The Economist USA - 21.09.2019

Arcticseaice,annual

minimumextent

Relative

volume

change

Observedtemperaturechangebylatitude,°C

2018,relativeto1951-1980average

TheArcticistheepicentreofglobalwarming

Sources:NSIDC;PIOMAS;NASA;CarbonBrief*Averageforice

thickerthan15cm, 2019 datafromAugust†MinimumatSept16th

‡1st-99thpercentileofvalueswithineachlatitudeband

RUSSIA

CANADA

GREENLAND

UNITED

STATES

ARCTIC

OCEAN

Arctic

Circle

66.5°N

NorthPole

1980

2000

2019†

2019†

4.2m

2000

6.4m

1980

7.9m

Annualminimum

extent, km2 2012 recordlow

90°S

45°S

45°N

0

90°N

02451 3

↑TheArcticis

warmingmuch

fasterthan

everywhereelse

Latitude

Equator

Antarctic

Arctic

Average Range‡

Icethickness*

metres

Annual

minimum

area

↑

Strongerjetstream

Weakerjetstream

Accordingtoone

theory,a bigtemperature

differenceyieldsa strong

jetstreamona relatively

straightpath.Thisformsa

barrierthatkeepscoldair

intheArctic

Smallertemperature

differencesproducea

slower,wavierjetstream

Coldairmovessouth

andwarmairmovesnorth.

Weakerwindsslowthe

movementofweather

systems,causing

heatwavesandcold

snapstolinger

Warmair

Warmair

Coldair

Coldair

1980

2000

2019

2.1

1.6

1.0

TheEconomistSeptember 21st 2019 93

A

round 320bc, a Greek merchant called

Pytheas set off for a long journey north.

He brought back reports of a land called

Thule, six days north of Scotland, “where...

there are no nights during the [summer]

solstice...and also no days during the win-

ter solstice”. It is unclear if Pytheas made it

there himself, or merely heard tales. But for

this and his account of a “congealed sea”, he

is said to have been one of the first Arctic

explorers. Were he to return today, he

would find a very different landscape.

Temperatures in the Arctic are warming

twice as fast as the global average. One

driver is the melting of floating sea ice.

When it vanishes, it exposes deep blue wa-

ters, which absorb more solar energy than

white ice does. In turn, this speeds up melt-

ing: a classic positive-feedback loop. The

ice recedes to an annual minimum extent

every September. The record low was set in

2012; 2007 and 2016 are joint second. This

year is expected to be level with them.

The best-known consequence of Arctic

heating is rising sea levels. Melting sea ice

does not raise the water level, for the same

reason that melting ice cubes do not make a

cup overflow. However, water trapped on

land in Greenland’s ice cap does increase

the sea level when it melts into the ocean.

Greenland has 2.85m cubic kilometres

of ice, enough to lift sea levels seven me-

tres. For now, it is melting slowly. Sea levels

are rising by an average of 3.3mm per year;

owing to an unusually hot summer in 2019,

Greenland will contribute about 1mm.

Another feedback loop involves frozen

soil. Normal garden soil consists of 5% car-

bon; soil in Arctic permafrost regions, rich

in organic matter, contains 20-50%. It is

thought to hold a total of 1.1-1.5trn tonnes of

carbon, more than the atmosphere and ten

times as much as the Amazon.

As the Arctic warms, bacteria in the soil

consume organic matter faster, releasing

more carbon dioxide and methane. These

gases can then speed up the greenhouse ef-

fect—heating the permafrost further and

causing more emissions. This July the Si-

berian tundra warmed and dried enough to

catch fire for weeks, a very unusual event.

The third threat posed by Arctic warm-

ing is less scientifically certain but more

immediate. Higher Arctic temperatures are

thought to affect weather patterns in the

northern mid-latitudes, where weather

systems form as a result of the temperature

gap between the hot tropics and cool pole.

The jet stream pushes them west to east.

Some evidence suggests that as the tem-

perature difference shrinks, the jet stream

weakens and its wavy pattern deepens.

This allows “tongues” of frigid air to reach

south, and warm pockets to approach the

Arctic Circle. It may also cause both storms

and clear skies to stay in place for longer,

leading to extended floods and dry spells.

Climate-change sceptics point to cold

snaps in North America as evidence that

concern about global warming is overheat-

ed. In fact such days, caused by chilly air es-

caping polar latitudes, may be a conse-

quence of Arctic warming. 7

The consequences of a rapidly warming

Arctic will be felt far afield

Ice would suffice

Graphic detailThe altered Arctic