Wednesday, October 24, 2018

Redrawing the Map:  How the World’s Climate Zones Are Shifting

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.

The tropics are expanding by half a degree per decade. (Credit: Click to Enlarge.
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
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.

Read more at Redrawing the Map:  How the World’s Climate Zones Are Shifting

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