‘Powerful Evidence’ of Global Warming’s Effect on Seasons Found in Troposphere
By Daisy Dunne
Scientists studying the troposphere—the lowest level of the atmosphere—have found "powerful evidence" that climate change is altering seasonal temperatures.
A study published in Science finds that climate change has caused an increase in the difference between summer and winter temperatures across North America and Eurasia over the past four decades.
This could be the result of summer temperatures warming at a faster rate than winter temperatures in these parts of the world, the researchers say.
The findings show the "substantial human influence on Earth's climate, affecting not only global averages, but also local and seasonal changes," another scientist says.
Most of the world's weather originates in the troposphere, a second scientist tells Carbon Brief, meaning that changes to seasonal temperatures could be affecting the likelihood of extreme weather events, such as flooding and drought.
Evidence shows that the seasons are changing. In Europe, for example, analysis of the first-emergence dates of more than 500 plant species shows that the first day of spring has advanced by six to eight days in the past three decades.
However, working out to what extent seasonal changes can be explained by climate change—rather than natural climate variability, caused by phenomena such as El Niño—presents more of a challenge.
To quantify the influence of climate change versus natural variability, scientists often carry out "attribution" studies.
The new study is the first to assess how climate change could be influencing seasonal temperature changes in the troposphere—a layer that covers roughly the first 17 kilometers (approximately 10.6 miles) of the atmosphere above the earth.
"Our results suggest that attribution studies with the changing seasonal cycle provide powerful evidence for a significant human effect on Earth's climate."
For the study, the researchers analyzed atmospheric temperature data recorded by satellites throughout the summer and winter months from 1979-2016. (Carbon Brief has published an explainer on how satellite temperature records compare with measurements at the earth's surface.)
The map below shows how the difference between summer and winter temperatures, expressed as an average per decade, has changed across the world.
On the map, dark red indicates where the difference between summer and winter temperatures have grown larger, while dark blue shows where the difference has grown smaller.
Temperature difference between summer and winter months (per decade) from 1979-2016. Red shows a large temperature difference between the seasons, while blue shows a small temperature difference. Source: Randel (2018) Data source: Santer et al. (2018)
The map shows that the difference between summer and winter temperatures has increased the most in mid-latitude regions, particularly in the northern hemisphere.
This is because, in these regions, atmospheric summer temperatures are increasing at a faster rate than winter temperatures—causing the disparity between the two seasons to grow larger and larger, the researchers say.
The northern hemisphere has experienced more summertime warming than the southern hemisphere because it contains more land, the researchers say. The presence of land means that less heat can be absorbed by the ocean—leading to amplified atmospheric warming.
The map also indicates that the eastern edges of both North America and Eurasia have experienced the greatest amounts of warming. This could be because, during the winter in these regions, warm air masses are carried from east to west—leading to milder winters along western continental margins, the researchers say.
Changes in temperatures are not as pronounced around the tropics because this region is not highly seasonal, the researchers say. Instead, the seasons in tropical regions are typically defined by rainfall ("wet" and "dry," for example), rather than by temperature.
In contrast, the difference between summer and winter temperatures around the poles has shrunk over the past four decades, the map shows. In these regions, winter temperatures are rising faster than summer temperatures, the research finds.
In the Arctic, this winter warming is partly influenced by the diminishing presence of sea ice during summer months, the researchers say. With less sea ice present, the ocean absorbs more heat—which is later released during the winter, the researchers say.
The reason that winter atmospheric warming may have accelerated above Antarctica is less clear. However, previous research suggests that the presence of polar clouds in the stratosphere (which sits above the troposphere) could be playing a role.
'Substantial Human Influence'
To understand to what extent the observed seasonal changes were influenced by human-caused climate change, the researchers compared the satellite results to climate models.
The model simulations, which ran from 1979-2016, included a range of natural factors that can influence tropospheric temperature, including the cooling effect of volcanic eruptions and aerosols. To include the impact of human-caused climate change, the researchers used a "business-as-usual" emissions scenario known as RCP8.5.
The results show that only simulations that include the impact of human-caused climate change could correctly predict the patterns of seasonal temperature change recorded by the satellites.
This indicates that humans are having a "substantial influence" on the temperatures in the troposphere, said Dr. William Randel, a senior scientist at the National Center for Atmospheric Research, who was not involved in the research. In an accompanying perspectives article, he writes:
"[The] findings provide further markers of a substantial human influence on Earth's climate, affecting not only global averages, but also local and seasonal changes."
However, it is not clear how temperature changes in the troposphere could affect conditions at the land surface, he adds:
"The connection between changes at the surface and the free troposphere awaits explanation."
One possible effect could be changes to the timing and likelihood of extreme weather events, said professor Neil Harris, head of the Centre for Environment and Agricultural Informatics at Cranfield University, who was not involved in the research. He told Carbon Brief:
"The paper significantly increases confidence that the observed changes in the troposphere—where all our weather is—are consistent with models and that they are of human origin. This gives more confidence in the existing findings of an increase frequency of extreme weather, such as more intense rainfall and more extreme summer hot spells."
Solar Eclipse: Why the Sun Is Not Responsible for Recent Climate Change https://t.co/LukE7BHXKE #SolarEclipse2017 #Eclipse2017 @NASA @SpaceX— EcoWatch (@EcoWatch)1503321539.0
Reposted with permission from our media associate Carbon Brief.
Hundreds of endangered sea turtles were stranded on beaches after suffering "cold stunning" in the waters off Cape Cod, Mass. Local rescuers and wildlife rehabilitators stabilized the turtles at the New England Aquarium (NEAQ) and National Marine Life Center and began treatment. Many of the sea turtles were transported by land or air to partner facilities around the Eastern Seaboard for longer-term care to make room for more incoming, cold-stunned animals.
Rehabilitators at The Turtle Hospital in the Florida Keys assess critically endangered, cold-stunned Kemp's ridley sea turtles flown in after rescue in New England. The Turtle Hospital<p>NEAQ and local rescuers begin seeing turtles every fall when water temperatures drop to that 50 degrees F threshold, and typically expect to find them into early January. After that, <a href="https://www.ecowatch.com/sea-turtle-cape-cod-weather-2621527394.html" target="_self">temperatures are so cold that any animals found are usually no longer alive</a>.</p><p>Merigo estimated that this year's cold season "looks very busy" and noted that local rescue efforts had already surpassed 400 turtles.</p><p>"It is a lot of animals. They're still coming in," she told EcoWatch as she surveyed 39 rescued turtles that day and 20 the day prior. "So far, this is a huge year."</p><p>At NEAQ, the turtles are gradually warmed up about five to 10 degrees F a day. More aggressive warming can cause serious damage and the turtle might not survive, Merigo said. Emergency treatments also include providing replacement fluids, balancing electrolytes and addressing pneumonia. Assessments take place for other serious problems too, such as shell or limb fractures, frostbite, emaciation and eye damage.<span></span></p><p>As local aquariums don't have the capacity to care for all the injured turtles, a group of private pilots called <a href="https://www.turtlesflytoo.org/" target="_blank">"Turtles Fly Too"</a> donated planes, fuel and time to transport some to various partner facilities around the country. Other turtles were driven to closer care facilities.</p><p>"We have a huge network of really great partners working with us, so if we can spread out the care, we can give better care to all the animals," Merigo said.</p><p>The 40 Kemp's ridley sea turtles recovering in The Turtle Hospital will continue to be treated and rehabilitated anywhere from 30 days to a year, depending on the severity of injuries, Zirkelbach said.</p><p>The turtle expert noted that while she's treated cold-stunned turtles from the north before, the newest arrivals were the most cold-stunned Kemp's ridleys ever received at one time.</p>
After rescue, cold-stunned sea turtles received immediate emergency care and assessments at the New England Aquarium. Caitlin Cunningham / New England Aquarium<p>In the past decade, the Gulf of Maine, which spans from Cape Cod to Nova Scotia, has warmed 99 percent faster than the rest of the ocean, Zirkelbach said. The warm water encourages turtles that migrate north along the Gulf Stream in warmer months to stay in the bay longer.</p><p>"Turtles that fail to migrate south get stuck in the unique horseshoe-shaped topography of the Cape Cod peninsula, and when temperatures drop, the bay becomes a death trap," she added.</p><p>Before ocean temperatures warmed, the waters of Maine were too cold for many of these sea turtles, Merigo echoed. Now, with warming sea surface temperatures, Maine can reach the high 70s to low 80s, which is "perfect turtle temperature," she said. The potential for more turtles getting trapped in the bay and then cold-stunned is nerve-racking for Merigo.</p><p>In addition to shifting habitats as waters warm, warming global temperatures also disrupt natural gender balance in sea turtles, Merigo warned. Gender is determined by the temperature of eggs in nests, and as the planet warms, it will result in all females at some point, she said.</p><p>"The turtles we work with are all endangered and threatened," Merigo said. "For sea turtles in general, the future is a little grim. Climate change is real; it does impact them."</p>
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