Yale Environment 360 - Nicola Jones
Rising global temperatures are altering climatic zones around the
planet, with consequences for food and water security, local economies,
and public health. Here’s a stark look at some of the distinct features
that are already on the move.
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A young boy herds his goats in the Ghat District of Libya, which has been converted largely to desert in the last 100 years.
TAHA JAWASHI/AFP/Getty Images
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As human-caused emissions change the planet’s atmosphere, and people
reshape the landscape, things are changing fast. The receding line of
Arctic ice has made headlines for years, as the white patch at the top
of our planet shrinks dramatically. The ocean is rising, gobbling up
coastlines. Plants, animals, and diseases are on the move as their
patches of suitable climate move too.
Sometimes, the lines on the map can literally be redrawn: the line of
where wheat will grow, or where tornadoes tend to form, where deserts
end, where the frozen ground thaws, and even where the boundaries of the
tropics lie.
Here we summarize some of the littler-known features that have
shifted in the face of climate change and pulled the map out from under
the people living on the edges. Everything about global warming is
changing how people grow their food, access their drinking water, and
live in places that are increasingly being flooded, dried out, or
blasted with heat waves. Seeing these changes literally drawn on a map
helps to hammer these impacts home.
The tropics are getting bigger at 30 miles per decade
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The tropics are expanding by half a degree per decade.
Source: Staten et al., Nature Climate Change, 2018. Graphic by Katie Peek.
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On an atlas, the boundary of the tropics is marked out by the Tropic
of Cancer and the Tropic of Capricorn, at about 23 degrees north and
south. These lines are determined by where the sun lies directly
overhead on the December and June solstices. But from a climate
perspective, most scientists draw the edges of the tropics instead at
the nearby boundary of the Hadley cell — a large-scale circulation
pattern where hot air rises at the equator, and falls back to earth,
cooler and drier, somewhere around 30 degrees latitude north (the top of
the Sahara desert and Mexico) and 30 degrees south (the bottom of the
Kalahari Desert).
The word “tropical” often brings to mind rainforests, colorful birds,
and lush, dripping foliage, but the vast majority of our planet’s
middle region is actually quite dry. “The ratio is something like 100 to
1,” says Jian Lu, a climate scientist at the Pacific Northwest National
Laboratory in Richland, Washington. About a decade ago, scientists
first noticed that this dry belt seemed to be getting bigger. The dry
edges of the tropics are expanding as the subtropics push both north and
south, bringing ever-drier weather to places including the
Mediterranean. Meanwhile, the smaller equatorial region with heavy rains
is actually contracting, Lu says: “People call it the
tropic squeeze.”
In a paper published in August, Lu and colleagues tracked how and why
the Hadley cell is expanding. They found that since satellite records
started in the late 1970s, the edges of the tropics have been moving at
about
0.2-0.3 degrees of latitude per decade
(in both the north and the south) .The change is already dramatic in
some areas, Lu says — the average over 30 years is about a degree of
latitude, or approximately 70 miles, but in some spots the dry expansion
is larger. The result is that the boundary between where it’s getting
wetter and where it’s getting drier is
pushing farther north,
making even countries as far north as Germany and Britain drier.
Meanwhile, already dry Mediterranean countries are really feeling the
change: In 2016, for example, the eastern Mediterranean region had its
worst drought in 900 years. The last time the tropics expanded northward (from
1568 to 1634, due to natural climate fluctuations), droughts helped to trigger the collapse of the Ottoman Empire.
There are several reasons for the shift in the Hadley cell, Lu’s team
reports, including the ozone hole in the Southern Hemisphere and
warming black soot in air pollution from Asia, along with rising air
temperatures from greenhouse gases. Changes in sea surface temperatures,
Lu says, seems to be causing at least half of the shift. That means
predicting future tropical expansion is difficult, says Lu. “We can’t
put a number on it, but we have a rough idea it will keep increasing.”
The Sahara desert has gotten 10 percent bigger since 1920 |
Since
1902, the Sahara Desert has grown 10 percent, advancing as much as
500
miles northward over the winter months in some spots.
Source: Thomas & Nigam, Journal of Climate, 2018. Graphic by Katie Peek.
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The world’s largest warm-weather desert is getting bigger. The Sahara
already covers a vast 3.6 million square miles — an area nearly as
large as the United States. The desert’s edges are defined by rainfall;
the line is usually drawn where the ground sees just 4 inches per year.
When Natalie Thomas and Sumant Nigam, ocean and atmospheric scientists
at the University of Maryland, looked at records stretching from 2013
back to 1920, they found that these boundaries for the Sahara had crept
both northward and southward, making the entire region about
10 percent larger.
The change, which is expected to reduce some countries’ ability to
grow food, hardly seems fair. “Morally, how do we deal with the fact
that developing countries are paying the price?” says Thomas. One study
in the 1990s showed that the limit of where plants could grow in the dry
southern edge of the Sahara had moved nearly
81 miles south in the 10 years between 1980 and 1990.
Across most of the Sahara the change is on the order of tens of miles
over the study period, but in other spots it’s far more dramatic: Libya
has gone from being mostly not desert in 1920, to mostly desert in
2013, as the line there has advanced a shocking 500 miles or so in
winter months. Lake Chad, which sits on the southern edge of the Sahara,
shrank dramatically from 9,600 square miles in the 1970s to less than
770 square miles in the 1990s, in part due to reduced rainfall in the
Sahel, the dry region just to the south of the Sahara.
Nigam and his colleague calculate that about two-thirds of the change
might be accounted for by natural climate cycles, such as the Atlantic
Multidecadal Oscillation and the Pacific Decadal Oscillation, which help
to determine rainfall. But the remaining third, they reckon, is down to
climate change — the northern edge of the desert, for example, seems to
be moving because of the climate-driven poleward creep of the tropics.
The 100th Meridian has shifted 140 miles east |
The
arid Western plains of North America meet the wetter, eastern region
near the 100th Meridian.
This climatic boundary has shifted about 140
miles east since 1980.
Source: Seager et al., Earth Interactions, 2018. Graphic by Katie Peek.
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Back in the 1870s, scientist and explorer John Wesley Powell noticed a
stark transition between the arid Western plains of North America and
the wetter, eastern region. As he wrote, “passing from east to west
across this belt a wonderful transformation is observed”: a “luxuriant
growth of grass” gives way to “naked” ground with the occasional cacti.
The line between the two regions goes from Mexico to Manitoba, cutting
right through the continent’s breadbasket. To the east, farmers grow
mainly rain-loving corn; to the west, mainly drought-resistant wheat.
This climatic transition has long been called the 100th Meridian,
after the longitudinal line that it roughly matches up with. But in
March, climate scientist Richard Seager of the Lamont–Doherty Earth
Observatory of Columbia University and colleagues published
papers showing the transition is
on the move.
The reasons for the existence of the line are many: the Rocky
Mountains force the wet air blowing in from the Pacific to rain out
before the winds reach the plains; Atlantic storms and winds from the
Gulf of Mexico bring moisture to the east. Now things are changing.
Rainfall hasn’t changed much in the northern plains, but rising
temperatures are increasing evaporation from the soil and drying things
out. Meanwhile, rainfall is diminishing further south due to shifts in
wind patterns. In total, that seems to have moved the line about 140
miles eastward since 1980, Seager calculated. The shift seen so far
might be due to natural variability, he says, but it’s in line with what
we expect to keep happening because of climate change. And it will keep
moving east as the planet keeps warming.
U.S. farmers don’t seem to report problems or changes yet, Seager
says, but he predicts that the country’s agriculture will eventually
have to adapt, by adding more irrigation, for example, using different
seeds, or shifting their crop entirely from one plant to another.
Tornado Alley has shifted 500 miles east in 30 years |
Hotspots for tornado formation in the U.S. have shifted east 500 miles since the mid-1980s,
along with shifts in temperatures.
Source: Agee et al, Journal of Applied Meteorology and Climatology, 2016. Graphic by Katie Peek.
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The author of the
Wizard of Oz likely chose Kansas for the
book’s setting for a reason: it was smack dab in the middle of “Tornado
Alley,” the stretch from South Dakota to Texas that’s infamous for
destructive storms. But things are changing; research shows that
tornados are now more likely to hit homes some 500 miles to the east in
Southern states, including Tennessee and Alabama.
Earth scientist Ernest Agee of Purdue University in Indiana and colleagues looked at
tornado activity going back to the 1950s
when modern tornado records began, and compared the first 30 years of
records to the next 30. This showed a clear shift in where tornadoes
were hitting hardest, both in terms of the total number of tornadoes and
the number of tornado days. In the first half of the study period, from
1954 to 1983, an area in Oklahoma was king, with a total of 477
tornadoes. But that area’s tornado count decreased dramatically, by 45
percent, in the second half of the study period, from 1984 to 2013.
Meanwhile, an equivalently sized area in northern Alabama bumped up 48
percent to 477 large tornadoes. Tennessee’s number of days of violent
tornadoes doubled, from 14 to 28 days, making the state arguably the new
heart of tornado activity, the authors argue.
The researchers don’t know exactly why the shift happened. Part of
the reason might be attributed to who is reporting tornados, notes
co-author Sam Childs, an atmospheric scientist at Colorado State
University. “The storm prediction center is based out of Oklahoma City.
There were a lot of reports there at first, and that’s broadening out
with time,” Childs says. “But there’s definitely a meteorological effect
too.” The shift in tornadoes matches up with a change in the weather,
he notes. The eastern half of the U.S. was about 1.2 degrees Fahrenheit
warmer during the second half of the study, making it likely that
climate had something to do with the move.
The general link between weather and tornadoes is fairly well
established. Tornadoes need several things to form, including warm, wet,
buoyant air and high wind shear. As the 100
th Meridian moves
eastward, it is pushing drier conditions further east (Oklahoma lies
right on that line). But it’s hard to say why Tennessee is seeing more
of them, and the future for tornado activity is hard to predict.
Plant Hardiness Zones are moving north in the U.S. at 13 miles per decade
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Hardiness
zones in the U.S., which track average low temperatures in winter,
have
all shifted northward by half a zone warmer since 1990.
Source: United States Department of Agriculture. Graphic by Katie Peek.
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As any gardener knows, the easiest way to keep track of which plants
will fare well where you live, or when to plant your tomatoes to avoid a
spring frost, is by taking note of your “
hardiness zone.”
In the frozen depths of Alaska and Siberia’s zone 1, you might want to
plant something like Yarrow to survive overwinter; in zone 5, which cuts
through the Corn Belt in the U.S. Midwest, you can plant asparagus in
March or April.
Hardiness maps are published around the world, but it’s easiest to
see change where the idea was first developed, in the United States. The
U.S. Department of Agriculture’s hardiness map, first published in
1960, is based on the average annual minimum temperature of any given
spot — a metric that plays a big part in determining if perennial crops
like orange trees will make it through the coldest months. Each zone
marks out a 10 degrees F band, from -60 to -50 degrees F in zone 1 to 60
to 70 degrees F in zone 13. When that map was last
updated,
in 2012, nearly half the country was upgraded to half a zone warmer
than it had been in 1990; in other words, all the lines shifted on
average a little to the north. That was partly thanks to more detailed
mapping techniques, the authors of the map reported, but also because
temperatures were warmer in the more recent data set.
The researchers who produced the 2012 revision stopped short of
saying the change was due to climate change, especially since the method
of how they produced the map changed so much from one version to the
next. But others have followed up on the same idea to show how climate
change, specifically, is shifting U.S. hardiness zones.
Lauren Parker and John Abatzoglou of the University of Idaho
tracked
what would happen to hardiness zones from 2041 to 2070 under future
global warming scenarios, and found the lines will continue to march
northward at a “climate velocity” of 13.3 miles per decade. That means
big changes in store for three major cash crops, they note. Almonds will
see their suitable growing range expand from 73 percent of the
continental U.S. from 1971-2000 to 93 percent from 2041–2070. Kiwifruit
will bump up from 23 percent to 32 percent during the same period, and
oranges from 5 percent to 8 percent.
So the shift in hardiness zones is good news for perennial cash crops
in the U.S., but not necessarily good news overall for food security in
North America, or globally. “On the plus side, if we can expand the
range over which we grow crops, that’s a good thing,” says Parker. But,
she adds, “On the flip side, you also allow for the expansion of weeds
and pests.”
The permafrost line has moved 80 miles north in 50 years in parts of Canada |
As
global air temperatures rise, permafrost is retreating north, moving as
far
as 80 miles poleward over a half-century in parts of Canada.
Source: Berkeley Earth. Graphic by Katie Peek.
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As the planet warms, the Arctic is feeling it the most: Temperatures
in northern regions are rising at about twice the global average. That’s
having a huge impact on the region’s permafrost, ground that typically
stays frozen all year round. As the line delineating an average
temperature of 0 degrees Celsius moves north, so too does the permafrost
line. “They roughly track together,” says Kevin Schafer, a permafrost
expert at the U.S. National Snow and Ice Data Center.
Permafrost isn’t particularly well documented: It’s underground, so
out of sight of satellites, and the Arctic is only sparsely covered with
meteorological stations. “There aren’t a lot of measurements that far
north,” says Schafer. That means much of the evidence of permafrost thaw
so far is either anecdotal or limited to specific well-monitored
regions. One study in northern Canada found that the permafrost around
James Bay had retreated 80 miles north over 50 years. Studies of ground
temperatures in boreholes have also revealed frightening rates of
change, says Schafer. “What we’re seeing is 20 meters down, it’s
increasing as high as 1-2 degrees C per decade,” he says. “In the
permafrost world that’s a really rapid change. Extremely rapid.”
The future looks similarly dire. One study predicts that by 2100, the
area covered by permafrost might shrink from nearly 4 million square
miles to
less than 0.4 million;
most of Alaska and the southern tip of Greenland would be permafrost-free.
The impacts are expected to be huge on both a local and global level.
Right now, permafrost acts like cement, keeping the ground firm and
impermeable to water. As it thaws, buildings and infrastructure
collapse. In the northern Russian city of
Norilsk, buildings are already tilting, cracking, and becoming condemned. In
Bethel,
Alaska, roads are buckling and homes collapsing. Many of the Arctic’s
uncountable small lakes will also drain away. “That’s going to have a
massive impact on the [region’s] ecology,” says Schafer. Meanwhile, the
thaw will also release vast amounts of climate-warming methane into the
atmosphere.
The Wheat Belt is pushing poleward at up to 160 miles per decade |
Between
1990 and 2015, production dropped in much of Australia's Wheat Belt
due
to drier than average conditions. The areas that disappear from this
map are
those where output dropped 50 percent or more.
Source: Hochman, Gobbett, & Horan, Global Change Biology, 2017. Graphic by Katie Peek.
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Australia, renowned for its interior deserts and coastal beaches, is also one of the planet’s largest
wheat exporters
— just after Canada, Russia, and the U.S. But the arable land at the
nation’s southern edge is shrinking, and its potential for growing wheat
declining.
In the 1860s, surveyor George Goyder drew a line to show where the
edge of Australia’s arable land ended. More than a century later,
Goyder’s line is still considered an important feature in determining
the country’s “cropping belt.” But climate change is making that land
drier, effectively
pushing the line further south.
Any given patch of land has a “theoretical potential” for the amount
of wheat it can support, given its soil, the climate, and other factors.
Reductions in rainfall and warmer temperatures have already reduced the
theoretical potential of southern Australia by
27 percent since 1990.
So far, farmers have managed to adapt to the changing conditions and
squeeze the same amount of wheat out of their lands. By tweaking things
such as their seeds and harvesting practices, they have gone from
harvesting 38 percent of their theoretical maximum in 1990 to 55 percent
in 2015. But that can only go on so long — farmers can typically only
reach about 80 percent of any given parcel of land’s maximum potential.
Once they hit that limit, Australian farmers probably won’t be able to
counteract the effects of the changing climate any longer. Zvi Hochman,
of Australia’s Commonwealth Scientific and Industrial Research
Organization (CSIRO), says he expects to see actual yields start to drop
around 2040. Places like the farming community of Orroroo, currently
right on top of Goyder’s line, will be “
significantly impacted,”
writes Julia Piantadosi of the University of South Australia in
Adelaide — they won’t be able to keep farming the way they are doing
today.
North America is seeing the opposite phenomenon: Its arable land is
romping northward, expanding the wheat belt into higher and higher
latitudes. Scientists project it could go from about 55 degrees north
today to as much as
65 degrees North
— the latitude of Fairbanks, Alaska — by 2050. That’s about 160 miles
per decade. That’s not all good news, as the southern edge gets drier,
hotter, and less agriculturally productive.
One study
showed that U.S. farmers will likely have to change the strains of
wheat they grow, while France and Turkey will have to invest heavily in
irrigation systems. In Asia, half of the Indo-Gangetic Plains, which
account for 15 percent of global wheat production, are predicted to
become
heat-stressed by 2050, significantly cutting yields.
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