This is the third in a 10-part series about the ongoing global impacts of climate change. These stories will look at the current effects of a changing planet, what the emerging science suggests is behind those changes and what we all can do to adapt to them.
Hurricane Harvey slammed into Houston, Texas, on August 25, 2017. Normally, hurricanes keep moving. Their high winds and torrential rains tend to last for only a brief time. But Harvey just sat over the city. For days. And it dumped a lot of rain. Really, a lot. By the time the storm had moved on, on August 29, it had drowned Houston with a whopping 164 centimeters (64.6 inches) of water, according to one rain gauge. That’s a record rainfall from one storm in one place in the continental United States. In fact, Harvey dumped so much rain that the National Weather Service had to add new colors to their rainfall maps of the event.
Rising waters inundated more than 300,000 homes. That drove around 40,000 residents to take refuge in shelters across Texas and Louisiana. And of the some 100 people who died during the storm, more than 65 perished from flooding. Including damage from strong winds, researchers estimate that the storm caused more than $125 billion in damage. That tally makes Harvey the second-costliest hurricane to hit the U.S. mainland.
Hurricanes are a normal part of summertime weather. Since 1966, when satellites began daily monitoring of the North Atlantic Ocean, there have been an average of six hurricanes — and never less than two — per year. But more and more studies are revealing that human-caused climate change is influencing the size and fury of these storms.
And heavier rains and stronger storms are not the only ways in which a warming world is making our weather weirder. Higher temperatures can trigger droughts. Heat waves become more likely, and droughts can make them even worse. There can be changes to both global and local weather patterns. And the effects won’t always be what’s expected. In one truly odd twist, the continuing loss of summertime sea ice in the Arctic Ocean — one big result of a warming world — could make Siberian winters colder. What could be wackier than that?
Whence the weather?
Take a look outside. Is it hot and dry or cool and rainy? Cloudy or clear? Windy or calm? These are all aspects of the weather where you are. Simply put, weather is a snapshot of what’s happening in the atmosphere at a particular time and place. Lots of things affect the weather. That’s why meteorologists may have difficulty predicting what next week’s weather may be in any particular spot. Climate scientists, in contrast, have a pretty good handle on overall patterns of global weather.
Differences in how some regions heat up compared to others have a big influence on the weather. For instance, near the poles, the sun almost always hugs the horizon. In some seasons, one polar region basks in round-the-clock sunshine while the other languishes in darkness. These sorts of heating differences can trigger the development of high- and low-pressure areas in the atmosphere, says Sean Sublette. He’s a meteorologist with Climate Central in Princeton, N.J. That organization brings together scientists and journalists who research and report facts about Earth’s changing climate and its impacts.
Because air flows away from areas of high pressure, heating differences ultimately create winds. Over broad regions and long distances, winds travel on slightly curving paths due to Earth’s rotation. In the Northern Hemisphere, winds travel clockwise around a high-pressure system. Air rushing into a low-pressure area north of the equator generally swirls in the other direction. (If you pay close attention to weather reports during the nightly news, you can see these motions depicted on weather maps.)
On a more local scale, weather is often affected by the clash of air masses, notes Sublette. Thunderstorms, for example, can erupt where warm, humid air collides with cooler air. This triggers water vapor to condense. Eventually it falls as rain.
In recent decades, scientists have noted that episodes of heavy rains have become more common. In large part, says Sublette, that trend has been triggered by climate change. Scientists have noted that global average temperature has been, for the most part, rising steadily throughout recent decades. And for every 0.6 degree Celsius (1 degree Fahrenheit) increase in air temperature, the amount of water vapor the air can hold increases about 4 percent.
“Climate change is supercharging the water cycle,” he explains. Warmer air causes more water to evaporate from lakes, seas and moist soil. Now there’s more moisture in the air that can condense into raindrops. This is one source of bigger downpours, he notes.
In the United States, yearly precipitation for the lower 48 states has increased about 4 percent since 1901, a recent report found. In particular, clusters of thunderstorms in the central United States during warm months have been developing more often and dumping more rain. That’s especially true since 1979, the researchers note.
David Easterling helped to put together the report. He’s a climate scientist at the National Centers for Environmental Information in Asheville, N.C. The frequency and intensity of storms are projected to increase nationwide for the rest of this century, he says. But not all areas of the country will suffer the same rates of increase. The U.S. Southwest will likely see smaller increases in rainfall than northern states. But maybe that’s not surprising, because southwestern states typically are drier than others.
Rainfall going to extremes
Across the Houston region, Hurricane Harvey dumped an average of about 84 centimeters (33 inches) of water. That’s probably more rain from one storm than the region had seen at one time in centuries, a new analysis suggests. What’s more, the study hints, that sort of furious rainfall could easily occur much more often in the future.
As our planet has heated up, so have its seas. The warmer those waters are, the more water that will evaporate from the ocean surface, notes Easterling. The energy stored in warm tropical waters fuels the tropical storms known as hurricanes, cyclones or typhoons, depending on location. The more moisture present in a storm’s clouds, the more that can fall as rain.
Kerry Emanuel is an atmospheric scientist at the Massachusetts Institute of Technology in Cambridge. Late last year, he used computer models of hurricanes to estimate how often Harvey-like rainfalls have deluged the Houston area. Then, he used the same computer programs to look into the future. And his results aren’t encouraging for the folks in Texas, where Harvey’s flooding displaced thousands of residents.
Emanuel started by creating thousands of cyber-storms. If one of the storms in these computer simulations passed within 300 kilometers (190 miles) of Houston and had winds higher than 74 kilometers (46 miles) per hour, he made an estimate of how much rain it would have dumped. If conditions were always like those in the late 20th century, he found, Houston likely would see storms dump 50 centimeters (20 inches) or more only once every 2,000 years.
Then, he ran the same simulations again. Now he used climate conditions expected to occur between 2081 and 2100. Greenhouse-gas levels might be a lot higher during those years. And during this period, the Houston area might experience major deluges once a century, not once every two millennia. Emanuel reported his findings November 13, 2017 in the Proceedings of the National Academy of Sciences.
People often think of climate change only as a problem for the future. But Emanuel’s analysis reveals that that’s not true. Over the course of 100 years, he found, the chances of extreme rainfall increased about 18 percent. So, if the risk of extreme rainfall in the Houston area is steadily increasing, then the chances of the region seeing Harvey-like precipitation is today about six times what is was just 20 years ago. And because temperatures are rising worldwide, on average that means the chances globally of extreme flooding are on the rise, too.
Droughts and heat waves
Studies have shown that droughts, like the one that plagued California from 2011 to 2017, also will occur more often in the future. So will heat waves. Again, both are due to climate change. But a new analysis hints at something even more dire: They will happen more frequently. And when these phenomena both occur at the same time, each will reinforce the effects of the other.
Heat waves are extended periods marked by temperatures much higher than average for that particular region. Consider the summer of 2018. In late June and early July, an excessive heat wave in the United Kingdom may have killed hundreds of people. In early July, a five-day heat wave — including one day of record-breaking temperatures — hit Montreal, Canada. Researchers estimate some 70 people there also died, due at least in part due to the heat. A few weeks later, Japan suffered a two-week-long heat wave. This heat spell killed dozens and sent thousands more to the hospital.
In the future, such heat waves will occur more frequently for two reasons. First, climate change has boosted temperatures in most regions. That means higher-than-normal temperatures are themselves more common. But climate change, in general, also has weakened a high-altitude river of strong winds. Known as the jet stream, it helps push weather systems from west to east. A weaker jet stream means that weather systems of all sorts, whether heat waves or flooding rains, move more slowly. So storms have a better chance of dropping more rain in one spot. And heat waves have a better chance of hanging around longer.
In a warming world where rainfall is gradually increasing, it doesn’t seem as if drought should be a problem. But oddly, Sublette observes, both rainfall and drought can both be on the rise. “You’d think everything would balance out, but that’s not the case,” he says. Warmer temperatures will increase evaporation. That pulls more moisture from soil and sends it into the atmosphere. When that moisture falls back to earth as rain, sometimes it can’t soak into the soil quickly enough. So it runs off into rivers and streams. That’s especially true when the rains fall in extreme amounts, he explains.
Droughts and heat waves by themselves are bad enough. But together, they can be especially devastating, says Felicia Chiang. She’s a hydrologist at the University of California, Irvine. (Hydrologists study the flow of water, especially in its liquid form, over and through Earth’s surface.) Chiang’s team recently looked at the combined effects of heat waves and droughts in the continental United States.
When water evaporates, or transforms from a liquid to a gas, it soaks up a lot of energy. Evaporation dries out the soil, of course. But it also cools the air and ground. (Think of this process as the planet sweating.) Now, consider what happens in an area that’s in the middle of a drought. With little or no moisture in the soil, a lot more of the sunlight’s energy goes toward heating the ground, says Chiang. That, in turn, heats the air a lot more than it would have if the soil had been moist.
In their new analysis, Chiang and her teammates looked at weather data collected in the lower 48 states during two 50-year-long windows of time. One lasted from 1902 through 1951. The other one stretched from 1965 through 2014. First, they calculated the temperature difference between these two eras. And the more recent era was, on average, 0.25 degree C (0.45 degree F) warmer than the early 20th century. That was due to climate change, she notes. Then, the team calculated the temperature difference between the two eras only for sites experiencing droughts — and then only during those droughts. The temperature increase during the later era at these sites during their droughts was 1.06 degrees C (1.9 degrees F). Chiang and her teammates reported their findings August 1 in Science Advances.
This trend will only worsen with time. The drought-induced boost in heating will have widespread effects, the researchers suggest. While cities swelter in extended heat, crops in the countryside may be stunted or fail. Wild animals as well as livestock will suffer. And the wildfires that plagued drought-stricken areas from California to Greece this summer may be just a preview of what’s in store.
In general, it makes sense that droughts could make a heat wave worse. But who would expect that in a warming world, some areas would actually get colder? That’s exactly what scientists have noticed in recent years about winters in Siberia. The odd weather pattern is so unusual — and has been so baffling for so long — that scientists have given it a name. They call it the “warm Arctic, cold Siberia,” or WACS. Now, researchers may have come up with an explanation for this puzzling trend.
Previous attempts to explain the WACS trend had been unsuccessful, notes Pengfei Zhang. He’s an atmospheric scientist now at the University of California, Santa Barbara. A number of teams had tried to duplicate the trend using computer models. But he says none were successful. Some people pointed to those failures as proof that climate models are no good. Others used the models’ findings to dismiss the idea of climate change being real, says Zhang. Those skeptics suggested that what many scientists call “global warming” was actually just natural variability in climate.
Zhang and his teammates decided to investigate more deeply.
Many previous teams had relied on climate models that lacked many details. In particular, he notes, depictions of the atmosphere above the troposphere (TROH-puh-sfeer) — the lowest level of the atmosphere — weren’t very complete. And that might be important, they realized. After all, the troposphere is where most weather takes place.
Those simple models run fast, Zhang notes. But they didn’t necessarily give them good answers.
For a new analysis, Zhang’s team crafted a model that included a simulated atmosphere that extended up from Earth’s surface for some 140 kilometers (87 miles). Plus, they divided the atmosphere into more than 60 layers. As such, it was very detailed. Its simulations took more than 2 million hours of computing time, says Zhang. But they also offered a much better simulation of the planet.
First, the team ran a simulation representing the climate and sea-ice conditions in the Arctic Ocean for the years 1980 through 1999. Those results set a baseline against which other simulations could be compared. Then, the team ran the same simulation but reduced the amount of Arctic Ocean sea ice along Russia’s northern coast to match modern-day conditions. Sea ice has been disappearing in this part of the Arctic, especially in the fall. That loss of sea ice meant the ocean north of Russia was losing extra heat to the atmosphere.
The computer model projected that such a sea-ice loss would change overall weather patterns here in a number of ways. First, a persistent high-pressure area over western Russia strengthened. A generally long-lived low-pressure area over eastern Russia also got stronger. These trends combined to bring frequent bursts of cold air to Siberia from December through February. That’s the same sort of unusual weather pattern that researchers for decades had been seeing in the real world.
Then, Zhang and his team tweaked their model. They had the sea ice match what’s now found in the Arctic. And they dialed back the simulation’s interaction between the troposphere and the stratosphere high above it. This didn’t let heat rising from the ocean warm up the stratosphere so much. The result: There were relatively few outbreaks of cold weather in Siberia. In other words, this model now predicted just what Siberia had been experiencing.
This finding shows the importance of including the stratosphere in the team’s computer model of climate, says Zhang. Indeed, when they restored the interaction between troposphere and stratosphere but left sea ice in their simulation, Siberian winters were colder than average but not nearly as cold as those observed in recent years.
Zhang and his colleagues described their results online in the July 25 Science Advances.
“People ask,” Sublette says, “‘How can any place in the world be getting colder, when the world overall is warming?’” These sorts of simulations describe one shift in weather patterns that could make that happen, he notes.
Such findings also bolster the notion that links between climate and weather, especially at the local level, can yield unexpected — and uncomfortable — results.
air masses Large volumes of air, sometimes covering many hundreds or thousands of square kilometers (square miles), that typically have a consistent temperature or water-vapor content. Air masses are often classified by their source, such as continental, arctic or tropical. Air masses and other weather systems are steered across Earth’s surface by jet streams and by differences in atmospheric pressure.
Arctic A region that falls within the Arctic Circle. The edge of that circle is defined as the northernmost point at which the sun is visible on the northern winter solstice and the southernmost point at which the midnight sun can be seen on the northern summer solstice. The high Arctic is that most northerly third of this region. It’s a region dominated by snow cover much of the year.
Atlantic One of the world’s five oceans, it is second in size only to the Pacific. It separates Europe and Africa to the east from North and South America to the west.
atmosphere The envelope of gases surrounding Earth or another planet.
average (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
climate The weather conditions that typically exist in one area, in general, or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
cloud A plume of molecules or particles, such as water droplets, that move under the action of an outside force, such as wind, radiation or water currents. (in atmospheric science) A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Its movement is driven by winds.
colleague Someone who works with another; a co-worker or team member.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
condense To become thicker and more dense. This could occur, for instance, when moisture evaporates out of a liquid. Condense can also mean to change from a gas or a vapor into a liquid. This could occur, for instance, when water molecules in the air join together to become droplets of water.
crop (in agriculture) A type of plant grown intentionally grown and nurtured by farmers, such as corn, coffee or tomatoes. Or the term could apply to the part of the plant harvested and sold by farmers.
cyber A prefix that refers to computers or to a type of system in which computerized or online communication occurs.
cyclone A strong, rotating vortex, usually made of wind. Notable examples include a tornado or hurricane.
dire An adjective that means grave, or hard to survive.
drought An extended period of abnormally low rainfall; a shortage of water resulting from this.
equator An imaginary line around Earth that divides Earth into the Northern and Southern Hemispheres.
evaporate To turn from liquid into vapor.
fossil fuel Any fuels — such as coal, petroleum (crude oil) or natural gas — that have developed within the Earth over millions of years from the decayed remains of bacteria, plants or animals.
frequency The number of times some periodic phenomenon occurs within a specified time interval.
gauge A device to measure the size or volume of something. For instance, tide gauges track the ever-changing height of coastal water levels throughout the day. Or any system or event that can be used to estimate the size or magnitude of something else. (v. to gauge) The act of measuring or estimating the size of something.
global warming The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.
hurricane A tropical cyclone that occurs in the Atlantic Ocean and has winds of 119 kilometers (74 miles) per hour or greater. When such a storm occurs in the Pacific Ocean, people refer to it as a typhoon.
jet stream A fast-flowing, high-altitude air current. On Earth, the major jet streams flow from west to east in the mid-latitude regions of the Northern and Southern Hemispheres.
link A connection between two people or things.
livestock Animals raised for meat or dairy products, including cattle, sheep, goats, pigs, chickens and geese.
meteorologist Someone who studies weather and climate events.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes. Or an individual that is meant to display how something would work in or look on others.
moisture Small amounts of water present in the air, as vapor. It can also be present as a liquid, such as water droplets condensed on the inside of a window, or dampness present in clothing or soil.
National Weather Service An agency of the National Oceanic and Atmospheric Administration. Created in 1870, its current role is to collect weather, precipitation and climate data. It also issues forecasts and warnings 24 hours a day for the entire United States, focusing on signs of possible conditions that could threaten lives and structures.
outbreak The sudden emergence of disease in a population of people or animals. The term may also be applied to the sudden emergence of devastating natural phenomena, such as earthquakes or tornadoes.
persistent An adjective for something that is long-lasting.
phenomena Events or developments that are surprising or unusual.
poles (in Earth science and astronomy) The cold regions of the planet that exist farthest from the equator; the upper and lower ends of the virtual axis around which a celestial object rotates. (in physics and electrical engineering) The ends of a magnet.
precipitation (in chemistry) The creation of a solid from a solution. This can occur if there is too much of a chemical to dissolve completely into a solution. It also can be a sign that some chemical reaction is taking place. (in meteorology) A term for water falling from the sky. It can be in any form, from rain and sleet to snow or hail.
pressure Force applied uniformly over a surface, measured as force per unit of area.
Proceedings of the National Academy of Sciences A prestigious journal publishing original scientific research, begun in 1914. The journal's content spans the biological, physical, and social sciences. Each of the more than 3,000 papers it publishes each year, now, are not only peer reviewed but also approved by a member of the U.S. National Academy of Sciences.
resident Some member of a community of organisms that lives in a particular place. (Antonym: visitor)
risk The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
Siberia A region in northern Asia, almost all of which falls within Russia. This land takes its name from the language of the Tatar people, where Siber means sleeping land. This region is vast. It has become famous for its long, harsh winters, where temperatures can fall to −68° Celsius (−90° Fahrenheit).
simulation (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.
stratosphere The second layer of the Earth’s atmosphere, just above the troposphere, or ground layer. The stratosphere stretches from 10 kilometers to 50 kilometers (about 6.2 to 31 miles) above sea level.
sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.
technology The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.
troposphere The lowest level of Earth's atmosphere. It runs from the planet's surface to a height of 6 to 18 kilometers (4 to 11 miles), depending on the latitude. It's the region where the air is thickest and where most weather occurs. Air currents moving through this region often flow not only horizontally, but often vertically (up and down).
typhoon A tropical cyclone that occurs in the Pacific or Indian oceans and has winds of 119 kilometers (74 miles) per hour or greater. In the Atlantic Ocean, such storm are referred to as hurricanes.
United Kingdom Land encompassing the four “countries” of England, Scotland, Wales and Northern Ireland. More than 80 percent of the United Kingdom’s inhabitants live in England. Many people — including U.K. residents — argue whether the United Kingdom is a country or instead a confederation of four separate countries. The United Nations and most foreign governments treat the United Kingdom as a single nation.
water vapor Water in its gaseous state, capable of being suspended in the air.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.
Journal: F. Chiang et al. Amplified warming of droughts in southern United States in observations and model simulations. Science Advances. Vol. 4. August 3, 2018. doi: 10.1126/sciadv.aat2380.
Journal: P. Zhang et al. A stratospheric pathway linking a colder Siberia to Barents-Kara Sea sea ice loss. Science Advances. Vol. 4. July 25, 2018. doi: 10.1126/sciadv.eaat6025.
Journal: K. Emanuel. Assessing the present and future probability of Hurricane Harvey’s rainfall. Proceedings of the National Academy of Sciences. Vol. 115. November 13, 2017 (online). doi: 10.1073/pnas.1716222114.
Report: D.R. Easterling et al. 2017: Precipitation change in the United States. Chapter 7 in Climate Science Special Report: Fourth National Climate Assessment, Vol. 1. November 2017. doi: 10.7930/j0h993cc.