Climate: quotes, facts and reflections - Page 2

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WonderMigration

The sardine run takes place off South Africa's east coast from about May to July, when vast shoals of sardines follow a band of cool water north along the shore. Dolphins, gannets, sharks, seals and whales gather to feed on them.

Sardines normally live in the cooler waters off South Africa's southern and western coasts. In some winters a narrow strip of cold water forms close to the shore of the Eastern Cape and KwaZulu-Natal, between the coast and the warm Agulhas Current, and the fish move north along it. The shoals can stretch for several kilometres and are followed from the air by spotter planes. Common dolphins herd the sardines into tight balls near the surface, Cape gannets plunge in from height, and sharks, Cape fur seals and Bryde's whales feed on the exposed fish. Scientists are still not sure what sets the run off: in some years it is weak or barely happens, and why is not fully understood.

KwaZulu-Natal sardine runRead card

WonderEvolution

Maize, sugarcane and sorghum grow well in heat and strong sun partly because they use C4 photosynthesis, a way of concentrating carbon dioxide inside the leaf. This method has evolved independently more than 60 times in different groups of plants.

Most plants use C3 photosynthesis, which relies on an enzyme called RuBisCO to capture carbon dioxide. In heat and drought, plants close the pores in their leaves to save water, carbon dioxide inside runs low, and RuBisCO grabs oxygen instead, wasting energy in a process called photorespiration. C4 plants first trap carbon dioxide in their outer leaf cells as a compound with four carbon atoms, which gives the pathway its name, then carry it to inner cells where RuBisCO works and release it there, so the enzyme is surrounded by carbon dioxide. With more than 60 separate origins, it is one of the most often repeated adaptations in plant evolution. Scientists are now trying to engineer the trait into rice.

C4 photosynthesisRead card

DriveEngineering

Hoover Dam, built between 1931 and 1936 in the Black Canyon of the Colorado River on the Nevada–Arizona border, controls floods, stores water in Lake Mead and generates hydroelectric power for the American Southwest.

The dam was built during the Great Depression, when thousands of men came to the desert looking for work. By the official count, 96 workers died in accidents during construction. At 221 metres it was the tallest dam in the world when finished, and it created Lake Mead, the largest reservoir in the United States by capacity. Its water is shared among seven US states under the 1922 Colorado River Compact, and with Mexico under a 1944 treaty. So much of the river is now taken for cities and farms that it rarely reaches the sea. In 2022, after years of drought, Lake Mead fell to its lowest level since it was first filled.

Hoover DamRead card

WonderArchaeology

Skara Brae, a village on Orkney off the north coast of Scotland, was built about 5,000 years ago, and its stone houses still hold their beds, hearths and dressers. It is one of the best-preserved Neolithic, or New Stone Age, villages in Europe.

Built in stone around 3180 BC and lived in for perhaps 600 years, Skara Brae is older than Stonehenge's famous stone circle and Egypt's Great Pyramid of Giza. A storm in 1850 stripped away the sand dune that had buried the village, revealing houses linked by covered passageways, each with a stone hearth, box beds and a dresser for displaying possessions, all built from slabs because Orkney has little timber. The archaeologist V. Gordon Childe carried out the first scientific excavations in the 1920s. The site now forms part of the Heart of Neolithic Orkney World Heritage Site, alongside the Ring of Brodgar and the Standing Stones of Stenness, two nearby stone circles from the same era.

Skara BraeRead card

WonderNature

The saiga, an antelope of the Central Asian steppes, has an enlarged, flexible nose that filters out dust in summer and warms and moistens freezing air in winter before it reaches the lungs.

Saigas live on the dry grasslands of Kazakhstan, Russia, Uzbekistan and Mongolia, and herds migrate hundreds of kilometres between summer and winter pastures. Their numbers collapsed after the Soviet Union broke up: poaching of males for their horns, used in traditional Chinese medicine, cut the population by about 95 per cent. In May 2015, during unusually warm, humid weather, a bacterium the animals normally carry harmlessly, Pasteurella multocida, caused a blood infection that killed about 200,000 saigas in Kazakhstan within a few weeks, more than half of all saigas then alive. Protection from poaching has since let the Kazakh herds recover so strongly that in 2023 the International Union for Conservation of Nature moved the species from Critically Endangered to Near Threatened.

Saiga antelopeRead card

StillnessNature

Biological soil crusts are living communities of cyanobacteria, lichens, fungi, mosses and algae that bind dryland soil and influence water and nutrient cycles.

Scientists often call these crusts biocrusts. The cyanobacterium Microcoleus vaginatus is usually among the first to settle: its sticky filaments glue sand grains together and prepare the ground for the lichens, mosses and algae that follow. Some crust organisms also fix nitrogen, taking it from the air and turning it into a form plants can use, which matters in poor desert soils. Growth is slow in dry ground, so a crushed crust can take years to regain its first layer and decades or even centuries to rebuild fully. In US national parks on the Colorado Plateau, such as Arches and Canyonlands, signs ask visitors to stay on the trails with the slogan 'Don't bust the crust'.

Biological soil crustRead card

StillnessNature

Cloud forests catch water straight from passing cloud: tiny droplets collect on leaves, mosses and branches, then drip to the ground, adding a significant share of the forest's water.

Scientists call this fog drip or horizontal precipitation, because the water arrives sideways on the wind rather than falling as rain. Epiphytes, plants such as mosses, ferns and orchids that grow on other plants instead of in soil, are especially good at it: their tangled surfaces multiply the area that catches droplets. In some cloud forests, fog supplies a significant share of the water that reaches the ground in the dry season, when rain is scarce. Monteverde in Costa Rica is one of the best-studied examples, and research published there in 1999 linked warming seas to clouds forming higher up the mountains, leaving less fog in the forest. Clearing the trees removes this water catcher along with the forest.

Cloud forestRead card

WonderNature

Some lodgepole pines keep their seeds inside cones sealed shut with resin. The heat of a forest fire melts the seal and releases the seeds onto freshly cleared, ash-rich ground.

Botanists call this trait serotiny, from a Latin word meaning 'late': the cones open late, once heat has melted their resin, typically at around 45 to 60 degrees Celsius. Sealed cones can keep seeds alive on the branches for many years, so a stand carries its own seed bank long before any fire arrives. The strategy became famous after the Yellowstone fires of 1988, when many burnt slopes filled with young lodgepole pines within a few years. Not every tree does it. In some populations most cones stay sealed, while in others most open each year like ordinary cones, a mix that tends to follow how often fire has struck the area.

SerotinyRead card

WonderOcean

Every night, vast numbers of sea animals rise from the depths towards the surface to feed, then sink back before dawn. This daily journey, called diel vertical migration, carries carbon down into the deep ocean.

The migration was first noticed by accident. In 1942 the scientists C. F. Eyring, R. J. Christensen and R. W. Raitt, working with sonar aboard the USS Jasper, found a false seafloor 300 to 500 metres down by day that sat shallower at night. The echo came from millions of animals, above all small fish of the dim middle depths whose gas-filled swim bladders reflect sound. The layer is now called the deep scattering layer. The animals rise under cover of darkness to feed on plankton and descend at dawn to hide from predators that hunt by sight. By mass it is often described as the largest migration on Earth, and it moves carbon downwards as the animals breathe, excrete and are eaten at depth.

Diel vertical migrationRead card

StillnessOcean

Marine snow is a continuous fall of organic particles, including plankton remains, mucus, faecal pellets and other debris, from upper waters into the deep sea.

Much of this material is eaten or broken down by bacteria on the way, and only a small fraction settles on the seabed, yet it is the main food supply for many deep-sea animals, far below where sunlight lets algae grow. Flakes grow as particles clump together and can sink tens to hundreds of metres a day. The name is often traced to the naturalist William Beebe, who saw particles drifting in his lights during bathysphere dives off Bermuda in the 1930s. Scientists measure the fall with sediment traps and underwater cameras, because it is a key part of the ocean's biological carbon pump: carbon taken up by plankton at the surface sinks, and what reaches the seabed can stay there for centuries or longer.

Marine snowRead card

WonderOcean

Coccolithophores are tiny single-celled algae that cover themselves in intricate plates of chalk (calcium carbonate). Drifting in sunlit seawater in vast numbers, they link microscopic life to ocean chemistry and the global carbon cycle.

Each plate, called a coccolith, is a few thousandths of a millimetre across; it is built inside the cell and then pushed out to its surface. When coccolithophores die, their plates sink, and over millions of years thick layers of them have hardened into chalk: the White Cliffs of Dover are made largely of coccoliths laid down in the Cretaceous period, roughly 100 to 66 million years ago. The commonest species today, Emiliania huxleyi, forms blooms that turn the sea milky turquoise and can be seen from space. Coccolithophores take up carbon through photosynthesis, and their sinking plates carry carbon to the seabed, where scientists read their fossil remains to reconstruct past climates.

CoccolithophoreRead card

WonderOcean

The Antarctic Circumpolar Current flows uninterrupted around Antarctica, connecting the Atlantic, Pacific and Indian oceans while transporting heat, nutrients and carbon.

The current flows eastwards, pushed by strong westerly winds, and is the strongest current system in the world's oceans. Estimates of its flow range from about 135 to 175 sverdrups (one sverdrup is a million cubic metres of water per second), well over a hundred times the flow of all the world's rivers combined. It could form only after South America and Australia pulled away from Antarctica, opening the Drake Passage and the Tasmanian Seaway; many scientists date its start to about 34 million years ago, though the timing is debated. By keeping warmer water away from the continent, the current helped Antarctica's great ice sheets grow, and today it carries heat and carbon between the Atlantic, Pacific and Indian oceans.

Antarctic Circumpolar CurrentRead card

WonderOcean

In polar seas, strong winds or warmer water welling up from below can keep patches of sea open where thick ice would normally form. These areas of open water or thin ice are called polynyas.

The name comes from a Russian word for a patch of open water in ice. Polynyas matter to wildlife: seals, walruses, whales and seabirds use them to breathe and feed. As new ice keeps forming at their edges, it leaves its salt behind in very dense water, which sinks and helps drive deep ocean currents. The North Water, between Greenland and Canada's Ellesmere Island, is one of the Arctic's richest wildlife areas; the Inuit call it Pikialasorsuaq, 'the great upwelling'. Other polynyas are rare: the Weddell Sea polynya in Antarctica, seen by satellite in the mid-1970s, opened again briefly in 2016 and 2017, and scientists are still working out what triggers it.

PolynyaRead card

WonderOcean

Along some coasts, winds push surface water out to sea and cold, nutrient-rich water rises from the depths to take its place. This upwelling feeds plankton and, through them, some of the richest fishing grounds on Earth.

Five coastal upwelling zones stand out: the Humboldt Current off Peru and Chile, the California Current, the Canary Current off north-west Africa, the Benguela Current off south-western Africa and the Somali Current off Somalia and Oman. Together they cover only about 5 per cent of the ocean, yet they yield roughly a quarter of the world's marine fish catch, much of it small fish such as anchovies and sardines. Off Peru, the anchoveta supports one of the largest single-species fisheries in the world. That richness is fragile. During an El Niño, warm water spreads along the South American coast and weakens the upwelling, so fewer nutrients reach the surface and anchovy numbers crash, along with the seabirds and sea lions that feed on them. The El Niño of 1972 to 1973, on top of heavy fishing, helped bring Peru's anchovy fishery to collapse.

UpwellingRead card

WonderClimate

Atmospheric rivers are long, narrow bands of moist air that carry huge amounts of water vapour, often from the tropics. When mountains force them upward over land, the vapour cools and falls as heavy rain or snow.

Researchers Reginald Newell and Yong Zhu of the Massachusetts Institute of Technology named them in the early 1990s, after finding that much of the water vapour moving through the atmosphere travels in a few narrow plumes, typically thousands of kilometres long but only a few hundred wide. A single strong one can carry more water, as vapour, than the Amazon carries as liquid. Those that reach the west coast of North America from the tropical Pacific near Hawaii are nicknamed the Pineapple Express. They bring much of California's rain and snow, and also many of its worst floods. Since 2019, scientists at the Scripps Institution of Oceanography have ranked them on a five-level scale, from weak and mainly beneficial to exceptional and mainly hazardous, based on how much vapour they carry and how long they last.

Atmospheric riverRead card

WonderClimate

Yedoma is ancient frozen ground in Siberia, Alaska and Canada's Yukon, packed with ice and with plant and animal remains tens of thousands of years old. When it thaws, microbes can finally break that old carbon down, releasing carbon dioxide and methane.

Yedoma formed during the Pleistocene, the ice-age epoch that ended about 11,700 years ago. Fine silt, carried by wind and water, piled up on cold plains that were never covered by glaciers, burying grasses and roots before they could rot, while huge wedges of ice grew in the ground. Ice now makes up 50 to 90 per cent of its volume, and the deposits, often tens of metres thick, cover more than a million square kilometres. Estimates of the carbon they hold range from about 210 to 500 billion tonnes, many times what burning fossil fuels releases each year. Because the ground is so icy, it slumps into pits and ponds, called thermokarst, as it thaws, exposing still more. Some of the carbon escapes as methane, a far more powerful greenhouse gas than carbon dioxide.

YedomaRead card

DriveClimate

A marine heatwave is a spell of unusually warm sea water lasting from days to months. It can kill kelp forests and corals, drive fish and other animals into new waters and upset fisheries.

Under a definition widely used since 2016, an event counts as a marine heatwave when the water stays warmer than 90 per cent of past readings for that place and time of year for at least five days. The term itself was coined after an unprecedented event off Western Australia in the summer of 2011, when warm water killed kelp forests, underwater forests of large brown seaweed, along hundreds of kilometres of coast. In many places the kelp did not come back: low, turf-like seaweeds and fish more typical of warmer waters took over, and the reef became a different kind of ecosystem. Another well-known case was the Blob, a mass of warm water that sat in the north-east Pacific from 2014 to 2016 and disrupted marine life and fisheries along the west coast of North America.

Marine heatwaveRead card

WonderClimate

Dust from the Sahara crosses the Atlantic and falls on the Amazon, bringing phosphorus that helps replace nutrients washed out of the rainforest's soils by rain.

Studies suggest that more than half of the dust reaching the Amazon comes from one place: the Bodélé Depression in Chad, the dry bed of a much larger ancient lake. Its surface is made largely of diatoms, the fossil shells of tiny algae, and strong winds lift them high into the air. Summer winds carry the dust west over the Atlantic; plumes take about five to seven days to reach the Caribbean. Most Amazon soils are old and heavily washed out, and short of phosphorus, a nutrient that limits how fast trees grow. Dust supplies only part of the phosphorus that falls on the basin, but the amount is comparable to what the forest loses each year to rain and rivers, so without it the soils would slowly run down.

Saharan dustRead card

StillnessClimate

Stalagmites and other mineral formations in caves grow in thin layers that can be dated precisely. The chemistry of each layer records how much rain fell above the cave thousands of years ago.

Scientists call these formations speleothems, from the Greek for 'cave deposit'. Rainwater picks up carbon dioxide from the soil, turns slightly acidic and dissolves limestone as it seeps down through cracks. Inside the cave, where the air holds less carbon dioxide, each drip gives up some of the gas and leaves a thin film of calcite behind; over centuries these films build a stalagmite. The calcite traps traces of uranium, which decays into thorium at a known rate, so layers can be dated precisely over hundreds of thousands of years. Scientists then measure oxygen isotopes, slightly heavier and lighter forms of oxygen atom, whose balance shifts with rainfall. Records from Chinese caves, for example, trace how the Asian monsoon strengthened and weakened through the last ice age.

SpeleothemRead card

WonderArchaeology

Doggerland was a stretch of low-lying land, now under the North Sea, that joined Britain to continental Europe after the last ice age, until rising seas slowly drowned it.

It was a land of rivers, marshes, lakes and woodland, rich in fish, birds and game, and home to Mesolithic (Middle Stone Age) hunter-gatherers. The first clear sign of people came in 1931, when the trawler Colinda dragged up a lump of peat about 40 kilometres off Norfolk with a barbed antler point inside, dated to between 10,000 and 4,000 BC. Fishing boats have since netted mammoth bones, tools and even a fragment of a Neanderthal skull, more than 40,000 years old, dredged off the Dutch coast. By about 6500 BC rising seas had cut Britain off from the continent, although the Dogger Bank survived as an island until at least 5000 BC. Around 6200 BC, a huge underwater landslide off Norway, the Storegga Slide, sent a tsunami across the North Sea; how much it destroyed is still debated.

DoggerlandRead card

WonderArchitecture

Chand Baori is a stepwell in Rajasthan, India: a well you reach by walking down stairs. Its 3,500 steps descend about 30 metres through 13 storeys to the water at the bottom.

Chand Baori stands in the village of Abhaneri and is said to be named after Raja Chanda, a local ruler of the Nikumbh dynasty. No inscription records its building, but its style dates it to the 8th or 9th century. In a dry region where most rain falls in a few monsoon months, stepwells stored water and let people reach it as the level dropped through the year. Here zigzag flights of steps line three sides of the square shaft, and the fourth side holds a pillared gallery with rooms. The depth also kept the air at the bottom noticeably cooler than at the surface. Beside the well stands the temple of Harshat Mata, a goddess of joy.

Chand BaoriRead card

WonderArchitecture

Shibam, a walled town in Yemen, is packed with about 500 mud-brick tower houses, some eleven storeys and over 30 metres tall. Built mostly in the 16th century, it is one of the oldest examples of a city planned to grow upwards.

People have lived at Shibam, in the long desert valley of Wadi Hadramawt, for about 1,700 years. A great flood in 1532–33 destroyed much of the earlier town, which was rebuilt on a rocky rise above the valley floor. Enclosed by a single defensive wall, it had little room to spread, so houses went up instead, with one or two rooms on each floor. Thick walls of sun-dried mud brick keep interiors cool in the desert heat, but they need constant upkeep: floods from a tropical storm in 2008 brought down several buildings. Shibam is often called the Manhattan of the Desert, and UNESCO listed it as a World Heritage Site in 1982.

Old Walled City of ShibamRead card

StillnessArchitecture

A mashrabiya is a projecting window enclosed by a carved wooden lattice, common in old Arab towns. The screen breaks up glare, lets breezes through and allows people inside to watch the street without being seen.

The earliest evidence of the mashrabiya in its present form comes from 12th-century Baghdad under the Abbasid caliphs. It spread to Egypt, the eastern Mediterranean, Arabia and beyond, reached its peak under the Ottomans, and goes by other names: shanashil in Iraq, rawashin in Jeddah. The lattice is usually finer at the bottom and more open at the top, so air moves fast above head height and gently lower down, and jars of water placed in the window cooled the breeze by evaporation as it passed over them. Privacy mattered too: the household could follow life in the street while staying out of sight. Mashrabiyas fell out of use in the 20th century, but architects now study them again as a way to cool buildings without machines.

MashrabiyaRead card

WonderClimate

In 1856 the American scientist and women's rights campaigner Eunice Newton Foote showed that a glass cylinder of carbon dioxide grew hotter in sunlight than one of ordinary air, and took longer to cool. She suggested that more of this gas in the air would warm the Earth.

Foote was an amateur scientist living in Seneca Falls, New York. In 1848 she signed the Declaration of Sentiments at the Seneca Falls Convention, the first meeting devoted to women's rights, and helped prepare its proceedings for publication. For her experiment she used an air pump, two glass cylinders and four thermometers, and compared dry air, moist air, carbon dioxide and hydrogen in sunshine and in shade. Her paper was read at the August 1856 meeting of the American Association for the Advancement of Science by Joseph Henry of the Smithsonian Institution; why she did not present it herself is unclear. Three years later John Tyndall measured how gases absorb infrared heat, the mechanism behind the greenhouse effect, and he received the credit. Foote's paper was largely forgotten until the 21st century.

Eunice Newton FooteRead card