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Climate of the past

Geosciences Column: How climate change put a damper on the Maya civilisation

Geosciences Column: How climate change put a damper on the Maya civilisation

More than 4,000 years ago, when the Great Pyramid of Giza and Stonehenge were being built, the Maya civilisation emerged in Central America. The indigenous group prospered for thousands of years until its fall in the 13th century (potentially due to severe drought). However, thousands of years before this collapse, severely soggy conditions lasting for many centuries likely inhibited the civilisation’s development, according to a recent study published in EGU’s open access journal Climate of the Past.

During their most productive era, often referred to as the Classic period (300-800 CE), Maya communities had established a complex civilisation, with a network of highly populated cities, large-scale infrastructure, a thriving agricultural system and an advanced understanding in mathematics and astronomy. However, in their early days, dating back to at least 2600 BCE, the Maya people were largely mobile hunter-gatherers, hunting, fishing and foraging across the lowlands.

Around 1000 BCE, some Maya communities had started to transition away from their nomadic lifestyles, and instead were moving towards establishing more sedentary societies, building small villages and relying more heavily on cultivating crops for their sustenance. However, experts suggest that agricultural practices didn’t gain momentum until 400 BCE, raising the question as to why Maya development was delayed for so many centuries.

By analysing two new palaeo-precipitation records, Kees Nooren, lead author of the study and a researcher at Utrecht University in the Netherlands, and his colleagues were able to gain insight into the environmental conditions during this pivotal time, and the impact that climate change could have had on the Maya society.

To determine the regional climate conditions during this period of time, the authors examined a beach ridge plain in the Mexican state of Tabasco, off the Gulf of Mexico, which contains a long-term record of ridge elevation changes for much of the late Holocene. Since precipitation has a large influence on the elevation of this beach ridge, this record is a good indicator of how much rainfall and flooding may have occurred during Maya settlement.

A large part of the central Maya lowlands (outlined with a black dashed line) is drained by the Usumacinta (Us.) River (a). During the Pre-Classic period this river was the main supplier of sand contributing to the formation of the extensive beach ridge plain at the Gulf of Mexico coast (b). Periods of low rainfall result in low river discharges and are associated with relatively elevated beach ridges. Taken from Nooren, K et al., 2018

Additionally, the researchers also assessed core samples taken from Lake Tuspan, a shallow body of water in northern Guatemala that is situated within the Central Maya Lowlands. Because the lake receives its water almost exclusively from a small section of the region (770 square kilometres), its sediment layers provide a good record of rainfall on a very local scale.

The image on p. 74 of the Dresden Codex depicts a torrential downpour probably associated with a destructive flood (Thompson, 1972). Taken from Nooren, K et al., 2018

The research team’s analysis suggested that, starting around 1000-850 BCE, the region shifted from a relatively dry climate, to a wetter environment. Such conditions would have made a farming in this region more difficult and less appealing compared to foraging and hunting. The researchers suggest that this change in climate could be one of the reasons why Maya agricultural development was at a standstill for such a long time.

The researchers also propose that this long-term climate trend could have been brought on by a shift of the Intertropical Convergence Zone (ITCZ), a region near the equator where northeast and southeast winds intermingle and where most of the Earth’s rain makes landfall. The position of this zone can move naturally in response to Earth’s changes in insolation, and a northerly shift of the ITCZ could help account for some of the morphological changes the authors observed in the precipitation records.

After more than 450 years of excessive rainfall and large floods, the records then suggest that the region experienced drier conditions once again. By this time period, the Maya populations began to rapidly intensify their farming efforts and develop major cities, further suggesting that the wet conditions may have helped delay such efforts.

This is not the first time the Nooren and his colleagues have found evidence of major environmental influence on the Maya civilisation. For example, earlier research led by Nooren suggests that, in the 6th century, the El Chichón volcano in southern Mexico released massive amounts of sulfur into the stratosphere, triggering global climate change that likely contributed to a ‘dark age’ in Maya history for several decades. During this time, often referred to as the “Maya Hiatus,’ Maya societies experienced stagnation, increased warfare and political unrest. The research results were presented at the 2016 General Assembly and later published in Geology.

The results of these studies highlight how changes in our climate have greatly influenced communities and at times even shaped the course of societal history, both for better and for worse.

By Olivia Trani, EGU Communications Officer

References

Ebert, C. et al.: Regional response to drought during the formation and decline of Preclassic Maya societies. Quaternary Science Reviews 173:211-235, 2017

Nooren, K., Hoek, W. Z., Dermody, B. J., Galop, D., Metcalfe, S., Islebe, G., and Middelkoop, H.: Climate impact on the development of Pre-Classic Maya civilization. Clim. Past, 14, 1253-1273, 2018

Nooren, K.: Holocene evolution of the Tabasco delta – Mexico : impact of climate, volcanism and humans. Utrecht University Repository (Dissertation). 2017

Nooren, K. et al.: Explosive eruption of El Chichón volcano (Mexico) disrupted 6th century Maya civilization and contributed to global cooling, Geology, 45, 175-178, 2016

Press conference: Volcanoes, climate changes and droughts: civilisational resilience and collapse. European Geosciences Union General Assembly 2016

Caltech Climate Dynamics Group, Why does the ITCZ shift and how? 2016

GeoSciences Column: The ‘dirty weather’ diaries of Reverend Richard Davis

GeoSciences Column: The ‘dirty weather’ diaries of Reverend Richard Davis

Researching the Earth’s climate of the past, helps scientists make better predictions about how the climate and our environment will continue to be affected by, change and adapt to rising temperatures.

One of the most invaluable sources of data, when it comes to understanding the Earth’s past climate, are historical meteorological records.

Accounts of weather and climate conditions for the Southern Hemisphere, prior to the 1850s, are particularly sparse. This makes the recently discovered, painstakingly detailed and richly descriptive weather diaries of a 19th Century missionary in New Zealand, incredibly valuable.

Researchers from the National Institute of Water and Atmospheric Research, In Auckland (New Zealand), poured over the contents of the diaries, which provide an eyewitness account to the end of the Little Ice Age (between 1300 and the 1870s winter temperatures – particularly in the Norther Hemisphere- were lower than those experienced throughout the 20th Century). The journals reveal that 19th century New Zealand experienced cooler winter temperatures and more dominant southerly winds when compared to the present day climatic conditions. The researchers present these and other findings in the open access journal of the EGU, Climate of the Past.

Print of a photomechanical portrait of Reverend Richard Davis taken ca. 1860, from the file print collection, Box 16. Ref: PAColl-7344-97, Alexander Turnbull Library, Wellington, New Zealand, sourced from http://natlib.govt.nz/records/23073407 (From A. M. Lorrey et al., 2016).

Print of a photomechanical portrait of Reverend Richard Davis taken ca. 1860, from the file print collection, Box 16. Ref:
PAColl-7344-97, Alexander Turnbull Library, Wellington, New Zealand, sourced from http://natlib.govt.nz/records/23073407 (From A. M. Lorrey et al., 2016).

The diaries were kept by Reverend Richard Davis, born in Dorset (England) in 1790. The Reverend was associated to the Church Mission Society (CMS) of England; an connection which lead him to settle in the blustery Northland Peninsula in the far north of New Zealand, back in 1831.

From 1839 to 1844, and then again from 1848 to 1851, Davis collected over 13,000 meteorological measurements and made detailed notes about the condition of the local environment.

The Reverend’s collection of data is remarkable, not only for its detail, but also because it is the earliest record of land-based meteorological measurements from New Zealand found to date.

He took twice daily temperature measurements – one at 9 a.m. and one at 12 noon – as well as noon pressure measurements. Qualitative observations included information about wind direction and strength, as well as detailed cloud cover descriptions, and notes on the occurrence of hail, frost, rainfall, snowfall, thunderstorms, lightning, sunsets and behavior of wildlife.

The journals also reveal that the Reverend was not keen on particularly gloomy days, when the winds were strong and blustery, cloud cover hung low and was often accompanied by rain.  On 67 separate occasions, Davis’ used the term “dirty weather” to described days like this.

It is important to assess the reliability of the measurements taken by Davis before drawing comparisons between 19th and 20th Century weather patterns for the island of the long white cloud; and especially if the data are to be integrated within past climate and weather reconstructions for New Zealand and the Southern Hemisphere.

To do so, the Auckland based researchers, compared the Reverend’s pressure measurements with observations made by ships travelling through New Zealand waters or stationed on the island (usually completing military operations), during the same time interval. The Reverend’s daily pressure observations are regularly lower (on average by -0:64 ± 0:10 inches of mercury) than those taken on board the ships. The offset is consistent with the change in altitude between the ships anchored in harbour versus the land-based measurements made by Davis; meaning the Reverend’s pressure measurements are robust.

Reverend Richard Davis pressure observation vs. expedition measurements (leader noted in parentheses) from USS Vincennes (Wilkes), the corvettes Astrolabe and Zelee (d’Urville) and the HMS Erebus (Ross). There are 29 pairs of daily observations and so the x axis simply shows the comparisons of Davis’ record to the three ships in a sequence with the specific intervals noted. (From A. M. Lorrey et al., 2016).

Reverend Richard Davis pressure observation vs. expedition measurements (leader noted in parentheses) from USS Vincennes (Wilkes), the corvettes Astrolabe and Zelee (d’Urville) and the HMS Erebus (Ross). There are 29 pairs of daily observations and so the x axis simply shows the comparisons of Davis’ record to the three ships in a sequence with the specific intervals noted. (From A. M. Lorrey et al., 2016).

There is no similar test which would verify the accuracy of Davis’ temperature measurements. However, the researchers argue that the (expected) annual cycles evident in his measurements, as well as the reliability of his other records mean that, at least some, of his readings are faithful to the local conditions. Not only that, Davis’ mean winter temperature anomalies are comparable to the temperatures reconstructed from tree rings and can be used by the researchers to gather information about the local atmospheric circulation at the time.

When compared to modern-day temperature measurements (from the Virtual Climate Station Network, VCSN), the journal data reveals that mid 1800s winters, at the far north of the island, were cooler. At present, atmospheric circulation over Northland means winds from the southwest are common, especially during the winter and spring. During the summer, easterly winds become dominant. There is a higher frequency of records of south and southwesterly winds in Davis’ diaries. Reconstructions of atmospheric flow over New Zealand in the 1800s, made with proxy tree-ring and coral data, also point towards more frequent south and southwesterly winds and cooler temperatures.

Not only that, the timing of monthly and seasonal climate anomalies, recorded both in tree-ring and the Davis diary data suggest that El Niño-Southern Oscillation (ENSO)-like conditions existed, in New Zealand, during the 1839-1851 time period. However, more work (and data) is needed in Australasia to corroborate the findings and define the extent of the ENSO conditions at the time.

With more data, better reconstructions of the atmospheric conditions in the southwest Pacific and Southern Hemisphere can be made. Combined with the newly found Davis’ records, these will make an important impact to the understanding of past weather and climate in the region.

By Laura Roberts Artal, EGU Communications Officer

The Waimate North mission house in the Far North of New Zealand where Davis lived (From: A. M. Lorrey et al., 2016).

The Waimate North mission house in the Far North of New Zealand where Davis lived (From: A. M. Lorrey et al., 2016).

References

Lorrey, A. M. and Chappell, P. R.: The “dirty weather” diaries of Reverend Richard Davis: insights about early colonial-era meteorology and climate variability for northern New Zealand, 1839–1851, Clim. Past, 12, 553-573, doi:10.5194/cp-12-553-2016, 2016.

Imaggeo on Mondays: what corals can tell us about past climate change

Imaggeo on Mondays: what corals can tell us about past climate change

Reconstructing past climates is a tricky task at the best of times. It requires an ample data set and a good understanding of proxies. Add into the mix some underwater fieldwork and the challenge got a whole lot harder! In today’s Imaggeo on Monday’s post, Isaac Kerlow explains how information locked in corals can tell the story of past climates and how important it is, not only to carry out the research, but to communicate the results to the public! If you stick with this post until the end you’ll be rewarded with a super informative video too!

One of the main science communication initiatives at the Earth Observatory of Singapore (EOS) is about producing short films that showcase the scientific research of the principal investigators. The collection of films created by the EOS Art+Media group is called the “Knowledge Capsules” and they are free to view and download on the internet. On this occasion the filmmaking team travelled to Checheng, Southern Taiwan, to document and explain the field methods of the Marine Geochemistry team.

Creating a successful science film for a mainstream audience requires an understanding of the scientists’ methods, theories and goals. During principal photography that takes place during expeditions the filmmaking team needs to stay a step ahead of the game in order to capture the critical moments such as this image where Dr. Nathalie Goodkin passes a sample of Porites coral (a type of stony, finger-like, coral) to a scientist aboard the research vessel.

The Marine Geochemistry team at EOS investigates Earth’s climate history through the study of corals. This region is where the Kuroshio Current intrudes into the South China Sea. The team extracted samples of the Porites coral species that are approximately 300 to 500 years old, as well as seawater in which these corals grow. Because the chemical composition of corals depends on the seawater in which they grow, analysing the coral samples can give an indication of the temperature and salinity of the surrounding seawater. With these results, the team is able to reconstruct global climate systems throughout several centuries.

By Isaac Kerlow, Earth Observatory of Singapore

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Geosciences Column: Pollen tells a 7300 year old story of Malta’s climate and vegetation

Geosciences Column: Pollen tells a 7300 year old story of Malta’s climate and vegetation

Figuring out what the climate was like, and how it changed, throughout Earth’s history is like trying to complete a 1000 piece puzzle. Except that scientists usually don’t have all the nuggets and building a comprehensive picture relies on a multidisciplinary approach in order to fill in the blanks.

This is particularly true during the Holocene, which spans the last 11,700 years of the Earth’s history, and began at the end of the last ice age. It marks a time of significant climate change across the planet, coupled with changing vegetation dynamics and the emergence of humans, who even at that early time, started leaving their mark on Earth. Bringing together all the evidence to build a complete picture of what the environment looked like is no easy task.

The Mediterranean, in particular, is considered a hot-spot for changing climate and biodiversity in the Holocene. The start of the epoch, in southern Europe, was characterised by a wet climate. The late Holocene, dominated by the presence of humans, is thought to have been warmer and drier. But disentangling the signature of naturally induced change vs. anthropogenically induced change continues to be difficult.

Queue the Maltese archipelago.

A small archipelago in the middle of the Mediterranean Sea

Located in the centre of the Mediterranean basin, approximately 96 km south of Sicily, Malta and a cluster of low-lying islands, including Gozo and Comino, are the focus of a recently published study in the open access journal Climate of the Past.

Study area. (a) Mediterranean region highlighting the Maltese Islands. Selected regional sites mentioned in text: 1: Lago Preola, 2: Gorgo Basso, 3: Biviere di Gela, 4: Lago Pergusa, 5: Lago Trifoglietti, 6: Lago Accesa, 7: Lago Ledro, 8: Tenaghi P., 9: SL152, 10: MNB-3, 11: NS14, 12: HCM2-22, 13: Soreq Cave; base map source: Arizona Geographic Alliance; (b) Maltese Islands: key sites mentioned in text; (c) average annual temperature and rainfall, based on Galdies (2011) data for the 30-year climatic period 1961–1990; (d) the topography and catchment area (blue) of Burmarrad.

Study area. (a) Mediterranean region highlighting the Maltese Islands. (Selected regional sites mentioned in text of the paper, but not mentioned in this blog post). (b) Maltese Islands. (Key sites mentioned in text of the paper, but not mentioned in this blog post). (c) average annual temperature and rainfall, based on Galdies (2011) data for the 30-year climatic period 1961–1990; (d) the topography and catchment area (blue) of Burmarrad. From B. Gambin et al. (2016). Click to enlarge.

It is their central Mediterranean location that makes the collection of isles so attractive, as they provide a good representation of the overall Mediterranean climate and can help decipher some of the questions regarding southern European climate during the Holocene.

The first human occupants sailed the short distance from Sicily, to settle in the region some 7200 years ago, introducing with them vegetation changes to the area. This well-defined date can be used as a possible marker to distinguish between naturally induced vegetation changes, brought about by climate variations, versus those triggered by the presence of humans.

In the paper, the researchers, led by B. Gambin, also present the first palaeoclimatic reconstruction for the Maltese islands and an updated palaeovegetation reconstruction.

But as encouraging as it sounds, using the Maltese archipelago as a study site for palaeoclimatic reconstructions has one, rather large, limitation. It has no peat bogs or lake deposits, which are the most suitable sites for the collection of data on old vegetation.

Using pollen to reconstruct the story of past climate

The spring and summer months are synonym of the onset of hay fever season for many. But the allergy triggering grains also play a surprisingly important role when it comes to reconstructing past climates, especially when there are no lakes or peat bogs around!

Palynology, the study of pollen grains, has been an important element in piecing together the history of our planet’s past climates since the early 20th Century.

Ancient pollen grains, extracted from sedimentary cores, can contribute to identifying changes in vegetation over time for a given region. If an area’s plant life changes to include more drought resistant varieties, it can point towards a warming climate in that region, whereas propagation of water-loving species might indicate wetter climes. Similarly, a sudden increase (or change) in the presence of pollen from cultivated taxa, like wheat, barely, olives, grapes, etc… highlights the presence of human (anthropogenic) influences in the region.

It is the pollen record, extracted from a core drilled in the region of Burmarrad in Northwest Malta, that the team of scientists used to compile their Maltese palaeoclimatic reconstruction.

Mediterranean climate and key events throughout the late Holocene

BM2 sedimentary profile and age–depth model interpo- lated curve. Dates on the core obtained via radiocarbon dating (for method and age detials, please see the paper). From B. Gambin et al. (2016).

BM2 sedimentary profile and age–depth model interpolated curve. Dates on the core obtained via radiocarbon dating (for method and age detials, please see the paper). From B. Gambin et al. (2016). Click to enlarge.

The 10m long BM2 drill core, extracted using a percussion corer, contained information on the climate, vegetation and precipitation history of the Maltese islands from the early Neolithic period (7280 before present,(BP)) through to the Roman age (1730 BP).

The early Neolothic

Pollen samples collected from the oldest section of the core indicate that from 7280 BP through to 6700 BP the Burmarrad region surrounded an ancient bay (in contrast to the present day agricultural plain setting) with the local vegetation affirming this. The researchers found pollen from non-arboreal (such as herbs and shrubs) taxa, as well as pollen from aquatic and marine species, such as Botryococcus, a common green algae species.

Drill core analysis also highlighted that average temperatures during this period were mild, stable and comparable to present-day values; while winter and summer precipitation levels were relatively high.

The early Neolithic period on the island coincided with the arrival of permanent settlers. But many of the pollen species which usually indicate the onset of anthropogenic influences are also native to Malta making it difficult for the scientists to draw conclusions as to whether the vegetation records show the arrival of the first human inhabitants.

A moister climate from 6700 BP resulted in the spread of Pistacia, a leafy green shrub, across the ancient bay. The damper conditions prevailed across the southern Mediterranean, with increased Pistacia populations found in Sicily and Spain too. The geographically wide-spread nature of the vegetation change indicates it was likely climate driven.

Templar period

By 6050 BP, human settlers started to make their presence felt on the tiny island. Communities started building free-standing stone temples, used for ritual purposes, which are unique to Malta. This settling period is accompanied by the rise in pollen from Olea plants – otherwise known as olive trees – herbaceous taxa such as Brassicaceae (which include mustard plants, radishes and cabbages), and fungus spores which occur in the dung of domestic livestock as well as wild herbivores.

Ggantija Temples of Malta. Image by Daniel Hausner. Attribution to Norum [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons

Ggantija Temples of Malta. Image by Daniel Hausner. Attribution to Norum [GFDL or CC-BY-SA-3.0], via Wikimedia Commons.

While individually these plants do not directly indicate vegetation change brought about by humans, when found together they do suggest human activity in the region, particularly the onset of grazing livestock.

The Bronze Age

The onset of the Bronze Age (4900 to 2650 BP) was marked by a drier climate where winter precipitation decreased and annual temperatures fluctuated between lows of 7°C and highs of 14°C. This coincides with the decrease in abundance of tree pollen found in the BM2 core and an increase in herbaceous taxa (plants which don’t have permanent woody stems).

Microcharcoal horizons become more prevalent in the core too, indicating an increase in fire activity. The drier climate and subsequent vegetation change might be a contributing factor to the increased burning in the region, but it is also likely that slash-and-burn farming was taking hold at this time. Further human activity is indicated by the rise in pollen from plants associated with pastoral activity and the increase in fungus spores commonly found in the dung of livestock.

The strain put on the environment of the Burmarrad Bay at this time is evident by the rise of algal spores and the Glomus fungus, typically associated with increased soil erosion rates. It is further supported by a reduction in overall pollen count in the core, indicating the land could support less plant-life.

The increased erosion rates had a significant impact on the landscape, with the marine lagoon slowly infilling with sediment and eventually becoming landlocked throughout the Bronze Age.

Roman occupation period

By 1972 BP the area became a well-developed fertile deltaic plain, so it’s not surprising that an increase in pollen concentrations from cultivated crop taxa, pointing towards a rise in agricultural activity, were found in the core. The area also benefited from a long period of stable climate with limited temperature and precipitation changes.

During this time, Malta became an important producer and exporter of olive oil, as evidenced by the extensive port-like remains, of this age, found across the island. This is supported by the Olea pollen count in the core, which is high too.

Synthesis of cultural phases, LPAZs (local pollen assemblage zones), sediment, vegetation dynamics, and climatic reconstruction: BM2 core, Malta. From B. Gambin et al. (2016).

Synthesis of cultural phases, LPAZs (local pollen assemblage zones), sediment, vegetation dynamics, and climatic reconstruction: BM2 core, Malta. From B. Gambin et al. (2016). Click to enlarge.

Climate- vs. human –driven environmental change in the Holocene

The research shows just how powerful palynology is as a tool for reconstructing past climates. The study of the BM2 core allowed scientists to put together a 7300 year history of climatic, vegetation and anthropogenic change in Malta.

At the same time it highlights the ongoing challenge of unravelling the signature of climate- vs. human –driven environmental change in the Holocene.

Taken as a whole, the researchers hope that the findings can be a starting point for further research into this subject, with hopes to gain better understanding of the factors and processes affecting past, present and future Mediterranean landscapes.

By Laura Roberts Artal, EGU Communications Officer

Reference

Gambin, B., Andrieu-Ponel, V., Médail, F., Marriner, N., Peyron, O., Montade, V., Gambin, T., Morhange, C., Belkacem, D., and Djamali, M.: 7300 years of vegetation history and climate for NW Malta: a Holocene perspective, Clim. Past, 12, 273-297, doi:10.5194/cp-12-273-2016, 2016.