Global sea level rise is complex as well. To begin with, it has multiple causes, including the thermal expansion of the ocean as it warms, runoff of meltwater from land-based ice sheets and mountain glaciers, and changes in water that’s stored on land. These factors combine to raise the height of our global ocean about 3.3 millimeters (0.13 inches) every year. That rate is accelerating by another 1 millimeter per year (0.04 inches per year) every decade or so.
Another factor that makes sea level rise complex is that it’s not uniform around the globe. If you look at a global map of sea level rise, you’ll find it’s happening rapidly in some places and more slowly in others. This means that although sea level rise affects coastal areas all over our ocean planet, some regions feel its effects sooner and more severely than others. This is reflected in future projections of sea level rise, with many cities in Asia expected to be among the hardest hit localities. Here in the United States, cities expected to see the worst impacts include New York, Miami and New Orleans, to name but a few.
Indeed, at any given place and time around our planet, sea level rise varies. But why is that? It turns out that when it comes to sea level rise, it’s all local. And it’s all relative.
Relative Sea Level
“Relative sea level” refers to the height of the ocean relative to land along a coastline. Common causes of relative sea level change include:
- Changes due to heating of the ocean, and changes in ocean circulation
- Changes in the volume of water in the ocean due to the melting of land ice in glaciers, ice caps, and ice sheets, as well as changes in the global water cycle
- Vertical land motion (up or down movements of the land itself at a coastline, such as sinking caused by the compaction of sediments, or the rise and fall of land masses driven by the movement of continental or oceanic tectonic plates)
- Normal, short-term, frequent variations in sea level that have always existed, such as those associated with tides, storm surges, and ocean waves (swell and wind waves). These variations can be on the order of meters or more (discussed in more detail in our previous blog post).
Let’s look at these factors more closely.
When you heat up water, it expands and takes up more space. How much it expands depends on how deep the warming occurs as well as the temperature of the water to begin with. For example, in Earth’s tropics, a 1-degree Celsius (1.8 degrees Fahrenheit) warming in the temperature of the top 100 meters (328 feet) of the ocean raises sea level there by about 3 centimeters (1.2 inches). This thermal expansion of the ocean is responsible for between one-third and one-half of the overall global sea level rise observed over the last two decades. Because Earth’s ocean isn’t warming at the same rate everywhere, it results in regional differences in relative sea level rise, with areas that are warming faster seeing faster sea level rise.
Changes in ocean circulation also contribute to regional sea level differences. For example, in the United States, El Niño, a cyclical, naturally-occurring ocean circulation pattern of warming (in the central and eastern tropical Pacific Ocean) and cooling (in the western tropical Pacific Ocean) can temporarily raise relative sea level along the West Coast by more than a foot for up to a couple of years. Similarly, along the U.S. East Coast, the speedup or slowdown of the major ocean current known as the Gulf Stream can temporarily add or subtract as much as 5 centimeters (2 inches) of sea level height to local coastlines.
Next, there’s melting land ice in the Greenland and Antarctic ice sheets and Earth’s glaciers and ice caps. The largest contribution is from Greenland, which loses hundreds of billions of tons of ice a year and is a major contributor to sea level rise across the globe. As Greenland loses ice, the land beneath its ice sheet rises as the weight of the ice sheet is removed. As a result, Greenland itself doesn’t see any local sea level rise.
But all of its melted ice — currently averaging 281 gigatons a year, as measured by the U.S./German Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-on (GRACE-FO) satellite missions — has to go somewhere. Gravity causes it to flow into the ocean, causing sea level to rise thousands of miles away. Data from GRACE-FO tell us that melting land ice in glaciers, ice caps, and ice sheets contributed about two-thirds of global sea level rise during the last decade.
As land ice in Greenland, Antarctica and elsewhere melts, it changes Earth’s gravity field and slightly shifts the direction of Earth’s rotation. This causes uneven changes in sea level across the globe. Each melting ice mass around the world creates its own unique pattern of sea level change in the global ocean. For example, when ice melts in Antarctica, the amount of sea level rise it generates in California and Florida is up to 52 percent greater in those locations than if the global ocean just filled up uniformly, like water in a bathtub. Scientists use gravity data from the GRACE-FO mission to calculate patterns of sea level change associated with the loss of ice from glaciers, ice caps and ice sheets, as well as from changes in land water storage.
Then there’s vertical land motion along coastlines. When land sinks (a process known as subsidence), it causes a relative increase in sea levels. When land rises (known as uplift), it results in a relative decrease in sea levels.
A number of factors, both natural and human-produced, cause land to rise or sink, including:
- Adjustments related to the rebound of land during and following the retreat of past ice sheets in North America and Eurasia at the end of the last Ice Age (known as isostatic, or post-glacial, rebound). The retreat of the ice sheets lightened the load of mass on the underlying mantle deep below Earth’s surface, causing Earth’s surface there to slowly rise. Land areas that were once near the edge of these ancient ice sheets, such as along the U.S. eastern seaboard, are today falling, exacerbating sea level rise there.
Plate tectonics. Earth is divided into multiple slowly moving tectonic plates that interact with each other along plate boundaries. At some plate boundaries, the motion of one plate under, over, or past another results in vertical uplift or subsidence of the land surface above.
Natural or human-produced compaction of sediments, such as those caused by pumping groundwater, or oil and gas. Subsidence related to groundwater withdrawal can be especially pronounced in areas with large populations and extensive agriculture. Sediments can also be compacted by construction activities by humans or by the natural settling of new soils. In the United States, subsidence is a major factor in relative sea level rise along parts of the Gulf and East Coasts.
Oceanographer and climate scientist Josh Willis of NASA’s Jet Propulsion Laboratory in Southern California says that when it comes to relative sea level rise at any particular coastal location, subsidence is the most immediate consideration.
“People in coastal areas need to know what the land is doing right now where they live,” he said. “Is it sinking? If so, how fast? When you combine a sinking coastline with sea level rise caused by other contributing factors, you’re in trouble. Remember, scientists are projecting feet of global-mean sea level rise in this century. But in some places, land can sink by one foot in a decade. We have to understand all of these pieces before we can project future sea level rise at a beach near you.”
Climate scientists will tell you a key challenge in studying climate change is the relative dearth of long-term monitoring sites around the world. The oldest continuously operating station — the Mauna Loa Observatory on Hawaii’s Big Island, which monitors carbon dioxide and other key constituents of our atmosphere that drive climate change — has only been in operation since the late 1950s.
This obstacle is even more profound in the world’s coastal areas. In the global open ocean, the international Argo program’s approximately 4,000 drifting floats have observed currents, temperature, salinity and other ocean conditions since the early 2000s. But near coastlines, the situation is different. While coastal weather stations are plentiful, their focus is to produce weather forecasts for commercial and recreational ocean users, which aren’t necessarily useful for studying climate. The relative lack of long-term records of surface and deep ocean conditions near coastlines has limited our ability to make accurate oceanographic forecasts.
A meteorological and oceanographic coastal station in the small Spanish coastal town of L’Estartit is a notable exception. Located in the Catalan Costa Brava region of the northwest Mediterranean Sea, the L’Estartit station has collected inland data on air temperature, precipitation, atmospheric pressure and humidity since 1969, and has also made oceanographic observations at least weekly since 1973. This makes L’Estartit the longest available uninterrupted oceanographic data time series in the Mediterranean. A new NASA-funded study presents a detailed analysis of the site, revealing climate trends for its Mediterranean coastal environment spanning nearly a half century.
“The long-term data set from L’Estartit is a treasure trove that’s useful for assessing the regional impacts of climate change and how it’s evolved over time."
The study, led by Jordi Salat of the Institut de Ciències del Mar (CSIC) in Barcelona, provides estimates of annual trends in sea and atmospheric temperature and sea level, along with seasonal trends. It also compares data from the site with previous and other estimates of climate trends in the region. Co-authors include Josep Pascual, also with CSIC; oceanographers Jorge Vazquez and Mike Chin of NASA’s Jet Propulsion Laboratory in Southern California; and Mar Flexas of Caltech, also in Southern California.
The Evolution of Modern Ocean Monitoring
The existence of the L’Estartit station reflects the results of decades of scientific research dating back to the 20th century. This body of work has established the vital role the ocean plays, in conjunction with our atmosphere, in shaping Earth’s global weather and climate. While sea level and sea state have been monitored regularly for some time, other measurements of oceanic conditions haven’t been as well-chronicled. In order to reconstruct the climate history of the ocean, scientists have typically relied on data from coastal tide gauges and stationary mooring stations, along with oceanographic cruises that weren’t generally part of any coordinated monitoring program.
By the 1980s, however, as Earth’s global climate warming trend became evident, scientists began to establish international programs to conduct long-term studies of the ocean. As a result, in recent years, scientists have increasingly acknowledged the value of having the oceanographic equivalent of weather forecasts. Maintaining regular, long-term records of air temperature, water temperatures at the surface and at various depths, winds, sea level, salinity, and other key oceanographic parameters gives scientists valuable information on long-term average values, how variable our climate is and on long-term changes and trends. Moreover, they help scientists better evaluate how humans are contributing to climate change.
Over the past 20 to 30 years, new technologies have given scientists the ability to monitor the ocean all the way from the sea surface to the ocean floor. These include satellites, drifters, gliders, moorings, buoys, Argo profilers and ship data. These data are used as inputs to computer models to estimate the state of the ocean, make ocean forecasts and estimate climate trends.
L’Estartit: Monitoring a Climate Hot Spot
Maintained by voluntary observer Josep Pascual in collaboration with CSIC and the authority of the marine protected area, the L’Estartit station is well positioned to monitor the Mediterranean, a region of our planet that’s significantly impacted by climate change. It lies at the southern end of a relatively narrow offshore continental shelf and along the coastal side of the Northern Current, the main along-slope ocean current in the northwestern Mediterranean.
You can think of the Mediterranean as sort of a miniature ocean, since most of the processes that take place in the global ocean also take place here, albeit at different time scales in some instances. Its relatively small size also makes it more accessible to monitoring than many other regions of the global ocean. Because it’s located in Earth’s mid latitudes, it experiences significant seasonal variations, which affect the way it exchanges heat with the atmosphere.
The L’Estartit site collects a broad array of oceanographic data. In addition to the data mentioned previously, the site began continuous measurements of potential daily evaporation in 1976; and has measured sea state, along with wind speed and direction, since 1988. With the installation of a tide gauge in the harbor in 1990, continuous sea level data have been collected. Also added in the 1990s were conductivity-temperature-depth (CTD) profiles and water samples to analyze the temperature and salinity of the water column.
L’Estartit’s long-term data record makes it possible for scientists to calculate trends for a variety of atmospheric and oceanic climate attributes, including air temperature, sea surface and sub-surface temperature to a depth of 80 meters (262 feet), air pressure, relative humidity, relative cloudiness, wind, salinity, changes in ocean stratification, estimates of favorable conditions for evaporation, sea level and precipitation.
“The long-term data set from L’Estartit is a treasure trove that’s useful for assessing the regional impacts of climate change and how it’s evolved over time,” said Vazquez. “The data can be used as reference for other areas in the Mediterranean. The strong agreement between the site’s measurements of sea surface temperatures and satellite data of sea surface temperatures demonstrates how L’Estartit can serve as both a long-term ground truth site to validate satellite observations and as a regional monitoring site for climate change.”
Vazquez says data from the site have been used in numerous climate research studies and have also been used to document a variety of extreme events, from cold spells and heat waves to storms.
A Half-Century of Climate Trends
The researchers’ analysis of the nearly 50-year data set reveals numerous climate trends. For example, air temperature has increased by an average of 0.05 degrees Celsius (0.09 degrees Fahrenheit) per year during this time. Sea surface temperature has increased by an average of 0.03 degrees Celsius (0.05 degrees Fahrenheit) per year, while the temperature of the ocean at a depth of 80 meters (262 feet) has increased by an average of 0.02 degrees Celsius (0.04 degrees Fahrenheit) per year.
While sea level in the Mediterranean decreased from the 1960s to the 1990s due to changes in the North Atlantic Oscillation (a multi-decadal cyclical fluctuation of atmospheric pressure over the North Atlantic Ocean that strongly influences winter weather in Europe, Greenland, northeastern North America, North Africa and northern Asia), it’s been on the rise since the mid-1990s. The L’Estartit data show that sea level at that site is currently rising at a rate of 3.1 millimeters (0.12 inches) per year.
The researchers found that some of the long-term climate trends they observed were more pronounced during some seasons than in others. For example, trends in air temperature and sea surface temperature were significantly stronger during spring, while the trend for ocean temperature at 80 meters was greatest during autumn. Among their other findings, they noted a small increase in the number of days per year that experience summer-like sea conditions. They also found an almost two day per year drop in conditions favorable for marine evaporation, which may be related to an observed decrease in springtime coastal precipitation.
Vazquez says the good statistical comparison between sea surface temperature values and trends from the L’Estartit data set and data from available satellite products is encouraging. “The long-term consistency of the direct measurements with our satellite data gives scientists the opportunity to validate climate trends across multiple decades,” he said. “Data from L’Estartit should serve as a wake-up call to the global climate science community to immediately begin similar initiatives and ensure their continuity over time.”
The L’Estartit data are available to the public free of charge. The digitized data are accessible at http://meteolestartit.cat/. The remote sensing data used in the study may be retrieved through NASA’s Physical Oceanography Distributed Active Archive Center (PO.DAAC) at http://podaac.jpl.nasa.gov.
Lots of forces are at work on the world’s ocean, and NASA studies them all. When it comes to sea level, NASA does much more than just measure it; they also seek to understand it. But for non-scientists, fathoming the forces that determine sea levels around the world can sometimes be a bit daunting, so here’s a little guide to some of the basics.
Let’s dive in.
Waves in the Bathtub
Most of the time, Earth’s ocean looks pretty darn flat to those of us here on the ground, like the water in a bathtub. If you’re on a boat at sea, the only topography you’re going to notice on the ocean is waves. Generated by the friction between wind and water, wind waves range from tiny ripples on a calm sea to storm-generated monsters that can tower more than 100 feet (30 meters) high. Some wind waves are generated locally. Others, called swells, which result from winds that blew somewhere else in the past, travel across the ocean surface.
But even in the absence of waves, it turns out the ocean isn’t really flat at all. It has hills and valleys just like land surfaces do, though they’re relatively small — up to about 2 meters (6.5 feet) high.
These small variations in ocean surface topography are influenced by many factors, including the temperature of the water, how much salt it contains (its salinity), the pressure of the atmosphere above the ocean surface, and ocean currents.
Currents move ocean waters around our planet over long distances, primarily in a horizontal direction, reshaping the ocean’s surface and causing it to tilt. They’re generated by various forces, including winds, breaking waves, ocean temperature, salinity, and a phenomenon known as the Coriolis effect (which causes water and wind to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere). Currents flow around the ocean’s hills and valleys, much like wind blows around areas of high and low pressure in our atmosphere.
Ocean currents happen in the open ocean and generally don’t have a big impact on coastlines, with a few major exceptions, such as the Gulf Stream in the Atlantic Ocean along the U.S. East Coast and a similar Pacific Ocean current off the coast of Japan called the Kuroshio, which transports water northward up Japan’s east coast and then due east. As our planet warms, it affects wind patterns that drive most of these currents, changing them.
While all of these factors are important drivers of ocean surface topography, there’s an even larger force working to shape the ocean: changes in Earth’s geoid. The geoid is the shape that Earth’s ocean surface would take if the only influences acting upon it were gravity and Earth’s rotation. Changes in the solid Earth affect Earth’s gravitational field, causing the height of Earth’s geoid to vary by up to 100 meters (328 feet) around the globe. For example, in places where Earth’s crust is thick and dense, the gravitational pull causes extra water to pile up. In addition, the shape of the geoid is partly determined by geologic features on the floor of the ocean, including seamounts (underwater mountains) and valleys, which pull the water due to the force of gravity.
Topographic features on the open ocean can only be seen from space, by specialized instruments called altimeters that precisely measure the height of the ocean surface.
Since 1992, NASA has partnered with other U.S. and European institutions on multiple satellite missions to map ocean surface topography. They include the joint NASA/Centre National d'Etudes Spatiales (CNES) Topex/Poseidon mission, which operated from 1992 to 2005; the NASA/CNES Jason-1, which operated from 2001 to 2013; the joint NASA/CNES/National Oceanic and Atmospheric Administration (NOAA)/European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Jason-2/Ocean Surface Topography Mission (OSTM), which operated from 2008 to 2019; and the current Jason-3, an international partnership led by NOAA with participation by NASA, CNES and EUMETSAT, launched in 2016. This November, Sentinel-6 Michael Freilich will launch to continue this long-term data set. The new mission is jointly developed by the European Space Agency (ESA), EUMETSAT, NASA and NOAA with funding support from the European Commission and support from CNES.
Measuring ocean surface topography allows us to understand ocean circulation (how our ocean stores energy from the Sun and moves it around our planet), accurately track changes in global sea level, and understand how the ocean joins forces with Earth’s atmosphere to create our weather and climate, including phenomena such as El Niño and La Niña and weather patterns such as hurricanes and other storms.
A Flattening Map
A look at a current map of trends in the nearly 30-year satellite record of global ocean surface topography reveals clear regional differences across the globe, with variations of up to 20 centimeters (8 inches) of sea level rise and fall from one place to another. But, says Josh Willis of NASA’s Jet Propulsion Laboratory in Pasadena, California, NASA’s project scientist for the Jason-2, Jason-3 and Sentinel-6 Michael Freilich missions, the map is getting flatter every year.
“Most of these 20-to-30 centimeter (8-to-12 inch) changes in sea level on the open ocean are cyclic, from natural things like El Niño and La Niña, or ocean currents speeding up or slowing down,” he said. “They've always been part of the story and always will be. But what really matters to people at the coast are long-term changes in their relative sea level – that is, the height of the ocean relative to the land. Those are caused by the overall rise due to global warming, and the movement of the land. And both of those are here to stay.”
Up next: why all sea level is "local."
Recently I stayed in a lovely vacation rental at the eastern end of Ocean Isle Beach, a small town on North Carolina’s southern coast not far from Myrtle Beach, South Carolina. Located on a 5-mile-long (8-kilometer-long) barrier island, the community is separated from the mainland by the Atlantic Intracoastal Waterway and marsh savannas. It’s a pleasant seaside resort, with restaurants, tourist amenities, and row upon row of stilt homes, many right on the beach. My bedroom looked out over small sand dunes to the shimmering Atlantic, which, at high tide, ebbed and flowed not much farther than a stone’s throw away. From my vantage point, there were no clues that the sea here might not always be a friendly neighbor.
A short stroll along the beach quickly provided a starkly different perspective. Just a few dozen yards away, huge sandbags were piled high, guarding a number of homes from the sea. As I continued walking, I soon found myself in front of homes that were perched literally above the waves at high tide.
I passed a woman walking her dog and asked her about the homes. “There used to be two streets of houses in front of these homes,” she told me. “Now they’re oceanfront.”
It turns out the homes at the east end of Ocean Isle Beach were victims of coastal erosion, which is common at most beaches in North Carolina and throughout the world. An eroding beach can lose several feet of sand a year. Of course, storms, including hurricanes, can result in rapid beach erosion. But there’s also chronic, long-term erosion, caused by changes in the supply of sand to a beach and in relative sea level (how much the height of the ocean rises or falls relative to the land at a particular location). Records show sea level in this part of coastal North Carolina has risen about 7.6 centimeters (3 inches) since the early 1980s. According to the U.S. Global Change Research Program, coastal erosion results in U.S. coastal property losses of about a half billion dollars each year in the form of damaged structures and lost land.
The case of Ocean Isle Beach illustrates a key paradox about sea level rise: since it occurs relatively slowly, it can be easy to think it’s not happening. But as oceanographer and climate scientist Josh Willis of NASA’s Jet Propulsion Laboratory in Pasadena, California, told me, if you’re not seeing it, you’re just not looking in the right place.
“Thanks to satellite and tide gauge data, we know that sea level is rising about 3.3 millimeters (0.13 inches) a year, a rate that grows by another 1 millimeter (0.04 inches) per year every decade or so,” Willis said. “Each year, global warming is currently adding about 750 gigatonnes of water to the ocean – enough to cover my home state of Texas about 1 meter (more than 3 feet) deep. We can’t really eyeball a few millimeters of sea level rise a year just by looking at the ocean because of waves, tides, etc. But we can definitely see the effects of it, both short- and long-term.”
Willis said sea level rise accelerates and exacerbates the natural coastal erosion that’s continually taking place in locations like Ocean Isle Beach. “Sea level rise literally eats away at a coastline, making it more vulnerable to floods,” he said. “While floods happen naturally, it’s sea level rise that causes them to gradually begin topping natural barriers—like wetlands, mangrove forests and saltwater marshes—and even human-built barriers that typically protect coastal areas around the world from flooding. All of a sudden, that flood that you used to be protected from is now wiping you out.”
It’s the same story all over the world. You may not be able to eyeball sea level rise at your local beach, but its effects are being felt in many ways. Willis says a good rule of thumb is that every inch of sea level rise results in the loss of about 2.5 meters (100 inches) of beach, though recent studies suggest beach losses around the globe could happen even faster.
In many places, sea level rise has rendered sea walls erected decades ago to handle 100-year floods inadequate. Floods that used to occur once a century are now happening once a decade. You can also see the impacts of sea level rise reflected in gradual damage to infrastructure, such as the condition of coastal roads like California’s Pacific Coast Highway, which is continually battling the effects of coastal erosion.
Another phenomenon many people are experiencing more frequently but may not necessarily think is an impact of sea level rise is high tide flooding, otherwise known as “nuisance,” or “sunny day” flooding. This type of flooding, which is generally low-level, occurs year-round during high tides. Its effects range from inconveniences to the public, such as the closure of roads, businesses and schools, to long-term infrastructure damage and overwhelmed storm drains. Climate-related sea level rise is a primary contributor to high tide flooding, as is the loss of natural coastal barriers. Another contributor is the sinking of coastal lands due to adjustments related to the end of the last Ice Age, tectonics, compaction of sediments, and other dynamic processes. In the United States, high tide flooding is especially common along the East and Gulf Coasts. Over the last two decades, their frequency is up by roughly 50 percent; 100 percent if you go back three decades.
Norfolk, Virginia, is a good case in point. Norfolk, home to the world’s largest Naval base, is one of several municipalities that comprise Virginia’s Hampton Roads region, which has a population of more than 1.8 million. In the 20th century, sea level relative to land in Norfolk rose between 4 and 5 millimeters (0.16 to 0.2 inches) a year, in part because the land in this region is sinking as it continues to adjust to the melting of the Laurentide ice sheet that covered it during the last Ice Age. Over the past couple of decades, high tide flooding here has accelerated rapidly, and now occurs about 10 days a year, causing flooding in downtown Norfolk.
“We’ve had such a large amount of sea level rise in the past century that we’re now nearing a tipping point,” said Ben Hamlington, a research scientist in JPL’s Sea Level and Ice Group. “When many coastal communities like Norfolk were established, developers took into account where historical high tide lines were, then added a safety gap to account for floods. But long-term climate change is narrowing that safety gap and a storm event is no longer required to cause significant flooding. The combination of long-term sea level rise and natural variations in sea level caused by climate cycles such as El Niño and the Pacific Decadal Oscillation (PDO) is leading to a dramatic increase in high-tide flooding events. As a result, coastal communities must now take these different natural climate cycles into account in their planning.”
Willis says sea level rise is causing some cities around the world to face the ultimate choice: spend huge sums of money to combat sea level rise, or literally abandon ship and move away. Last year, Indonesia announced plans to move its capital inland from Jakarta, a city of 10 million that’s sinking and challenged by sea level rise. In cities all over the world, local officials are confronting their own battles. Even relatively affluent metropolitan areas like Southern California aren’t immune. But many places, such as Bangladesh, parts of southeast Asia and small island nation states, simply don’t have the resources.
Willis says California and the U.S. West Coast have been spared the worst effects of sea level rise over the past 20 years. But that may be about to change.
“Over the past 15 to 20 years, we’ve been watching warm waters in the Pacific Ocean move away from the West Coast due to a shift in the PDO, a long-term ocean fluctuation pattern that’s similar to the El Niño/La Niña cycles but that operates on a much larger scale, waxing and waning about every 20 to 30 years,” he said. “This has served to counteract the effects of global sea level rise, so that along the U.S. Pacific Coast, we’ve seen almost no sea level rise over that time. But those days are over. Since the major El Niño of 2015-16, the PDO has shifted and the West Coast is likely to see faster-than-average sea level rise in the next 20 years. We’re already beginning to see this. California, in particular, needs to prepare. We could see increases up to 1 centimeter (0.4 inch) a year, more than three times the global rate.”
Such a rate of sea level rise would equate to more than 20 centimeters (8 inches) in the next two decades. To put that into perspective, over the past century, sea level along California’s coast has gone up about 23 centimeters (9 inches). This will pose major challenges for many parts of the Golden State, from San Francisco and San Diego Bays, to the Ports of Long Beach and Los Angeles, and cities in coastal Orange County, to name a few. “We think of California as having a lot of cliffs,” says Willis. “It does, but in between, there are lots of low-lying areas where sea level rise is going to cause problems.”
Whether you want to see it or not, sea level rise is a global problem. And that’s no day at the beach.
When NASA climate scientists speak in public, they’re often asked about possible connections between climate change and extreme weather events such as hurricanes, heavy downpours, floods, blizzards, heat waves and droughts. After all, it seems extreme weather is in the news almost every day of late, and people are taking notice. How might particular extreme weather and natural climate phenomena, such as El Niño and La Niña, be affected by climate change, they wonder?
There’s no easy answer, says Joao Teixeira, co-director of the Center for Climate Sciences at NASA’s Jet Propulsion Laboratory in Pasadena, California, and science team leader for the Atmospheric Infrared Sounder (AIRS) instrument on NASA’s Aqua satellite. “Within the scientific community it’s a relatively well-accepted fact that as global temperatures increase, extreme precipitation will very likely increase as well,” he says. “Beyond that, we’re still learning.”
“Within the scientific community it’s a relatively well-accepted fact that as global temperatures increase, extreme precipitation will very likely increase as well. Beyond that, we’re still learning.”
While there’s not yet a full consensus on the matter, in recent years a body of evidence linking extreme weather with climate change has begun to emerge. Evidence from satellites, aircraft, ground measurements and climate model projections are increasingly drawing connections. Quantifying those interconnections is a big challenge.
“All our available tools have pros and cons,” says Teixeira. “Rain gauges, for example, provide good measurements, but they’re local and spread far apart. In contrast, satellites typically measure climate variables (such as precipitation, temperature and humidity) indirectly and don’t yet have long enough data records to establish trends, though that’s beginning to change. In addition, representing small-scale processes of the atmosphere that are key to extreme weather events in climate models, such as turbulence, convection and cloud physics, is notoriously difficult. So, we’re in a bit of a conundrum. But great progress is being made as more studies are conducted.”
A simple analogy describes how difficult it is to attribute extreme weather to climate change. Adding fossil fuel emissions to Earth’s atmosphere increases its temperature, which adds more energy to the atmosphere, supercharging it like an athlete on steroids. And just as it’s difficult to quantify how much of that athlete’s performance improvement is due to steroid use, so too it’s difficult to say whether extreme weather events are definitively due to a warmer atmosphere.
Are Supercharged Atlantic Hurricane Seasons a Case in Point?
Take hurricanes, for example. A hot topic in extreme weather research is how climate change is impacting the strength of tropical cyclones. A look at the 2019 Atlantic hurricane season provides a case in point.
After a quiet start to the 2019 season, Hurricane Dorian roared through the Atlantic in late August and early September, surprising many forecasters with its unexpected and rapid intensification. In just five days, Dorian grew from a minimal Category 1 hurricane to a Category 5 behemoth, reaching a peak intensity of 185 miles (295 kilometers) per hour when it made landfall in The Bahamas. In the process, Dorian tied an 84-year-old record for strongest landfalling Atlantic hurricane and became the fifth most intense recorded Atlantic hurricane to make landfall, as measured by its barometric pressure.
Two weeks later the remnants of Tropical Storm Imelda swamped parts of Texas under more than 40 inches (102 centimeters) of rain, enough to make it the fifth wettest recorded tropical cyclone to strike the lower 48 states. Fueled by copious moisture from a warm Gulf of Mexico, the slow-moving Imelda’s torrential rains and flooding wreaked havoc over a wide region.
Then in late September, Hurricane Lorenzo became the most northerly and easterly Category 5 storm on record in the Atlantic, even affecting the British Isles as an extratropical cyclone.
Earth’s atmosphere and oceans have warmed significantly in recent decades. A warming ocean creates a perfect cauldron for brewing tempests. Hurricanes are fueled by heat in the top layers of the ocean and require sea surface temperatures (SSTs) greater than 79 degrees Fahrenheit (26 degrees Celsius) to form and thrive.
Since 1995 there have been 17 above-normal Atlantic hurricane seasons, as measured by NOAA’s Accumulated Cyclone Energy (ACE) Index. ACE calculates the intensity of a hurricane season by combining the number, wind speed and duration of each tropical cyclone. That’s the largest stretch of above-normal seasons on record.
So while there aren’t necessarily more Atlantic hurricanes than before, those that form appear to be getting stronger, with more Category 4 and 5 events.
NASA Research Points to an Increase in Extreme Storms Over Earth’s Tropical Oceans
What does NASA research have to say about extreme storms? One NASA study from late 2018 supports the notion that global warming is causing the number of extreme storms to increase, at least over Earth’s tropical oceans (between 30 degrees North and South of the equator).
A team led by JPL’s Hartmut Aumann, AIRS project scientist from 1993 to 2012, analyzed 15 years of AIRS data, looking for correlations between average SSTs and the formation of extreme storms. They defined extreme storms as those producing at least 0.12 inches (3 millimeters) of rain per hour over a certain-sized area. They found that extreme storms formed when SSTs were hotter than 82 degrees Fahrenheit (28 degrees Celsius). The team also saw that for every 1.8 degrees Fahrenheit (1 degree Celsius) that SST increased, the number of extreme storms went up by about 21 percent. Based on current climate model projections, the researchers concluded that extreme storms may increase 60 percent by the year 2100.
Thanks to weather satellites, scientists have identified possible correlations between the extremely cold clouds seen in thermal infrared satellite images (called deep convective clouds) and extreme storms observed on the ground under certain conditions, especially over the tropical oceans. When precipitation from these clouds hits the top of Earth’s lowest atmospheric layer, the troposphere, it produces torrential rain and hail.
AIRS can’t measure precipitation directly from space, but it can measure the temperature of clouds with extraordinary accuracy and stability. Its data can also be correlated with other climate variables such as SSTs, for which scientists maintain long data records.
To determine the number of extreme storms, Aumann’s team plotted the number of deep convective clouds each day against measurements of sea surface temperature. They found that the number of these clouds correlated with increases in sea surface temperature.
The results of this study reflect a long line of AIRS research and three previously published papers. The researchers say large uncertainties and speculations remain regarding how extreme storms may change under future climate scenarios, including the possibility that a warming climate may result in fewer but more intense storms. But the results of this study point to an intriguing direction for further research.
What Lies Ahead?
Aumann is confident future studies will reveal additional insights into how severe storms detected as individual deep convective clouds coalesce to form tropical storms and hurricanes. He notes that if you look at these clouds over the global ocean, they frequently occur in clusters.
“AIRS sees hurricanes as hundreds of these clusters,” he said. “For example, it saw Hurricane Dorian as a cluster of about 150 deep convective clouds, while Hurricane Katrina contained about 500. If you look at a weather satellite image, you’ll see the severe storms that make up a hurricane are not actually contiguous. In fact, they’re uncannily similar to the stars within the spiral arms of a galaxy. It’s one severe thunderstorm after another, each dumping a quantity of rain on the ground.
“AIRS has 2,400 different frequency channels, so it’s a very rich data set,” he said. “In fact, there’s so much data, our computer capabilities aren’t able to explore most of it. We just need to ask the right questions.”