date: 20 Jun 2007 12:21:16 -0400 from: Gavin Schmidt subject: Re: Wengen section to: Phil Jones 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. 1240­1248., 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, 281­299, 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, > >99­113, 2006. > > > >Haigh, J. D., The impact of solar variability on climate, Science, 272, > >981­984, 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, 2425­2428, 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, 837­842, 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, 261­293, 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, 7193­7210, 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 | > > > >| | > > > >| gschmidt@giss.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.jones@uea.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.jones@uea.ac.uk > NR4 7TJ > UK > ---------------------------------------------------------------------------- > >