All the reasons why global warming hasn't stopped
- 15 Jan 2013, 11:00
- Carbon Brief Staff
Accusations of scientific misconduct flowed from skeptics and
some news outlets last week after the Met Office revised downwards
decadal prediction of global temperature rise up to 2017.
Although the Met Office has explained why this change to their
short-term forecast doesn't affect their view of the likely
long-term warming trend, this didn't stop the Mail on Sunday
resurrecting one of its
favourite arguments - that global warming has "stopped".
This is not a new claim. In the
article on Sunday, climate skeptic
journalist David Rose claimed the Met Office's new decadal forecast
proves global warming "stopped" 16 years ago -
contrary to the Met Office itself. This is a claim
making for well over a year - he dismissed the
rebuttals that followed initial
claims about the Met Office's new data as
the "Stalinist way the Green Establishment tries to stifle
Well, the argument that a slowdown in temperature rise in recent
years shows global warming has "stopped" certainly isn't new - and
has been extensively picked apart, discussed, rebutted and
critiqued many, many times online. Here, for your amusement, are a
selection of responses.
Natural climate fluctuations can slow temperature
Climate skeptics often claim that scientists ignore the effect
of natural changes on the climate - this is the "The climate has
always changed!" argument.
However, the scientific literature is full of discussion of
natural fluctuations in the climate - and scientists believe it's
such natural processes which are currently masking the full extent
of human-induced warming - making global temperature rise slower
than in previous decades. On Tuesday last week, the Met Office said
"Small year to year fluctuations such as
those that we are seeing in the shorter term five year predictions
are expected due to natural variability in the climate system, and
have no sustained impact on the long term warming."
Analysis which strips out known natural influences on global
temperature over the last 30 years suggests there is no evidence
that human-induced global warming has slowed down - let alone
stopped, as this
video from blog Skeptical Science shows.
Meanwhile, Met Office chief scientist Julia Slingo told the
programme on Friday:
"Our latest forecast for the next five
years show that earth will continue to be at record warm levels
similar to those that we've seen over the last decade, and with a
fair chance that a new record will be made during that period".
Making judgements about global temperature usually boils down to
how long you measure temperatures for. Others have given helpful
explanations of the difference between short-lived natural climate
variability and longer term climate change. Scientists Gavin
Schmidt and Stefan Rahmstorf said in a
post for science blog Real Climate in 2008 that considering
only 10-15 years of temperature is like "analysing the temperature
observations from 10-17 April to check whether it really gets
warmer during spring."
To demonstrate this, Rahmstorf and Schmidt show the graph
below. The red line represents annual global temperature from 1977
to 2007 and each blue line is the average of an eight-year period
of data. As the scientists explain, the trends
over short periods are variable; sometimes small, sometimes large,
sometimes negative - depending on which year you start with. But
over a longer time period, the upward trend in global temperatures
Even prominent US climate skeptic Dr Pat Michaels told an
audience that the global warming has stopped argument is "a
little bit unfair to the data". He suggested not using it because
it is ultimately easy to disprove - temperatures will go up more in
the future - which could undermine credibility.
Finally, need a more facebook-friend illustration of why
short-term fluctuations in the weather don't negate longer term
trends in the climate? Try this video about taking a dog for
Cherry-picking data can flatten out
Other responses to the "global warming has stopped" argument
deal with so-called "cherry-picking" - selectively citing data to
support a particular point. In the case of global temperatures,
people have been known to suggest 1998 as the year when climate
change "stopped". 1998 was a particularly warm year due to a
natural climate cycle known as
El Niño. Comparing with this particularly warm year can be used
to make subsequent temperature change appear negligible.
In a previous
comment on this issue, the Met Office explained why this can be
a problem, and why this leads them to talk about temperature change
between decades, rather than years:
"[C]hoosing a starting or end point on
short-term scales can be very misleading. [...] If you use a longer
period from [temperature dataset] HadCRUT4 the trend looks very
different. [...] Looking at successive decades over this period,
each decade was warmer than the previous...Eight of the top ten
warmest years have occurred in the last decade".
Dr Myles Allen at Oxford University told the
Science Media Centre last week:
"Comparing the expected temperature for
2013-2017 with a single exceptionally warm year (1998), as some
reports have done, is just daft."
Of course, the argument to look at long term trends can also
cast over doubt statements about global temperature rise
"accelerating" when there are a series of particularly hot years.
Dr Allen points out that just as the current - and temporary
- cooling effect of natural factors is not evidence that global
warming has slowed, a series of warm years shouldn't be used to
suggest global warming has accelerated. He says:
"[A] lot of people (not the IPCC) were
claiming, in the run-up to the Copenhagen 2009 conference, that
'warming was accelerating and it is all worse than we thought'.
What has happened since then has demonstrated that it is
foolish to extrapolate short-term climate trends. We did see
unexpectedly fast warming from the mid-1990s to the early-2000s,
but the IPCC, quite correctly, did not suggest this was evidence
Like to look at data more closely? Blogger and climate
statistician Tamino took a more
detailed statistical look at temperature data up to 2011.
Or if you prefer fewer graphs and more pictures, try here
for visual instructions on how to cherry pick data.
Such 'slowdowns' are not unprecedented
There is scientific
research showing this isn't the first time in recent history
natural fluctuations in the climate have had a masking effect on
greenhouse gas warming. What's more, we'll probably see similar
periods in the future. As the Met Office explained
"Over the last 140 years global surface
temperatures have risen by about 0.8ºC. However, within this record
there have been several periods lasting a decade or more during
which temperatures have risen very slowly or cooled. The current
period of reduced warming is not unprecedented and 15 year long
periods are not unusual."
The oceans are a big heat sink
Finally, the vast majority of the warming of the earth's system
since 1955 has gone
into the oceans, as these two very simple graphs from the blog
Skeptical Science show,
The vast majority of warming in the last half century has
gone into the oceans. Source: http://www.skepticalscience.com/
Atmospheric temperatures are only a small (but important) part
of the picture. Professor
Chris Rapley of University College London told the Science
Media Centre last week:
"90% of the energy imbalance enters the
ocean and is not visible to the global mean surface temperature
Oil companies sometimes get a bad rep for not talking enough
about the causes and risks of climate change. But David Hone,
Shell's climate change advisor, appears to have gone out of his way
to dispel the myth that global warming has stopped, telling the
"[R]eal "global warming" is far broader
than [atmospheric temperatures] and includes ocean heating (surface
and deep ocean) and land ice melting".
"To simply argue that "global warming
has stopped" is short sighted. The evidence to support such a claim
is not there."
We wrote a bit more about
just how much heat the ocean take up in
response to yet another Mail on Sunday article.
New story, old argument
So, we've seen another round of "global warming has stopped"
stories in certain sections of the media. But as Dr Richard
Allan of the University of Reading told the Science Media
"Nothing in [the Met Office's] data
leads me to think that global warming due to human influence has
stopped, or is irrelevant. It hasn't, and it isn't."