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.”
In the last few months, a number of questions have come in asking if NASA has attributed Earth’s recent warming to changes in how Earth moves through space around the Sun: a series of orbital motions known as Milankovitch cycles.
What cycles, you ask?
Milankovitch cycles include the shape of Earth’s orbit (its eccentricity), the angle that Earth’s axis is tilted with respect to Earth’s orbital plane (its obliquity), and the direction that Earth’s spin axis is pointed (its precession). These cycles affect the amount of sunlight and therefore, energy, that Earth absorbs from the Sun. They provide a strong framework for understanding long-term changes in Earth’s climate, including the beginning and end of Ice Ages throughout Earth’s history. (You can learn more about Milankovitch cycles and the roles they play in Earth’s climate here).
But Milankovitch cycles can’t explain all climate change that’s occurred over the past 2.5 million years or so. And more importantly, they cannot account for the current period of rapid warming Earth has experienced since the pre-Industrial period (the period between 1850 and 1900), and particularly since the mid-20th Century. Scientists are confident Earth’s recent warming is primarily due to human activities — specifically, the direct input of carbon dioxide into Earth’s atmosphere from burning fossil fuels.
So how do we know Milankovitch cycles aren’t to blame?
First, Milankovitch cycles operate on long time scales, ranging from tens of thousands to hundreds of thousands of years. In contrast, Earth’s current warming has taken place over time scales of decades to centuries. Over the last 150 years, Milankovitch cycles have not changed the amount of solar energy absorbed by Earth very much. In fact, NASA satellite observations show that over the last 40 years, solar radiation has actually decreased somewhat.
Second, Milankovitch cycles are just one factor that may contribute to climate change, both past and present. Even for Ice Age cycles, changes in the extent of ice sheets and atmospheric carbon dioxide have played important roles in driving the degree of temperature fluctuations over the last several million years.
The extent of ice sheets, for example, affects how much of the Sun’s incoming energy is reflected back to space, and in turn, Earth’s temperature.
Then there’s carbon dioxide. During past glacial cycles, the concentration of carbon dioxide in our atmosphere fluctuated from about 180 parts per million (ppm) to 280 ppm as part of Milankovitch cycle-driven changes to Earth’s climate. These fluctuations provided an important feedback to the total change in Earth’s climate that took place during those cycles.
Today, however, it’s the direct input of carbon dioxide into the atmosphere from burning fossil fuels that’s responsible for changing Earth’s atmospheric composition over the last century, rather than climate feedbacks from the ocean or land caused by Milankovitch cycles.
Since the beginning of the Industrial Age, the concentration of carbon dioxide in Earth’s atmosphere has increased 47 percent, from about 280 ppm to 412 ppm. In just the past 20 years alone, carbon dioxide is up 11 percent.
Scientists know with a high degree of certainty this carbon dioxide is primarily due to human activities because carbon produced by burning fossil fuels leaves a distinct “fingerprint” that instruments can measure. Over this same time period, Earth’s global average temperature has increased by about 1 degree Celsius (1.8 degrees Fahrenheit), and is currently increasing at a rate of 0.2 degrees Celsius (0.36 degrees Fahrenheit) every decade. At that rate, Earth is expected to warm another half a degree Celsius (almost a degree Fahrenheit) as soon as 2030 and very likely by 2040.
This relatively rapid warming of our climate due to human activities is happening in addition to the very slow changes to climate caused by Milankovitch cycles. Climate models indicate any forcing of Earth’s climate due to Milankovitch cycles is overwhelmed when human activities cause the concentration of carbon dioxide in Earth’s atmosphere to exceed about 350 ppm.
Scientists know of no natural changes to the equilibrium between the amount of solar radiation absorbed by Earth and the amount of energy radiated back to space that can account for such a rapid period of global warming. The amount of incoming solar radiation has increased only slightly over the past century and is therefore not a driver of Earth’s current climate warming.
Since 1750, the warming driven by greenhouse gases coming from the human burning of fossil fuels is over 50 times greater than the slight extra warming coming from the Sun itself over that same time interval. If Earth’s current warming was due to the Sun, scientists say we should expect temperatures in both the lower atmosphere (troposphere) and the next layer of the atmosphere, the stratosphere, to warm. Instead, observations from balloons and satellites show Earth’s surface and lower atmosphere have warmed but the stratosphere has cooled.
Finally, Earth is currently in an interglacial period (a period of milder climate between Ice Ages). If there were no human influences on climate, scientists say Earth’s current orbital positions within the Milankovitch cycles predict our planet should be cooling, not warming, continuing a long-term cooling trend that began 6,000 years ago.
There’s nothing cool about that.
"Pink elephant in the room" time: There is no impending “ice age” or "mini ice age" to be caused by an expected reduction in the Sun’s energy output in the next several decades.
Through its lifetime, the Sun naturally goes through changes in energy output. Some of these occur over a regular 11-year period of peak (many sunspots) and low activity (fewer sunspots), which are quite predictable.
But every so often, the Sun becomes quieter, experiencing much fewer sunspots and giving off less energy. This is called a "Grand Solar Minimum," and the last time this happened, it coincided with a period called the "Little Ice Age" (a period of extremely low solar activity from approximately AD 1650 to 1715 in the Northern Hemisphere, when a combination of cooling from volcanic aerosols and low solar activity produced lower surface temperatures).
Some scientists have suggested that the relatively small magnitude of the last solar cycle (SC 24) presages a new Grand Solar Minimum in the next few decades.
But how big of an effect might a Grand Solar Minimum have? In terms of climate forcing – a factor that could push the climate in a particular direction – solar scientists estimate it would be about -0.1 W/m2, the same impact of about three years of current carbon dioxide (CO2) concentration growth.
Thus, a new Grand Solar Minimum would only serve to offset a few years of warming caused by human activities.
What does this mean? The warming caused by the greenhouse gas emissions from the human burning of fossil fuels is six times greater than the possible decades-long cooling from a prolonged Grand Solar Minimum.
Even if a Grand Solar Minimum were to last a century, global temperatures would continue to warm. Because more factors than just variations in the Sun’s output change global temperatures on Earth, the most dominant of those today being the warming coming from human-induced greenhouse gas emissions.
As 2019 comes to an end and a new decade approaches, we look back at all the important Earth science NASA has revealed. This is a time to take stock in all that we have learned and to use those insights to better understand and reliably predict the many ways our planet is changing in the coming decades.
Our planet is an interconnected system, and every new discovery leads to new understandings and new avenues of exploration. Our climate is changing, and NASA spacecraft and science over the past decade have studied it from numerous angles and perspectives. Below are highlights of the science NASA spacecraft have enabled over the past decade, and important topics to study over the decades to come.
In 2012, the Arctic sea ice cover reached the smallest point observed from space yet, and in the years since, scientists have watched it shrink further. Studying the sea ice loss has brought new insights to the feedback loops that climate change has set in motion. The sea ice cover in the Arctic has, in the past, protected that area of the world from warming. Because ice is a much brighter surface than the dark ocean, it reflected more light and heat. With the loss of that ice, the ocean now absorbs that heat and speeds up warming in the Arctic.
This sea ice loss has also been found to affect the average age of the sea ice, with less and less of the seasonal cover lasting multiple years. Now that much of the multiyear ice has been lost, further changes in ice thickness and age will happen more slowly, as the majority of the sea ice now is seasonal, melting with the summer. Continuing to monitor the Arctic sea ice in the coming years, from airborne and spaceborne platforms, will be critical to understanding the effects of climate change in that region.
In May of 2013, the average global CO2 concentration broke the 400 parts per million (ppm) level, and has since continued to rise. In 2017, the annual minimum CO2 concentration also reached the 400 ppm level, further cementing the importance of tracking the carbon dioxide levels in our atmosphere. Satellite missions like the Orbiting Carbon Observatory 2 (OCO-2) have added space-based global measurements of atmospheric CO2 with the precision, resolution, and coverage needed to characterize sources and sinks (fluxes) on regional scales. The Orbiting Carbon Observatory 3 (OCO-3) was launched in 2019 to expand the horizons of OCO-2 , focusing its instrument on large cities.
When OCO-2 and OCO-3 aren’t studying carbon dioxide, they’re still hard at work measuring plant growth! The OCO-2 science team found that their instrument could also be used to track the tiny amount of glow plants give off when they photosynthesize, and have been studying the health and stress of plants in tandem with the project’s other science objectives
For a student-friendly explanation of the greenhouse effect and how that contributes to rising carbon levels in the atmosphere, visit Climate Kids!
NASA has been continuously studying the ozone hole since it was first discovered, and 2019 has shown the smallest hole yet. Typically, it grows to a size of 8 million square miles, but this past year its maximum was 6.3 million miles. While this is good news for the ozone hole, it is not caused entirely by the repair that began with the signing of the Montreal Protocol on Substances that Deplete the Ozone Layer. With no other systems at play, a decrease in chlorofluorocarbons (CFCs) in the atmosphere after they were banned in 1987 would result in the ozone hole reaching its past levels around 2070.
The Antarctic ozone hole forms during the Southern Hemisphere’s late winter as the returning Sun’s rays start ozone-depleting reactions. These reactions involve chemically active forms of chlorine and bromine derived from man-made compounds. The chemistry that leads to their formation involves chemical reactions that occur on the surfaces of cloud particles that form in cold stratospheric layers, leading ultimately to runaway reactions that destroy ozone molecules. In warmer temperatures, fewer polar stratospheric clouds form and they don’t persist as long, limiting the ozone-depletion process.
The smaller hole is in part because of the decrease in CFCs in the atmosphere, but also because of warming temperatures. In warmer temperatures, fewer polar stratospheric clouds form, and they don’t persist as long, limiting the ozone-depletion process.
NASA and the National Oceanic and Atmospheric Administration (NOAA) study the ozone hole in complementary methods. A trio of NASA satellites measure ozone from space, and Aura’s Microwave Limb Sounder instrument estimates levels of chlorine in the atmosphere. NOAA staff, meanwhile, launch weather balloons from the ground that carry ozone-measuring instruments, providing another set of data to pair with the space-based record.
Both agencies will continue to study the ozone hole and how it is affected by climate change in the coming years.
From a 2015 study showing that fire seasons are growing longer, to the satellite record displaying the similar effects from space, it is becoming clear that the longer and fiercer fire seasons are also an effect of climate change. The satellite record of the past 20 years has shown a large-scale trend of increased fire activity in places experiencing warming temperatures and a drying climate. A drier climate leads to an increase in burning fuel as plants die out and dry up. Further, warmer temperatures at night result in fires lasting multiple days, where cooler temperatures may have suppressed the fire and kept it from spreading as drastically.
NASA is continuing to study fires from space, and has also launched airborne and ground-based Earth Expeditions, such as FIREX-AQ to further study the effects of more frequent fires on our planet and on human health.
Looking for a student-friendly explanation of wildfires? Check out this video: https://scijinks.gov/wildfires/
In May 2019, after the wettest 12 months ever recorded in the Mississippi River Basin, the region was bearing the weight of 8 to 12 inches (200 to 300 millimeters) more water than average. New data from NASA's Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission, which launched in May 2018, showed that there was an increase in water storage in the river basin, extending east around the Great Lakes.
GRACE-FO is a follow-on mission from GRACE, which built a 15-year data record of tracking water mass movement, studying floods, droughts, and ice melt. Some of GRACE’s findings include measuring the melting of both the Greenland and Antarctic ice sheets. The Greenland mass loss trend from April 2002-March 2009 (7 years) is -219 Gigatonnes/year (Gt/yr). The Greenland mass loss trend for the next 7 years, from 2009 to 2016, is -319 Gt/yr. Antarctica’s melt is smaller in magnitude, but there is a more distinct acceleration happening there. For the same seven years, between 2002 and 2009, Antarctica’s trend is -73 Gt/yr. Then, between 2009 and 2016, the trend is -165 Gt/yr. If you’d like to explore and see GRACE’s work yourself, you can access and download the data here.
The twin GRACE-FO spacecraft are used to measure the change in the mass of water across the planet, providing scientists, decision makers and resource managers with an accurate measure of how much water is retained - not only on Earth's surface, but also in the soil layer and below ground in aquifers. Monitoring these changes provides a unique perspective of Earth's climate and has far-reaching benefits for humankind, such as understanding both the possibility and the consequences of floods and droughts.
The continuing work from GRACE-FO will be important in the coming decade as climates around the world change, helping scientists monitor the movement of water around the globe.
2020 and Beyond: Upcoming Missions
NASA’s Earth Science program is ever-evolving, and there are many missions being currently built. Of the many in development, two to keep an eye on are the Surface Water and Topography (SWOT) and NASA-ISRO Synthetic Aperture Radar (NISAR) Missions, both of which will be launching in the next few years. Each new mission being developed will provide a new angle on Earth science, and will help provide a better understanding of the complicated and interconnected systems that govern our planet.
Want to see how your home planet is changing? Explore NASA’s “Images of Change” gallery to see different locations on Earth, showing change over time periods ranging from days to decades.
In March, a NASA-led research team announced that Jakobshavn Isbrae, Greenland's fastest-flowing and thinning glacier over the past two decades, is now flowing more slowly, thickening and advancing toward the ocean instead of retreating farther inland.
On the surface, that sounds like great news. After all, if this glacial behemoth, which drains seven percent of Greenland, is slowing, certainly that must mean that global warming is also slowing, right?
Wrong. The findings have been interpreted that way by some, suggesting that the study results were evidence that global warming is slowing or stopping. However, the facts paint a different picture, as a quick review of the study’s key findings illustrates. To recap:
- The recent changes in Jakobshavn, located on Greenland’s west coast, are attributed to the 2016 cooling of an ocean current that carries water to the glacier’s ocean face, likely due to a shift in the North Atlantic Oscillation (NAO) that took place in 2015. The NAO is an oceanic climate pattern that causes northern Atlantic water temperatures to alternate between warm and cold every five to 20 years. The glacier’s dramatic slowdown coincided with the arrival of the cooler waters near Jakobshavn that summer.
- Water temperatures near the glacier are now colder than they’ve been since the mid-1980s. The colder water isn’t melting the ice at the front of and beneath the glacier as quickly as the warmer water did.
- Jakobshavn’s changes are temporary. When the NAO flips again, the glacier will most likely resume accelerating and thinning, as warm waters return to continue melting it from beneath.
Following the study’s publication, additional analyses show Jakobshavn grew thicker by 22 and 33 yards (20 to 30 meters) each year from 2016 to 2019.
How Ocean Temperatures Impact Greenland’s Glaciers
Many factors can speed up or slow down a glacier’s rate of ice loss. These include the shape of the bedrock under it and along its sides, short-term variations in ocean temperature and circulation, air temperature and precipitation and climate change. To better understand the role ocean temperatures play, four years ago NASA launched the Oceans Melting Greenland (OMG) campaign to measure ocean temperature and salinity around Greenland.
While Greenland is an island, it’s surrounded by a continental shelf beneath the ocean surface. The shelf forms a natural barrier that keeps the deeper, warmer waters of the Atlantic from reaching parts of the Greenland coast. Near the coast, the average ocean depth is about 1,300 to 1,600 feet (400 to 500 meters), whereas in the deep ocean, 30 to 200 miles (50 to 320 kilometers) offshore, waters usually reach depths of around 13,100 feet (4,000 meters).
However, deep underwater canyons cut through the continental shelf, allowing the faces of many Greenland glaciers to sit in warm, deep water. A key OMG objective has been to conduct the most comprehensive mapping to date of the sea floor around Greenland to see where these canyons are located. As a result, we now know just how many glaciers sit in deep water, how deep the water is, and how fjords around Greenland connect to warm offshore waters.
“We’ve filled in huge gaps in our knowledge of the sea floor depth around Greenland,” said OMG Principal Investigator and study co-author Josh Willis of NASA’s Jet Propulsion Laboratory in Pasadena, California. “Some of the glaciers sit in about 3,300 feet (1,000 meters) of water, the equivalent of 10 football fields below the surface. In fact, everything we’ve found suggests Greenland’s glaciers are more threatened than we expected.”
"Everything we’ve found suggests Greenland’s glaciers are more threatened than we expected."
Parsing Out the Facts About Jakobshavn
While Jakobshavn’s behavior may be confusing to some, there is no evidence that its growth is indicative of any slowdown in global warming. Global carbon dioxide concentrations aren’t dropping, global atmospheric and ocean temperatures aren’t dropping and global sea levels aren’t falling. In fact, all evidence points strongly in the opposite direction.
What the current events at Jakobshavn do show us is that, in addition to the longer-term changes happening to Earth due to human-produced emissions of greenhouse gases, natural processes, such as ocean oscillations, also play key roles in the shorter-term changes we’re observing on our planet.
“The NAO is a cycle that’s been going back and forth for centuries,” said Willis. “There’s no evidence that it or other climate cycles like the Pacific Decadal Oscillation or El Niño are going to stop. The last time the NAO switched to a warm phase was in the mid- to early-90s. So we expect it to switch again, sometime between now and the next 15 years. That’s one of the reasons why studies like OMG are so important. At the end of the day, Greenland is still losing ice, other Greenland glaciers are still retreating and the oceans are warming.”
The bottom line for Jakobshavn is that it is still a major contributor to sea level rise and it continues to lose more ice mass than it’s gaining.
What’s Ahead for OMG?
In early August, the OMG team arrived in Greenland to begin its fourth year of ocean surveys to see how the water is changing. The start of this year’s survey came on the heels of a record melting event in late July and early August. The team again dropped sensors in front of Jakobshavn to see if the water is still cold and whether we can expect another year of growth, or for it to resume retreating. The investigation also examined whether the NAO shift is impacting other glaciers.
Within the next year and a half, the OMG team will complete its comprehensive categorization of all of Greenland’s 200-plus glaciers to quantify the role the ocean is playing in their retreat and how much ice the island is losing because of it. Willis says the team also plans to look at data from the NASA/German Gravity Recovery and Climate Experiment (GRACE) Follow-On mission to see whether the NAO’s impact is big enough to affect the ice sheet’s overall mass balance.
“If we’re lucky, OMG may also catch the reversal of the cooling signal now impacting Jakobshavn,” he said. “That will tell us what happens when the glaciers start to retreat again as warm water comes back, and just how sensitive the whole thing is to the water. Understanding these natural fluctuations will help us calibrate how Greenland’s ice is going to behave in the long run.”
The Sun powers life on Earth; it helps keep the planet warm enough for us to survive. It also influences Earth’s climate: We know subtle changes in Earth’s orbit around the Sun are responsible for the comings and goings of the past ice ages. But the warming we’ve seen over the last few decades is too rapid to be linked to changes in Earth’s orbit, and too large to be caused by solar activity.1
The Sun doesn’t always shine at perpetually the same level of brightness; it brightens and dims slightly, taking 11 years to complete one solar cycle. During each cycle, the Sun undergoes various changes in its activity and appearance. Levels of solar radiation go up or down, as does the amount of material the Sun ejects into space and the size and number of sunspots and solar flares. These changes have a variety of effects in space, in Earth’s atmosphere and on Earth’s surface.
The current solar cycle began January 4, 2008, and appears to be headed toward the lowest level of sunspot activity since accurate recordkeeping began in 1750. It’s expected to end sometime between now and late 2020. Scientists don’t yet know with confidence how strong the next solar cycle may be.
What Effect Do Solar Cycles Have on Earth’s Climate?
According to the United Nations’ Intergovernmental Panel on Climate Change (IPCC), the current scientific consensus is that long and short-term variations in solar activity play only a very small role in Earth’s climate. Warming from increased levels of human-produced greenhouse gases is actually many times stronger than any effects due to recent variations in solar activity.
For more than 40 years, satellites have observed the Sun's energy output, which has gone up or down by less than 0.1 percent during that period. Since 1750, the warming driven by greenhouse gases coming from the human burning of fossil fuels is over 50 times greater than the slight extra warming coming from the Sun itself over that same time interval.
Are We Headed for a ‘Grand Minimum’? (And Will It Slow Down Global Warming?)
As mentioned, the Sun is currently experiencing a low level of sunspot activity. Some scientists speculate that this may be the beginning of a periodic solar event called a “grand minimum,” while others say there is insufficient evidence to support that position. During a grand minimum, solar magnetism diminishes, sunspots appear infrequently and less ultraviolet radiation reaches Earth. Grand minimums can last several decades to centuries. The largest recent event happened during the “Little Ice Age” (13th to mid-19th century): the “Maunder Minimum,” an extended period of time between 1645 and 1715, when there were few sunspots.
Several studies in recent years have looked at the effects that another grand minimum might have on global surface temperatures.2 These studies have suggested that while a grand minimum might cool the planet as much as 0.3 degrees C, this would, at best, slow down (but not reverse) human-caused global warming. There would be a small decline of energy reaching Earth, and just three years of current carbon dioxide concentration growth would make up for it. In addition, the grand minimum would be modest and temporary, with global temperatures quickly rebounding once the event concluded.
Some people have linked the Maunder Minimum’s temporary cooling effect to decreased solar activity, but that change was more likely influenced by increased volcanic activity and ocean circulation shifts.3
Moreover, even a prolonged “Grand Solar Minimum” or “Maunder Minimum” would only briefly and minimally offset human-caused warming.
More about solar cycles:
Periodically, we receive queries asking if Earth is cooling. Although multiple lines of converging scientific evidence show conclusively that our climate is warming, stories sometimes appear in the media calling that into question. New studies are interpreted as contradicting previous research, or data are viewed to be in conflict with established scientific thinking.
Last spring, for example, a number of media outlets and websites reported on a story that looked at data acquired from NASA’s Goddard Institute for Space Studies (GISS) Surface Temperature Analysis (GISTEMP), which estimates changes in global surface temperature. The article discussed a short-term cooling period that showed up in the data in 2017 and 2018 and correctly stated that short-term cooling cycles are “statistical noise compared to the long-term trend.”
Afterward, we received some queries from readers who wanted to know if this finding meant a significant period of global cooling either could be or already was under way.
The answer is no. This story is a great example of why focusing on just a short period of time – say, one, two or even several years — doesn’t tell you what’s really going on with the long-term trends. In fact, it’s likely to be misleading.
So, what’s really important to know about studying global temperature trends, anyway?
Well, to begin with, it’s vital to understand that global surface temperatures are a “noisy” signal, meaning they’re always varying to some degree due to constant interactions between the various components of our complex Earth system (e.g., land, ocean, air, ice). The interplay among these components drive our weather and climate.
For example, Earth’s ocean has a much higher capacity to store heat than our atmosphere does. Thus, even relatively small exchanges of heat between the atmosphere and the ocean can result in significant changes in global surface temperatures. In fact, more than 90 percent of the extra heat from global warming is stored in the ocean. Periodically occurring ocean oscillations, such as El Niño and its cold-water counterpart, La Niña, have significant effects on global weather and can affect global temperatures for a year or two as heat is transferred between the ocean and atmosphere.
This means that understanding global temperature trends requires a long-term perspective. An examination of two famous climate records illustrate this point.
You may be familiar with the Keeling Curve (above), a long-term record of global carbon dioxide concentrations. It’s not a straight line: The curve jiggles up and down every year due to the seasonal cycling of carbon dioxide. But the long-term trend is clearly up, especially in recent decades. As countries around the world rapidly develop and gross domestic products increase, human-produced emissions of carbon dioxide are accelerating.
During fall and winter in the Northern Hemisphere, when trees and plants begin to lose their leaves and decay, carbon dioxide is released in the atmosphere, mixing with emissions from human sources. This, combined with fewer trees and plants removing carbon dioxide from the atmosphere, allows concentrations to climb in winter, reaching a peak by early spring. During spring and summer in the Northern Hemisphere, plants absorb a substantial amount of carbon dioxide through photosynthesis.
Similarly, the above graph of long-term independent global temperature records maintained by NASA, NOAA and the UK’s Climatic Research Unit doesn’t show perfectly straight lines, either. There are ups and downs, and depending on when you start and stop, it’s easy to find numerous periods spanning multiple years where no warming occurred or when global temperatures even decreased. But the long-term trend is clearly up. To learn more about the relationship between carbon dioxide and other greenhouse gases and climate change, visit NASA’s Global Climate change website.
Growing Confidence in Earth Temperature Measurements
Scientists continue to grow increasingly confident that measurements of Earth’s long-term temperature rise in recent decades are accurate. For example, an assessment published earlier this year1 of the agency’s GISTEMP record of global temperatures found that NASA’s estimate is accurate to within less than one-tenth of a degree Fahrenheit in recent decades. They concluded that Earth’s approximately 1 degree Celsius (2 degrees Fahrenheit) global temperature increase since 1880 can’t be explained by any uncertainty or data error. The recent trends were also validated with data from the Atmospheric Infrared Sounder (AIRS) instrument on NASA’s Aqua satellite.
Global Warming Is 'Global'
What’s perhaps most important to remember about global surface temperature fluctuations is that despite short-term ups and downs, the evidence shows that our planet is steadily accumulating heat. Scientists assessing global warming study Earth’s entire heat content, not just what happens in one part of the atmosphere or one component of the Earth system. And what they have found is that the balance of energy in the Earth system is out of whack: Our lower atmosphere is warming, the ocean is accumulating more energy, land surfaces are absorbing energy, and Earth’s ice is melting.
A study by Church et al. (2011) found that since 1970, Earth’s heat content has risen at a rate of 6 x 1021 Joules a year. That’s the equivalent of taking the energy output of about 190,000 nuclear power plants and dumping it into the ocean every year.
Despite short-term decreases in global temperature, the long-term trend shows that Earth continues to warm.
- Lenssen, N., G. Schmidt, J. Hansen, M. Menne,A. Persin,R. Ruedy, and D. Zyss, 2019: Improvements in the GISTEMP uncertainty model. J. Geophys. Res. Atmos., early view, doi:10.1029/2018JD029522.
We often get questions from readers about Earth’s sea ice in the Arctic and the Antarctic, and the differences between those areas. Arctic sea ice has declined over the past five decades, while Antarctic sea ice has increased, and then declined. Why do they behave differently?
How They’re Different
The primary difference between the Arctic and Antarctica is geographical. The Arctic is an ocean, covered by a thin layer of perennial sea ice and surrounded by land. ("Perennial" refers to the oldest and thickest sea ice.) Antarctica, on the other hand, is a continent, covered by a very thick ice cap and surrounded by a rim of sea ice and the Southern Ocean.
The Arctic Ocean is very deep and closely linked with the climate systems around it, making it more sensitive to climate changes than Antarctica.
During the centuries of human exploration in the Arctic, sea ice covered the Arctic Ocean well year-round, up until recent decades. But satellite observations show that Arctic sea ice has been declining in extent*, thickness and volume since 1979.1 Average Arctic sea ice extent is at its lowest since 1850.
During the summer melt season, the sea ice’s edge retreats toward the North Pole, only to re-grow during the Arctic winter. As a result of ongoing warming driven by human activities, the trend toward summer sea ice loss (from July to September, followed by a winter re-growth) continues.
Recent research suggests that there is a relationship between Arctic sea ice losses and the human burning of fossil fuels in all months.2 Aerosols (tiny particles suspended in the atmosphere) tied to human activities have offset some of the Arctic sea ice extent loss trend; a reduction in aerosol pollution will likely see a sea ice loss acceleration.3 Ice loss at the sea ice’s margins** results in winds driving warmer water beneath the Arctic sea ice, increasing the amount of heat the Arctic Ocean stores 4 and priming conditions for further sea ice loss.
A figure showing current Arctic sea ice extent can be found here.
Antarctic Sea Ice
Antarctic sea ice expands during the winter, only to melt back largely to the continent’s edge in summer.
Antarctic sea ice extent is currently below the long-term average of all decades prior since 1979. Previously, Antarctic sea ice extent had been above that long-term average due to long-term, large-scale wind circulation patterns that drove sea ice away from Antarctica5, making room for more sea ice to form nearer to the continent.6 Climate models, or computer simulations that incorporate all the factors that affect Earth’s climate, predicted this behavior.7 These long-term wind patterns reversed several years ago, resulting in a significant sea ice decline surrounding Antarctica. A figure showing current Antarctic sea ice extent can be found here.
Why Sea Ice Matters
Some of the questions we receive ask why we should care about the polar regions. These regions are very important in regulating global temperature. Because sea ice has a bright surface, 50-70 percent of incoming energy is reflected back into space. As sea ice melts in the summer, it exposes the dark ocean surface. Instead of reflecting 50-70 percent of the sunlight, it absorbs 90 percent of the sunlight. As the ocean warms, global temperatures rise further.
Also, what happens in the polar regions doesn’t stay in those regions. Their changes affect global temperatures and can even change ocean circulation. Earth’s sea ice is very attuned and responsive to even small changes in global surface and ocean temperatures.
- New Year Lows Once Again, NSIDC; Kwok, R. (2018), Arctic sea ice thickness, volume, and multiyear ice coverage: Losses and coupled variability (1958 – 2018). Environ. Res. Lett. 13 (2018) 105005 https://doi.org/10.1088/1748-9326/aae3ec; and
Arctic Sea Ice Volume Anomaly, Polar Ice Center
- Julienne Stroeve and Dirk Notz, Changing state of Arctic sea ice across all seasons, Environmental Research Letters, Volume 13, Number 10
- B. L. Mueller, Attribution of Arctic Sea Ice Decline from 1953 to 2012 to Influences from Natural, Greenhouse Gas, and Anthropogenic Aerosol Forcing, https://doi.org/10.1175/JCLI-D-17-0552.1
- Mary-Louise Timmermans, John Toole and Richard Krishfield, Warming of the interior Arctic Ocean linked to sea ice losses at the basin margins, Science Advances 29 Aug 2018: Vol. 4, no. 8
- All About Sea Ice, NSIDC
- Gerald A. Meehl et al, Antarctic sea-ice expansion between 2000 and 2014 driven by tropical Pacific decadal climate variability, https://doi.org/10.1038/ngeo2751
- A Tale of Two Poles, Earth Observatory, 2014
*Sea ice extent is a measurement of the area of ocean where there is at least some sea ice.
**Margins are transition regions between the ice-covered and ice-free portions of the ocean.
On February 27, 2014, a Japanese rocket launched NASA’s latest satellite to advance how scientists study raindrops from space. The satellite, the Global Precipitation Measurement (GPM) Core Observatory, paints a picture of global precipitation every 30 minutes, with help from its other international satellite partners. It has provided innumerable insights into Earth’s precipitation patterns, severe storms, and the rain and snow particles within clouds. It has also helped farmers trying to increase crop yields, and aided researchers predicting the spread of fires.
In honor of GPM’s fifth anniversary, we’re highlighting some of our favorite and most unique Earth Observatory stories, as made possible by measurements taken by GPM.
The second wettest October in Texas ever
In Fall 2018, storm after storm rolled through and dumped record rainfall in parts of Texas. When Hurricane Willa hit Texas around October 24, the ground was already soaked. One particularly potent cold front in mid-October dropped more than a foot of rain in areas. By the end of the month, October 2018 was the second wettest month in Texas on record.
GPM measured the total amount of rainfall over the region from October 1 to October 31, 2018. The brightest areas reflect the highest rainfall amounts, with many places receiving 25 to 45 centimeters (10 to 17 inches) or more during this period. The satellite imagery can also be seen from natural-color satellite imagery.
Observing rivers in the air
With the GPM mission’s global vantage point, we can more clearly see how weather systems form and connect with one another. In this visualization from October 11-22, 2017, note the long, narrow bands of moisture in the air, known as “atmospheric rivers.” These streams are fairly common in the Pacific Northwest and frequently bring much of the region’s heavy rains and snow in the fall and winter. But this atmospheric river was unusual for its length—extending roughly 8,000 kilometers (5,000 miles) from Japan to Washington. That’s about two to three times the typical length of an atmospheric river.
Since atmospheric rivers often bring strong winds, they can force moisture up and over mountain ranges and drop a lot of precipitation in the process. In this case, more than four inches of rain fell on the western slopes of the Olympic Mountains and the Cascade Range, while areas to the east of the mountains (in the rain shadow) generally saw less than one inch.
Increasing crop yield for farmers in Pakistan
Knowing how much precipitation is falling or has fallen is useful for people around the world. Farmers, in particular, are interested in knowing precipitation amounts so they can prevent overwatering or underwatering their crops.
The Sustainability, Satellites, Water, and Environment (SASWE) research group at the University of Washington has been working with the Pakistan Council of Research in Water Resources (PCRWR) to bring this kind of valuable information directly to the cell phones of farmers. A survey by the PCRWR found that farmers who used the text message alerts reported a 40 percent savings in water. Anecdotally, many farmers say their income has doubled because they got more crops by applying the correct amount of water.
The map above shows the forecast for evapotranspiration for October 16-22, 2018. Evapotranspiration is an indication of the amount of water vapor being removed by sunlight and wind from the soil and from plant leaves. It is calculated from data on temperature, humidity, wind speed, and solar radiation, as well as a global numerical weather model that assimilates NASA satellite data. The team also looks at maps of precipitation, temperature and wind speed to help determine crop conditions. Precipitation data comes from GPM that is combined with ground-based measurements from the Pakistan Meteorological Department.
Precipitation can drastically affect the spread of a fire. For instance, if a region has not received normal precipitation for weeks or months, the vegetation might be drier and more prone to catching fire.
NASA researchers recently created a model that analyzes various weather factors that lead to the formation and spread of fires. The Global Fire Weather Database (GFWED) accounts for local winds, temperatures, and humidity, while also being the first fire prediction model to include satellite–based precipitation measurements.
The animation above shows GFWED’s calculated fire danger around the world from 2015 to 2017. The model compiles and analyzes various data sets and produces a rating that indicates how likely and intense fire might become in a particular area. It is the same type of rating that many firefighting agencies use in their day–to–day operations. Historical data are available to understand the weather conditions under which fires have occurred in the past, and near–real–time data are available to gauge current fire danger.
Automatically detecting landslides
In this mountainous country of Nepal, 60 to 80 percent of the annual precipitation falls during the monsoon (roughly June to August). That’s also when roughly 90 percent of Nepal’s landslide fatalities occur. NASA researchers have designed an automated system to identify potential landslides that might otherwise go undetected and unreported. This information could significantly improve landslide inventories, leading to better risk management.
The computer program works by scanning satellite imagery for signs that a landslide may have occurred recently, looking at topographical features such as hill slopes.
The left and middle images above were acquired by the Landsat 8 satellite on September 15, 2013, and September 18, 2014—before and after the Jure landslide in Nepal on August 2, 2014. The image on the right shows that 2014 Landsat image processed with computer program. The red areas show most of the traits of a landslide, while yellow areas exhibit a few of the proxy traits.
The program also uses data from GPM to help pin when each landslide occurred. The GPM core satellite measures rain and snow several times daily, allowing researchers to create maps of rain accumulation over 24-, 48-, and 72-hour periods for given areas of interest—a product they call Detecting Real-time Increased Precipitation, or DRIP. When a certain amount of rain has fallen in a region, an email can be sent to emergency responders and other interested parties.
The GPM Core Observatory is a joint satellite project by NASA and the Japan Aerospace Exploration Agency. The satellite is part of the larger GPM mission, which consists of about a dozen international satellite partners to provide global observations of rain and snow.
To learn more about GPM’s accomplishments over the past five years, visit https://pmm.nasa.gov/resources/featured-articles-archive.
To learn more about the GPM mission, visit https://www.nasa.gov/mission_pages/GPM/main/index.html.
This piece was originally published on the NASA Earth Observatory "Earth Matters" blog.
Lately, it feels like we’re hearing about wildfires erupting in the western United States more often. But how have wildfire occurrences changed over the decades?
Researchers with the NASA-funded Rehabilitation Capability Convergence for Ecosystem Recovery (RECOVER) have analyzed more than 40,000 fires from Colorado to California between 1950 to 2017 to learn how wildfire frequency, size, location, and a few other traits have changed.
Here are six trends they have observed in the western United States:
1. There are more fires.
Over the past six decades, there has been a steady increase in the number of fires in the western U.S. In fact, the majority of western fires—61 percent—have occurred since 2000 (shown in the graph below).
2. And those fires are larger.
Those fires are also burning more acres of land. The average annual amount of acres burned has been steadily increasing since 1950. The number of megafires—fires that burn more than 100,000 acres (156 square miles)—has increased in the past two decades. In fact, no documented megafires occurred before 1970.
The recent increase in fire frequency and size is likely related to a few reasons, including the rise of global temperatures since the start of the new millennia. Seventeen of the 18 warmest years on record have occurred since 2001.
Global temperatures can affect local fire conditions. Amber Soja, a wildfire expert at NASA’s Langley Research Center, said fire-weather conditions—high temperatures, low relative humidity, high wind speed, and low precipitation—can increase dryness and make vegetation in the west easier to burn. “Those fire conditions all fall under weather and climate,” Soja said. “The weather will change as Earth warms, and we’re seeing that happen.”
3. A small percentage of the West has burned.
Even though fire frequency and size has increased, only a small percentage of western lands— 11 percent—has burned since 1950. In this map, wildfires are shown in orange. Private lands are shown in purple while public lands are clear (no color). The location of wildfires was random; that is, there was no bias toward fires affecting private or public land.
Keith Weber, a professor at Idaho State University who led the analysis, was surprised at the 11 percent figure. There’s no clear reason yet for why more of the region hasn’t burned. “Some of the 89% may not burn because it has low susceptibility—not dry enough or it has low fuel (vegetation),” said Weber. “Some areas may be really ripe for a fire, but they have not had an ignition source yet.”
4. The same areas keep burning.
How has only 11 percent of the west burned, yet the annual number of acres burned and the frequency of fire increased? It turns out that many fires are occurring in areas that have already experienced fires, known as burn-on-burn effects. About 3 percent—almost a third of the burned land—has seen repeated fire activity.
The map here shows the locations of repeated fire activity. While you can’t see it at this map’s resolution, some areas have experienced as many as 11 fires since 1950. In those areas, fires occurred about every seven years, said Weber, which is about the amount of time it takes for an ecosystem to build up enough vegetation to burn again.
5. Recent fires are burning more coniferous forests than other types of landscape.
Since 2000, wildfires have shifted from burning shrub-lands to burning conifers. The Southern Rocky Mountains Ponderosa Pine Woodland landscape has experienced the most acres burned—more than 3 million.
The reason might lie within the tree species. Ponderosa Pine is a fire-adapted species. With its thick and flaky bark, the tree can withstand low-intensity surface fires. It also drops branches lower as they age, which deters fire from climbing up the tree and burning their green needles. “The fire will remove forest undergrowth, but will be just fine for the pines,” said Weber. “We are starting to see Ponderosa Pines thrive in those areas.”
6. Wildfires are going to have a big impact on our future.
Research suggests that global warming is predicted to increase the number of very large fires (more than 50,000 acres) in the western United States by the middle of the century (2041-2070).
The map below shows the projected increase in the number of “very large fire weeks”—periods where conditions will be conducive to very large fires—by mid-century (2041-2070) compared to the recent past (1971-2000). The projections are based on scenarios where carbon dioxide emissions continue to increase.
According the Fourth National Climate Assessment, wildfires are expected to affect human health and several industries:
- Wildfires are expected to further stress our nation’s “aging and deteriorating infrastructure.”
- Smoke from wildfires is expected to impair outdoor recreational activities.
- Wildfires on rangelands are expected to disrupt the U.S.’s agricultural productivity, creating challenges to livestock health, declining crop yields and quality, and affecting sustainable food security and price stability.
- Increased wildfire activity is “expected to decrease the ability of U.S. forests to support economic activity, recreation, and subsistence activities.”
More about the source data:
Unless otherwise stated in the article, these data come from NASA’s Rehabilitation Capability Convergence for Ecosystem Recovery. RECOVER is an online mapping tool that pulls together data on 26 different variables useful for fires managers, such as burn severity, land slope, vegetation, soil type, and historical wildfires. In the past, fire managers might need several days or weeks to assemble and present such a large amount of information. RECOVER does so in five minutes, with the help of sophisticated server technologies that gather data from a multitude of sources. Funded by NASA’s Applied Science Program, RECOVER provides these data on specific fires to help fire managers to start rehabilitation plans earlier and implement recovery efforts quickly.
The researchers used the data layer showing historical fires since 1950, which were compiled from comprehensive databases by the U.S. Geological Survey Geospatial Multi-Agency Coordination, National Interagency Fire Center, Bureau of Land Management, U.S. Forest Service, and various state agencies such as the California Department of Forestry and Fire Protection. The historical fires do not include prescribed fires and undocumented fires. Learn more about the RECOVER program and its recent involvement with the Woosley Fire.
The piece was originally published on the NASA Earth Observatory "Earth Matters" blog.