Scientists set straight the latest Mail on Sunday climate contortion
- 19 Mar 2013, 14:00
- Carbon Brief Staff
Why does the Mail on Sunday claim that recent measurements
of global temperature provide "irrefutable evidence that official
predictions of global climate warming have been catastrophically
flawed"? The scientists we spoke to don't think so.
The Mail on Sunday
article is the latest in
series of pieces by journalist and climate skeptic campaigner
David Rose, who has previously penned articles claiming that
stopped 16 years ago.
Rose presents a graph comparing the latest projections of
temperature rise from climate models with actual temperature
measurements. The headline claims because the graph shows the two
differ over the last decade, there is "hard proof that global
warming forecasts … were wrong all along".
seems the graph is taken from
a post on the 'Climate Lab Book' blog by Ed Hawkins,
a research fellow at the University of Reading, although the
article doesn't attribute it.
The graph compares recent global temperatures from the Met
dataset with projected temperatures from a set of climate models
which will be used in the next IPCC
report, called CMIP5.
CMIP5 includes a range of different models, which vary in how
they represent some aspects of the climate system. Scientists run
the models repeatedly with slightly different - but equally
plausible - starting conditions. This produces quite a spread of
projected future temperatures, which can be interpreted to give an
idea of the likely range of future temperatures.
Hawkins's comparison shows global temperatures are tracking the
bottom of the range in which 90 per cent of the model simulations
lie. In other words, global temperatures are, broadly speaking,
represented by a model simulation that is cooler than about 90% of
the CMIP5 model runs.
More simply, most of the models predict warmer temperatures than
we've seen in the past decade.
Observations vs model data
Based on this assessment of the past decade, Rose claims "the
speed of warming has been massively overestimated". Rather than
showing temperatures "steadily climbing", the graph "confirms there
has been no statistically significant increase in the world's
average temperature since January 1997", he says.
The argument that slowed temperature rise in recent years means
global warming has stopped certainly isn't new. And scientists and
commentators have extensively picked apart, discussed and critiqued
many times online.
But what about the argument that temperatures over the past
decade show that climate models are flawed? As Hawkins writes in
another blog post, there are three possible reasons for the
mismatch between climate models and measurements.
The first is natural variation in the climate. Small changes in
solar radiation, volcanic eruptions and ocean circulation patterns
can affect global temperatures, producing short-lived warming or
cooling effects. These natural processes mean temperatures bounce
around over the short term.
Scientists believe such natural processes may be masking the
full extent of human-induced warming currently - making global
temperature rise slower than in previous decades. Hawkins
"[A] decade with no global warming (or
even a cooling) is not implausible - various
that around 5% of decades should exhibit a cooling trend globally,
perhaps because the
warming is in the deeper ocean."
As the Met Office
explained back in January, such variability is not evidence
global warming has stopped. It says:
"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."
And as Dr Richard Allan, climate scientist at the University of
Reading, tells Carbon Brief, surface temperature is not the only
indicator of how the climate is changing. He says:
"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
We've written more about
Climate models are constantly being
refined to better account for natural variability, but they are
not perfect. Over the short term - a decade, for example - natural
variability may lead to mismatches between model and observed
Some climate models are projecting too high
Natural variability may not give the whole story, however.
Hawkins also suggests climate models that project the highest
temperature rise may be getting it wrong.
Professor Matt Collins from the University of Exeter tells us
this could be down to assumptions the climate models make about
tiny particles in the atmosphere, known as aerosols, which provide
a cooling effect which suppresses greenhouse gas warming. He
"[T]here are assumptions about the
declining role of atmospheric aerosols from 2005 onwards which are
As tighter controls on pollution are introduced, aerosol
emissions should reduce, and this could produce an additional
warming effect. But if climate models are overestimating how much
aerosol pollution has been reduced in recent times, that could
explain why they project more rapid warming than we can observe at
A third possibility is that climate models are overestimating
how much the climate would warm if carbon dioxide levels doubled -
what scientists call climate sensitivity. The IPCC estimates a
likely range for climate sensitivity of between two and 4.5
degrees, but hasn't ruled out higher values. Recently, however,
scientists have suggested that values higher than this range are appearing less
likely. This is an area of continuing scientific debate.
Significant climate change
The difficulty, as Hawkins explains, is disentangling the
different reasons why models might overestimate recent temperature
rise. This isn't a simple task. James Annan, a climate scientist
quoted in the Mail on Sunday article, tells us:
"In my opinion, the most obvious reason
for the moderate model-data mismatch would be that the models are a
little bit too sensitive overall. However a detailed physical
explanation of why this is so would be harder to discern."
Annan is quoted in the Mail on Sunday article as having said
that the "true figure [for climate sensitivity is] likely to be
about half of the IPCC prediction...". But he writes
in a blog post today:
"[This is] not something I can imagine
having said, or being likely. I do think the IPCC range is a bit
high, especially the 17 per cent probability of sensitivity greater
than 4.5 degrees. But their range, or best estimate, is certainly
not something I would disagree with by a factor of two."
Professor Piers Forster from Leeds University is also quoted in
the article. In a
response to the article, he says that even if it turns out that
very high values of climate sensitivity can be ruled out, this only
means higher estimates of temperature rise are less likely - not
that future temperature rise will be insignificant.
"Even with a suggested [climate
sensitivity] of around 2.5 Celsius or so we can end up with a very
significant climate change by 2100 if we don't do something.
Therefore,I think the tone of the article in terms of its
implications for the IPCC, climate science and the climate
itself are all wrong."
Climate forecasting is complicated, and scientific debate about
why models don't generally match the last decade or so of
observations was in full flow before David Rose's latest article.
While the climate scientists we spoke to said it'll take a few
decades more data until we know what's causing the current
mismatch, none have argued this is anything close to
"irrefutable evidence that official predictions of global climate
warming have been catastrophically flawed", as the Mail on Sunday
Rather, the Mail on Sunday has spun up an extreme interpretation
of a scientific discussion as part of a wider project to challenge
the government's green policies. It's not the first time, and it
won't be the last.