Earth, weather and climate

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• Watch Try it What If? Lab What if Earth had no atmosphere? Earth's air works like a jacket: it keeps heat in and carries it around. Take the air away in our model of a sandy patch of ground on the equator, and the coldest part of the night drops from about 13 °C to about 2 °C.
• Watch Try it What If? Lab What if there were no greenhouse gases? Greenhouse gases such as carbon dioxide and water vapor hold some of Earth's heat in, like a blanket. Take them all away in our energy-balance model and Earth's average temperature falls from about 15 °C to about −19 °C, well below freezing.
• Watch Try it What If? Lab What if there were more greenhouse gases? Adding greenhouse gases makes it harder for Earth's heat to escape, so the planet warms until it balances again. In our model a small change in how freely heat escapes warms Earth by about 2.6 °C, and the oceans make the warming take years.
• Watch Try it What If? Lab What if Earth were covered in ice? White ice bounces sunlight back to space, so an icy planet soaks up less of the Sun's warmth. In our energy-balance model, making Earth reflect 60 percent of its sunlight instead of 30 percent drops its average temperature from about 15 °C to about −23 °C.
• Watch Try it What If? Lab What if glaciers melted? Glaciers are huge, slow-moving masses of ice on land. When land ice melts, its water runs into the ocean and raises the sea. In our model, melting every mountain glacier on Earth would lift the sea by about 40 centimeters; scientists' best estimate is a little less, about 32.
• Watch Try it What If? Lab What if sea levels rose? When the sea rises, the water's edge moves inland. How far depends on the land: on a steep cliff it barely moves, but on a flat, gentle coast a rise of 76 centimeters in our model moves the shore about 760 meters.
• Watch Try it What If? Lab What if the oceans became warmer? Water expands a little when it warms. The ocean is so deep and wide that this small stretch adds up: in our model, warming the top 700 meters by 2 °C raises the sea about 28 centimeters, with no ice melting at all.
• Watch Try it What If? Lab What if an island experienced more erosion? Wave power grows with the square of wave height, so a wave about three times taller carries about 11 times the power. In our model, swapping 1.5-meter waves for 5-meter storm waves makes a sandy island wear away about 11 times faster, though plant roots help hold it together.
• Watch Try it What If? Lab What if a beach lost its dune plants? Grasses such as marram grass and sea oats grow on sand dunes, and their spreading roots help hold the sand in place. In our coast model, taking away all the plant cover makes ordinary waves wear a sandy shore about 1.7 times faster.
• Watch Try it What If? Lab What if the water cycle changed? Warm air can hold more water vapor, about 7 percent more for each degree. In our model of the whole planet's water cycle, a world 2 °C warmer evaporates about 14 percent more water each year if rain kept up with that rule, or about 4 percent more at 2 percent per degree, a rate within the range climate models find.
• Watch Try it What If? Lab What if rain grew more slowly than water vapor? Warm air can hold about 7 percent more water vapor for each degree. Climate models find that rain around the world grows only about 1 to 3 percent per degree. In this lab, warm the world by 2 °C and compare the two rates: if rain lags behind, water spends longer in the air.
• Watch Try it What If? Lab What if you lived where the Sun never sets? North of the Arctic Circle, Earth's tilt keeps the Sun above the horizon all day and all night around midsummer: the midnight Sun. In our sunlight model, moving from 40° north to 69.5° north on June 21 turns a 14.8-hour day into 24 hours of daylight.
• Watch Try it What If? Lab What if you lived on the equator? On the equator the Sun is up for about 12 hours every single day of the year, and it always climbs high at noon. In our sunlight model, moving from London to the equator on December 21 turns a 7.6-hour winter day into a 12-hour day. Near the equator, seasons are mostly about rain, not cold.
• Watch Try it What If? Lab What if June were winter? Earth's two halves take turns leaning toward the Sun, so they have opposite seasons. In our sunlight model, on June 21 London gets 16.4 hours of daylight and a high Sun, while Sydney, south of the equator, gets just 9.7 hours and a low Sun: its shortest day of the year.
• Watch Try it What If? Lab What if the Sun stayed low in the sky? A low Sun spreads its light thinly over more ground, and in winter it is up for fewer hours too. In our sunlight model, London's noon Sun drops from about 62 degrees in June to about 15 degrees in December, and its daily sunlight falls to about one seventh.
• Watch Try it What If? Lab What if you could stand where airliners fly? Air gets colder and thinner the higher you go. In our model of the standard atmosphere, climbing from sea level to 11 kilometers, near where airliners cruise, cools the air from 15 °C to about −56 °C and leaves less than a quarter of the air pressure.
• Watch Try it What If? Lab What if you counted the seconds after lightning? You see lightning almost at once, but its thunder travels at the speed of sound, about 343 meters per second. The National Weather Service says thunder takes about 5 seconds to travel a mile, so counting the seconds tells you how far away the lightning was. In our model, thunder from 1,700 meters away arrives after about 5 seconds.
• Watch What If? Lab What if all trees disappeared? Trees take in carbon dioxide and store carbon in their wood, put water into the air, hold soil in place and give homes to most kinds of land animals. Follow what would happen without them, and find out why the ocean still makes about half of our oxygen.
• Watch What If? Lab What if it rained every day? Rain is wonderful: it waters plants and fills rivers. But what if it rained every day? Soil can only soak up so much, so more water would run into rivers. Some rainforests and mountain slopes get rain very often, and life there is built for it. Explore what daily rain would change.
• Watch What If? Lab What if there were no clouds? Clouds shade the ground by day, send heat back down at night and are where rain comes from. Imagine a sky with no clouds at all, and follow what might happen to hot days, cold nights, rain and even the colors of the sky.
• Watch What If? Lab What if snow fell in the desert? A desert is a dry place, not always a hot one. Clear desert skies let heat escape at night, some deserts are icy cold, and snow has even dusted sand dunes in the Sahara. Explore what happens when snow falls in a desert, and how desert plants cope with the cold.
• Watch What If? Lab What if Earth had no mountains? Mountains push moving air upward so it drops rain and snow on one side, leaving the other side dry. Many rivers begin high in the mountains, and mountains are home to many plants and animals. Imagine flattening them all, and follow what might change.
• Watch What If? Lab What if volcanoes stopped erupting? Volcanoes build new land, like the island of Surtsey near Iceland, and their rock slowly breaks down into some of the richest soil on Earth. Over millions of years, gases from volcanoes even helped give Earth its air and oceans. Imagine every volcano going quiet, and follow what might change.
• Watch What If? Lab What if earthquakes could be predicted? No scientist has ever predicted a major earthquake, and USGS says nobody knows how. What scientists can do is work out the chances of a quake in an area, send a warning seconds after one starts, and help design buildings that shake less. Imagine we could know the exact day, and follow what might change.
• Watch What If? Lab What if the wind stopped blowing? Wind is air rushing from places of high pressure to places of low pressure. It carries heat, water and weather around the world, lifts seeds to new homes and has pushed sailing ships across oceans. Imagine all the air going still, and follow what might change.
• Watch What If? Lab What if lightning never struck? Lightning is a giant electric spark from a storm cloud. It's powerful, but it also does useful jobs: it turns some nitrogen in the air into plant food, and its fires help some forests renew. Imagine a world where lightning never struck, and see what would change and what would carry on.
• Watch What If? Lab What if rainbows appeared at night? A rainbow appears when light shines into drops of water from behind you. Sunlight usually does the job, but bright moonlight can too, making a faint “moonbow”. Explore moonbows, why the light must be behind you, and the ghostly white fog bow.
• Watch What If? Lab What if every snowflake were the same? Each snowflake grows as it falls through air that keeps changing in temperature and dampness, so no two take exactly the same path or end up exactly alike. Imagine a world where every snowflake were identical, and discover what that would take and what scientists would lose.
• Watch What If? Lab What if Earth had no deserts? About one third of Earth's land is desert. Deserts form where sinking air dries out the sky or mountains block the rain, and they are home to animals found nowhere else. Their dust even blows across an ocean to feed the Amazon rainforest. Imagine Earth with no deserts and see what would change.
• Watch What If? Lab What if there were no soil? Soil is broken-down rock mixed with rotted plants and animals, plus water, air and huge numbers of living things. It holds plant roots, feeds them and soaks up rain, and UNEP says about 95 percent of our food comes from it. Under good conditions, one inch of topsoil can take hundreds of years to form. Imagine a world with no soil, and follow what might change.
• Watch What If? Lab What if Earth had no ozone layer? High above the clouds, a gas called ozone soaks up most of the Sun's harmful ultraviolet light before it reaches us. Chemicals made by people thinned it, so in 1987 countries agreed to phase them out. Imagine Earth with no ozone layer at all, and follow what might change.
• Watch What If? Lab What if an ice age came back? About 20,000 years ago, ice sheets more than 3,000 meters thick covered much of North America, and the sea was about 125 meters lower than today. Ice ages come and go over tens of thousands of years as Earth's orbit and tilt slowly change. Imagine an ice age returning, and follow what might change.
• Watch What If? Lab What if the continents joined together again? Earth's outer shell is broken into plates that move a few centimeters a year. Long ago the continents were joined in one supercontinent called Pangaea, which USGS says began to break up about 225 to 200 million years ago. Imagine the continents joining together again, and follow what might change for land, climate and animals.
• Watch What If? Lab What if we could see the wind? Wind is moving air, and air is invisible. So people watch what the wind moves: smoke, leaves, flags, clouds and kites. Scientists even make wind maps from satellite pictures. Imagine the wind suddenly becoming visible, and follow the swirls, maps and kites that might appear.
• Watch What If? Lab What if there were no wetlands? Marshes, swamps and bogs are wetlands: places where water covers the soil or sits just below it. EPA compares them to natural sponges that soak up rain and floodwater and let it go slowly. They also clean water and shelter many birds and young fish. Imagine a world with no wetlands, and follow what might change.
• Watch What If? Lab What if a town used up its groundwater? Below the ground, water fills the tiny gaps between grains of rock and sand. In the United States, this groundwater is the drinking water for about half of all people. Rain and snow refill it, but often slowly. Imagine a town pumping faster than nature refills it, and follow what might change and how people help.
• Watch What If? Lab What if the Sahara turned green? Thousands of years ago, the Sahara Desert was a land of grass, wetlands and lakes, where cave paintings show hippos in watering holes. A slow wobble of Earth on its axis brought stronger monsoon rains. Find out why the Sahara was green, why it dried, and what might change if it turned green again.
• Watch What If? Lab What if clouds were made of candyfloss? Puffy clouds look like giant balls of candyfloss (cotton candy), but they are really made of tiny water droplets or ice crystals, and the water in a big one weighs about as much as 100 big African elephants. Imagine clouds spun from sugar instead, and find out what real clouds are made of.
• Watch What If? Lab What if pollution disappeared from cities? Vehicles, factories and other sources can put pollution into city air. Since 1970, EPA says total emissions of six main air pollutants in the United States have dropped by 78 percent, and cities such as Beijing have cut their pollution too. Imagine city air with no pollution at all, and follow what might change.
• Watch What If? Lab What if a drought lasted for years? A drought is a long stretch with much less rain or snow than normal, long enough that water runs short. Droughts can last a few weeks, a few months or several years. Imagine a land where the dry years keep coming, and follow its rivers, the ways people save water, and what it takes for a drought to end.
• Watch What If? Lab What if a city had no trees? Cities are full of roads, roofs and parking lots that soak up the Sun's heat. Street trees shade them, cool the air, catch rain and give birds a home. Imagine a city with every tree gone: hotter streets, more rain rushing down the drains and fewer birds. Then see how people plant trees to help.
• Watch What If? Lab What if a mountain wore away? Even the tallest mountains are slowly broken apart by ice, rain, plant roots and wind, and the pieces are carried away by rivers and glaciers. Imagine following a mountain as it wears away: its rock might end up as sand on a beach, mud on a farm or new land at a river's mouth, while moving plates push other mountains up.
• Watch What If? Lab What if a rainforest had little rain? Rainforests are tall, green forests that get a great deal of rain. In 2005 and 2010, big droughts hit the Amazon rainforest, and satellites watched what happened to its trees. Use those real events to imagine a rainforest with little rain: drier leaves, hungry animals and a forest that might change.
• Watch What If? Lab What if a river changed its course? A winding river wears away the outside of each bend and piles sand on the inside, so slowly, year by year, it moves. Sometimes it cuts off a loop and leaves a lake behind. Imagine a big river changing its course: what would happen to the old loops, and to the towns and bridges beside it?
• Watch What If? Lab What if a river flooded every year? For thousands of years, the Nile in Egypt flooded every year, spreading fresh mud across its banks. That mud made the land rich for farming, in the middle of a desert. Imagine a river that flooded every year: you could plan for it, farm on it, and build banks called levees to control it. Many of the earliest towns grew up beside rivers like this.
• Watch What If? Lab What if cold wind blew over a warm lake? In late fall and winter, the Great Lakes are often much warmer than the freezing air that often blows in from Canada. As cold wind crosses the open water, it picks up warmth and moisture, rises and builds narrow bands of clouds that can drop 2 to 3 inches of snow an hour, while a town a mile or two away stays sunny. This thought experiment follows the air across the lake.
• Watch What If? Lab What if every day had the same weather? Weather changes because the Sun heats some places more than others, and big bodies of warm or cold, wet or dry air keep moving around. Imagine a world where every day had the same weather: no need to check a forecast, but plants and animals that follow the seasons might be confused. Some places near the equator come surprisingly close.
• Watch What If? Lab What if fog never lifted? Fog is a cloud resting on the ground, made of tiny water droplets floating in the air. Most morning fog disappears as the Sun warms the ground. Imagine fog that never lifted: travel would be slower, but in some dry places people already use nets to catch drinking water from fog.