Our work is unlikely to be last word on slowdown, say authors of new paper
- 20 Nov 2013, 15:00
- Roz Pidcock
Recent research suggesting the so-called slowdown in surface
warming might be less than previously thought has been met with
interest from both climate scientists and skeptics. We hear from
the authors about some of the queries raised and why they think
their research is not the final say on whether or not the
"slowdown" is real.
A fresh look at global temperature
released last week challenged the idea put forward in the most
recent IPCC report that there has been a so called "slowdown" in
surface warming recently, saying temperature rise in the last
decade and a half may be nothing unusual after all.
The authors of the new research used satellite data to
essentially plug the holes in the Met Office's HadCrut4 temperature
record where on-the-ground measurements are sparse. We reported on
the new research
The image below shows warming after 1997 in the original
HadCrut4 data (thin red line) compared to the new, updated data
(thick red line). The warming trend in the last decade or so is
about two and a half times greater than without the corrections,
say the authors.
Source: Cowtan & Way (
Hot and cold reception
With a figure of 0.12 degrees per decade, the paper puts
temperature rise since 1997 right in line with the warming we've
seen since the middle of last century. This is up from just
0.05 degrees in the last IPCC report.
Parts of the media
hailed the new research as showing the "pause" has all but
disappeared. The Guardian calls it
"barely a speed bump" on the long term global temperature
According to the
Independent, the new research
undermines attempts by climate change skeptics to discredit
human caused climate change. The newspaper says:
"It was the evidence that climate
change sceptics loved to cite. ... critics pointed time and again
to graphs showing the rise in the world's average surface
temperatures has slowed down since 1998 - a fact extensively
interpreted by many vocal opponents as a fundamental failure in the
basic science of climate change."
It's perhaps no great surprise then that climate change skeptics
have reacted coolly to the paper. It's worth noting lead author
Kevin Cowtan from the University of York is not a climate scientist
trained in theoretical physics and computational analysis.
The Met Office's Hadcrut4 data is freely
available for anyone to download. Cowtan took the data and
carried out his own analysis on it, the kind of non-expert take on
temperature data that climate change skeptics usually
The new paper is the first to try to fill in the gaps in the
temperature record this way. Cowtan tells us:
"What we do is essentially to take the
satellite data and 'anchor' it to the nearest surface observations
… I don't think anyone has attempted this."
With such a new approach to what's become a hot topic in climate
science it's worth investigating some of the main queries about the
paper in more detail.
The most significant "gap" in the HadCrut4 data is in the
Arctic. Temperatures in the Arctic are rising at about
twice the speed of global average so it follows that missing
data from this region could mean an underestimation of how much
global temperatures are changing.
Cowtan and colleague Robert Way deal with this problem by using
satellite data instead of land measurements to reconstruct surface
temperatures in the region. There's a good description from the
authors of how they do that,
The new, updated data in the bottom image shows faster warming
in the Arctic than the rest of the world. The white areas in the
top image show where data gaps exist in the HadCrut4 data. Source:
Cowtan & Way (
One criticism raised is that satellite data is unreliable at
high latitudes. But such claims are overstated,
say the authors:
"The satellite blind spot at the poles
is very small and can easily be interpolated over … We have
validated our results against the International Arctic Buoy Program
data and three weather models for the North Pole, and against the
Amundsen Scott South Pole Station record, which lies at the centre
of the Antarctic blind spot."
Indeed, Cowtan says the hard part of their research has been
making sure their approach stands up to rigorous scrutiny. He tells
"[M]ost of our work has been devoted to
testing whether what we are doing is valid. One way we do this by
leaving out surface observations and seeing if we can predict what
they would have been. This tells us that we can use the satellite
data, at least over land."
The authors have written a more detailed Q & A about their
here, which is well worth a read.
Some other critics have argued that even with the new
corrections to the observational record, temperatures are still
running at the low end of climate model projections for the last
decade or so.
Research by Ed Hawkins, climate scientist at the University of
Reading, includes an ongoing comparison of model projected
temperatures against observations. He's just added an update
comparing the new, corrected HadCrut4 data - so we can see what the
The following graph on his
blog is an updated version of one from the IPCC's latest
report. Model projections are within the upper and lower grey
lines, the original Hadcrut4 data is the black line and the Cowtan
& Way data is the blue line.
Source: Ed Hawkins,
Climate Lab Book
This graph shows that models are still overestimating recent
temperatures - albeit by slightly less than before the new analysis
- but the new observations are just within the uncertainty limits
of the models, indicated by the light grey shading.
We've written more about why most models could be running higher
Hawkins tells us it's important to note that the new
results also fall within the uncertainty limits of the Hadcrut4
data, shown by the red lines. These uncertainty limits were
calculated to take into account the known - but then unquantified -
issues with incomplete coverage.
Not the final word
While their research has ignited plenty of interest, the authors
keen to stress they see it as a contribution towards building a
more complete picture of recent temperature change, not as the
final word on the "slowdown".
say it's likely there has still been a slowdown in surface
warming, albeit less than the original version of the Hadcrut
temperature data suggests - a point the
The bottom line from the research should not be what
temperatures over the last 15 years can tell us about climate
change, say the authors:
"Our results highlight the dangers of
drawing conclusions from short term trends. This type of argument
has dominated the public discourse, but is in our view a misleading
approach to evaluating climate science."
Instead, Kevin Cowtan tells us he hope his research will mean
scientists and the public will be more aware of the issue of data
coverage - and how it may affect our understanding of how far and
how fast global temperatures are changing.