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The IPCC report on extreme climate and weather events

Filed under: — rasmus @ 19 November 2011

The IPCC recently released the policy-maker’s summary (SREX-SPM) on extreme weather and climate events. The background for this report is a larger report that is due to be published in the near future, and one gets a taste of this in the ‘wordle‘ figure below. By the way, the phrase ‘ET’ in this context does not refer ‘extra-terrestrial’, and ‘AL’ is not a person, but these refer to the way of citing many scholars: ‘et al.

Fig. 1. The text analysis according to

The fact that the summary is released before the main report is bound to cause some confusion, and has lead to a number of false allegations in the past, such as the main report being written to suit the conclusions of the summary. This is not the case, but I personally think that the IPCC handles the release of these reports in a strange way.

The main report has already been written, but there are some fine details that need to be approved by the member states before it is finalized. My understanding is that the whole process will be open and transparent, and that the previous drafts and review comments will be available in time. Those who already have read the main report are not supposed to cite it before it’s out.

I must also confess that one of the aspects that I’m most curious about concerns tropical cyclones (TCs). Hence, these phenomena was one of the things I looked at first. Here are some quotes:

Average tropical cyclone maximum wind speed is likely to increase, although increases may not occur in all ocean basins. It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged.

The message from the summary of policy-makers is therefore that it is likely [66-100% probability] that there will be fewer or same number but more intense tropical cyclones (including tropical storms, hurricanes, and typhoons) in the future. This conclusion is not new, however, as it was also the concusion of the AR4, as well as the most recent WMO consensus statement on tropical storms.

A combination of stronger tropical cyclone maximum winds but fewer tropical cyclones is nevertheless quite interesting. My feeling is that this statement is still a bit premature, as it surely is based on projections made with global climate models (GCMs). The tropical cyclones are represented differently in the GCMs compared to real world measurements, where the wind speed changes continuously in space.

The message from the SREX-SPM is similar to that of a 2010 study from Nature Geoscience, for which the abstract reads (my outline):

Whether the characteristics of tropical cyclones have changed or will change in a warming climate — and if so, how — has been the subject of considerable investigation, often with conflicting results. Large amplitude fluctuations in the frequency and intensity of tropical cyclones greatly complicate both the detection of long-term trends and their attribution to rising levels of atmospheric greenhouse gases. Trend detection is further impeded by substantial limitations in the availability and quality of global historical records of tropical cyclones. Therefore, it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes. However, future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%. Balanced against this, higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre. For all cyclone parameters, projected changes for individual basins show large variations between different modelling studies.

In GCMs, these phenomena appear as vortex-like features in the discrete representation of the flow represented by neighboring grid boxes. It’s quite remarkable that these phenomena are present at all in these models (sometimes they are not, though), even though they may have too weak or exaggerated features. In the real world, the definition of a tropical storm is a synoptic scale low-pressure system with maximum sustained surface wind speed greater than 17 m/s, and in hurricanes greater than 33 m/s.

If we look at wind speed measurements at a given location, we see that there are relatively few days with zero wind, more often there are moderate wind speeds, and it is typically rare when the wind speed exceed the threshold defining a tropical storm or a hurricane. In statistical terms, the wind speed may be described by a distribution function – e.g. a Weibull distribution (e.g. here and here). The situation is illustrated below showing wind speed statistics, where the curve is the probability distribution function (pdf) for the wind speed and where the x-axis represents the wind speed and the y-axis the likelihood (frequency) of occurrence. The threshold marking tropical storms is shown as the first vertical line (the others mark typical TC categories), and the area under the curve to the left of this treshold (denoted “a” in the diagram) is proportional to number of observations (e.g. days) with no tropical storms. The area above (“A”) is proportional to the frequency of tropical cyclone occurrence.

Fig. 2. Wind speed statistics and tropical cyclones.

Let’s consider the implication of fewer tropical cyclones but an increase in their intensity. In terms of wind speed statistics, this suggests a shift in the pdf (grey gurve in Fig. 3), with an increase in the area under the curve with wind speeds lower than 17 m/s (“a”). This also implies a decrease in the area under the curve for which wind speeds exceed 17 m/s (“A”), as the area under the total curve of a pdf must be constant (unity by definition). But if the tropical cyclones are getting more intense (increased mean TC maximum wind speed), there must be a second threshold, e.g. 33 m/s for which the area under the curve for the new pdf is greater than for the old curve.

Fig 3. The grey curve illustrates a perceived shift in the pdf describing the wind speed statistics

It is certainly possible that the requirement in these changes in the wind speed statistics can occur, but the question is whether it is likely and whether we are able to detect this. If the shape of the wind speed is constrained to being a Weibull type, then it is easy to simulate the probability that the area under the curve is greater both for the portion of the curve with wind speeds lower than say 17 m/s and greater than 33 m/s (Monte-Carlo simulations – R-script). The fraction of Weibull shapes satisfying this, accoring to a simple Monte-Carlo simulation, is 1.9% (i.e. not very likely). Another issue is the required size of a statistical sample to be able to detect such changes, and the GCMs’ ability to provide such details (there are not that many simulations with high-resolution GCMs, the number of TCs is sensitive to a number of factors, such as ENSO, AMO, MJO, and the annual cycle – I must admit that I don’t know if the GCMs capture these dependencies well).

An analysis of empirical data provides strong indications that the number of TCs, N, varies with the surface area of warm surface (warmer than 26C). However, N also depends on the number of triggering events (organized convection e.g. African easterly waves in the North Atlantic), the wind shear, and the convective available potential energy (CAPE). The question about future trends in number and intensity of TCs depend on these aspects, in addition to the magnitude of the sea surface temperature.

It is well-known that tropical cyclones are influenced by a number of factors, such as El Nino Southern Oscillation (ENSO), the Madden Julian Oscillation (MJO), wind shear, organized convection, and sea surface temperatures. The GCMs, however, provide different accounts as to which direction the ENSO takes, struggle with reproducing the MJO, and may have biases with respect to the sea surface temperatures and wind shear. There are also some problems, as they produce a spurious “double” inter-tropical convergence zone (ITCZ), as well as biases in sea surface temperatures and wind shear.

Furthermore, small-scale processes may still not be sufficiently resolved by the GCMs used for projecting the future climates. Having said that, high-resolution global atmosphere models provide realistic-looking pictures of tropical cyclones, and the question is not whether the models in principle are capable to capture these events, but rather whether the current set-up of GCM experiments is sufficient for providing reliable information about how these will evolve in the future. The main report may shed more light on this, and we should keep in mind that the models must be evaluated against the past and reproduce known dependencies, in addition to reproducing the wind speed distributions.

There is still a debate about the past trends (see previous discussions here, here, here, and here). Has the tropical cyclone activity or the number of cyclones increased? Note, the trends may not not necessarily linear, and if one tries to fit a straight line in time, it may not provide the best picture of the situation. As long as we have no reliable records on tropical cyclones for the past, I’d argue that we don’t know how well our models are able to capture long-term changes in tropical cyclones. However, this is only one small issue (see Fig. 1), and the SREX-SPM covers many other topics on which I have little expertise.

68 Responses to “The IPCC report on extreme climate and weather events”

  1. 51
    Pete Dunkelberg says:

    @ 49: “Nothing in the paper [Dan H.] linked to even addresses the effect of anthropogenic global warming on hurricanes.”

    That’s not clear to me, and not just because I read the latter part of the paper. Our topic is planetary physics. In physics on a heated rotating sphere, everything pushing on everything else directly or indirectly. In addition much energy is advected by fluids and may escape to space or change back into a pushing force at some distant spot. All is unavoidably related, and trends in SSTs, discussed in the paper, are surely related to trends in tropical cyclones.

  2. 52
    Hank Roberts says:

    > surely related
    But not in a simple way; pointing to a chart and saying one line goes down so another one must also is the WTF approach to analysis, not worth porting over here uncritically except to divert and delay.

    Try something serious; Isaac Held has quite a bit on his blog, including a mention of this summary of the complexities beginning to emerge:

    PERSPECTIVES CLIMATE CHANGE Whither Hurricane Activity?
    Gabriel A. Vecchi, Kyle L. Swanson, Brian J. Soden
    (the abstract there is a machine translation, mostly word salad; look for the original, which was published in Science, I think in 2008)
    Alternative interpretations of the relationship between sea surface temperature and hurricane activity imply vastly different future Atlantic hurricane activity. …”

  3. 53
  4. 54
    Aaron Lewis says:

    In the past, wind and barometric pressure were good proxies for storm intensity. I think the lesson from 2011 is that in a world of AGW, we need new proxies for how much damage a storm is likely to inflict.

    The PDF for wind speed does not really matter if most of the damage will be from flooding resulting from rain.

    The useful thing to know is: how much rain should be anticipated as a base of enginering and building design (for long term infrastructure)?

  5. 55
    prokaryotes says:

    The 2011 Eastern Pacific hurricane season featured a well below-average number of named storms–eleven (fifteen is average). However, all but one of these storms reached hurricane strength, the highest proportion of hurricanes in a single season ever recorded.

  6. 56
    Pete Dunkelberg says:

    Aaron Lewis gets the big picture. The problem is, Aaron, the quantitative answer to your question is not bounded above compatibly with business as usual. Your question can be answered after we stop burning carbon, or are stopped by the consequences.

  7. 57
    David B. Benson says:

    Aaron Lewis & Pete Dunkelberg — The SREX-SPM does attempt to delineate the increased risk of significant rainfall; rather high IMO.

  8. 58
    Urs Neu says:

    The Eastern Pacific often reacts contrary to the Atlantic, e.g. in response of ENSO. It is interesting that this year also the proportion of hurricanes (in relation with total tropical cyclones) is in the opposite direction (record high proportion in the Eastern Pacific, very low proportion in the Atlantic). I had not the time yet to look if this might be a regular feature. If yes, this might be a point for interesting conclusions.

  9. 59
    Dan H. says:

    Thank you for the link.
    We are certainly at an impasse with regards to future hurricane activity. The differential in SST appears to come from Latif’s work, were he determined wind shear was a primary forcing.
    Based on what I have read, it appears that hurricane development and intensity is related to SST up to 28C, after which, wind shear is the dominant forcing.

  10. 60
    Paul C says:

    “The ideas of eliminating or controlling CO2 production are actually ideas whose end game is one of making man live as he did 200 years ago.”

    To be more accurate, the idea of managing CO2 production is an idea whose end game is one of ensuring humanity can live well 200 years in the future.

  11. 61
    Hank Roberts says:


    Mesospheric temperature trends at mid-latitudes in summer
    Key Points
    For the first time a model explains large trends observed in the mesosphere
    Trends are not uniform in time and are largest (3–5 K/decade) from 1979–1997
    Trends in the mesosphere are significantly influenced by stratospheric ozone

    The Leibniz-Institute Middle Atmosphere Model LIMA is used to study mesospheric temperature trends in summer during the last 5 decades (1961–2009). … the period from 1979–1997 shows large mesospheric cooling of 3–5 K/decade. This large cooling is primarily caused by long-term changes of ozone in the upper stratosphere in combination with a CO2 increase. For the first time, modeling of mesospheric temperature trends confirm the extraordinarily large trends from observations….”

  12. 62
    Hank Roberts says:

    Dan, that’s not an impasse; you’re looking at two items from an older paper. More factors are involved, and they vary over time (perhaps independently, perhaps as functions of something else). That’s the point of Vecchi et al.

    What you call “an impasse” because the simple older explanation didn’t suffice is what science calls progress. That often involves new complications.

    A paper on climate sensitivity just out from AGU is another illustration of the same point — data and models give somewhat different results. This isn’t an impasse, this is how science makes progress. One world climate; many approximations in our understanding of it.

    Sensitivity of distributions of climate system properties to the surface temperature dataset
    Key Points
    Model parameter estimates are sensitive to the dataset used for evaluation
    Climate sensitivity distributions are most sensitive to the surface dataset
    The range of TCR is within the IPCC AR4 range but shifted to lower values

    “… Estimates of effective climate sensitivity mode and mean differ by as much as 1 K between the datasets, with an overall range of 1.2 to 5.3 K. Ocean effective diffusivity distributions are poorly constrained by any dataset. The overall range of net aerosol forcing values, −0.19 to −0.83 Wm−2, is small compared to other uncertainties in climate forcings. Transient climate response (TCR) estimates derived from these distributions range between 0.87 and 2.41 K and the shapes of individual TCR distributions depend on the surface dataset. Understanding the differences in parameter distributions and climate system properties derived from them is critical for understanding the full range of uncertainty involved in climate model calibration and prediction results….”

    One of the illustrations:

  13. 63
    Hank Roberts says:

    Also instructive as to how science progresses:

    Deep Sea Research Part II: Topical Studies in Oceanography
    Volume 58, Issues 17-18, September 2011, Pages 1880-1894
    Climate and the Atlantic Meridional Overturning Circulation

    A perspective on decadal climate variability and predictability

    Mojib Latif, Noel S. Keenlyside


    The global surface air temperature record of the last 150 years is characterized by a long-term warming trend, with strong multidecadal variability superimposed. Similar multidecadal variability is also seen in other (societal important) parameters such as Sahel rainfall or Atlantic hurricane activity. The existence of the multidecadal variability makes climate change detection a challenge, since global warming evolves on a similar timescale. The ongoing discussion about a potential anthropogenic signal in the Atlantic hurricane activity is an example. A lot of work was devoted during the last years to understand the dynamics of the multidecadal variability, and external and internal mechanisms were proposed. This review paper focuses on two aspects….”

    No impasse on this subject.
    Don’t rely on people citing old papers; check for recent work.

  14. 64
    Dan H. says:

    I was just arguing this same position with Tamino recently (I believe you even supplied a link to the post). I supported the position of Latif and Keenlyside against Tamino, who argued against these multidecadal variations.

  15. 65
    alynnw says:

    Even the Intergovernmental Panel on Climate Change acknowledges that the study of climate change and its consequences is unparalleled in the scientific arena; therefore, the “expert judgment (from the scientific community) on the correctness and completeness of current scientific understanding” on climate change is strongly extrapolative.This is valid because the future outcomes of climate change are not as easy to study as the effects of tobacco or the outcome of eating fatty foods. Consequently, it is feasible that climate change is indeed cyclic and/or will not pose as great a threat to humanity as currently reported.
    Nevertheless, putting aside all the uncertainty surrounding climate change, humans should without the need of persuasion or influence, already possess reverence for their home, a home that needs to sustain life for future generations.

  16. 66
    jimb says:

    Sorry to be late to the post, but I wanted to add some comments to the thread talking about ‘far north’ farming, i.e post 45. Here in central Alberta in November, (not that far north) I am told that the average high temperature is 4C, and so far we have had temperatures from -20C to +10C, and the temperatures will drop and then recover to that point probably in late March. A 4C rise in temperature isn’t going to help crops much. Here they do a great job of producing crops in the one season they have, but you are not going to get a two crop season even with a 4C rise.
    The Canadian Shield was properly noted (post 48) as not having much topsoil, but much of the far north is muskeg- great for methane production but not much for crop growths far as I know, but I stand to be corrected on that. Between here and the ‘far north’ is the boreal forest- if you are going to farm that you are going to have to do a lot of de-foresting, that is if the northern pine beetles don’t do that for you. (they love the warmer winters we have been having lately)

  17. 67
    Dan H. says:

    I am not sure about “far north” farming. Alaskans use the near-constant summer daylight to grow some of the largest crops in the world, prompting the following joke: A man walks into an Alaskan grocery store and asks for 10-lbs of potatoes. The grocery replies, “sorry, but we don’t cut our vegetables here.”

    You probably know better that I about the change in Albertan agriculture through the years and points farther north, but here in Michigan, we have seen a two-week increase in the growing season in the past several decades, resulting in more abundant harvests. Neither the recent increase, nor a 4C increase will likely result in a two-crop growing season. However, I did just brush some snow off my broccoli yesterday, before picking it for dinner.

  18. 68
    jimb says:

    Thanks for the comment Dan H.. I expect that here, as in Michigan, we could expect a longer growing season that would prevent the kind of crop degradation that early frosts cause. Late frost this year helped to make up for late planting due to unusually heavy rains in the spring/early summer.
    Snow here too, but no broccoli.