14 | NewScientist | 15 March 2014

THIS WEEK

Michael Le Page

FORGET the “pause” in global warming. The climate is highly sensitive to greenhouse gases after all, and global temperatures could still shoot up this century.

Some recent analyses had suggested that the climate does not respond much to carbon dioxide, but three new studies say this hopeful scenario is unlikely, and Earth reacts strongly to CO2.

“All the lines of evidence now point to the high end,” says Drew Shindell of the NASA Goddard Institute for Space Studies in New York. “They’re all telling us the exact same thing.”

Climate sensitivity is a measure of how much Earth will heat up, on various timescales, if CO2 levels in the atmosphere double. The latest Intergovernmental Panel on Climate Change report says the medium-term warming will be between 1.5 °C and 4.5 °C. Studies of past climates usually point to the high end of this range, as do climate models.

But it has been hard to reconcile this with the slow warming over the past decade. Most climate models projected more warming

than observed, suggesting they are too sensitive to CO2. What’s more, the slowdown meant that simple estimates of climate sensitivity based on the recent temperature record were lower.

But new analyses can explain

these discrepancies. In February, Shindell and colleagues showed that models inflated the warming because they were fed the wrong data. The modellers had assumed that solar output, volcanic activity and aerosol pollution would stay at 1990s levels. But solar output fell, while volcanic activity and aerosol pollution rose. All that had a cooling effect. Shindell reran the models with the correct data, and most of the mismatch vanished (Nature Geoscience, doi.org/rsh).

Now Shindell has found a flaw in simple estimates of climate sensitivity that use recent temperature data. These underestimate the global cooling effects of aerosol pollution, so also underestimate sensitivity. That means the short-term sensitivity is between 1.3 °C and 3.2 °C – most likely 1.7 °C (Nature Climate Change, doi.org/rtm).

It also explains why a much-trumpeted study from 2013, from a team that included Shindell, used the temperature record and concluded that the short-term sensitivity to CO2 was just 1.3 °C (Nature Geoscience, doi.org/mj7).

A high short-term sensitivity suggests that the medium-term sensitivity, which builds on the initial warming, is also high.

That’s not the only evidence for high sensitivity. Steven Sherwood of the University of New South Wales in Sydney, Australia, and his colleagues looked at why different models give different estimates of sensitivity. They found that the way clouds behave in the models explains half of the differences. Models that simulate clouds best also predict higher medium-term sensitivity (Nature, doi.org/rsj).

“Sensitivity is not down at the low end,” Shindell says. “We can’t take any solace from warming being slower of late.”

That means we must cut our emissions fast, he says, or the planet could warm 6 °C by 2100. ■

Our planet reveals its sensitive side

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“Cognitive tests and brain scans suggest addiction and OCD share a lack of control of behaviour“

–On the road to warming–

ARE people with obsessive

compulsive disorder addicted to

their repetitive behaviours?

In a test designed to measure

decision-making, individuals with

OCD performed much like gambling

addicts, suggesting their underlying

brain problems may be similar.

OCD makes people worry

obsessively, compelling them to

OCD makes people behave like gamblers

carry out rituals like repeated hand

washing. It affects about one in 50

people and can take over their lives.

Because sufferers get anxious if they

can’t complete their rituals, OCD is

usually treated as an anxiety disorder

with talking therapies to relieve

distress or anti-anxiety drugs. These

approaches reduce symptoms but

only a minority of people are cured.

In the new study, 80 people – half of

whom had OCD – had to choose cards

from four decks, winning or losing

money in the process. Two decks were

rigged to produce big wins but even

bigger losses. The people without OCD

learned to choose from the two safer

decks but those with the disorder

were consistently less likely to make

good judgements and finished with

a significantly lower final score.

Drug and gambling addicts also

perform poorly on the test. That

doesn’t prove OCD is an addiction but

a growing body of work, including

brain scans and other cognitive tests,

suggest it should be recast in this way,

says Naomi Fineberg of the University

of Hertfordshire in Welwyn Garden

City. Both addiction and OCD “share a

lack of control of behaviour”, she says.

People with OCD might benefit

from talking therapies and drugs

designed for addictions, says Giacomo

Grassi at the University of Florence

in Italy, who recently presented the

work at the European Congress of

Psychiatry in Munich, Germany.

For example, a compound called

N-acetylcysteine, which looks

promising as a treatment for drug

and gambling addiction, also seems

to help in OCD. Clare Wilson ■