Monday, April 30, 2012

3636.txt

date: Sat, 19 Jun 1999 22:34:40 +0100 (BST)
from: Simon Tett <sfbtettatXYZxyzo.gov.uk>
subject: [schlesin@uiatma.atmos.uiuc.edu: Commentary on your Nature paper]
to: k.briffaatXYZxyz.ac.uk


Keith -- I guess a little in confidence but what is the quality of the
paleo data for the "65-70" nrth atl oscillation the Michael
Schlesinger is going on about.
Simon
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Date: Sat, 19 Jun 1999 15:07:07 -0500
From: Michael Schlesinger <schlesinatXYZxyztma.atmos.uiuc.edu>
Subject: Commentary on your Nature paper
To: sfbtettatXYZxyzo.gov.uk
Cc: schlesin@uiatma.atmos.uiuc.edu, Natasha@uiatma.atmos.uiuc.edu,
jfbmitchellatXYZxyzo.gov.uk
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Dear Simon:

Below is a Scientific Correspondence that I and Natalia Andronova have
submitted to Nature commenting on your recent paper therein. Our
commentary is not critical of your paper, rather, it proposes a possible
alternate explanation for the warming of the first half of this century, a
possibility that you did not exclude in your paper, as we note in the
commentary.

Cheers,
Michael Schlesinger
_________________________________________________________
Cause of the Warming During the First Half of the Twentieth Century -
Implications for Climate Sensitivity and Detection

Tett et al. [1] find that the increase in global-mean near-surface
temperature during the first half of the twentieth century may be due to
variations in the sun's irradiance. While this supports the earlier
findings of Kelly and Wigley [2], Schlesinger and Ramankutty [3] and
others, there are two reasons for being cautious about this finding.
=46irst, there are no reliable measurements of solar irradiance prior to the
beginning of satellite observations in 1978. Second, an oscillation with a
period of about 65-70 years has been found in the near-surface temperature
over the North Atlantic which appears to explain the observed global-mean
warming during the first half of the twentieth century. Below we briefly
expand on these two factors and highlight the importance of their
resolution in terms of climate sensitivity and detection.
Ground-based observations of solar irradiance, such as those of
Abbott from 1908 to 1952, are unable to detect irradiance variations less
than about 1% [4], while the much more precise solar-irradiance
observations by satellites began only in 1978 [5]. Thus we know
observationally that solar irradiance did not vary by more than about 1%
over the four '11-year' sunspot cycles observed by Abbott, and that solar
irradiance did vary by about 0.1% over the two sunspot cycles since 1978
[6]. We do not know observationally, however, whether solar irradiance
before 1978 varied by more than it did after 1978. Tett et al. [1] used
putative solar-irradiance variations constructed using 'surrogates' by Hoyt
and Schatten [7] (the fraction of penumbral spots, solar-cycle length,
equatorial rotation rate, decay rate of the solar cycle, and mean level of
solar activity) and Lean et al. [8] (sunspot areas and locations, He 1083
nm emission, group sunspot numbers, and Ca emissions from the sun and
sunlike stars). The resulting constructions of solar-irradiance variations
are not in agreement with each other. Thus both cannot be completely
correct, while both could be wrong.
In contrast, there is a growing body of evidence that the warming
during the first half of the twentieth century was the result, at least in
part, of an oscillation in the surface temperature of the North Atlantic
Ocean with a period of about 65-70 years, a possibility not excluded by
Tett et al. [1]. This oscillation was found by Schlesinger and Ramankutty
[9] in their analysis of the instrumental near-surface temperature record,
first from its manifestation in the global-mean temperature, and second
from the geographical distribution of temperature, the latter showing the
oscillation to be located only over the North Atlantic and its surrounding
continental margins. Subsequent analyses of much longer paleoclimate proxy
data (tree rings, ice cores, ice melt, lake varves, coral, historical data)
have shown the existence of this oscillation during the pre-instrumental
period, going back as far as the paleodata allow, to the year 500 [10-12].
Moreover, simulations by the coupled atmosphere-ocean general circulation
model of the Geophysical Fluid Dynamics Laboratory have shown such an
oscillation in the surface temperature of the North Atlantic, initially
with a period of 40-50 years [13], but as the length of the simulation was
increased from 300 to 2000 years, with a longer period of about 40-80 years
[14]. A version of this model with doubled horizontal resolution and 50%
greater vertical resolution simulates the oscillation with a period of
about 60-80 years [Delworth, personal communication].
It is important to determine which of these two explanations for
the early twentieth century warming is correct, if either. Our
maximum-likelihood estimation of the climate sensitivity - the change in
the earth's equilibrium near-surface temperature in response to a radiative
forcing equivalent to that for a doubling of the pre-industrial CO2
concentration, =90T2x - indicates that if the solar irradiance varied as
suggested by Hoyt and Schatten [7] or Lean et al. [8], then the value of
=90T2x required to best represent the instrumental near-surface temperature
changes is about 40% smaller than if there was no variation in solar
irradiance [15]. Clearly, whether or not this is so can have a significant
impact on decisions about climate-change policy. Conversely, if the more
correct answer is the 65-70 year oscillation in North Atlantic near-surface
temperature, then any coupled atmosphere-ocean model that either fails to
simulate this oscillation or simulates it too weakly would underestimate
the internal variability of the climate system [16]. This in turn would
overestimate the signal-to-noise ratio for the observed near-surface
temperature changes during the second half of the twentieth century, and
thereby contribute to a possibly spurious claim of the detection of climate
change and its attribution to anthropogenic causes.
It is therefore of paramount importance to learn whether the
observed warming during the first half of the twentieth century was caused
by variations in the sun's irradiance, the 65-70 year oscillation in North
Atlantic surface temperature, their combined effect, or something as yet
undiscerned.

Michael E. Schlesinger and Natalia G. Andronova
Department of Atmospheric Sciences
University of Illinois @ Urbana-Champaign
105 S. Gregory Avenue
Urbana, IL 61801 USA
e-mail: schlesinatXYZxyzos.uiuc.edu
1. Tett, S.F.B., Stott, P.A., Allen, M.R., Ingram, W.J. & Mitchell,
J.F.B. Nature (1999).
2. Kelly, P.M. & Wigley, T.M.L. Nature 360, 328-330 (1992).
3. Schlesinger, M.E. & Ramankutty, N. Nature 360, 330-333 (1992).
4. Fr=F6hlich, C. in The Solar Output and its Variation (ed. White,
O.R.) 93-109 (Colorado Associated University Press, Boulder, 1977).
5. Willson, R.C. in The Sun as a Variable Star, International
Astronomical Union Colloquium 143 Proceedings (eds. Pap, J., Fr=F6hlich, X.,
Hudson, H. & Solanki, K.) 54-62 (Cambridge Univ. Press, New York, 1994).
6. Fr=F6hlich, C. & Lean, J. Geophys. Res. Lettr. 25, 4377-4380 (1998).
7. Hoyt, D.V. & Schatten, K.H. J. Geophys. Res. 98, 18,895-18,906 (1993).
8. Lean, J., Beer, J. & Bradley, R. Geophys. Res. Lettr. 22, 3195-3198
(1995).
9. Schlesinger, M.E. & Ramankutty, N. Nature 367, 723-726 (1994).
10. Mann, M.E., Park, J. & Bradley, R.S. Nature 378, 266-270 (1995).
11. Mahasenan, N., Watts, R. & Dowlatabadi, H. Geophys. Res. Lettr. 24,
563-566 (1997).
12. Shabalova, M.V. & Weber, S.L. J. Geophys. Res. , In press (1999).
13. Delworth, T., Manabe, S. & Stouffer, R.J. J. Climate 6, 1993-2011
(1993).
14. Delworth, T.L., Manabe, S. & Stouffer, R.J. Geophys. Res. Lettr.
24, 257-260 (1997).
15. Andronova, N.G. & Schlesinger, M.E. J. Geophys. Res. , (in
preparation) (1999).
16. Barnett, T.P. J. Climate 12, 511-518 (1999).

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Dear Simon:


Below is a Scientific Correspondence that I and Natalia Andronova have
submitted to Nature commenting on your recent paper therein. Our
commentary is not critical of your paper, rather, it proposes a
possible alternate explanation for the warming of the first half of
this century, a possibility that you did not exclude in your paper, as
we note in the commentary.


Cheers,

Michael Schlesinger

_________________________________________________________

<bold><fontfamily><param>Times</param><bigger>Cause of the Warming
During the First Half of the Twentieth Century - Implications for
Climate Sensitivity and Detection


</bigger></fontfamily></bold><fontfamily><param>Times</param>Tett et
al. [1] find that the increase in global-mean near-surface temperature
during the first half of the twentieth century may be due to variations
in the sun's irradiance. While this supports the earlier findings of
Kelly and Wigley [2], Schlesinger and Ramankutty [3] and others, there
are two reasons for being cautious about this finding. First, there
are no reliable measurements of solar irradiance prior to the beginning
of satellite observations in 1978. Second, an oscillation with a
period of about 65-70 years has been found in the near-surface
temperature over the North Atlantic which appears to explain the
observed global-mean warming during the first half of the twentieth
century. Below we briefly expand on these two factors and highlight
the importance of their resolution in terms of climate sensitivity and
detection.

Ground-based observations of solar irradiance, such as those of Abbott
from 1908 to 1952, are unable to detect irradiance variations less than
about 1% [4], while the much more precise solar-irradiance observations
by satellites began only in 1978 [5]. Thus we know observationally
that solar irradiance did not vary by more than about 1% over the four
'11-year' sunspot cycles observed by Abbott, and that solar irradiance
did vary by about 0.1% over the two sunspot cycles since 1978 [6]. We
do not know observationally, however, whether solar irradiance before
1978 varied by more than it did after 1978. Tett et al. [1] used
putative solar-irradiance variations constructed using 'surrogates' by
Hoyt and Schatten [7] (the fraction of penumbral spots, solar-cycle
length, equatorial rotation rate, decay rate of the solar cycle, and
mean level of solar activity) and Lean et al. [8] (sunspot areas and
locations, He 1083 nm emission, group sunspot numbers, and Ca emissions
from the sun and sunlike stars). The resulting constructions of
solar-irradiance variations are not in agreement with each other. Thus
both cannot be completely correct, while both could be wrong.

In contrast, there is a growing body of evidence that the warming
during the first half of the twentieth century was the result, at least
in part, of an oscillation in the surface temperature of the North
Atlantic Ocean with a period of about 65-70 years, a possibility not
excluded by Tett et al. [1]. This oscillation was found by Schlesinger
and Ramankutty [9] in their analysis of the instrumental near-surface
temperature record, first from its manifestation in the global-mean
temperature, and second from the geographical distribution of
temperature, the latter showing the oscillation to be located only over
the North Atlantic and its surrounding continental margins. Subsequent
analyses of much longer paleoclimate proxy data (tree rings, ice cores,
ice melt, lake varves, coral, historical data) have shown the existence
of this oscillation during the pre-instrumental period, going back as
far as the paleodata allow, to the year 500 [10-12]. Moreover,
simulations by the coupled atmosphere-ocean general circulation model
of the Geophysical Fluid Dynamics Laboratory have shown such an
oscillation in the surface temperature of the North Atlantic, initially
with a period of 40-50 years [13], but as the length of the simulation
was increased from 300 to 2000 years, with a longer period of about
40-80 years [14]. A version of this model with doubled horizontal
resolution and 50% greater vertical resolution simulates the
oscillation with a period of about 60-80 years [Delworth, personal
communication].

It is important to determine which of these two explanations for the
early twentieth century warming is correct, if either. Our
maximum-likelihood estimation of the climate sensitivity - the change
in the earth's equilibrium near-surface temperature in response to a
radiative forcing equivalent to that for a doubling of the
pre-industrial CO2 concentration, =90T2x - indicates that if the solar
irradiance varied as suggested by Hoyt and Schatten [7] or Lean et al.
[8], then the value of =90T2x required to best represent the
instrumental near-surface temperature changes is about 40% smaller than
if there was no variation in solar irradiance [15]. Clearly, whether
or not this is so can have a significant impact on decisions about
climate-change policy. Conversely, if the more correct answer is the
65-70 year oscillation in North Atlantic near-surface temperature, then
any coupled atmosphere-ocean model that either fails to simulate this
oscillation or simulates it too weakly would underestimate the internal
variability of the climate system [16]. This in turn would
overestimate the signal-to-noise ratio for the observed near-surface
temperature changes during the second half of the twentieth century,
and thereby contribute to a possibly spurious claim of the detection of
climate change and its attribution to anthropogenic causes.

It is therefore of paramount importance to learn whether the observed
warming during the first half of the twentieth century was caused by
variations in the sun's irradiance, the 65-70 year oscillation in North
Atlantic surface temperature, their combined effect, or something as
yet undiscerned.


</fontfamily><bold>Michael E. Schlesinger and Natalia G. Andronova

</bold><italic>Department of Atmospheric Sciences

University of Illinois @ Urbana-Champaign

105 S. Gregory Avenue

Urbana, IL 61801 USA

e-mail:
<underline><fontfamily><param>Times</param><color><param>0000,0000,00FF</par=
am>schlesin@atmos.uiuc.edu

</color></fontfamily></underline></italic><fontfamily><param>Times</param>1.=
Tett,
S.F.B., Stott, P.A., Allen, M.R., Ingram, W.J. & Mitchell, J.F.B.
<italic>Nature</italic> (1999).

2. Kelly, P.M. & Wigley, T.M.L. <italic>Nature
</italic><bold>360</bold>, 328-330 (1992).

3. Schlesinger, M.E. & Ramankutty, N. <italic>Nature
</italic><bold>360</bold>, 330-333 (1992).

4. Fr=F6hlich, C. in <italic>The Solar Output and its Variation</italic>
(ed. White, O.R.) 93-109 (Colorado Associated University Press,
Boulder, 1977).

5. Willson, R.C. in <italic>The Sun as a Variable Star, International
Astronomical Union Colloquium 143 Proceedings</italic> (eds. Pap, J.,
=46r=F6hlich, X., Hudson, H. & Solanki, K.) 54-62 (Cambridge Univ. Press,
New York, 1994).

6. Fr=F6hlich, C. & Lean, J. <italic>Geophys. Res. Lettr.
</italic><bold>25</bold>, 4377-4380 (1998).

7. Hoyt, D.V. & Schatten, K.H. <italic>J. Geophys. Res.
</italic><bold>98</bold>, 18,895-18,906 (1993).

8. Lean, J., Beer, J. & Bradley, R. <italic>Geophys. Res. Lettr.
</italic><bold>22</bold>, 3195-3198 (1995).

9. Schlesinger, M.E. & Ramankutty, N. <italic>Nature
</italic><bold>367</bold>, 723-726 (1994).

10. Mann, M.E., Park, J. & Bradley, R.S. <italic>Nature
</italic><bold>378</bold>, 266-270 (1995).

11. Mahasenan, N., Watts, R. & Dowlatabadi, H. <italic>Geophys. Res.
Lettr. </italic><bold>24</bold>, 563-566 (1997).

12. Shabalova, M.V. & Weber, S.L. <italic>J. Geophys. Res.</italic> ,
In press (1999).

13. Delworth, T., Manabe, S. & Stouffer, R.J. <italic>J. Climate
</italic><bold>6</bold>, 1993-2011 (1993).

14. Delworth, T.L., Manabe, S. & Stouffer, R.J. <italic>Geophys. Res.
Lettr. </italic><bold>24</bold>, 257-260 (1997).

15. Andronova, N.G. & Schlesinger, M.E. <italic>J. Geophys.
Res.</italic> , (in preparation) (1999).

16. Barnett, T.P. <italic>J. Climate </italic><bold>12</bold>, 511-518
(1999).</fontfamily>

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