Has global warming stopped?
Over the last few weeks Carbon Brief has analysed some of the
key scientific arguments made by the UK's climate skeptics.
These arguments appear online, have been quoted in the media,
and are even on occasion repeated in Parliament. But we have found
that many of them cannot be supported by the scientific evidence
base.
Instead, the arguments rely on cherry-picking data, ignoring
scientific evidence and in some cases misrepresentating data or
research. In a series of blog posts we're examining some of
the most prominent assertions. (h/t for inspiration to Skeptical
Science).
The first counter-science argument, and the one we've found
repeated most frequently,
was recently noted by science journalist Simon Singh on
his blog:
Lord Lawson seems particularly keen to
focus on the first decade of the 21st century in
order to argue that manmade climate has ended or never happened …
Why does the GWPF fixate on just a few years of data when we can
look at decades, centuries or millennia of data? GWPF appears to
have a "less is more" (or "homeopathic") approach to data.
"Global warming has stopped"
The argument that global warming has stopped is typified by this
quote from Benny Peiser, Director of the Global Warming Policy
Foundation, in the Daily Telegraph last December:
"The data on 2010 confirms that since
1998 there has been no overall warming… Climate theory is at a loss
to explain this."
The assertion that "global warming has stopped" over the last
10-15 years is
frequently
stated by climate skeptics.
It has been featured by the UK media - for example, in a
Daily Mail article subtitled "The truth is global warming has
halted", in the
Daily Telegraph, the
New Statesman and the
Spectator. In
Parliament Lord Lawson suggested that "there has been no
warming so far this century at all," and it is even alluded to in
the masthead graph
of the Global Warming Policy Foundation's website:
Temperature rise
Those using argument that 'global warming has stopped' point to
where the global average temperature measurements conducted by
scientists show temperatures falling over time. Skeptics often
point to the year 1998 as a turning point, after which global
temperatures have fallen.
However, this argument relies on considering global temperature
over very short periods of time - ten or fifteen years. There
are good reasons to not draw conclusions from such short time
periods.
The world's temperature varies in response to natural cycles, in
particular the three-seven year
El Nino/ La Nina cycle and the eleven year
solar cycle, as well as occasional one-off events like volcanic
eruptions. For example, 1998 was hot in part because of a
particularly strong El Nino effect.
Over ten or fifteen year timescales natural variation can
dominate temperature behaviour, masking the longer-term trend in
rising temperatures.
This is evident from the header on the Global Warming Policy
Foundation's website, which charts global temperature data over
just 10 years:
Over ten years - a brief snapshot of climate history - there's
no clear up or down trend. However, by increasing the amount
of data they consider, scientists are able to see clear trends and
exclude the "noise" of natural variablity.
The past hundred years have seen significant periods of time
when temperatures have not risen. But by looking at the bigger
picture we get a more informed view of what's going on.
Global surface temperature as measured by the four main
datasets since 1880. Source: NASA
earth observatory
The Met Office itself has
specifically indicated that short-term observations of its data
do not indicate that 'warming has stopped':
Over the last ten years, global
temperatures have warmed more slowly than the long-term trend. But
this does not mean that global warming has slowed down or even
stopped. It is entirely consistent with our understanding of
natural fluctuations of the climate within a trend of continued
long-term warming.
As one scientist
put it, 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."
Statistically flawed
Because it relies on looking at temperature behaviour over only
short periods of time, the argument that "global warming has
stopped" has been labelled statistically flawed.
In 2008, the Associated Press (AP) gave four statistical experts
global temperature data which had been 'anonymised' so the analysts
wouldn't know what the data represented.
According to AP:
Statisticians who analyzed the data
found a distinct decades-long upward trend in the numbers, but
could not find a significant drop in the past 10 years in either
data set. The ups and downs during the last decade repeat random
variability in data as far back as 1880.
One of the experts, a professor of statistics at the University
of South Carolina, concluded:
"If you look at the data and sort of
cherry-pick a microtrend within a bigger trend, that technique is
particularly suspect."
This argument has even been rejected by Dr Pat Michaels, one of
the most prominent US climate skeptics. In this
video, he urges the audience at a skeptic conference not to use
the argument that global warming has stopped, because it is so easy
to disprove it undermines their credibility.
Global temperatures
The data shows that temperatures have been rising since the
1880s. There has been
roughly 0.75°C warming since the beginning of the 20th Century.
The period 2000-2010 was the
hottest decade on record.
There has been a slow-down in the rate of warming
over the last decade. Whilst the average temperature rose at
about 0.16-0.17 degrees per decade since the
late 1970s, the temperature rise through the 2000s has been between
0.05-0.13 degrees per decade. This slow-down has been attributed to
natural variability.
The lower end of this estimates comes from the Met Office. The other main
datasets which use surface temperature measurements (held by NASA
and the NCDC) show greater temperature rise over the last ten
years.
The difference may be because the Met Office leaves areas of the
Arctic ocean out of their calculations, whilst the other two
datasets do not. The rate of Arctic sea
ice loss indicates that the Arctic climate is changing rapidly,
and would support suggestions that the Met Office method may
underestimate temperature rise in the Arctic. Climate skeptics
usually ignore the data from the other datasets.
Finally, it's worth pointing a wider point: arguing 'global
warming has stopped' based on only the temperature record would be
ignoring all of the other
indicators of warming.
To support their claim that 'global warming has stopped'
climate skeptics cherry-pick a small sample of years from a single
dataset. This approach has been labelled 'particularly suspect' by
independent statistical experts, and rejected by the scientific
bodies which produce the temperature datasets.