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IPCC in action: Part II

Filed under: — rasmus @ 15 March 2005

The primary purpose of the Intergovernmental Panel for Climate Change (IPCC) is to assess the available scientific knowledge about climate change, not to initiate new research. The next IPCC report (Assessment Report 4, or AR4) is due in 2007, and in order to update of the state of knowledge it will only consider papers published in peer-review scientific journals between 2000 and papers submitted by May 1st 2005 (must be accepted before December 2005). It is essential that the papers be published in scientific quality journals in order to ensure the credibility of the results. Nevertheless the IPCC reports undergo several additional reviews and revisions involving a large number of independent referees. Thus, the IPCC reports undergo a more stringent review process than common papers in the scientific literature.

Independent people are assessing the performance of the models experiments intended for the AR4. Recently, a number of modelling groups around the world have put a great deal of effort into producing model results for a set of emission scenarios (pre-industrial control, 20th century, ‘comittment’, SRES B1, SRES A1b, SRES A2, 1% compound CO2 emission and doubled CO2 stabilisation). The modelling endevour is unprecedented in terms of number of participants and and the degree of complexity that must be coordinated. An archive of the model results has been established at the PCMDI.

Although there is still some disagreement in the preliminary results (eg the description of polar ice caps), a lot of things appear to be quite robust as the climate models for instance indicate consistent patterns of surface warming and rainfall trends: the models tend to agree on a stronger warming in the Arctic and stronger precipitation changes in the Topics (see crude examples for the SRES A1b scenarios given in Figures 1 & 2; Note, the degrees of freedom varies with latitude, so that the uncertainty of these estimates are greater near the poles). Some model differences in the stratospheric response to e.g. volcanos may be related to how the volcanic forcing is represented in the model experiments. Clouds are still one of the most difficult aspects of climate modelling, and a great deal of effort is going into resolving uncertainties concerning clouds.

Fig. 1: Zonal mean temperature change for AR4 multi-model ensemble

Fig. 2: Zonal mean precipitation change for AR4 multi-model ensemble

Climate models may over-estimate the so-called ‘water vapour feedback‘, they nevertheless seem to capture the right sign of the water vapour feedback. However, it appears increasingly unlikely that the net water vapour feedback could be negative, as proposed by Lindzen’s with his ‘Iris Effect’. Observations of the humidity in the upper troposphere and its relation with sea surface temperature in areas of deep convection point to an overall positive climate feedback by water vapour in the upper troposphere, which is inconsistent with the Iris effect. The Iris effect has been a controversial topic in the climate debate and has been used as an argument against a significant clobal warming, e.g. in Lomborg’s ‘the Skeptical Environmentalist’.

Another hot topics in the context of climate change concerns the temperature trends in the free troposphere (lower part of the atmosphere) and their relationship with the sufrace trends. There has been a debate on the trend estimates from a number of different studies based on the Microwave Sounding Unit (MSU) instrument carried by a number of satellites, and different researchers have come up with different trend estimates depending on how they have carried out the analysis. The study often cited by the people who do not ‘believe’ in a global warming is the one by Christy and Spencer that indicates a weak trend, whereas other studies ( Mears et al.; Vinnikov & Grody Science 2003 vol 302; Prabhakara et al. 1998; Fu et al. Nature 2004 vol 429) indicate a somewhat more rapid warming that is more in line with the models. It has been argued that not taking the cooling in the stratosphere properly into account can be one explanation for the trend differences. It also seems to be tricky to reconcile radiosonde data with the climate models for the 1979-2000 period, although the agreement between surface and upper air trends is considered to be good for the 1958-2000 (Angell, 2003). Recent work by Santer et al. (J. Geophys. Res. 2004, vol 109) provide support for claims that human activities have warmed the troposphere and cooled the lower stratosphere (a region of the atmosphere above the troposphere), hence increasing the height of the tropospause (the top of the troposphere). Ongoing work presented at the IPCC meeting is further adressing this issue (e.g. further analysis & studying radio occulatation).

Acknowledgement: The figures were made using Ferret. Credit must also be given to the of the various modelling groups making their results available on the PCMDI data base and the people at PCMDI.


7 Responses to “IPCC in action: Part II”

  1. 1

    A small correction: Lindzen’s “Iris” speculation is not the same as his earlier proposal that water vapor should have a zero or negative feedback. The water vapor feedback proposal has gotten a lot of study over the years, and does not seem to be credible at this point. The “Iris” proposal deals with changes in high cloud cover, rather than water vapor. A number of studies have appeared that have questioned the mechanism and the data analysis on which it is based, but the work referred to in this otherwise excellent precis of the IPCC meeting would not have a direct bearing on the IRIS mechanism.

    Unsurprisingly, Lomborg cites the IRIS paper as if it were the gospel truth and last word on clouds, and doesn’t give any attention to the fact that it is only one among many proposals about how cloud feedbacks might operate, and far from the best supported among those.

    I haven’t seen anything that very strongly supports the IRIS idea, but I do concur with one idea buried in the paper: that the parameterization of fractional cloud cover in GCM’s is not based on very clear physical principles, and could operate in many different ways — some of which, I think, could make climate sensitivity considerably greater than the midrange model of the current crop.

    [Response: Thanks! -rasmus]

  2. 2
    Mike Mayson says:

    At the October 2004 SORCE meeting there was a presentation by Enric Palle ( Big Bear Solar Observatory ) on “Decadal Variability in the Earth’s Reflectance as Observed by Earthshine”.
    You can find it here: http://lasp.colorado.edu/sorce/2004ScienceMeeting/SORCE%20WORKSHOP%202004/SESSION_4/4_12_Palle.pdf
    It suggests that natural changes in earth’s albedo of 3% have occurred in the period 1984-2003.
    They say that this is equivalent to 6 W/m2 change in energy flux compared to 2.4 W/m2 for accumulated GHG to date.
    This seems to be a potentially major climate variable and I wonder if it is taken into account in current models or is it subsumed under ‘cloud’ effects.

    [Response:Thanks for you pointing this out! Enric Palle presents some interesting ideas. At this point, these are still novel and it is hard to incorporate effects into climate models when the physical mechanisms are not well-understood. Remember, the previous IPCC report stated that representing clouds in climate models are among the greatest challenge in present-day climate modelling. If Palle manages to convince the research community and if we get a better grasp on what's actually going on, then hopefully his ideas will become part of the scientific knowledge in the future and advance our understanding of climate change. I feel it's too early to say now. -rasmus]

    [Response: I would add that the planetary albedo is a principally a consequence of the climate, rather than an independent driver. For instance, increasing cloud cover due to global warming may change the albedo, but this would be a feedback to a larger warming effect, rather than a cooling. Some forcings can affect the albedo directly (like volcanic aerosols), and some may do so semi-directly (aerosols that change cloud properties). Having said that, the changes implied by Palle do seem rather large, and so further work is clearly required to understand what is going on here. -gavin]

  3. 3
    Jeremy says:

    “Unsurprisingly, Lomborg cites the IRIS paper as if it were the gospel truth and last word on clouds….”

    Curse you, Lomborg! Mine ancient enemy strikes again….

    :)

  4. 4

    Comment #2 says “It suggests that natural changes in earth’s albedo of 3% have occurred in the period 1984-2003.” This isn’t right. If the measurements are correct they say only that “changes” have occurred. There is nothing regarding those observations that can distinguish between natural and anthropogenic changes in the Earth’s albedo.

    For example, all models predict that clouds change as the climate changes. I don’t think any of them gives an albedo change of as much as 3% in 20 years, but one cannot discount the possibility that some part of the albedo change is connected with anthropogenic forcings.

    As rasmus says, an albedo change is something that would be modeled as part of the basic physics process of a model, including aerosols and clouds. It wouldn’t be imposed as part of the climate forcings. As a diagnostic study, though, if these albedo monitoring techniques are borne out by further study, they could provide another tool for studying the climate’s response to radiative perturbations.

  5. 5
    Mike Mayson says:

    Re comment #4: “There is nothing regarding those observations that can distinguish between natural and anthropogenic changes in the Earth’s albedo”.
    I should have been more accurate – the presentation says “reversibility suggests natural variations” and I presume this is because over the period 1984-2003 the albedo is seen to both decrease and increase and there is an assumption that an anthropogenic variation would be one-way.
    It would be good to see the paper from which this presentation sprang.

  6. 6
    Mike Mayson says:

    Following on from #5: I have found a paper by Palle et al, “Changes in Earth’s Reflectance over the Past Two Decades” in Science May 2004 and it can be seen here http://www.sciencemag.org/cgi/reprint/304/5675/1299.pdf?ijkey=HhdOjAJ0NDCDc&keytype=ref&siteid=sci

    They say;

    “However, the reflectance increase from 1999 to 2003 would be difficult to attribute to monotonically increasing atmospheric greenhouse gases. Natural variability is a much more plausible explanation, given the size and time scale of the proxy changes.”

    [Response: There is also another link to a NASA feature on 'earthshine'. I am not sure how one can use these the 'earthshine' to estimate a net radiative effect, as I don't see how the reflected light can tell anything about whether they are from low or high clouds (which are believed to have different net radiative effects). I must admit that I haven't read the Science article and that I don't have access to the on-line version, so I don't know whether the researchers address this issue (perhaps extra information from the ISCCP?). Furthermore, it would be interesting to know how eg stratospheric dust from say volcanic eruptions show up in the 'earthshine'. I can't see any clear volcanic response in the curves shown in Enric Palle's presentation on the above URL. It is also intrieging that the albedo reached a grand minimum (the axis is inverted on the graph) in 1997 and subsequently rise whereas the ISCCP clouds data (shown on a few slids later) continue to show a negative trend in cloudiness.
    -rasmus]

    [Response:One thing to bear in mind is that the ISCCP observations of alebdo are based on radiative transfer modelling that use a fixed background aerosol field. Thus, no Pinatubo effect, since that was related to a change in aerosols, rather than clouds. On another note, I have been cautioned by the ISCCP team that the long term trends in D2 cloudiness are somewhat suspect, and are undergoing a lot of further investigation to eliminate possible sources of systematic errors. I would be very careful in relying on them to prove very much (at least for the time being). - gavin]

  7. 7
    Mike Mayson says:

    Rasmus
    You can find a copy of the Science paper here: http://www.bbso.njit.edu/~epb/papers.html

    There is a 2004 overview paper on the EarthShine Project here: http://solar.njit.edu/preprints/shumko1268.pdf

    [Response:Thanks! -rasmus]

    [Response:I have now read the paper and think it is very interesting. However, I have one comment about it. I may not have understood the analysis correctly, but if I'm right, the results presented are a bit weird. The correlation between ISCCP-reconstructed (delta-p-star in equation 1) and observed apparent albedo is 0.6 and the relationship between the reconstructed and observed apparent albedo/ES look more like a cloud with a great deal of scatter rather than a straight line indicative of a close relationaship (PallĀ“e et al. Fig. 2). A correlation of 0.6 suggests that the ISCCP can account for only about 36% of the variance of the albedo - and 64% of the variance cannot be accounted for using this linear ISCCP-based model (i.e. not very good). Their Fig. 3 shows, despite this fact, an apparently very good correspondence between observed ES and the reconstruction (delta-p-star). It seems to be too good considering the relatively large scatter in Fig. 2! If the reconstruction were to account for 36% of the variance, I would expect to see a greater difference in the variations in the two curves in Fig. 3.
    I think it is very important that this kind of activity is continued and that ES is monitored over time so that we can get long time series. It's only when we have a long record of high quality data that we can use them to test hypotheses. Hopefully, a longer ES record will allow us to resolve my query as well. -rasmus]


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