2019-04-20_New_Scientist

8 | NewScientist | 20 April 2019

WE ARE starting to understand why a

tender caress feels different to other

forms of touch.

Parts of the skin that have hairs on

them, such as the backs of hands but

not the palms, have nerve fibres that

respond to gentle touch. Normally,

when mammals are touched, these

fibres send a signal through the spinal

cord to a part of the brain called the

primary somatosensory cortex, which

reacts to changes on the surface of

the body.

But for pleasurable touch, this

signal takes a detour to a part of the

brain called the insular cortex first.

To understand how these pleasure

signals are processed, Louise Kirsch

at Sorbonne University in France and

her colleagues compared the touch

responses of 59 people who had

experienced a stroke with those of

20 people who hadn’t.

The team touched the participants

with a soft-bristled brush on their

forearms at two speeds; the slower

of the two is known to stimulate the

pleasure-sensing nerve fibres. They

asked everyone to rate how intensely

they felt the touch and how pleasant

it was.

All of the participants found the

slower brush speed more pleasurable,

but some of those who had

experienced a stroke rated it less

pleasant than the control group did

(BioRxiv, doi.org/c4hb).

When the team analysed the brain

scans of the participants, they found

that over 80 per cent of people who

had had a stroke and reported less

sensitivity to pleasant touch on their

arm had a lesion on their insular

cortex, suggesting that enjoying

pleasurable touch less is tied to

damage in this part of the brain.

We have previously found that the

nerve fibres involved in pleasure are

related to the insular cortex, but this

study gives more concrete evidence,

says Francis McGlone at Liverpool

John Moores University in the UK.

Chelsea Whyte ■

How the brain

processes a

pleasant touch

NEWS & TECHNOLOGY

Adam Vaughan, Vienna

WE KNOW climate change made

the heatwave that swept the

northern hemisphere last year

more probable, but can we say

that it actually caused it?

In a bold claim, a group of

researchers are suggesting that

the extent of the heatwave would

have been impossible without the

carbon dioxide we have pumped

into the atmosphere. Other

scientists have cautioned against

unequivocally pinning the blame

on climate change.

Many regions were hit by

extreme heat between May

and July 2018. An area of 5 million

square kilometres was affected by

“hot days” over the period, which

is around 22 per cent of populated

and agricultural land areas in the

northern hemisphere. There were

record temperatures in Japan, and

wildfires in Sweden.

“The area affected could not

have occurred without climate

change,” says Martha Vogel at

ETH Zurich in Switzerland.

Vogel and her colleagues

modelled the extent of areas

concurrently affected by heat

in a world without the 1°C rise

of warming that humanity has

caused since the days of the

industrial revolution.

When the team compared

this with the size of observed

heatwave areas since the 1950s,

they found the two were mostly

in line. But their simulations

couldn’t replicate the size of

the area affected in 2018. The

largest region they could reach

was just 20 per cent of the actual

affected area.

When the researchers

reintroduced our warming

impact, they found comparable

heatwaves could occur every six

years. “So it is not unlikely to

have such an event like last year,”

says Vogel.

The team’s research suggests

that if temperatures rise by a

further 1°C, as they are on track

to do, heatwaves like that in 2018

could occur every year. Vogel

presented the findings at the

European Geosciences Union

General Assembly in Vienna.

Other efforts to attribute

specific extreme weather events

to climate change look at how

much more probable an event

was made by the warming we

have caused. For example, an

assessment last year found

that the 2018 heatwave was five

times more probable because

of climate change.

“Climate models aren’t good

at simulating heatwaves, so

people are usually cautious

about reporting the results as

having been impossible,” says

Geert Jan van Oldenborgh of the

Royal Netherlands Meteorological

Institute in De Bilt.

The research also set a relatively

low threshold for what constitutes

a hot day: temperatures above the

90th percentile of the historical

long-term average. A higher bar,

closer to what most people would

define as a hot day, would yield a

different result, says Oldenborgh.

The time frame of May to July

also excludes August, meaning

around half the heatwaves of a

typical year are left out.

“There are many uncertainties

involved in attribution studies

which make concrete statements

very difficult to defend,” says

Hannah Cloke at the University

of Reading, UK.

It is reasonable to measure the

area affected by hot days but it

isn’t the only way of looking at

heatwaves. Equally, there are

many different ways of defining

what extreme heat is, Cloke says. ■

Heatwave blamed

on climate change

TO

M^ N

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N/

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P/R

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/SH

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TO

CK

The sun sets over London on the

hottest day of 2018–

“The many uncertainties in

attribution studies make

concrete statements

very difficult to defend”