What's causing the surface warming slowdown? Scientists tell us what they think
- 08 May 2013, 14:15
- Roz Pidcock
Despite greenhouse gas emissions continuing to rise fairly
steadily, earth's surface - that's the land and top of the oceans -
has warmed relatively slowly over the past fifteen years or so.
We asked climate scientists to give us their thoughts about
what's causing the recent slower pace of surface warming. Here's
what they told us.
Recent slower warming isn't unusual
To draw conclusions about climate change, climate scientists
tend to look at long time periods - temperatures measured over
decades, or whole centuries. The last 15 years or so is a
relatively short amount of time to measure temperatures over.
There are good reasons for taking a longer view. As professor
Gabriele Hegerl from the University of Edinburgh tells us,
surface temperatures bounce around from one year to the next
because of natural fluctuations in the climate:
"Climate change becomes more visible
against [natural fluctuations when] the longer periods are
considered ... To see the long-term change, long periods need to be
considered, such as multiple decades."
Periods of slow surface temperature rise
aren't particularly unusual. In the twentieth century there
were several decades in which surface temperature rise slowed,
despite greenhouse gases in the atmosphere continuing at
fairly steady rates.
Michael Mann from Pennsylvania State University tells us:
"[W]hen we look at periods as short as a
decade over the past century, its easy to find time intervals where
natural factors ... obscure the warming trend."
Looking at temperatures over the course of the whole century,
trend is one of warming.
While the scientists we spoke to didn't think a period of slower
surface temperature rise was particularly surprising, they still
want to understand the mechanics of it in more detail. Hegerl tells
"[T]he hiatus certainly has lasted a bit
now, and raises interesting questions."
So what's causing it?
Natural variability in the climate system is the most
Natural changes in the sun's energy, volcanic eruptions and
ocean cycles such as the El Niño Southern Oscillation (ENSO) influence global
temperature from year to year.
Most of the scientists we spoke told us the main reason for a
slowdown in surface warming is that natural variability is
currently obscuring the full extent of greenhouse gas warming.
One team of scientists made an effort to strip
out the known effects of natural variation from the temperature
record. When they did so, the warming trend due to human activity
in the past few decades was pretty
clear - as the video below from Skeptical Science shows.
Professor Mann explains:
"[W]hen we account for the effects of
natural factors like El Niño and La Niña, volcanic eruptions, etc.
we find that the warming trend is proceeding steadily at about 0.15
degrees Celsius warming per decade."
So why does natural variability cause slower surface warming?
The scientists we spoke to explained the likely reason is that
natural variability is redistributing the sun's heat into different
parts of the climate system, notably the oceans.
The oceans are warming more, which might explain why
there's less surface warming
Surface temperature is, relatively-speaking, a small part of
climate change. The oceans absorb more than
90 per cent of the sun's energy that hits earth.
Temperature measurements show the deep oceans have warmed
substantially in the last half century - and this warming has
in the last decade.
The ocean has experienced accelerated warming since 2000 at
a rate unprecedented in the last 50 years - shown in purple.
Source: Balmaseda et al. (
If more heat is being diverted to the deep oceans, we'd expect
to see less surface warming.
Knutti from the Institute for Atmospheric and Climate Science
in Zurich tells us that less warming in one part of the climate
system is balanced out by more warming in another.
"[T]he Earth is warming and taking up
heat, sea level is rising, ice is melting, it's just that much of
the heat goes into the ocean at the moment."
research suggests that
other decades in the 20th century with little surface warming
also saw ocean warming increase.
So what drives such shifts between the atmosphere and ocean?
Trenberth from the US National Centre for Atmospheric Research
"A key pattern [of natural variability]
is the one called the Pacific Decadal Oscillation, which relates to
the magnitude and frequency of El Niño [and La Niña] events ... One can argue
that surface temperature rise was increased from about 1975 to 2000
or so but now has gone in the other direction owing to these
Other signs of climate change aren't
It's not just about the atmosphere and the oceans - there are
many other indicators of the changing climate which show little
sign of pausing.
Arctic sea ice is diminishing
more rapidly than scientists expected, while ice loss from the
Greenland and Antarctic
ice sheets is accelerating.
Dr Richard Allan, climate scientist at the University of
Reading, points out that taken as a whole, there's little sign
climate change is slowing:
"Some aspects are changing more quickly
than predicted by climate simulations (e.g. Arctic ice) and others
are slower than the projections (e.g. surface temperature over last
Satellite data show that from 1992 to 2011, both ice sheets
lost mass. In Greenland, the rate of loss is now five times higher
than it was in the 1990s. Loss has been slower in Antarctica, still
showing a doubling since the 1990s. Source: Shepherd
et al., (2012)
It's understandable that there's a focus on what surface
temperatures are doing - it's what we experience most directly. But
surface temperatures are just one part of the climate system.
Recent surface temperatures don't change our best
estimate of climate sensitivity
proposed one reason surface temperatures haven't risen as much
as expected recently could be because the climate isn't as
sensitive to greenhouse gases as scientists previously thought. So
is it evidence that "climate sensitivity" - how much the planet
will warm as we put more carbon dioxide into the atmosphere - is
lower than thought?
The current best estimate of climate sensitivity comes from the
2007 Intergovernmental Panel on Climate Change (IPCC) report, which
put forward a "likely" range for climate sensitivity of between
two and 4.5 degrees Celsius. But the organisation didn't rule out
values up to six or seven degrees, although they were seen as less
Parts of the media have cited
papers supporting low values of climate sensitivity to suggest
scientists are considering whether the IPCC's "likely" range should
be revised downwards.
But as far as we can see, scientists are only
discussing whether the so-called "
long tail" of climate sensitivity - values above 4.5 degrees -
Climate sensitivity estimates based on climate models still
support the higher end of the IPCC's likely scale - which means it
be revised any time soon.
This is all quite complex stuff. But as discussions move into
the media, some of this scientific detail is
being lost. Professor Knutti tells us:
"Some [studies] are pointing [to the
possibility that some models overestimate climate sensitivity] but
I don't think this is the main factor ... But even if it were, that
does not mean that climate change is not happening. It does not
change the fact that the earth has warmed, the fact that the
changes are attributed to human influence, nor the fact that it
will continue to change"
The impression from the scientists we spoke to is that it has
climate sensitivity probably isn't that relevant in explaining the
recent period of slower surface warming. Refining the IPCC's likely
range, if and when that happens, will depend on more than just the
last decade and a half of surface temperatures.
"My personal view is that our overall
assessment [of the value of climate sensitivity] hasn't changed
So from the scientists we spoke with, the view is that slower
warming is most likely down to natural variability redistributing
heat between the atmosphere and the oceans. Such natural variation
is by its nature short-term, and scientists expect the long term
warming trend to continue.