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How kings created Angkor Wat—then lost it
The major temples seem much more interesting than what also appears on the landscape: apparently random mounds of earth.
Between the 9th and 15th centuries, the Khmer Empire created some of the most spectacular architecture in history, including one of the largest religious monuments in the world: Angkor Wat. This magnificent stone temple is one of the most famous archaeological sites in the world, visited by over 2.6 million tourists a year. A nearby temple, Ta Prohm, was featured in the movie Tomb Raider.
At first glance, the most spectacular and interesting things about the Khmer Empire are these major temples, along with the massive agricultural system built by the empire's kings to support the people and increase their wealth. With this infrastructure, the medieval Khmer people transformed a giant floodplain into a highly engineered landscape over the course of hundreds of years.
The scale of the hydraulic system is perhaps unparalleled in the preindustrial world. They built channels that were over 20 km in length and 40–60 m wide, above-ground reservoirs thousands of acres in size, and a vast network of walled fields used for flooded rice agriculture. The largest of these were constructed by kings, who claimed credit for them in inscriptions etched on the walls of temples and commemorative stone slabs called stelae.
Most academic accounts of agriculture at Angkor have focused on this major infrastructure, and this is perfectly understandable. On the face of it, the reservoirs and major temples seem much more interesting than what also appears on the landscape: apparently random mounds of earth.
But analyses of these mounds, reaching far beyond the walls of Angkor Wat, are now revealing an important story about the development of the Khmer Empire. Archaeological investigations by our team and others reveal that a diverse network of small-holder farms was subsumed by bigger farms owned by elite, richer farmers until mass centralization took over. This happened just decades before the area slipped into decline and might have been one of the reasons for it.
It's intriguing to see similar patterns in the United States today, as small farms are subsumed by ever bigger agribusinesses. In general, this scaling up tends to improve efficiency but reduce diversity, and can introduce risks into the food supply chain. Some of these risks may be tested in the coming months and years as the modern food system is strained by everything from climate change to the current coronavirus pandemic.
For decades, archaeologists have been mapping the regions around the great temples of Angkor using remote sensing and ground surveys. This has worked well for most of the Greater Angkor region, but some areas—especially in the urban core—are now hidden by dense forest. In 2012, the Khmer Archaeology Lidar Consortium was formed to organize a campaign of lidar (a 3D laser light scanning technique) across 370 km2 of Cambodia, including the forested areas at the center of Angkor. The resulting images revealed the surface lying beneath the vegetation.
The Bayon temple at the famous temple area of Angkor Archeological Park.
Ian Walton/Getty Images
The generated maps reveal both areas of dense occupation with city blocks and streets, and lower-density areas with scattered community temples, sometimes marked by little more than a scatter of bricks or just a faint impression of a mound with a moat around it. These community temples probably served a somewhat similar function as churches in the agricultural communities of modern America do: not just to promote religion but also to facilitate social networking and help neighbors coordinate their activities. When growing rice, it's important to coordinate and manage water collaboratively with your neighbors. If one farm hoards all the water, neighboring farms may have to let their fields go fallow. When that happens, pests take over and devastate everyone's crops.
Our team realized that the key to cracking the code of Angkorian agriculture was to understand these community temples. The new maps showed where the temples were on the landscape, but we needed to figure out when they were built.
In a study published in 2018, we used machine learning (a form of artificial intelligence) to estimate the age of temples that didn't have inscriptions or artistic elements to date them. In the end, we were able to assign dates to over 5,000 community temples, reservoirs, ponds, and moats.
Our results indicated that, unsurprisingly, the construction of small temples blossomed around new water sources: Small-holder farms took advantage of good growing conditions fostered by various kings' construction of large hydraulic features.
What was strange was that we found a severe decrease in the number of new temple foundations on the landscape during the 11th and 12th centuries, right when the kings were constructing major projects like Angkor Wat, hospitals, and extensive road networks.
Why was that?
We pondered this strange observation for months, until we had a eureka moment. We were at a conference in Poznan, Poland, in July 2017, where me met colleagues who had noted an intriguing pattern in inscriptions relating to land ownership and disputes.
The landowners mentioned in the earlier inscriptions tended to be of all ranks in society, from lower class to higher class. However, by the mid-11th century, free males of middle rank (vāp) were no longer referenced in the context of land transactions or the foundations for new temples. By the 12th century, even free males of higher rank (loñ) were referred to as temple personnel or workers rather than as landowners. As we published in a paper this year, it seems the land was increasingly concentrated into the hands of the evermore wealthy. First the middle class was squeezed out, and then even the upper class lost their lands to the state.
This period of rapid change in agriculture in the 12th and 13th centuries coincides with a period of urbanization: the emergence of very large populations in epicenters. This was the apogee of Angkor's political power.
But the rise of kingly centralization ushered in an extended period of political and economic decline that lasted for centuries. It seems that the concentration of land ownership and management, along with rapid growth in the population of non–rice-producing citizens in the urban core, conspired to make Greater Angkor more vulnerable to climatic and social challenges. When the political regime shifted and the city was faced with a series of extreme monsoons and droughts, the centralized system may have had a hard time coping.
Plenty of other societies and empires throughout human history have fallen prey to the problems of over-centralization. As communities grow larger, sometimes the diversity of crops grown or diverse traditions get lost. A larger, more cohesive and more uniform system can be very efficient at feeding a large population. But, if and when something goes wrong, a system can collapse quickly if it relies on a single way of doing things. Similar phenomena have been documented in the pre-Hispanic U.S. Southwest and with the Maya in Central America.
Something similar may be going on in the United States and in other nations today. Over time, much of the small-holder farmland has been bought up by big agricultural businesses. Of course, there are plenty of differences: Modern America is an entirely different place from Angkor—economically, socially, politically, and environmentally. But there are parallels. What scholars have seen in America and elsewhere is the promotion of efficiency at the cost of diversity. That introduces risk and fragility into a complex system.
Resilient systems are those that are flexible enough to respond to challenges. Global food supply networks and infrastructure will be strained by the current COVID-19 crisis as food production workers fall ill, transport falters, and borders close. Is the global food system flexible enough to meet the new demands and respond to this unprecedented challenge? Angkor's did not seem to be. Only time will tell what happens to the world's modern empires.
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When it comes time for humanity to pick a new home, where will we go?
- Regardless of whether you think the Earth will suffer some catastrophe or not, most individuals believe that humanity will eventually have to live on another planet.
- There is no nearby planet that can support human life, however; we'll have to pick a good candidate and terraform it.
- Each celestial body presents its own unique challenges and requirements. Some need more carbon dioxide, others need less; some would become water worlds, others more Earth-like; and so on.
Whether you're feeling optimistic or pessimistic about humanity's long-term chances on Earth, most of us agree that we should colonize other planets. Whether that's out of humanity's sheer pioneering spirit or the pragmatic survival instinct to spread out so that a catastrophe on Earth doesn't wipe out the species, establishing a colony on a nearby planet seems like a must.
Trouble is, our neighboring celestial bodies are constantly bombarded by deadly radiation, lack water or oxygen, rain sulfuric acid, swing from extreme heat to cold, and possess many other inhospitable characteristics. No matter where we go in our solar system, we'll have to engage in one of the largest projects imaginable: terraforming. Depending on the environment we want to transform into a more Earth-like one, the nature of this project will vary tremendously. Here's some examples from some of the most likely candidates for terraforming in our solar system.
An artist's depiction of Mars' gradual transformation via terraforming.
Mars has always been an appealing target for terraforming, as it is arguably the most Earth-like planet in the solar system. It goes through similar seasons to Earth, has a relatively similar atmospheric composition, its day-night cycle is extremely close to our own, it possesses abundant water in the form of ice, and it lies in the Sun's habitable zone.
But the biggest problem with Mars is that it has no magnetosphere. Without an envelope of shielding magnetism, solar wind will blow away any atmosphere before it can accumulate. Proposals to create the right kind of atmosphere on Mars — like Elon Musk's flashy idea of nuking the polar ice caps to release stored CO2 and water vapor, thereby heating the planet up — won't work long term without a magnetosphere to protect the planet against solar wind. With Mars' current, flimsy atmosphere, between 1 and 2 kilograms of gas are lost to space every second. Not to mention that the lack of this protective magnetosphere also exposes the planet and all life on it to deadly radiation from the sun.
One proposal is to place a gigantic magnetic shield in orbit between Mars and the Sun to recreate the effects produced by, for instance, Earth's rotating iron outer core. This would be an incredible engineering task, likely requiring regular maintenance and fuel to keep the magnet powered. But it would be the first step to ensuring that Mars could be made habitable. Even prior to that point, Mars gradual growth of an atmosphere would make future exploration on the red planet easier and easier.
An artist's depiction of Venus if it were terraformed.
Compared to Mars, Venus has very little going for it. The surface temperature is 462°C, or 864°F; it has the opposite problem as Mars, with an atmosphere more than 90 times as dense as that of the Earth; and it's got no breathable oxygen. Not to mention that it's covered in volcanos and rains sulfuric acid. On the other hand, it's our closest planetary neighbor, and its gravity is about 90 percent that of Earth's compared to Mars' 38 percent, meaning our muscles and bones wouldn't atrophy while living there.
While Venus also suffers from a lack of a sufficiently strong magnetosphere, it's abundance of atmosphere means that concern can be put aside for a while in our hypothetical terraforming project. Venus's major problem is its excess of CO2, which makes the surface of the planet too hot for life and too heavy for humans.
One approach would be to use autonomous robots to expose Venus's underground deposits of calcium and magnesium, resulting in a chemical reaction that would store CO2 in a magnesium carbonate. This would need to be supplement by a bombardment of those elements mined from asteroids as well in order to remove enough carbon from the atmosphere for human life.
There are a variety of other methods, but they all rely on removing CO2 from the atmosphere rapidly. Seeing as how our inability to do that on Earth may be one of the biggest reasons to find another planet, Venus may not be the ideal target for terraforming in the future. An alternative to terraforming, however, would be to build a floating city in the Venusian clouds, a feat that isn't too far-fetched technologically.
A full-color image of Callisto as captured by NASA's Galileo spacecraft/
NASA/JPL/ DLR(German Aerospace Center)
Many of the Jupiter's Galilean moons are attractive targets for terraforming due to their high abundance of water, but only Callisto lies far enough away from the radiation belts generated by Jupiter's magnetosphere. On Earth, we're exposed to about 0.066 rems of radiation per day. In contrast, Ganymede receives 8 rems of radiation per day, Europa receives 540 rems per day, and Io receives a whopping 3,600 rems. Callisto, in contrast, is exposed to about 0.01 rems per day, which humans can tolerate.
The process of terraforming these moons would all follow essentially the same recipe. First, heat up their icy surfaces either through giant mirrors, nuclear devices, or some other method. Then, let the radiation from Jupiter split the resulting water vapor into hydrogen and oxygen — the hydrogen will be blown into space by solar wind, while the oxygen will settle close to the surface. Use bacteria to convert the moons' ammonia into nitrogen, and there's a breathable atmosphere.
Of course, these planets would be completely covered in oceans hundreds of kilometers deep, and Callisto wouldn't have its own magnetosphere to keep that atmosphere in place long term, but their abundance of water makes it an attractive target nonetheless. More concerning is the possibility that life already exists beneath the Galilean moons' icy surfaces, in the warm waters by thermal vents. If we were to discover such life, would it be ethical to disrupt the only alien life we have ever known?
A composite image of Titan in infrared as seen by NASA's Cassini spacecraft. Because Titan's atmosphere is so hazy, viewing it in the wavelengths of visible light is not possible. Using the infrared spectrum enables us to see through the clouds to the moon's surface.
The appeal of terraforming Titan lies in its vast reservoir of resources. Its hydrocarbon reserves (such as petroleum) are several hundred times greater than all known reserves on Earth. It's covered in a wide variety of organic compounds, particularly methane and ammonia, as well as a great deal of water. And its atmosphere is primarily nitrogen as well — a composition that scientists believe resembles that of early Earth's.
Together, these ingredients would be of significant benefit to any terraforming project. If Titan's atmosphere does resemble early Earth's, then transitioning to an atmosphere that resembles modern Earth would be (relatively) straightforward. One proposal would be to position mirrors in orbit to direct focused sunlight onto the moon's surface. Since the surface ice contains many greenhouse gases, this could warm Titan up considerably, releasing water vapor and consequently oxygenating the atmosphere. It also spends most of its time within Saturn's magnetosphere, protecting its atmosphere from the solar wind.
But perhaps more so than any other body in our solar system, Titan could already have extraterrestrial life owing to its abundance of organic chemicals. And, if all of Titan's ice were melted, it would become an ocean planet 1700 km deep, or over 1,000 miles deep, making the establishment of fixed, permanent structures a challenge.
There are challenges common to all of these potential candidates for terraforming. The big one, of course, is getting there. Many of these targets are incredibly distant. For a comparison, it took Voyager 1 a little over three years to get to Saturn, where Titan, the most distant candidate, is located, and a ship with all of the necessary equipment, people, and resources would be significantly slower than a lightweight probe. Then, there's the issue of establishing a semipermanent colony while the long work of terraforming goes on. It's difficult to speculate about the capabilities we'll have at our disposal when terraforming a planet becomes a feasible project, but it could be hundreds, possibly thousands of years before any of these planets are completely terraformed. And these are just some of the known issues: a project of this scale is bound to have unexpected problems and consequences. Despite these major challenges, the vast majority of humanity believes that establishing a second home in our solar system is a necessity — the question is, which will it be?
The author of 'How We Read' Now explains.
During the pandemic, many college professors abandoned assignments from printed textbooks and turned instead to digital texts or multimedia coursework.
As a professor of linguistics, I have been studying how electronic communication compares to traditional print when it comes to learning. Is comprehension the same whether a person reads a text onscreen or on paper? And are listening and viewing content as effective as reading the written word when covering the same material?
The answers to both questions are often “no," as I discuss in my book “How We Read Now," released in March 2021. The reasons relate to a variety of factors, including diminished concentration, an entertainment mindset and a tendency to multitask while consuming digital content.
Print versus digital reading
The benefits of print particularly shine through when experimenters move from posing simple tasks – like identifying the main idea in a reading passage – to ones that require mental abstraction – such as drawing inferences from a text. Print reading also improves the likelihood of recalling details – like “What was the color of the actor's hair?" – and remembering where in a story events occurred – “Did the accident happen before or after the political coup?"
Studies show that both grade school students and college students assume they'll get higher scores on a comprehension test if they have done the reading digitally. And yet, they actually score higher when they have read the material in print before being tested.
Educators need to be aware that the method used for standardized testing can affect results. Studies of Norwegian tenth graders and U.S. third through eighth graders report higher scores when standardized tests were administered using paper. In the U.S. study, the negative effects of digital testing were strongest among students with low reading achievement scores, English language learners and special education students.
My own research and that of colleagues approached the question differently. Rather than having students read and take a test, we asked how they perceived their overall learning when they used print or digital reading materials. Both high school and college students overwhelmingly judged reading on paper as better for concentration, learning and remembering than reading digitally.
The discrepancies between print and digital results are partly related to paper's physical properties. With paper, there is a literal laying on of hands, along with the visual geography of distinct pages. People often link their memory of what they've read to how far into the book it was or where it was on the page.
But equally important is mental perspective, and what reading researchers call a “shallowing hypothesis." According to this theory, people approach digital texts with a mindset suited to casual social media, and devote less mental effort than when they are reading print.
Podcasts and online video
Given increased use of flipped classrooms – where students listen to or view lecture content before coming to class – along with more publicly available podcasts and online video content, many school assignments that previously entailed reading have been replaced with listening or viewing. These substitutions have accelerated during the pandemic and move to virtual learning.
Surveying U.S. and Norwegian university faculty in 2019, University of Stavanger Professor Anne Mangen and I found that 32% of U.S. faculty were now replacing texts with video materials, and 15% reported doing so with audio. The numbers were somewhat lower in Norway. But in both countries, 40% of respondents who had changed their course requirements over the past five to 10 years reported assigning less reading today.
A primary reason for the shift to audio and video is students refusing to do assigned reading. While the problem is hardly new, a 2015 study of more than 18,000 college seniors found only 21% usually completed all their assigned course reading.
Maximizing mental focus
Researchers found similar results with university students reading an article versus listening to a podcast of the text. A related study confirms that students do more mind-wandering when listening to audio than when reading.
Results with younger students are similar, but with a twist. A study in Cyprus concluded that the relationship between listening and reading skills flips as children become more fluent readers. While second graders had better comprehension with listening, eighth graders showed better comprehension when reading.
Research on learning from video versus text echoes what we see with audio. For example, researchers in Spain found that fourth through sixth graders who read texts showed far more mental integration of the material than those watching videos. The authors suspect that students “read" the videos more superficially because they associate video with entertainment, not learning.
The collective research shows that digital media have common features and user practices that can constrain learning. These include diminished concentration, an entertainment mindset, a propensity to multitask, lack of a fixed physical reference point, reduced use of annotation and less frequent reviewing of what has been read, heard or viewed.
Digital texts, audio and video all have educational roles, especially when providing resources not available in print. However, for maximizing learning where mental focus and reflection are called for, educators – and parents – shouldn't assume all media are the same, even when they contain identical words.
Humans may have evolved to be tribalistic. Is that a bad thing?
- From politics to every day life, humans have a tendency to form social groups that are defined in part by how they differ from other groups.
- Neuroendocrinologist Robert Sapolsky, author Dan Shapiro, and others explore the ways that tribalism functions in society, and discuss how—as social creatures—humans have evolved for bias.
- But bias is not inherently bad. The key to seeing things differently, according to Beau Lotto, is to "embody the fact" that everything is grounded in assumptions, to identify those assumptions, and then to question them.
Ancient corridors below the French capital have served as its ossuary, playground, brewery, and perhaps soon, air conditioning.