Monday, March 12, 2012

2444.txt

date: 20 Jun 2007 12:21:16 -0400
from: Gavin Schmidt <gschmidtatXYZxyzs.nasa.gov>
subject: Re: Wengen section
to: Phil Jones <p.jonesatXYZxyz.ac.uk>

yeah, I've been noticing... Well, just let me know if I can do anything
- even if it's just sending the occasionally nice email!

Gavin


On Wed, 2007-06-20 at 05:59, Phil Jones wrote:
> Gavin,
> Thanks. Yours was the nicest email I got overnight.
> Cheers
> Phil
>
>
> At 20:02 19/06/2007, you wrote:
> >Refs for my section - note that the first Goosse reference should be
> >Goosse et al 2006, and the second was in error and shouldn't be there
> >any way.
> >
> >
> >Gavin
> >
> >
> >
> >References
> >
> >Collins, W. D., et al., Radiative forcing by well-mixed greenhouse
> >gases: Estimates from climate models in the Intergovernmental
> > Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), J.
> >Geophys. Res, 111, 2006.
> >
> >Dickinson, R., Solar variability and the lower atmosphere, Bull. Amer.
> >Meteor. Soc., pp. 1240�1248., 1975.
> >
> >Gerber, S., F. Joos, P. P. Bruegger, T. F. Stocker, M. E. Mann, and S.
> >Sitch, Constraining temperature variations over the last
> > millennium by comparing simulated and observed atmospheric CO2, Clim.
> >Dyn., 20, 281�299, 2003.
> >
> >Goosse, H., O. Arzel, J. Luterbacher, M. E. Mann, H. Renssen, N.
> >Riedwyl, A. Timmermann, E. Xoplaki, and H. Wanner, The origin
> > of the European "Medieval Warm Period", Climate of the Past, 2,
> >99�113, 2006.
> >
> >Haigh, J. D., The impact of solar variability on climate, Science, 272,
> >981�984, 1996.
> >
> >Houghton, J. T., Y. Ding, D. J. Griggs, M. Nouger, P. J. van der Linden,
> >X. Dai, K. Maskell, and C. A. Johnson, Climate Change
> > 2001: The scientific basis, Cambridge Univ. Press, New York, 2001.
> >
> >Lean, J., Evolution of the sun's spectral irradiance since the Maunder
> >Minimum, Geophys. Res. Lett., 27, 2425�2428, 2000.
> >
> >LeGrande, A. N., G. A. Schmidt, D. T. Shindell, C. Field, R. L. Miller,
> >D. Koch, G. Faluvegi, and G. Hoffmann, Consistent simulations
> > of multiple proxy responses to an abrupt climate change event, Proc.
> >Natl. Acad. Sci., 103, 837�842, 2006.
> >
> >Oman, L., A. Robock, G. Stenchikov, G. A. Schmidt, and R. Ruedy,
> >Climatic response to high-latitude volcanic eruptions, J. Geophys.
> > Res., 110, 2005.
> >
> >Ruddiman, W. F., The anthropogenic greenhouse era began thousands of
> >years ago, Clim. Change, 61, 261�293, 2003.
> >
> >Shindell, D. T., G. A. Schmidt, R. L. Miller, and D. Rind, Northern
> >hemisphere winter climate response to greenhouse gas, ozone,
> > solar and volcanic forcing, J. Geophys. Res., 106, 7193�7210, 2001.
> >
> >Shindell, D. T., G. Faluvegi, R. L. Miller, G. A. Schmidt, J. E. Hansen,
> >and S. Sun, Solar and anthropogenic forcing of tropical
> > hydrology, Geophys. Res. Lett., 33, 2006.
> >
> >
> >
> >On Tue, 2007-05-29 at 05:55, Phil Jones wrote:
> > > Gavin,
> > > Thanks for this. I'll incorporate this into a revised draft
> > > later this week
> > > and then send around. Gene has sent me something as well.
> > > Can you send the refs if you have them?
> > >
> > > Thorsten will likely send a reminder around as he's being
> > > pressurized by Larry from EPRI.
> > >
> > > Cheers
> > > Phil
> > >
> > >
> > > At 09:51 28/05/2007, you wrote:
> > >
> > > >Hi Phil, sorry for the long delay. But here is a first draft of the
> > > >forcings and models section I was supposed to take the lead on.
> > > >Hopefully, we can merge that with whatever Caspar has.
> > > >
> > > >Thanks
> > > >
> > > >Gavin
> > > >
> > > >================
> > > >
> > > >4 Forcing (GS/CA/EZ) 4-5pp
> > > >
> > > >Histories (CA)
> > > >How models see the forcings, especially wrt aerosols/ozone and
> > > >increasing model complexities (GS)
> > > >
> > > >An important reason for improving climate reconstructions of the past few
> > > >millenia is that these reconstructions can help us both evaluate
> > > >climate model responses and sharpen our understanding of important
> > > >mechanisms and feedbacks. Therefore, a parallel task to improving
> > > >climate reconstructions is to assess and independently constrain
> > > >forcings on the climate system over that period.
> > > >
> > > >Forcings can generically be described as external effects on a
> > > >specific system. Responses within that system that also themselves
> > > >have an impact on its internal state are described as feeebacks. For
> > > >the atmosphere, sea surface temperature changes could
> > > >therefore be considered a forcing, but in a coupled ocean-atmosphere
> > > >model they could be a feedback to another external factor or be
> > > >intrinsic to the coupled system. Thus the distinction between forcings and
> > > >feedbacks is not defined a priori, but is a function of the scope of
> > > >the modelled system. This becomes especially important when dealing
> > > >with the bio-geo-chemical processes in climate that effect the
> > > >trace gas concentrations (CO2 and CH4) or
> > aerosols. For example, if a model
> > > >contains a carbon cycle, than the CO2 variations as a function of
> > > >climate will be a feedback, but for a simpler physical model, CO2 is
> > > >often imposed directly as a forcing from observations, regardless of
> > > >whether in the real world it was a feedback to another change, or a
> > > >result of human industrial activity.
> > > >
> > > >It is useful to consider the pre-industrial period (pre-1850 or so)
> > > >seperately from the more recent past, since the human influence on
> > > >many aspects of atmospheric composition has increased dramatically in
> > > >the 20th Century. In particular, aerosol and land use changes are
> > > >poorly constrained prior to the late 20th Century and have large
> > > >uncertainties. Note however, there may conceivably be a role for human
> > > >activities even prior to the 19th Century due to early argiculatural
> > > >activity (Ruddiman, 2003; Goosse et al, 2005).
> > > >
> > > >In pre-industrial periods, forcings can be usefully separated into
> > > >purely external changes (variations of solar activity, volcanic
> > > >eruptions, orbital variation), and those which are intrinsic to the
> > > >Earth system (greenhouse gases, aerosols, vegetation etc.). Those
> > > >changes in Earth system elements will occur predominantly as feedbacks
> > > >to other changes (whether externally forced or simply as a function of
> > > >internal climate 'noise'). In the more recent past, the human role in
> > > >affecting atmospheric composition (trace gases and aerosols) and land
> > > >use have dominated over natural processes and so these changes can, to
> > > >large extent, be considered external forcings as well.
> > > >
> > > >Traditionally, the 'system' that is most usually implied when talking
> > > >about forcings and feedbacks are the 'fast' components atmosphere-land
> > > >surface-upper ocean system that, not coincidentally, corresponds to
> > > >the physics contained within atmospheric
> > general circulation models (AGCMs)
> > > >coupled to a slab ocean. What is not
> > included (and therefore considered as a
> > > >forcing according to our previous definition) are 'slow' changes in
> > > >vegetation, ice sheets or the carbon cycle. In the real world these
> > > >features will change as a function of other climate changes, and in
> > > >fact may do so on relatively 'fast' (i..e multi-decadal)
> > > >timescales. Our choice then of the appropriate 'climate system' is
> > > >thus slightly arbitrary and does not give a complete picture of the
> > > >long term sensitivity of the real climate.
> > > >
> > > >These distinctions become important because the records available for
> > > >atmospheric composition do not record the distinction between feedback
> > > >or forcing, they simply give, for instance, the history of CO2 and
> > > >CH4. Depending on the modelled system, those records will either be a
> > > >modelling input, or a modelling target.
> > > >
> > > >While there are good records for some factors (particularly the well
> > > >mixed greenhouse gases such as CO2 and CH4), records for others are
> > > >either hopelessly incomplete (dust, vegetation) due to poor spatial or
> > > >temporal resolution or non-existant (e.g. ozone). Thus estimates of
> > > >the magnitude of these forcings can only be made using a model-based
> > > >approach. This can be done using GCMs that include more Earth system
> > > >components (interactive aerosols, chemistry, dynamic vegetation,
> > > >carbon cycles etc.), but these models are still very much a work in
> > > >progress and have not been used extensively for paleo-climatic
> > > >purposes. Some initial attempts have been made for select feedbacks
> > > >and forcings (Gerber et al, 2003; Goosse et al 2006) but a
> > > >comprehensive assessment over the millennia prior to the
> > > >pre-industrial does not yet exist.
> > > >
> > > >Even for those forcings for which good records exist, there is a
> > > >question of they are represented within the models. This is not so
> > > >much of an issue for the well-mixed greenhouse gases (CO2, N2O, CH4)
> > > >since there is a sophisticated literature and history of including
> > > >them within models (IPCC, 2001) though some aspects, such as minor
> > > >short-wave absorption effects for CH4 and N2O are still not
> > > >universally included
> > > >(Collins et al, 2006). However, solar effects have been treated in
> > > >quite varied ways.
> > > >
> > > >The most straightforward way of including solar irradiance effects on
> > > >climate is to change the solar 'constant' (preferably described as
> > > >total solar irradiance - TSI). However, observations show that solar
> > > >variability is highly dependent on wavelength with UV bands having
> > > >about 10 times as much amplitude of change than TSI over a solar cycle
> > > >(Lean, 2000). Thus including this spectral variation for all solar
> > > >changes allows for a slightly different behaviour (larger
> > > >solar-induced changes in the stratosphere where the UV is mostly
> > > >absorbed for instance). Additionally, the changes in UV affect ozone
> > > >production in both the stratosphere and troposphere, and this
> > > >mechanism has been shown to affect both the total radiative forcing
> > > >and dynamical responses (Haigh 1996, Shindell et al 2001;
> > > >2006). Within a chemistry climate model this effect would potentially
> > > >modify the radiative impact of the original solar forcing, but could also
> > > >be included as an additional (parameterised) forcing in standard GCMs.
> > > >
> > > >There is also a potential effect from the indirect effect of solar
> > > >magnetic variability on the sheilding of cosmic rays, which have been
> > > >theorised to affect the production of cloud condensation nuclei
> > > >(Dickinson, 1975). However, there have been no quantitative
> > > >calculations of the magnitude of this effect (which would require a
> > > >full study of the relevant aerosol and cloud microphysics), and so its
> > > >impact on climate is not (yet) been included.
> > > >
> > > >Large volcanic eruptions produce significant amounts of sulpher
> > > >dioxide (SO2). If this is injected into the tropical stratosphere
> > > >during a particularly explosive eruption, the resulting sulphate can
> > > >persist in the atmosphere for a number of years (e.g. Pinatubo in
> > > >1991). Less explosive, but more persistent eruptions (e.g. Laki in
> > > >1789??) can still affect climate though in a more regional way and for
> > > >a shorter term (Oman et al, 2005). These aerosols have both a
> > > >shortwave (reflective) and longwave (absorbing) impact on the
> > > >radiation and their local impact on stratospheric heating can have
> > > >important dynamical effects. It is therefore better to include the
> > > >aerosol absorber directly in the radiative transfer code. However, in
> > > >less sophisticated models, the impact of the aerosols has been
> > > >parameterised as the equivalent decrease in TSI. For extreme eruptions
> > > >it has been hypothesised that sulphate production might saturate the
> > > >oxidative capacity of the stratosphere leaving significant amounts of
> > > >residual SO2. This gas is a greenhouse gas and would have an opposite
> > > >effect to the cooling aerosols. This effect however has not yet been
> > > >quantified.
> > > >
> > > >Land cover changes have occured both due to deliberate modification by
> > > >humans (deforestation, imposed fire regimes, arguculture) as well as a
> > > >feedback to climate change (the desertification of the Sahara ca. 5500
> > > >yrs ago). Changing vegetation in a standard model affects the seasonal
> > > >cycle of albedo, the surface roughness, the impact of snow,
> > > >evapotranspiration (through different rooting depths) etc. However,
> > > >modelling of the yearly cycle of crops, or incorporating the effects
> > > >of large scale irrigation are still very much a work in
> > > >progress.
> > > >
> > > >Aerosol changes over the last few milllenia are very poorly
> > > >constrained (if at all). These might have arisen from climatically
> > > >or human driven changes in dust emissions, ocean biology feedbacks
> > > >on circulation change, or climate impacts on the emission volatile
> > > >organics from plants (which also have an impact on ozone
> > > >chemistry). Some work on modelling a subset of those effects has
> > > >been done for the last glacial maximum or the 8.2 kyr event
> > > >(LeGrande et al, 2006), but there have been no quantitative
> > > >estimates for the late Holocene (prior to the industrial period).
> > > >
> > > >Due to the relative expense of doing millennial simulations with
> > > >state-of-the-art GCMs, exisiting simulations have generally done the
> > > >minimum required to include relevant solar, GHG and volcanic
> > > >forcings. Progress can be expected relatively soon on more
> > > >sophisticated treatments of those forcings and the first
> > > >quantitative estimates of additional effects.
> > > >
> > > >=============
> > > >
> > > >
> > > >*--------------------------------------------------------------------*
> > > >| Gavin Schmidt NASA/Goddard Institute for Space Studies |
> > > >| 2880 Broadway |
> > > >| Tel: (212) 678 5627 New York, NY 10025 |
> > > >| |
> > > >| gschmidtatXYZxyzs.nasa.gov http://www.giss.nasa.gov/~gavin |
> > > >*--------------------------------------------------------------------*
> > >
> > > 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 p.jonesatXYZxyz.ac.uk
> > > NR4 7TJ
> > > UK
> > >
> > ----------------------------------------------------------------------------
> >
> > >
>
> 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 p.jonesatXYZxyz.ac.uk
> NR4 7TJ
> UK
> ----------------------------------------------------------------------------
>
>

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