Hothouse Earth could
soon be unavoidable
Tim Radford
Climate News Network
Researchers say the world may be approaching a tipping point, followed by a dan-
gerous slide towards Hothouse Earth, an overheated planet.
H
uman actions threaten to push the
planet into a new state, called Hot-
house Earth. In such a world, global
average temperatures could stabilise
at 4°C or even 5°C higher than they have been
for most of human history.
Global sea levels, too, would rise, by 10 me-
tres, or even as much as 60 metres, to drown
all the world’s great coastal cities. Such a tran-
sition might happen “in only a century or two”,
but once started, there might be no stopping
it.
It would be uncontrollable and dangerous to
many and “it poses severe risks for health,
economies, political stability … and ultimately
the habitability of the planet for humans.”
And, say scientists who have completed a
survey of the research landscape, there is no
knowing how close the threshold of dramat-
ic change might be. The planet has already
warmed by 1°C in the last century, and the
thermometer is climbing at a rate of 0.17°C
per decade.
Even at the ambitious target temperature rise
of no more than 2°C by the end of the century
– a target endorsed by 195 nations in Paris in
2015 – humans might already have triggered a
cascade of feedbacks that would set the plan-
et sliding to a point hotter than at any time in
the last 10 million years.
“These tipping elements can potentially act like
AgriKultuur |AgriCulture
a row of dominoes. Once one is pushed over,
it pushes Earth towards another. It may be
very difficult or impossible to stop the whole
row of dominoes from tumbling over”
Researchers, led by Will Steffen of the Austra-
lian National University and backed by some
of the big names of European climate science,
report in the Proceedings of the National
Academy of Sciences that they considered 10
natural processes, among them a number of
tipping points that could lead to change once
a certain temperature threshold had been
crossed.
These feedbacks could turn what are, right
now, carbon sinks – stores of atmospheric car-
bon locked away in the soils and the forests
– into sources of greenhouse gases that could
accelerate global warming.
These future hazards include thawing of the
permafrost, the loss of methane hydrates
stored in the ocean floor, the weakening of
carbon stores both on land and in the oceans,
increasing bacterial activity in the seas, die-
back in the tropical Amazon forest and in the
cool forests of the north, the loss of sea ice in
the Arctic summer, and the loss of Antarctic
sea ice and the polar ice sheets.
“These tipping elements can potentially act
like a row of dominoes. Once one is pushed
over, it pushes Earth towards another. It may
be very difficult or impossible to stop the
whole row of dominoes from tumbling over.
34