Dunia H. Urrego

ENSO Drove 2500-Year Collapse of Eastern Pacific Coral Reefs

A Long Collapse Coral reefs are threatened by global warming and ocean acidification, and so it is important to understand better how and why environmental changes have affected them in the past. Toth et al. (p. 81) present a 6000-year-long record of coral reefs off the coast of Panama, Central America. The reefs effectively stopped growing for approximately 2600 years, beginning around 4000 years ago. This collapse of the coral reef system was probably caused by increased variability of ENSO, the El Nino–Southern Oscillation. If the strength or frequency of ENSO were to increase, the viability of these and other reef systems in the Pacific could be put further at risk. A 6000-year record captures the influence of the El Niño–Southern Oscillation on coral reefs off the coast of Panama. Cores of coral reef frameworks along an upwelling gradient in Panamá show that reef ecosystems in the tropical eastern Pacific collapsed for 2500 years, representing as much as 40% of their history, beginning about 4000 years ago. The principal cause of this millennial-scale hiatus in reef growth was increased variability of the El Niño–Southern Oscillation (ENSO) and its coupling with the Intertropical Convergence Zone. The hiatus was a Pacific-wide phenomenon with an underlying climatology similar to probable scenarios for the next century. Global climate change is probably driving eastern Pacific reefs toward another regional collapse.

Orbital-scale climate forcing of grassland burning in southern Africa.

Although grassland and savanna occupy only a quarter of the worlds vegetation, burning in these ecosystems accounts for roughly half the global carbon emissions from fire. However, the processes that govern changes in grassland burning are poorly understood, particularly on time scales beyond satellite records. We analyzed microcharcoal, sediments, and geochemistry in a high-resolution marine sediment core off Namibia to identify the processes that have controlled biomass burning in southern African grassland ecosystems under large, multimillennial-scale climate changes. Six fire cycles occurred during the past 170,000 y in southern Africa that correspond both in timing and magnitude to the precessional forcing of north–south shifts in the Intertropical Convergence Zone. Contrary to the conventional expectation that fire increases with higher temperatures and increased drought, we found that wetter and cooler climates cause increased burning in the study region, owing to a shift in rainfall amount and seasonality (and thus vegetation flammability). We also show that charcoal morphology (i.e., the particles length-to-width ratio) can be used to reconstruct changes in fire activity as well as biome shifts over time. Our results provide essential context for understanding current and future grassland-fire dynamics and their associated carbon emissions.

Millennial-Scale Ecological Changes in Tropical South America Since the Last Glacial Maximum

An analysis of rates of ecological change (RoC) from thirteen pollen records from tropical South America is presented here. The analysis aims to identify the periods of fastest change since the last glacial maximum (LGM) and possible driving mechanisms. Despite rapid cooling periods, region-wide profound droughts, fire and human disturbances, RoC analysis showed that the speed of these climate changes never exceed the species response capabilities. Our results legitimize concerns regarding the resilience of species to accommodate future change and emphasize the urgency for integrative environmental measures.

Vegetation and Fire at the Last Glacial Maximum in Tropical South America

This chapter aims to review current knowledge of the key vegetation types, and their composition, structure, distribution, and fire regime across the South American tropics during the global Last Glacial Maximum ca. 21,000 cal yr BP (calendar years before present). We do this by synthesising previously published Last Glacial Maximum fossil pollen and charcoal data as well as Last Glacial Maximum vegetation model simulations, in comparison with ecoregion/biome maps of present day vegetation. Both model simulations and empirical data suggest that there were no large-scale differences in major biome distributions between the Last Glacial Maximum and present (notwithstanding the Atlantic forests of SE Brazil), with biome shifts largely associated with ecotonal areas – downslope expansion of montane grasslands in the Andes at the expense of montane forest, and savanna expansion at the expense of rainforest and gallery forest at the Amazon basin margins. However, species composition and structure of these Last Glacial Maximum forests was quite different from those of today. At the Last Glacial Maximum, pollen data show that montane Andean taxa descended into the lowlands to form novel non-analogue forest communities with lowland Amazonian taxa, whilst vegetation model simulations show that carbon limitation caused by low atmospheric CO2 likely produced forest communities with reduced canopy density and hence lower biomass than present-day forests. These pollen data-model comparisons show that although Amazonia was probably still dominated by closed forest at the Last Glacial Maximum, its carbon store may have been only 50% of present. Most charcoal records show reduced burning during the Last Glacial Maximum compared with today, most likely due to the significantly colder temperatures.

Accuracy and Consistency of Grass Pollen Identification by Human Analysts Using Electron Micrographs of Surface Ornamentation

Premise of the study: Humans frequently identify pollen grains at a taxonomic rank above species. Grass pollen is a classic case of this situation, which has led to the development of computational methods for identifying grass pollen species. This paper aims to provide context for these computational methods by quantifying the accuracy and consistency of human identification. Methods: We measured the ability of nine human analysts to identify 12 species of grass pollen using scanning electron microscopy images. These are the same images that were used in computational identifications. We have measured the coverage, accuracy, and consistency of each analyst, and investigated their ability to recognize duplicate images. Results: Coverage ranged from 87.5% to 100%. Mean identification accuracy ranged from 46.67% to 87.5%. The identification consistency of each analyst ranged from 32.5% to 87.5%, and each of the nine analysts produced considerably different identification schemes. The proportion of duplicate image pairs that were missed ranged from 6.25% to 58.33%. Discussion: The identification errors made by each analyst, which result in a decline in accuracy and consistency, are likely related to psychological factors such as the limited capacity of human memory, fatigue and boredom, recency effects, and positivity bias.

Holocene land–sea climatic links on the equatorial Pacific coast (Bay of Guayaquil, Ecuador)

We analyzed the pollen content of a marine core located near the Bay of Guayaquil in Ecuador to document the link between sea surface temperatures (SSTs) and changes in rainfall regimes on the adjacent continent during the Holocene. Based on the expansion/regression of five vegetation types, we observe three successive climatic patterns. In the first phase, between 11,700 and 7700 cal. yr BP, the presence of a cloud (Andean) forest in the mid altitudes and mangroves in the estuary of the Guayas basin, were associated with a maximum in boreal summer insolation, a northernmost position of the Intertropical Convergence Zone (ITCZ), a land–sea thermal contrast, cloud dripping, and dry edaphic conditions. Between 7700 and 2850 cal. yr BP, the expansion of the coastal vegetation and the regression of the mangrove indicate a drier climate with weak ITCZ and low El Niño Southern Oscillation (ENSO) variability while austral summer insolation gradually increased. The interval between 4200 and 2850 cal. yr BP was marked by the coolest and driest climatic conditions of the Holocene because of the weak influence of the ITCZ and a strengthening of the Humboldt Current. After 2850 cal. yr BP, high variability and amplitude of the Andean forest changes occurred when ENSO frequency and amplitude increased, indicating high variability in land–sea connections. The ITCZ reached the latitude of Guayaquil only after 2500 cal. yr BP inducing the bimodal precipitation regime we observe today. Our study shows that besides insolation, the ITCZ position, and ENSO frequency, changes in eastern equatorial Pacific SSTs play a major role in determining the composition of the ecosystems and the hydrological cycle of the Ecuadorian Pacific coast and the Western Cordillera in Ecuador.

The legacy of 4,500 years of polyculture agroforestry in the eastern Amazon

The legacy of pre-Columbian land use in the Amazonian rainforest is one of the most controversial topics in the social1–10 and natural sciences11,12. Until now, the debate has been limited to discipline-specific studies, based purely on archaeological data8, modern vegetation13, modern ethnographic data3 or a limited integration of archaeological and palaeoecological data12. The lack of integrated studies to connect past land use with modern vegetation has left questions about the legacy of pre-Columbian land use on the modern vegetation composition in the Amazon, unanswered11. Here, we show that persistent anthropogenic landscapes for the past 4,500 years have had an enduring legacy on the hyperdominance of edible plants in modern forests in the eastern Amazon. We found an abrupt enrichment of edible plant species in fossil lake and terrestrial records associated with pre-Columbian occupation. Our results demonstrate that, through closed-canopy forest enrichment, limited clearing for crop cultivation and low-severity fire management, long-term food security was attained despite climate and social changes. Our results suggest that, in the eastern Amazon, the subsistence basis for the development of complex societies began ~4,500 years ago with the adoption of polyculture agroforestry, combining the cultivation of multiple annual crops with the progressive enrichment of edible forest species and the exploitation of aquatic resources. This subsistence strategy intensified with the later development of Amazonian dark earths, enabling the expansion of maize cultivation to the Belterra Plateau, providing a food production system that sustained growing human populations in the eastern Amazon. Furthermore, these millennial-scale polyculture agroforestry systems have an enduring legacy on the hyperdominance of edible plants in modern forests in the eastern Amazon. Together, our data provide a long-term example of past anthropogenic land use that can inform management and conservation efforts in modern Amazonian ecosystems.Fossil records suggest that the Amazon rainforest in the pre-Columbian era was home to polyculture agroforestry, with multiple annual crops providing subsistence for indigenous groups who shaped the Amazon as early as 4,500 years ago.

Pollen from the Deep-Sea: A Breakthrough in the Mystery of the Ice Ages

Pollen from deep-sea sedimentary sequences provides an integrated regional reconstruction of vegetation and climate (temperature, precipitation, and seasonality) on the adjacent continent. More importantly, the direct correlation of pollen, marine and ice indicators allows comparison of the atmospheric climatic changes that have affected the continent with the response of the Earth’s other reservoirs, i.e., the oceans and cryosphere, without any chronological uncertainty. The study of long continuous pollen records from the European margin has revealed a changing and complex interplay between European climate, North Atlantic sea surface temperatures (SSTs), ice growth and decay, and high- and low-latitude forcing at orbital and millennial timescales. These records have shown that the amplitude of the last five terrestrial interglacials was similar above 40°N, while below 40°N their magnitude differed due to precession-modulated changes in seasonality and, particularly, winter precipitation. These records also showed that vegetation response was in dynamic equilibrium with rapid climate changes such as the Dangaard-Oeschger (D-O) cycles and Heinrich events, similar in magnitude and velocity to the ongoing global warming. However, the magnitude of the millennial-scale warming events of the last glacial period was regionally-specific. Precession seems to have imprinted regions below 40°N while obliquity, which controls average annual temperature, probably mediated the impact of D-O warming events above 40°N. A decoupling between high- and low-latitude climate was also observed within last glacial warm (Greenland interstadials) and cold phases (Greenland stadials). The synchronous response of western European vegetation/climate and eastern North Atlantic SSTs to D-O cycles was not a pervasive feature throughout the Quaternary. During periods of ice growth such as MIS 5a/4, MIS 11c/b and MIS 19c/b, repeated millennial-scale cold-air/warm-sea decoupling events occurred on the European margin superimposed to a long-term air-sea decoupling trend. Strong air-sea thermal contrasts promoted the production of water vapor that was then transported northward by the westerlies and fed ice sheets. This interaction between long-term and shorter time-scale climatic variability may have amplified insolation decreases and thus explain the Ice Ages. This hypothesis should be tested by the integration of stochastic processes in Earth models of intermediate complexity.

New Insights From Pre-Columbian Land Use and Fire Management in Amazonian Dark Earth Forests

Anthropogenic climate change driven by increased carbon emissions is leading to more severe fire seasons and increasing the frequency of mega-fires in the Amazon. This has the potential to convert Amazon forests from net carbon sinks to net carbon sources. Although modern human influence over the Earth is substantial, debate remains over when humans began to dominate Earth’s natural systems. To date, little is known about the history of human land use in key regions like the Amazon. Here, we examine the history of human occupation from a ~8,500 year-old sediment core record from Lake Carana (LC) in the eastern Amazon. The onset of pre-Columbian activity at LC (~4,500 cal yr B.P.) is associated with the beginning of fire management and crop cultivation, later followed by the formation of Amazonian Dark Earth soils (ADEs) ~2,000 cal yr B.P. Selective forest enrichment of edible plants and low-severity fire activity altered the composition and structure of forests growing on ADEs (ADE forests) making them more drought susceptible and fire-prone. Following European colonization (1661 A.D.), the Amazon rubber boom (mid-1800s to 1920 A.D.) is associated with record-low fire activity despite drier regional climate, indicating fire exclusion. The formation of FLONA Reserve in 1974 A.D. is accompanied by the relocation of traditional populations and a fire suppression policy. Despite suppression efforts, biomass burning and fire severity in the past decade is higher than any other period in the record. This is attributed to combined climate and human factors which create optimal conditions for mega-fires in ADE forests and threatens to transform the Amazon from a net carbon sink to a net carbon source. To help mitigate the occurrence of mega-fires, a fire management policy reducing fire-use and careful fire management for farming may help to reduce fuel loads and the occurrence of mega-fires in fire-prone ADE forests. As both natural and anthropogenic pressures are projected to increase in the Amazon, this study provides valuable insights into the legacy of past human land use on modern ADE forest composition, structure, and flammability that can inform ecological benchmarks and future management efforts in the eastern Amazon.

Holocene book review: La Cordillera Oriental Colombiana Transecto Sumapaz

Modelling earth surface processes, by which we generally understand the geomorphological action of the agents of ice, water, air and soil, is not a mature subject. The grand synthetic descriptions of landform assemblages enunciated in the first half of the last century remain valid targets for modellers, even while satellites gather terabytes of data. Whether these are the right data often remains the key issue. More precisely: do these data define the ‘state’ of the system sufficiently well to make predictions, in the same way as the measurements of atmospheric pressure are used to reset the state of the atmosphere every six hours in numerical weather prediction? The answer to the last question is a qualified ‘not yet’. The degree to which this is true in the disparate areas comprising quantitative modelling of earth surface processes circumscribes the problem for a textbook writer. Quantitative modeling of earth surface processes essentially comprises a series of case studies of the application of techniques to these different areas. These case studies are illustrative and quite fun, but the book is not a highly lengthened review article of the state of the art in different subject areas and it cannot really be considered a reference book. The book comprises 221 pages of main text, eight chapters, seven appendices containing C-code for particular problems and 12 pages of references. The eight chapters cover a brief introduction to earth surface systems (fluvial, aeolian and glacial); the diffusion equation, which has fundamental relevance to many mass transport systems; flow routing, which is the direction of flows by a DEM; the advection equation, which here refers to the advection of knick points in bedrock erosion; flexural isostasy, as a response to erosion; non-Newtonian flows, with applications to glaciology and to thrust sheets; instabilities, with applications to dunes, drumlins and meanders; and stochastic processes, with a diversity of applications. The purpose of including the C-code in the appendices seems questionable, especially as the code is easily available electronically and is not analysed line-by-line in the text. The programs call Numerical Recipes (Press et al., 1992) and provide no graphics capabilities themselves. I suspect anyone who will be able to overcome these problems would probably want to write the algorithms as well. Who might use this book? Some of the chapters, for example the flow routing, the diffusion equation and stochastic processes chapters, could be used in a final year undergraduate course. Others are restatements of the author’s research work and will be useful as starting points for graduate researchers. However, in summary, the book veers too much towards a research monograph rather than a reference book for me to really enthusiastically recommend it as a teaching resource. For researchers, the material is a little selective, and whether they find that idea or reference to a key idea is likely to be a bit hit and miss.