropical convergence zone bias in modern climate models. Tropical sectors (or global zonal means) with low tropical modality are characterized by monsoonal seasonal variations (i.e., seasonal migrations of rainbands following the sun). In sectors with high tropical modality we identify three important seasonal modes: (i) migration of the precipitation distribution toward the warmer hemisphere, (ii) variation in the latitudinal separation between hemispheric rainbands, and (iii) seesaw variation in the intensity of the hemispheric rainbands. In high tropical modality sectors, due to contrasting shifts of the migration and separation modes, counter to general wisdom, seasonal migrations of tropical rainbands cannot be generally assumed to follow the sun.</p

Climatic and tectonic drivers shaped the tropical distribution of coral reefs

Today, warm-water coral reefs are limited to tropical-to-subtropical latitudes. These diverse ecosystems extended further poleward in the geological past, but the mechanisms driving these past distributions remain uncertain. Here, we test the role of climate and palaeogeography in shaping the distribution of coral reefs over geological timescales. To do so, we combine habitat suitability modelling, Earth System modelling and the ~247-million-year geological record of scleractinian coral reefs. A broader latitudinal distribution of climatically suitable habitat persisted throughout much of the Mesozoic–early Paleogene due to an expanded tropical belt and more equable distribution of shallow marine substrate. The earliest Cretaceous might be an exception, with reduced shallow marine substrate during a ‘cold-snap’ interval. Climatically suitable habitat area became increasingly skewed towards the tropics from the late Paleogene, likely steepening the latitudinal biodiversity gradient of reef-associated taxa. This was driven by global cooling and increases in tropical shallow marine substrate resulting from the tectonic evolution of the Indo-Australian Archipelago. Although our results suggest global warming might permit long-term poleward range expansions, coral reef ecosystems are unlikely to keep pace with the rapid rate of anthropogenic climate change

Hydrological and associated biogeochemical consequences of rapid global warming during the Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM) hyperthermal, ~ 56 million years ago (Ma), is the most dramatic example of abrupt Cenozoic global warming. During the PETM surface temperatures increased between 5 and 9 °C and the onset likely took < 20 kyr. The PETM provides a case study of the impacts of rapid global warming on the Earth system, including both hydrological and associated biogeochemical feedbacks, and proxy data from the PETM can provide constraints on changes in warm climate hydrology simulated by general circulation models (GCMs). In this paper, we provide a critical review of biological and geochemical signatures interpreted as direct or indirect indicators of hydrological change at the PETM, explore the importance of adopting multi-proxy approaches, and present a preliminary model-data comparison. Hydrological records complement those of temperature and indicate that the climatic response at the PETM was complex, with significant regional and temporal variability. This is further illustrated by the biogeochemical consequences of inferred changes in hydrology and, in fact, changes in precipitation and the biogeochemical consequences are often conflated in geochemical signatures. There is also strong evidence in many regions for changes in the episodic and/or intra-annual distribution of precipitation that has not widely been considered when comparing proxy data to GCM output. Crucially, GCM simulations indicate that the response of the hydrological cycle to the PETM was heterogeneous – some regions are associated with increased precipitation – evaporation (P – E), whilst others are characterised by a decrease. Interestingly, the majority of proxy data come from the regions where GCMs predict an increase in PETM precipitation. We propose that comparison of hydrological proxies to GCM output can be an important test of model skill, but this will be enhanced by further data from regions of model-simulated aridity and simulation of extreme precipitation events

The Cenozoic history of palms:Global diversification, biogeography and the decline of megathermal forests

10.1111/geb.13436GLOBAL ECOLOGY AND BIOGEOGRAPHY313425-43

Terrestrial environmental change across the onset of the PETM and the associated impact on biomarker proxies:A cautionary tale

The following supplementary information includes one dataset which contains 3 tables: Biomarker distributions and proxies at Cobham, UK Bulk and compound specific isotope data at Cobham (UK) Model-derived mean annual surface temperature and precipitation estimates as a function of CO2 at Cobham (UK)

Simulating the Climate Response to Atmospheric Oxygen Variability in the Phanerozoic

Abstract. The amount of dioxygen (O2) in the atmosphere may have varied from as little as 10 % to as high as 35 % during the Phanerozoic eon (541 Ma–Present). These changes in the amount of O2 are large enough to have lead to changes in atmospheric mass, which may alter the radiative budget of the atmosphere, leading to this mechanism being invoked to explain discrepancies between climate model simulations and proxy reconstructions of past climates. Here we present the first fully 3D numerical model simulations to investigate the climate impacts of changes in O2 during different climate states using the HadGEM3-AO and HadCM3-BL models. We show that simulations with an increase in O2 content result in increased global mean surface air temperature under conditions of a pre-industrial Holocene climate state, in agreement with idealised 1D and 2D modelling studies. We demonstrate the mechanism behind the warming is complex and involves trade-off between a number of factors. Increasing atmospheric O2 leads to a reduction in incident shortwave radiation at Earth's surface due to Rayleigh scattering, a cooling effect. However, there is a competing warming effect due to an increase in the pressure broadening of greenhouse gas absorption lines and dynamical feedbacks, which alter the meridional heat transport of the ocean, warming polar regions and cooling tropical regions. Case studies from past climates are investigated using HadCM3-BL which show that in the warmest climate states, increasing oxygen may lead to a temperature decrease, as the equilibrium climate sensitivity is lower. For the Maastrichtian (72.1–66.0 Ma), increasing oxygen content leads to a better agreement with proxy reconstructions of surface temperature at that time irrespective of the carbon dioxide content. For the Asselian (298.9–295.0 Ma), increasing oxygen content leads to a warmer global mean surface temperature and reduced carbon storage on land, suggesting that high oxygen content may have been a contributing factor in preventing a Snowball Earth during this period of the early Permian. These climate model simulations reconcile the surface temperature response to oxygen content changes across the hierarchy of model complexity and highlight the broad range of Earth system feedbacks that need to be accounted for when considering the climate response to changes in atmospheric oxygen content. </jats:p