When Charles Dickens, the English novelist, was detailing the "soft black drizzle" of pollution over London, he might inadvertently have been chronicling the early signs of global warming.
New research led by Australian scientists has pegged back the timing of when humans had clearly begun to change the climate to the 1830s.
A team comprised of geological and environmental science researchers from Stanford University has been studying the impact that early European exploration had on the New World and have found evidence that they say suggests the European cold period from 1500 to 1750, commonly known as the Little Ice Age, was due to the rapid decline in native human populations shortly after early explorers arrived.
Following up on their paper published in 2008, the team has now brought their findings before the Geological Society of America. The researchers say that the population decrease, which came about due to the introduction of previously unknown diseases, led to the rapid reforestation of the Americas. This led to a sudden increase in the amount of carbon dioxide being pulled from the air, which meant the atmosphere wasn’t able to hold as much heat, which led to colder air covering Europe.
The team, led by visiting scholar Richard Nevle, came to this conclusion after analyzing charcoal remnants in soil and lake sediments left behind by early American inhabitants as they burned forests to make room for farmland. They found that starting approximately 500 years ago, the charcoal accumulations came to a virtual standstill, coinciding with the death of native peoples.
Nevle et al then got out their calculators and crunched the numbers. They estimate that for a population of some 40 to 80 million indigenous people, the total amount of deforested land would likely have amounted to something the size of California. And since most estimates suggest that close to 90 percent of the native peoples died or were killed after the Europeans arrived, that meant most of that land returned to forest. That many trees, they say, all of a sudden appearing, almost as if out of nowhere, could have resulted in a loss of some 2 to 17 billion tons of carbon dioxide from the air.
The high frequency and magnitude of volcanic eruptions could have been the cause of the progressive cooling of ocean surfaces over a period of 1,800 years. This is made apparent in an international study published recently in the journal Nature Geoscience, involving researcher P. Graham Mortyn of the Institute for Environmental Science and Technology (ICTA-UAB) and the UAB Department of Geography.
The study emphasises that this trend came to an end with the beginning of the Industrial Revolution and the resulting global warming caused by human activity. It further shows that the lowest temperatures in the first 1,800 years of the Common Era were recorded between the 16th and the 18th centuries, a period known as the "Little Ice Age".
Earlier research had already shown that volcanic explosions cause the atmosphere to cool. The present study demonstrates that the oceans can absorb and capture more heat than the atmosphere over longer periods of time, thus attenuating global temperature changes in the short term. These alterations in temperature can be prolonged when the volcanic eruptions are concentrated into a short space of time.
These findings bring a new perspective to the study of regional and global variations in ocean-surface temperature over the centuries, before the appearance of anthropogenic (human activity-induced) climate change.
The researchers combined, for the first time, 57 previous works on sea-surface temperature changes, which are calculated from fossil remains extracted from oceans all over the world, from the Poles to the Tropics. The results were compared with data from terrestrial indicators, such as tree rings or ice cores. These revealed a similar cooling trend.
Counting sunspots over time helps in knowing the activity of our star but the two indices used by scientists disagree on dates prior to 1885. Now an international team of researchers has tried to standardise the historical results and has discovered that, contrary to what one may think, the solar activity of our times is very similar to that of other times, such as the Enlightenment.
Scientists have been counting sunspots since 1610 with small telescopes. Thus it has been verified that the Sun's activity increases every eleven years, according to the interval in the growth of the number of darker and colder spots in comparison with the rest of its surface. The more spots that appear, the more luminous the surrounding areas are, and our star shines brighter.
Nonetheless, the eleven-year cycles do not always have the same intensity. The more intense peaks of the Sun's luminosity were produced in the 20th century, which experts have called the 'modern maximum'. However, an international team of scientists has reviewed the historical data and has verified that there were also elevated values in other periods.
"It has been a huge surprise to observe that in the 18th century the levels of the Sun's activity were practically the same as they are now," points out José M. Vaquero, researcher at the University of Extremadura (Spain) and co-author of the research, a review of the number of sunspots recorded in the last 400 years.
Volcano-induced regime shifts in millennial tree-ring chronologies from northeastern North America
Authors:
Gennaretti et al
Abstract:
Dated records of ice-cap growth from Arctic Canada recently suggested that a succession of strong volcanic eruptions forced an abrupt onset of the Little Ice Age between A.D. 1275 and 1300 [Miller GH, et al. (2012) Geophys Res Lett 39(2):L02708, 10.1029/2011GL050168]. Although this idea is supported by simulation experiments with general circulation models, additional support from field data are limited. In particular, the Northern Hemisphere network of temperature-sensitive millennial tree-ring chronologies, which principally comprises Eurasian sites, suggests that the strongest eruptions only caused cooling episodes lasting less than about 10 y. Here we present a new network of millennial tree-ring chronologies from the taiga of northeastern North America, which fills a wide gap in the network of the Northern Hemisphere's chronologies suitable for temperature reconstructions and supports the hypothesis that volcanoes triggered both the onset and the coldest episode of the Little Ice Age. Following the well-expressed Medieval Climate Anomaly (approximately A.D. 910–1257), which comprised the warmest decades of the last millennium, our tree-ring-based temperature reconstruction displays an abrupt regime shift toward lower average summer temperatures precisely coinciding with a series of 13th century eruptions centered around the 1257 Samalas event and closely preceding ice-cap expansion in Arctic Canada. Furthermore, the successive 1809 (unknown volcano) and 1815 (Tambora) eruptions triggered a subsequent shift to the coldest 40-y period of the last 1100 y. These results confirm that series of large eruptions may cause region-specific regime shifts in the climate system and that the climate of northeastern North America is especially sensitive to volcanic forcing.
Evidence of strong storm events possibly related to the little Ice Age in sediments on the southerncoast of Brazil
Authors:
Oliveira et al
Abstract:
Late Holocene environmental changes on the southeastern Brazilian coast were assessed using a high-resolution paleoproductivity proxy record from a sediment core collected at 14 m water depth in the Pântano do Sul Inlet. Mollusk shells from the core were AMS dated, and sediment grain size, concentrations of organic carbon and total nitrogen, and δ13C and δ15N values were determined to investigate changes in paleoceanographic and paleoclimatic conditions over the depositional period. Most of the parameters showed strong fluctuations in the depth interval corresponding to the Little Ice Age, between 1560 and 1700 AD, that were marked by first an increase and then a decrease of input of terrigenous sediments to the inlet. Proxies also indicate that sedimentary conditions were more stable, before and after this period. The strong sedimentary changes observed in the Pântano do Sul Inlet may be related to climatic changes reported elsewhere in South America during 1550 and 1800 AD and to severe storm events associated with the enhanced cold fronts that occurred in the southern littoral region during this period.
Surface changes in the Eastern Labrador Sea around the onset of the Little Ice Age
Authors:
Moffa-Sánchez et al
Abstract:
Despite the relative climate stability of the present interglacial, it has been punctuated by several centennial time scale climatic oscillations, the latest of which are often colloquially referred to as the Medieval Climatic Anomaly (MCA) and the Little Ice Age (LIA). The most favored explanation for the cause of these anomalies is that they were triggered by variability in solar irradiance and/or volcanic activity and amplified by ocean-atmosphere-sea ice feedbacks. As such, changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) are widely believed to have been involved in the amplification of such climatic oscillations. The Labrador Sea is a key area of deep water formation. The waters produced here contribute approximately one-third of the volume transport of the deep limb of the AMOC and drive changes in the North Atlantic surface hydrography and subpolar gyre circulation. In this study, we present multi-proxy reconstructions from a high-resolution marine sediment core located south of Greenland that suggest an increase in the influence of polar waters reaching the Labrador Sea close to MCA-LIA transition. Changes in freshwater forcing may have reduced the formation of Labrador Sea Water and contributed towards the onset of the LIA cooling.
A mysterious, centuries-long cool spell, dubbed the Little Ice Age, appears to have been caused by a series of volcanic eruptions and sustained by sea ice, a new study indicates.The research, which looked at chemical clues preserved in Arctic vegetation as well as other data, also pinpointed the start of the Little Ice Age to the end of the 13th century.During the cool spell, which lasted into the late 19th century, advancing glaciers destroyed northern European towns and froze the Thames River in London and canals in the Netherlands, places that are now ice-free. There is also evidence it affected other continents."This is the first time anyone has clearly identified the specific onset of the cold times marking the start of the Little Ice Age," said Gifford Miller, a geological sciences professor at the University of Colorado, Boulder, and the lead study researcher. "We also have provided an understandable climate feedback system that explains how this cold period could be sustained for a long period of time."The cause appears to have been massive tropical volcanic eruptions, which spewed tiny particles called aerosols into the atmosphere. While suspended in the air, the aerosols reflect solar radiation back into space, cooling the planet below.The cooling was sustained after the aerosols had left the atmosphere by a sea-ice feedback in the North Atlantic Ocean, the researchers believe. Expanding sea ice would have melted into the North Atlantic Ocean, interfering with the normal mixing between surface and deeper waters. This meant the water flowing back to the Arctic was colder, helping to sustain large areas of sea ice, which, in turn, reflect sunlight back into the atmosphere. The result was a self-sustaining feedback loop.Miller and colleagues came to these conclusions by looking at radiocarbon dates — based on how much of the radioactive form of carbon they contain — from dead plants revealed by melting ice on Baffin Island, in the Canadian Arctic. Their analysis found that many plants at both high and low altitudes died between A.D. 1275 and A.D. 1300 — evidence that Baffin Island froze over suddenly. Many plants also appeared to have died at around A.D. 1450, an indication of a second major cooling.These periods coincide with two of the most volcanically active half centuries in the past millennium, according to the researchers.
The power of viruses is well documented in human history. Swarms of little viral Davids have repeatedly laid low the great Goliaths of human civilization, most famously in the devastating pandemics that swept the New World during European conquest and settlement.
In recent years, there has been growing evidence for the hypothesis that the effect of the pandemics in the Americas wasn't confined to killing indigenous peoples. Global climate appears to have been altered as well.
Stanford University researchers have conducted a comprehensive analysis of data detailing the amount of charcoal contained in soils and lake sediments at the sites of both pre-Columbian population centers in the Americas and in sparsely populated surrounding regions. They concluded that reforestation of agricultural lands-abandoned as the population collapsed-pulled so much carbon out of the atmosphere that it helped trigger a period of global cooling, at its most intense from approximately 1500 to 1750, known as the Little Ice Age.
"We estimate that the amount of carbon sequestered in the growing forests was about 10 to 50 percent of the total carbon that would have needed to come out of the atmosphere and oceans at that time to account for the observed changes in carbon dioxide concentrations," said Richard Nevle, visiting scholar in the Department of Geological and Environmental Sciences at Stanford. Nevle and Dennis Bird, professor in geological and environmental sciences, presented their study at the annual meeting of the American Geophysical Union on Dec. 17, 2008.
Nevle and Bird synthesized published data from charcoal records from 15 sediment cores extracted from lakes, soil samples from 17 population centers and 18 sites from the surrounding areas in Central and South America. They examined samples dating back 5,000 years.
What they found was a record of slowly increasing charcoal deposits, indicating increasing burning of forestland to convert it to cropland, as agricultural practices spread among the human population-until around 500 years ago: At that point, there was a precipitous drop in the amount of charcoal in the samples, coinciding with the precipitous drop in the human population in the Americas.
To verify their results, they checked their fire histories based on the charcoal data against records of carbon dioxide concentrations and carbon isotope ratios that were available.
"We looked at ice cores and tropical sponge records, which give us reliable proxies for the carbon isotope composition of atmospheric carbon dioxide. And it jumped out at us right away," Nevle said. "We saw a conspicuous increase in the isotope ratio of heavy carbon to light carbon. That gave us a sense that maybe we were looking at the right thing, because that is exactly what you would expect from reforestation."
