[Vision2020] NASA's Goddard Climate Scientist G. Schmidt: 2008 Temperature Summaries And Spin

[Ted Moffett]

Gavin Schmidt's Bio:

http://www.realclimate.org/index.php?p=46
------------------

http://www.realclimate.org/index.php/archives/2008/12/2008-temperature-summaries-and-spin/#more-632
 16 December 2008 2008 temperature summaries and spin Filed under:

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

— gavin @ 1:56 PM

The great thing about complex data is that one can basically come up with
any number of headlines describing it - all of which can be literally true -
but that give very different impressions. Thus we are sure that you will
soon read that 2008 was warmer than any year in the 20th Century (with the
exception of 1998), that is was the coolest year this century (starting from
2001), and that 7 or 8 of the 9 warmest years have occurred since 2000.
There will undoubtedly also be a number of claims made that aren't true;
2008 is not the coolest year this decade (that was 2000), global warming
hasn't 'stopped', CO2 continues to be a greenhouse gas, and such variability
is indeed predicted by climate models. Today's post is therefore dedicated
to cutting through the hype and looking at the bigger picture.

As is usual, today marks the release of the 'meteorological year' averages
for the surface temperature records
(GISTEMP<http://data.giss.nasa.gov/gistemp>,
HadCRU <http://www.cru.uea.ac.uk/cru/data/temperature/#datdow>,
NCDC<http://lwf.ncdc.noaa.gov/oa/climate/research/anomalies/anomalies.html#anomalies>).
This time period runs from December last year through to the end of November
this year and is so-called because of the fact that it is easier to dice
into seasons than the calendar year. That is, the met year consists of the
average of the DJF (winter), MAM (spring), JJA (summer) and SON (autumn)
periods (using the standard shorthand for the month names). This makes a
little more sense than including the JF from one winter and the D from
another as you do in the calendar year calculation. But since the
correlation between the D-N and J-D averages is very high (r=0.997), it
makes little practical difference. Annual numbers are a little more useful
than monthly anomalies for determining long term trends, but are still quite
noisy.

The bottom line: In the GISTEMP, HadCRU and NCDC analyses D-N 2008 were at
0.43, 0.42 and 0.47ºC above the 1951-1980 baseline (respectively). In
GISTEMP both October and November came in quite warm (0.58ºC), the former
edging up slightly on last month's estimate as more data came in. This puts
2008 at #9 (or #8) in the yearly rankings, but given the uncertainty in the
estimates, the real ranking could be anywhere between #6 or #15. More
robustly, the most recent 5-year averages are all significantly higher than
any in the last century. The last decade is by far the warmest decade
globally in the record. These big picture conclusions are the same if you
look at any of the data sets, though the actual numbers are slightly
different (relating principally to the data extrapolation - particularly in
the Arctic<http://www.realclimate.org/index.php/archives/2008/11/mind-the-gap/>).


So what to make of the latest year's data? First off, we *expect* that there
will be oscillations in the global mean temperature. No climate model has
ever shown a year-on-year increase in temperatures because of the currently
expected amount of global warming. A big factor in those oscillations is
ENSO<http://www.realclimate.org/index.php/archives/2008/07/global-trends-and-enso/>-
whether there is a a warm El Niño event, or a cool La Niña event makes
an
appreciable difference in the global mean anomalies - about 0.1 to 0.2ºC for
significant events. There *was* a significant La
Niña<http://www.cdc.noaa.gov/people/klaus.wolter/MEI/>at the beginning
of this year (and that is fully included in the D-N annual
mean), and that undoubtedly played a role in this year's relative coolness.
It's worth pointing out that 2000 also had a similarly sized La Niña but was
notably cooler than this last year.

While ENSO is one factor in the annual variability, it is not the only one.
There are both other sources of internal variability and external forcings.
The other internal variations can be a little difficult to characterise (it
isn't as simple as just a super-position of all the climate acronyms you
ever heard of NAO+SAM+PDO+AMO+MJO etc.), but the external (natural) forcings
are a little easier. The two main ones are volcanic variability and solar
forcing. There have been no climatically significant volcanoes since 1991,
and so that is not a factor. However, we are at a solar
minimum<http://www.pmodwrc.ch/tsi/composite/pics/org_comp2_d41_61_0810.png>.
The impacts of the solar cycle on the surface temperature record are
somewhat disputed, but it might be as large as 0.1ºC from solar min to solar
max, with a lag of a year or two. Thus for 2008, one might expect a
deviation below trend (the difference between mean solar and solar min, and
expecting the impact to not yet be fully felt) of up to 0.05ºC. Not a very
big signal, and not one that would shift the rankings significantly.

There were a number of rather overheated
claims<http://www.dailytech.com/Temperature+Monitors+Report+Worldwide+Global+Cooling/article10866.htm>earlier
this year that 'all the global warming had been erased' by the La
Niña-related anomaly. This was always ridiculous, and now that most of that
anomaly has passed, we aren't holding our breath waiting for the 'global
warming is now back' headlines from the same sources.

Taking a longer perspective, the 30 year mean trends aren't greatly affected
by a single year (GISTEMP: 1978-2007 0.17+/-0.04ºC/dec; 1979-2008
0.16+/-0.04 - OLS trends, annual data, 95% CI, no correction for
auto-correlation; identical for HadCRU); they are still solidly upwards. The
match of the Hansen et al
1988<http://www.realclimate.org/index.php/archives/2007/05/hansens-1988-projections/>scenario
B projections are similarly little affected (GISTEMP 1984-2008
0.19+/-0.05 (LO-index) 0.22+/-0.07 (Met-station index); HansenB 1984-2008
0.25+/-0.05 ºC/dec) - the projections run slightly warmer as one would
expect given the slightly greater (~10%) forcing in the projection then
occurred in reality. This year's data then don't really change our
expectations much.

Finally, as we've discussed
before<http://www.realclimate.org/index.php/archives/2008/05/what-the-ipcc-models-really-say/>,
what climate models did or did not predict is available for all to
see<http://climexp.knmi.nl/>.
Despite many cautions about using short-term changes to imply something
about the long-term trend, these comparisons will still be made. So just for
fun, here is a comparison of the observations with the model projections
from 1999 to 2008 using 1999 as a baseline. The answer might be surprising
for some:

[image: 1999-2008 model and data trends]

You can get slightly different pictures if you pick the start year
differently, and so this isn't something profound. Picking *any* single year
as a starting point is somewhat subjective and causes the visual aspect to
vary - looking at the trends is more robust. However, this figure does show
that in models, as in data, some years will be above trend, and some will be
below trend. Anyone who expresses shock at this is either naive or … well,
you know.

As for the next few years, our expectations are not much changed. This
coming winter is predicted to be ENSO neutral, so on that basis one would
expect a warmer year next year than this year (though probably not quite
record breaking). Barring any large volcanic eruption, I don't see any
reason for the decadal trends to depart much from the anticipated
~0.2ºC/decade.

*Update:* Just FYI, the same figure as above baselined to
1990<http://www.realclimate.org/images/2008_from1990.jpg>,
and 1979 <http://www.realclimate.org/images/2008_from1979.jpg>.

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Vision2020 Post: Ted Moffett
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