date: Tue, 20 Nov 2007 16:03:03 +1100
from: David Thompson <davet@atmos.colostate.edu>
subject: The volcanoes...
to: Phil Jones <p.jones@uea.ac.uk>

   Hi Phil,

   I'm really glad your interested. And thanks for the comments.

   First a general comment about the direction of the paper. Then some specific thoughts on
   the volcano results.

   A general comment:

   The paper has two main parts: 1) clarifying the importance of the step in 1945 and 2)
   providing new insight into the climatic impacts of volcanic eruptions.

   I've been emailing with John Kennedy about how to approach the dip in 1945. It's now seems
   clear the dip is due to uncorrected data issues. But the step contributes substantially to
   the appearance of  global-mean cooling in the middle of the 20th century. So correcting the
   dip is almost certainly going to impact the appearance of the global-mean time series. It's
   good to have the data experts on board to help frame the discussion carefully and
   appropriately!

   The attached Fig. 4 includes a few new figures you might not have seen. SSTs diverge
   rapidly from land data in 1945 in the tropics. I'll keep iterating with the Hadley Centre
   on this and keep you posted ...

   The volcano results:

   Thanks for pointing me to the monograph. I thought I was up to date on the literature, but
   I missed that one. I've ordered the monograph from the library and it will arrive in a
   couple days.

   In the meantime: please see the attached Figures 2-3. They clarify some of my thinking on
   the topic.

   The left column of Fig 2 shows the composite for the largest 5 tropical eruptions
   (Krakatoa, Santa Maria, Agung, El Chichon, Pinatubo) for the globe (top), global land
   (middle) global ocean (bottom). The anomalies are shown with respect to the 4 year period
   before the eruption (ie, the mean for years -4 to 0 is zero).

   The middle column shows the exact same results, but for the residual data. The key here is
   that the volcanic signal is much clearer in the residual data, both in terms of the signal
   after the eruption date, but also in terms of less volcanic noise during the rest of the
   composite period. You can see the improvement in the global mean, in the land data, and  in
   the SST data.

   It's also clear from both the left and middle panels that there is a lingering trend in the
   composites. For example: if you look at the top left panel (the composite of the raw global
   mean), the anomalies ~8 years before the eruptions are more pronounced than the anomalies
   after the eruptions. This doesn't make physical sense, and reflects the fact the eruptions
   are superposed on the global-mean warming of the past century.

   So it occurred to us that the global warming of the past century is likely biasing the
   volcanic signal. And ....

   The right hand panel shows the residual composite after the trend calculated for the decade
   preceding the eruptions has been removed from the 20yr composite period. In this case, the
   prevolcanic period is very quiescent, as you'd expect. But the volcanic signal is very,
   very pronounced, and seems to linger for more than a decade.

   That volcanos may have a >10 year timescale is supported by the recent Church et al paper
   in Nature (attached as a PDF) on changes in ocean heat content after volcanic eruptions.
   And I think it makes physical sense. See, for example, the attached Fig. 3.

   Fig. 3 shows the simulated output to a 3 W/m2 forcing imposed over two years. The result is
   based on the simple Hasselman climate model, that is:

   c*dT/dt = Forcing - alpha*T

   where T is temperature, Forcing is the 3 W/m2 for two years, alpha*T is Newtonian cooling,
   and c is the heat capacity of the atmosphere+the depth of the ocean to which the signal
   penetrates.

   The model is extremely simple, but shows that if the signal penetrates the top 100-150m of
   the ocean mixed layer, then the e-folding damping timescale of the volcanos is at least 6-8
   years. If you use a shallower mixed layer (say 10m), then you get a shorter damping
   timescale, but the amplitude of the response is much too large (~1 K). So the argument is
   that to get the right amplitude (~0.3 K), then you need a mixed layer response of at least
   100m.

   I think that a reader could argue about how we detrend the data. And so I don't think we
   can "prove" that the volcanic signal lasts for a decade or more based on observations and
   our simple model alone. But I do think we can make a compelling case that the data are
   suggestive of a long time scale, and we can certainly show that a long time scale is
   consistent with changes in ocean heat content down to 100 m. So the paper could get people
   chatting about the possibility.

   What do you think?

   -Dave

   ps I understand what you mean about centering about January (to account for the midwinter
   warming). It turns out that centering matters for a few months during NH midwinter (as you
   know). But it doesn't change the general appearance of Fig. 3.

   Attachment Converted: "c:\eudora\attach\Figures_VNov141.pdf" Attachment Converted:
   "c:\eudora\attach\Church_etal_Nature2006.pdf"

   On Nov 20, 2007, at 3:47 AM, Phil Jones wrote:

      Dave,
         I've had a read of the email now and looked at the pictures
      over the weekend. It seems very interesting and I like to be
      involved.
         The volcano response looks clear, but this may be a result
      of the way you've done superposed epoch analysis - and maybe
      on the residuals (i.e. without ENSO and COWL). Anyway have
      you seen this paper?
      Jones, P.D., Moberg, A., Osborn, T.J. and Briffa, K.R., 2003: Surface climate responses
     to explosive volcanic eruptions seen in long European temperature records and
     mid-to-high latitude tree-ring density around the Northern Hemisphere, In (A. Robock and
     C. Oppenheimer, Eds.) Volcanism and the Earths Atmosphere. American Geophysical Union,
     Washington D.C. 239-254.
      I would normally send you a pdf, but for some reason when AGU allowed us to
      get them in 2004 they were enormous - 40Mb, so too big to email. It is
      worth you getting hold of this and comparing to what we've al;ready done there.
      We used longer European records, but for the NH we used 5 large tropical
      eruptions in the 20th century. I don't think your 10 years before/after
      (as opposed to our 5) is the cause of the difference. It could be your
      extraction of trends, or could just be your working with residual T.
      The paper will show you how to calculate significance levels for the
      volcanic signal.  I hope you've always used the January of the eruption
      year to designate the 4 volcanoes you've chosen.
      I can't see in your volcano plot the greater cooling in summers (NH
      mainly) cf NH winters. This was a clear signal in the work we did.
      Some other thoughts:
      1.  The COWL series looks to have smaller variance before about 1925.
       Also COWL variance looks more one-sided in recent decades.
      2. As you or David said, the 1945 drop could be that 1940-44 are too warm.
      Still think that it doesn't look natural.
      Cheers
      Phil
     At 06:44 14/11/2007, David Thompson wrote:

     Dear David, Phil and John,
     (This is a bit of a long email, so you might want to grab a cup of
     coffee - or tea - before reading on...)
     Thanks again for the quick and helpful responses last week.
     Mike and I would be happy to include John as a coauthor on our paper.
     And David, Phil: we understand if you are too busy to join another
     project. But if you are interested in joining the paper, too, that
     would be great. The goal of the paper is to clarify some key aspects
     of 20th century temperature variability, and the study would
     certainly benefit from your expertise.
     Before I get too far ahead of myself, let me review the main points
     of the paper as it currently stands. I've attached 3 pages of figures
     (the figures will evolve as the writing evolves, but as of now it
     appears the paper will end up being short and punchy)
     Figure 1 includes 2 panels. The top time series in the top panel
     shows the global-mean temperature time series. The next two time
     series show the linear fit of ENSO and the COWL (cold-ocean/warm- land) time series to
     the global-mean. The ENSO time series is found
     as a damped thermal response to variations in the cold-tongue (this
     gives a marked improvement in the representation of ENSO in global- mean temperatures).
     The COWL time series represents the effects of
     random fluctuations in climate acting on the different heat
     capacities of the ocean and land (eg: periods of warm advection over
     land/cold advection over the ocean lead to warmer than normal global- mean temperatures
     by virtue of the fact that the continents have a
     lower heat capacity).
     I'll provide more details of the ENSO and COWL methodologies in a
     future email, but the main point is that a lot of the high frequency
     'noise' in global-mean temperatures can be accounted for on the basis
     of two simple, physically based time series.
     The bottom panel in Fig. 1 includes a reproduction of the global-mean
     time series (top) and also shows the residual global mean time
     series, which is found by removing the ENSO and COWL time series from
     the global-mean time series.
     The bottom panel of Fig. 1 is the key figure in the paper. We think
     it's remarkable how well the fitting 'cleans up' the global-mean time
     series. If you look closely, you'll see that the major volcanoes of
     the past century (marked by solid vertical lines) are much, much
     clearer in the residual time series. But the fitting not only
     isolates the volcanoes, it also isolates the very large drop in Aug
     1945. The Aug 1945 drop is about 0.3 K, almost twice as large as the
     response to Pinatubo.
     The residual time series also suggests a slightly different view of
     20th century temperature variability. The canonical view is that
     temperatures warmed in the 20s, settled from the 40s-70s, and warmed
     from the 70s-the present. But if you stare at the residual time
     series, you get the impression that global-mean temperatures have
     actually risen steadily over the past century, but that the warming
     has been disturbed by several discrete and abrupt drops in temperature.
     As for the volcanos:
     In figure 2 we're exploiting the fitting procedure to provide a
     'cleaned up' version of the volcanic response in surface
     temperatures. The figure shows the composite temperature response for
     the 4 largest tropical volcanoes of the 20th century. The composite
     is done such that the 10 year period before each volcano has a trend
     of zero and mean of zero. (If you don't remove the trend then the
     long term warming biases the composite). The response in the residual
     data is surprisingly coherent (much more so than in raw data). But we
     think what's most striking is that surface temperatures do not appear
     to fully recover for up to a decade after each volcano (the composite
     is limited to 9 years after the eruptions since the eruption of
     Pinatubo occurred 9 years after the eruption of El Chichon).
     You can see the long timescale of the volcanoes in the residual
     global-mean time series: if you follow the temperature time series
     before, say, Agung or Pinatubo, you can see that it takes a long,
     long time for global-mean temperatures to catch up to where they
     would have been, assuming they would have continued to warm...
     And as for the dip in 1945:
     Fig 2 shows the residual (ie with the COWL or ENSO time series
     removed) global-mean land and ocean time series. The point here is
     that the large dip in Aug 1945 does not show up in the land data.
     My impression from your emails last week is that the dip is almost
     certainly due to changes in instrumentation during the war. But it's
     also my impression that the specific reasons for the dip are not yet
     known. It's possible that the dip is offset by spurious rises in SSTs
     at the start of the war. But this isn't certain. And even so, there
     is a large drop in SSTs between the period before the war (1939) and
     the period after the war (1945). To my eye, the residual ocean time
     series in Fig. 3 suggests temperatures ratcheted downwards spuriously
     in 1945.
     In our view, the dip in Aug 1945 is very important and warrants being
     highlighted in the literature. In fact, once you know it's there,
     it's hard to view  any time series of global-mean temperatures and
     not wonder how different it would appear if the dip was not there.
     For example, Fig. 4 shows the raw and residual global-mean
     temperature time series assuming the 0.3 K drop in Aug 1945 is
     spurious. I realize the figure is crude, and it might not make it
     into the paper. But the point is that you would get a very, very
     different impression of 20th century temperature trends if the dip
     proves to be an artifact of the end of the war.
     So our main point regarding the drop in Aug 1945 is this: if we
     assume the dip is spurious, then the global-warming of the past
     century would be at least ~0.3 K larger than currently thought, and
     global-mean temperatures would have risen steadily throughout the
     past century.
     That's enough for now. I'm working on the writing and our goal is to
     submit something by Xmas. Please let me know what you think, and if
     you are interested in continuing to interact with Mike and I on the
     paper.
     And again: thanks for your time and interest.
     -Dave
     ￼
     --------------------------------------------------------------------
     --------------------------------------------------------------------
     David W. J. Thompson
     [1]www.atmos.colostate.edu/~davet
     Dept of Atmospheric Science
     Colorado State University
     Fort Collins, CO 80523
     USA
     Phone: 970-491-3338
     Fax: 970-491-8449

     Dear David, Phil and John,
     (This is a bit of a long email, so you might want to grab a cup of coffee - or tea -
     before reading on...)
     Thanks again for the quick and helpful responses last week.
     Mike and I would be happy to include John as a coauthor on our paper. And David, Phil:
     we understand if you are too busy to join another project. But if you are interested in
     joining the paper, too, that would be great. The goal of the paper is to clarify some
     key aspects of 20th century temperature variability, and the study would certainly
     benefit from your expertise.
     Before I get too far ahead of myself, let me review the main points of the paper as it
     currently stands. I've attached 3 pages of figures (the figures will evolve as the
     writing evolves, but as of now it appears the paper will end up being short and punchy)
     Figure 1 includes 2 panels. The top time series in the top panel shows the global-mean
     temperature time series. The next two time series show the linear fit of ENSO and the
     COWL (cold-ocean/warm-land) time series to the global-mean. The ENSO time series is
     found as a damped thermal response to variations in the cold-tongue (this gives a marked
     improvement in the representation of ENSO in global-mean temperatures). The COWL time
     series represents the effects of random fluctuations in climate acting on the different
     heat capacities of the ocean and land (eg: periods of warm advection over land/cold
     advection over the ocean lead to warmer than normal global-mean temperatures by virtue
     of the fact that the continents have a lower heat capacity).
     I'll provide more details of the ENSO and COWL methodologies in a future email, but the
     main point is that a lot of the high frequency 'noise' in global-mean temperatures can
     be accounted for on the basis of two simple, physically based time series.
     The bottom panel in Fig. 1 includes a reproduction of the global-mean time series (top)
     and also shows the residual global mean time series, which is found by removing the ENSO
     and COWL time series from the global-mean time series.
     The bottom panel of Fig. 1 is the key figure in the paper. We think it's remarkable how
     well the fitting 'cleans up' the global-mean time series. If you look closely, you'll
     see that the major volcanoes of the past century (marked by solid vertical lines) are
     much, much clearer in the residual time series. But the fitting not only isolates the
     volcanoes, it also isolates the very large drop in Aug 1945. The Aug 1945 drop is about
     0.3 K, almost twice as large as the response to Pinatubo.
     The residual time series also suggests a slightly different view of 20th century
     temperature variability. The canonical view is that temperatures warmed in the 20s,
     settled from the 40s-70s, and warmed from the 70s-the present. But if you stare at the
     residual time series, you get the impression that global-mean temperatures have actually
     risen steadily over the past century, but that the warming has been disturbed by several
     discrete and abrupt drops in temperature.
     As for the volcanos:
     In figure 2 we're exploiting the fitting procedure to provide a 'cleaned up' version of
     the volcanic response in surface temperatures. The figure shows the composite
     temperature response for the 4 largest tropical volcanoes of the 20th century. The
     composite is done such that the 10 year period before each volcano has a trend of zero
     and mean of zero. (If you don't remove the trend then the long term warming biases the
     composite). The response in the residual data is surprisingly coherent (much more so
     than in raw data). But we think what's most striking is that surface temperatures do not
     appear to fully recover for up to a decade after each volcano (the composite is limited
     to 9 years after the eruptions since the eruption of Pinatubo occurred 9 years after the
     eruption of El Chichon).
     You can see the long timescale of the volcanoes in the residual global-mean time series:
     if you follow the temperature time series before, say, Agung or Pinatubo, you can see
     that it takes a long, long time for global-mean temperatures to catch up to where they
     would have been, assuming they would have continued to warm...
     And as for the dip in 1945:
     Fig 2 shows the residual (ie with the COWL or ENSO time series removed) global-mean land
     and ocean time series. The point here is that the large dip in Aug 1945 does not show up
     in the land data.
     My impression from your emails last week is that the dip is almost certainly due to
     changes in instrumentation during the war. But it's also my impression that the specific
     reasons for the dip are not yet known. It's possible that the dip is offset by spurious
     rises in SSTs at the start of the war. But this isn't certain. And even so, there is a
     large drop in SSTs between the period before the war (1939) and the period after the war
     (1945). To my eye, the residual ocean time series in Fig. 3 suggests temperatures
     ratcheted downwards spuriously in 1945.
     In our view, the dip in Aug 1945 is very important and warrants being highlighted in the
     literature. In fact, once you know it's there, it's hard to view  any time series of
     global-mean temperatures and not wonder how different it would appear if the dip was not
     there. For example, Fig. 4 shows the raw and residual global-mean temperature time
     series assuming the 0.3 K drop in Aug 1945 is spurious. I realize the figure is crude,
     and it might not make it into the paper. But the point is that you would get a very,
     very different impression of 20th century temperature trends if the dip proves to be an
     artifact of the end of the war.
     So our main point regarding the drop in Aug 1945 is this: if we assume the dip is
     spurious, then the global-warming of the past century would be at least ~0.3 K larger
     than currently thought, and global-mean temperatures would have risen steadily
     throughout the past century.
     That's enough for now. I'm working on the writing and our goal is to submit something by
     Xmas. Please let me know what you think, and if you are interested in continuing to
     interact with Mike and I on the paper.
     And again: thanks for your time and interest.
     -Dave
     --------------------------------------------------------------------
     --------------------------------------------------------------------
     David W. J. Thompson
     [2]www.atmos.colostate.edu/~davet
     Dept of Atmospheric Science
     Colorado State University
     Fort Collins, CO 80523
     USA
     Phone: 970-491-3338
     Fax: 970-491-8449

     Prof. Phil Jones
     Climatic Research Unit        Telephone +44 (0) 1603 592090
     School of Environmental Sciences    Fax +44 (0) 1603 507784
     University of East Anglia
     Norwich                          Email    [3]p.jones@uea.ac.uk
     NR4 7TJ
     UK
     ----------------------------------------------------------------------------

   --------------------------------------------------------------------
   --------------------------------------------------------------------
   David W. J. Thompson
   www.atmos.colostate.edu/~davet
   Dept of Atmospheric Science
   Colorado State University
   Fort Collins, CO 80523
   USA
   Phone: 970-491-3338
   Fax: 970-491-8449

