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Friday, October 4, 2013

Lacking confidence at WUWT

Sou | 4:55 AM Go to the first of 4 comments. Add a comment

Today's denier flavour is confidence.  Around the traps deniers are wondering if the IPCC has reduced "alarm" about climate change.  WUWT and other denier blogs have referred to Table 12.4 (see below), trying to claim that the only thing to worry about is the loss of Arctic sea ice - and in their delusion they are not worried about that because it will lower shipping costs and let everyone go into the Arctic to spill oil - and they reckon the polar bears can always find somewhere else to live, or some such argument.

Here is what Barry Brill writes today at WUWT (archived here) in relation to Table 12.4 (my bold italics):
And the other good news is that every one of the “substantial disruption” possibilities are seen as “unlikely” by the IPCC except* Arctic Sea Ice melting. This is mainly positive in opening up new sea lanes – while albedo effects have low significance in a slow-warming world.

Here is what Judith Curry wrote (archived here) (my bold italics):
But the bottom line is this.  The only one of these changes likely to occur in the 21st century is disappearance of the summer sea ice, and by ‘disappearance’ I assume they mean what they usually do when they say this:  < 1 M sq km left.... 

The reference by Barry and Judith to "likely" and "unlikely" is not correct in my view.  Confidence is on a different scale to the likelihood scale.  Take a look at the Guidance Note for Lead Authors of the IPCC Fifth Assessment Report on Consistent Treatment of Uncertainties.  I don't think that the fake sceptics are interpreting these properly.

The particular aspect that seems to be wrongly interpreted are the expressions of confidence.  The Guidance Note has this diagram relating to confidence:


So if something has low confidence it is generally because there is low agreement or limited evidence or both.  Low confidence doesn't mean that something is thought unlikely to happen.  It means that there is low agreement as to whether it will happen or not.  Or there is limited evidence to indicate one way or another.  In other words, it's more like "maybe but don't really know" than "it probably won't happen".  At least that's how I read it.  Confidence and likelihood are described separately by the IPCC.  Here are two short extracts from the Guidance Note:
  • A level of confidence is expressed using five qualifiers: “very low,” “low,” “medium,” “high,” and “very high.” It synthesizes the author teams’ judgments about the validity of findings as determined through evaluation of evidence and agreement....
  • Likelihood, as defined in Table 1, provides calibrated language for describing quantified uncertainty. It can be used to express a probabilistic estimate of the occurrence of a single event or of an outcome (e.g., a climate parameter, observed trend, or projected change lying in a given range). Likelihood may be based on statistical or modeling analyses, elicitation of expert views, or other quantitative analyses....

Okay, let's test it out.  Fake skeptics today are saying that none of the things in Table 12.4 (refer page 12-78 of the full WG1 report) below pose any real danger.

Table 12.4: Components in the Earth system that have been proposed in the literature as potentially being susceptible to abrupt or irreversible change. Column 2 defines whether or not a potential change can be considered to be abrupt under the AR5 definition. Column 3 states whether or not the process is irreversible in the context of abrupt change, and also gives the typical recovery time scales. Column 4 provides an assessment, if possible, of the likelihood of occurrence of abrupt change in the 21st century for the respective components or phenomena within the Earth system, for the scenarios considered in this chapter.

Change in climate
system component
Potentially
abrupt (AR5
definition)
Irreversibility if
forcing reversed
Projected likelihood of 21st century change in
scenarios considered
Atlantic MOC collapse
Yes
Unknown Very unlikely that the AMOC will undergo a rapid transition (high confidence)
Ice sheet collapse
No
Irreversible for
millennia
Exceptionally unlikely that either Greenland or West Antarctic Ice sheets will suffer near-complete disintegration (high confidence)
Permafrost carbon release
No
Irreversible for
millennia
Possible that permafrost will become a net source of atmospheric greenhouse gases (low confidence)
Clathrate methane release
Yes
Irreversible for
millennia
Very unlikely that methane from clathrates will undergo catastrophic release (high confidence)
Tropical forests dieback
Yes
Reversible within
centuries
Low confidence in projections of the collapse of large areas of tropical forest
Boreal forests dieback
Yes
Reversible within
centuries
Low confidence in projections of the collapse of large areas of boreal forest
Disappearance of summer
Arctic sea ice
Yes
Reversible within
years to decades
Likely that the Arctic Ocean becomes nearly ice-free in September before mid-century under high forcing scenarios such as RCP8.5 (medium confidence)
Long-term droughts
Yes
Reversible within
years to decades
Low confidence in projections of changes in the frequency and duration of megadroughts
Monsoonal circulation
Yes
Reversible within
years to decades
Low confidence in projections of a collapse in monsoon circulations

The thing is that low confidence doesn't mean it won't happen.  Here is what the report states about permafrost.  I've removed all the literature references to make it easier to read.  You can see the full text with references on page 12-80 of the full report:
The conjunction of a long carbon accumulation time scale on one hand and potentially rapid permafrost thawing and carbon decomposition under warmer climatic conditions on the other hand suggests potential irreversibility of permafrost carbon decomposition (leading to an increase of atmospheric CO2 and/or CH4 concentrations) on timescales of hundreds to thousands of years in a warming climate. Indeed, recent observations suggest that this process, induced by widespread permafrost warming and thawing, might be already occurring. However, the existing modelling studies of permafrost carbon balance under future warming that take into account at least some of the essential permafrost-related processes do not yield coherent results beyond the fact that present-day permafrost might become a net emitter of carbon during the 21st century under plausible future warming scenarios (low confidence). This also reflects an insufficient understanding of the relevant soil processes during and after permafrost thaw, including processes leading to stabilization of unfrozen soil carbon, and precludes a firm assessment of the amplitude of irreversible changes in the climate system potentially related to permafrost degassing and associated global feedbacks at this stage.
In other words, there is at this stage insufficient knowledge to say definitely one way or another.  Hence the low confidence.

What about long term droughts?  Here is what the IPCC reports:
As noted in Sections 5.5.2.4 and 5.6.2, long-term droughts (often called megadroughts, see Glossary) are a recurring feature of Holocene paleoclimate records in North America, east and south Asia, Europe, Africa and India. The transitions into and out of the long-term droughts take many years. Since the long-term droughts all ended they are not irreversible. Nonetheless transitions over years to a decade into a state of long-term drought would have impacts on human and natural systems.
AR4 climate model projections (Milly et al., 2008) and CMIP5 ensembles (Figure 12.23) both suggest widespread drying and drought across most of southwestern North America and many other subtropical regions by the mid to late 21st century (see Section 12.4.5), although without abrupt change. Some studies suggest that this subtropical drying may have already begun in southwestern North America. More recent studies suggest that regional reductions in
precipitation are primarily due to internal variability and that the anthropogenic forced trends are currently weak in comparison.
While previous long-term droughts in southwest North America arose from natural causes, climate models project that this region will undergo progressive aridification as part of a general drying and poleward expansion of the subtropical dry zones driven by rising greenhouse gases. The models project the aridification to intensify steadily as radiative forcing and global warming progress without abrupt changes. Because of the very long lifetime of the anthropogenic atmospheric CO2 perturbation, such drying induced by global warming would be largely irreversible on millennium time scale (see Sections 12.5.2 and 12.5.4). For example, Solomon et al. (2009) found in a simulation where atmospheric CO2 increases to 600 ppm followed by zero emissions, that the 15% reduction in precipitation in areas such as southwest North America, southern Europe and western Australia would persist long after emissions ceased. This however is largely a consequence of the warming persisting for centuries after emissions cease rather than an irreversible behaviour of the water cycle itself.

As for boreal forest:
Evidence from field observations and biogeochemical modelling make it scientifically conceivable that regions of the boreal forest could tip into a different vegetation state under climate warming, but uncertainties on the likelihood of this occurring are very high (Lenton et al., 2008; Allen et al., 2010). This is mainly due to large gaps in knowledge concerning relevant ecosystemic and plant physiological responses to warming (Niinemets, 2010). The main response is a potential advancement of the boreal forest northward and the potential transition from a forest to a woodland or grassland state on its dry southern edges in the continental interiors, leading to an overall increase in herbaceous vegetation cover in the affected parts of the boreal zone (Lucht et al., 2006). The proposed potential mechanisms for decreased forest growth and/or increased forest mortality are: increased drought stress under warmer summer conditions in regions with low soil moisture (Barber et al., 2000; Dulamsuren et al., 2009; Dulamsuren et al., 2010); desiccation of saplings with shallow roots due to summer drought periods in the top soil layers, causing suppression of forest reproduction (Hogg and Schwarz, 1997); leaf tissue damage due to high leaf temperatures during peak summer temperatures under strong climate warming; increased insect, herbivory and subsequent fire damage in damaged or struggling stands (Dulamsuren et al., 2008). The balance of effects controlling standing biomass, fire type and frequency, permafrost thaw depth, snow volume and soil moisture remains uncertain. While the existence of, and the thresholds controlling, a potential critical threshold in the Boreal forest are extremely uncertain, its existence cannot at present be ruled out.

The discussion on monsoons in various parts of the world is mixed.  It does state at the end that in regard to the Indian monsoon:
Given that the effect of increased atmospheric regional loading of aerosols is opposed by the concomitant increases in greenhouse gas concentrations, it is unlikely that an abrupt transition to the dry summer monsoon regime will be triggered in the 21st century.

The point of all this is that one has to be careful how to read the report.  Low confidence doesn't mean the same thing as low likelihood.

If anyone has a different take on this, let me know.

PS A final thought - for all her posts about "uncertainty" you'd think that Judith Curry would know the difference between "unlikely" knowns and "known unknowns".  Apparently not!

4 comments:

  1. I think it likely that India will reduce the regional aerosol loading this century, if only because the rich suffer from it as well as the little people. Lets hope the monsoon cycle doesn't actually depend on it.

    ReplyDelete
  2. I would say, that the climate ostriches are at least consistent. They always act as if stating that the uncertainties are high mean that nothing will happen.

    ReplyDelete
    Replies
    1. Yep, so true. Unfortunately the way the IPCC works tends to act as an enabler for this behaviour: uncertain things are things scientists are likely to disagree on, and then a consensus position is one not stating anything risqué about these things. It's easy to be right if you're not saying much :-(

      Delete
  3. The keyword here is abrupt. Some of these things are thought likely to happen to some degree, just not abruptly. For instance on AMOC (WG1 - TS p35-37) :

    "It remains very likely that the AMOC will weaken over the 21st century relative to preindustrial values with a best estimate decrease in 2100 of about 20–30% for the RCP4.5 scenario and 36–44% for the RCP8.5 scenario, but there is low confidence on the magnitude of weakening. It also remains very unlikely that the AMOC will undergo an abrupt transition or collapse in the 21st century for the scenarios considered (high confidence)"

    Or for the Greenland ice sheet:
    The available evidence indicates that global warming beyond a threshold would lead to the near-complete loss of the Greenland Ice Sheet over a millennium or longer, causing a global mean sea level rise of approximately 7 m. Studies with fixed present-day ice sheet topography indicate that the threshold is greater than 2°C but less than 4°C of global mean surface temperature rise with respect to preindustrial.

    So it could collapse but the thermal mass means it won't happen abruptly (i.e. entirely) within the 21st century.

    What I take from this is that just because something isn't abrupt doesn't mean it isn't a problem.

    ReplyDelete

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