[Vision2020] RealClimate.org, 10/6/2009: Climate Change Slowing or Stopping?: 371 Responses!

[Ted Moffett]

http://www.realclimate.org/index.php/archives/2009/10/a-warming-pause/#more-1265
 A warming pause? Filed under:

   - Climate Science<http://www.realclimate.org/index.php/archives/category/climate-science/>
   - Communicating
Climate<http://www.realclimate.org/index.php/archives/category/communicating-climate/>
   - Instrumental
Record<http://www.realclimate.org/index.php/archives/category/climate-science/instrumental-record/>
   - skeptics<http://www.realclimate.org/index.php/archives/category/communicating-climate/skeptics/>

— stefan @ 6 October 2009

Stefan Rahmstorf's bio:

http://www.realclimate.org/index.php/archives/2004/12/stefan-rahmstorf/

The blogosphere (and not only that) has been full of the “global warming is
taking a break” meme lately. Although we have
discussed<http://www.realclimate.org/index.php/archives/2008/12/2008-temperature-summaries-and-spin/>
this <http://www.realclimate.org/index.php/archives/2008/11/mind-the-gap/>
topic<http://www.realclimate.org/index.php/archives/2008/04/model-data-comparison-lesson-2/>
repeatedly<http://www.realclimate.org/index.php/archives/2008/01/uncertainty-noise-and-the-art-of-model-data-comparison/>,
it is perhaps worthwhile reiterating two key points about the alleged pause
here.

(1) This discussion focuses on just a short time period – starting 1998 or
later – covering at most 11 years. Even under conditions of anthropogenic
global warming (which would contribute a temperature rise of about 0.2 ºC
over this period) a flat period or even cooling trend over such a short time
span is nothing special and has happened repeatedly before (see 1987-1996).
That simply is due to the fact that short-term natural variability has a
similar magnitude (i.e. ~0.2 ºC) and can thus compensate for the
anthropogenic effects. Of course, the warming trend keeps going up whilst
natural variability just oscillates irregularly up and down, so over longer
periods the warming trend wins and natural variability cancels out.

(2) It is highly questionable whether this “pause” is even real. It does
show up to some extent (no cooling, but reduced 10-year warming trend) in
the Hadley Center data, but it does not show in the GISS data, see Figure 1.
There, the past ten 10-year trends (i.e. 1990-1999, 1991-2000 and so on)
have all been between 0.17 and 0.34 ºC per decade, close to or above the
expected anthropogenic trend, with the most recent one (1999-2008) equal to
0.19 ºC per decade – just as predicted by IPCC as response to anthropogenic
forcing.

[image: GISS temperature
trends]<http://www.realclimate.org/wp-content/uploads/GISStrends.jpg>

*Figure 1. Global temperature according to NASA GISS data since 1980. The
red line shows annual data, the larger red square a preliminary value for
2009, based on January-August. The green line shows the 25-year linear trend
(0.19 ºC per decade). The blue lines show the two most recent ten-year
trends (0.18 ºC per decade for 1998-2007, 0.19 ºC per decade for 1999-2008)
and illustrate that these recent decadal trends are entirely consistent with
the long-term trend and IPCC predictions. Even the highly “cherry-picked”
11-year period starting with the warm 1998 and ending with the cold 2008
still shows a warming trend of 0.11 ºC per decade (which may surprise some
lay people who tend to connect the end points, rather than include all ten
data points into a proper trend calculation).*

Why do these two surface temperature data sets differ over recent
years? We analysed
this <http://www.realclimate.org/index.php/archives/2008/11/mind-the-gap/> a
while ago here, and the reason is the “hole in the Arctic” in the Hadley
data, just where recent warming has been greatest.

[image: Mean temperature difference between the periods 2004-2008 and
1999-2003] <http://www.realclimate.org/wp-content/uploads/ncepawrming.gif>
*Figure 2. The animated graph shows the temperature difference between the
two 5-year periods 1999-2003 and 2004-2008. The largest warming has occurred
over the Arctic in the past decade and is missing in the Hadley data.*

If we want to relate global temperature to global forcings like greenhouse
gases, we’d better not have a “hole” in our data set. That’s because global
temperature follows a simple planetary heat budget, determined by the
balance of what comes in and what goes out. But if data coverage is not
really global, the heat budget is not closed. One would have to account for
the heat flow across the boundary of the “hole”, i.e. in and out of the
Arctic, and the whole thing becomes ill-determined (because we don’t know
how much that is). Hence the GISS data are clearly more useful in this
respect, and the supposed pause in warming turns out to be just an artifact
of the “Arctic hole” in the Hadley data – we don’t even need to refer to
natural variability to explain it.

Imagine you want to check whether the balance in your accounts is consistent
with your income and spendings – and you find your bank accounts contain
less money than you expected, so there is a puzzling shortfall. But then you
realise you forgot one of your bank accounts when doing the sums – and
voila, that is where the missing money is, so there is no shortfall after
all. That missing bank account in the Hadley data is the Arctic – and we’ve
shown <http://www.realclimate.org/index.php/archives/2008/11/mind-the-gap/>that
this is where the “missing warming” actually is, which is why there is
no shortfall in the GISS data, and it is pointless to look for explanations
for a warming pause.

It is noteworthy in this context that despite the record
low<http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant>in
the brightness of the sun over the past three years (it’s been at its
faintest since beginning of satellite measurements in the 1970s), a number
of warming records <http://www.ncdc.noaa.gov/sotc/> have been broken during
this time. March 2008 saw the warmest global land temperature of any March
ever recorded in the past 130 years. June and August 2009 saw the warmest
land and ocean temperatures in the Southern Hemisphere ever recorded for
those months. The global ocean surface temperatures in 2009 broke all
previous records for three consecutive months: June, July and August. The
years 2007, 2008 and 2009 had the lowest summer Arctic sea ice cover ever
recorded, and in 2008 for the first time in living memory the Northwest
Passage and the Northeast Passage were simultaneously ice-free. This feat
was repeated in 2009. Every single year of this century (2001-2008) has been
warmer than all years of the 20th Century except 1998 (which sticks out well
above the trend line due to a strong El Niño event).

The bottom line is: the observed warming over the last decade is 100%
consistent with the expected anthropogenic warming
trend<http://www.pik-potsdam.de/~stefan/update_science2007.html>of 0.2
ºC per decade, superimposed with short-term natural variability. It
is no different in this respect from the two decades before. And with an El
Niño developing <http://www.elnino.noaa.gov/> in the Pacific right now, we
wouldn’t be surprised if more temperature records were to be broken over the
coming year or so.

*Update:* We were told there is a new
paper<http://www.agu.org/journals/pip/jd/2009JD012442-pip.pdf>by
Simmons et al. in press with JGR that supports our analysis about the
Hadley vs GISS trends (sorry, access to subscribers only).

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Vision2020 Post: Ted Moffett
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