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Climate Hustle

Factcheck: Are climate models ‘wrong’ on rainfall extremes?

Posted on 7 April 2016 by Guest Author

This is a re-post from Robert McSweeney at Carbon Brief

Several media outlets are reporting that new research shows climate model projections of rainfall extremes may be “flawed” or “wrong”.

The study, published yesterday in Nature, reconstructed periods of wet and dry extremes for the last 12 centuries. The paper says that large extremes of wet and dry conditions that climate models simulate for the 20th century aren’t found in the reconstruction.

The paper prompted a MailOnline headline of, “Projections of global drought and flood may be flawed”, while the Australian followed suit with, “Climate model projections on rain and drought wrong, study says”. Elsewhere, a brief news story in today’s Times – subsequently bumped from the second edition – claims that “climate scientists have wrongly blamed manmade emissions for droughts and floods”, under the headline “Climate change row”.

But other scientists tell Carbon Brief that the discrepancy between the data reconstruction and model simulations is more likely because the reconstruction underestimates climate extremes, not that models overestimate them.

MailOnline article 6 April 2016

MailOnline, 6 April 2016.

Proxy records

The focus of the new study is how researchers pieced together a record of extreme wet and dry periods across the northern hemisphere for the past 1,200 years.

To understand how climate changed before anyone was actively taking records, scientists use techniques known collectively as “palaeoclimatology”. These involve looking for climate information in places where it is stored indirectly, such as in tree rings, lake sediments and ice cores.

The researchers used 196 of these “proxy” records to create a “hydroclimate reconstruction” of the northern hemisphere back to the 9th century.

Now, when scientists build climate models, they test them by running them for the past and seeing if the models replicate what actually happened. So, the researchers compared model simulations of past climate against their newly-created reconstruction.

The Times, first edition, 7 April 2016

The Times, first edition, 7 April 2016.

The study finds that the proxy data and model simulations of wet and dry periods match well for most of the past 1,200 years. However, the same isn’t the case for the 20th century, the researchers say.

Lead author Dr Fredrik Charpentier Ljungqvist, a medieval historian and palaeoclimatologist at Stockholm University, explains to Carbon Brief:

“[T]he climate models do simulate centennial-scale hydroclimate variability almost surprisingly well from the 10th to the 18th centuries. The ‘problem’ in the models is the too strong intensification of wet and dry extremes in the 20th century.”

The implication of these findings is that if climate models don’t tally with past climate, this questions how well they can project future climate, Ljungqvist says:

“Our study indicates that climate models might have a more limited ability to predict which regions will get drier and which regions will get wetter with global warming than previously assumed.”

However, Prof Michael Mann, distinguished professor of atmospheric science at Penn State University, who wasn’t involved in the study, says the researchers may have got their conclusion the wrong way around. In a post on his Facebook page, Mann writes:

“The discrepancy could arise, of course, from the opposite problem: that the palaeoclimate proxy data are underestimating hydroclimatic extremes. In my view, that is a far more likely explanation.”

Less variability

Mann suggests that differences between the palaeo record and model simulations are a result of shortcomings in the proxy data, not flaws in climate models, as he explains to Carbon Brief:

“[M]y own extensive work using these [palaeo] data has led me to the conclusion that they are not well suited for reconstructing past climate extremes. Tree rings and many other chemical and biological climate proxy records, by their nature, tend not to record very large short-term fluctuations, and for this reason they are likely to show less variability than actually exists in the climate record.”

Prof Steven Sherwood, director of the Climate Change Research Centre at theUniversity of New South Wales, who also wasn’t involved in the study, agrees:

“[The Nature study is] heavily smoothing the data so as to look only at centennial-scale shifts, not what we usually think of as droughts or rainfall extremes, which would be scales of days to at most a decade or two.”

Indeed, for the 20th century, scientists also have the meteorological records they have collected directly. And these show that wet and dry extremes have been intensifying, says Prof Brian Soden, professor of meteorology and physical oceanography at the University of Miami. He tells Carbon Brief:

“The proxy records used in this study appear to contradict other studies that have used direct observations of precipitation from rain gauges…All else being equal, I would trust direct rainfall measurements over proxy records.”

Big challenge

So, does the study show that climate models are “wrong” on rainfall, as the Australian headline suggests?

In an accompanying News & Views article in Nature, Prof Matthew Kirby, a professor of palaeoclimatology at California State University who wasn’t involved in the study, tackles this exact question:

“Do their results invalidate current predictive models? Certainly not. But they do highlight a big challenge for climate modellers, and present major research opportunities both for modellers and for climate scientists who work with proxy data.”

Sherwood agrees:

“If this paper’s conclusion about model overprediction holds up to further scrutiny it will be extremely interesting…But…I am not convinced that this particular conclusion will hold up.”
The Australian article, 7 April 2016

The Australian, 7 April 2016.

Data gathering

Despite reservations about the paper’s conclusions about climate models, there is one element of the study that scientists can be sure about: that gathering more proxy data like this is a good thing.

For example, in the maps below from the paper, you can see the sparseness of the proxy records that the researchers had available to create their rainfall (left map) and temperature (right map) records.

Location map of the a) 196 rainfall proxy records and b) 128 temperature records used in the study. All the records cover at least the period 1000-1899. The colour of the dots indicates the type of proxy record

Location map of the a) 196 rainfall proxy records and b) 128 temperature records used in the study. All the records cover at least the period 1000-1899. The colour of the dots indicates the type of proxy record (see key). Source: Ljungqvist et al. (2016).

Proxy data covers key regions, such as China, Europe, and much of Central America and North America, Ljungqvist says, but many areas lack data:

“Unfortunately, the geographical coverage of hydroclimate proxy records is still very limited – or nonexistent – in many parts of the world.”

Prof Richard Harding from the Centre for Ecology & Hydrology in Wallingford says he hopes the study will encourage more palaeo reconstructions to fill the gaps in the record:

“The long-term observational record of main components of the hydrological cycle – rainfall, evaporation, river runoff and soil moisture – is woefully incomplete, particularly in sensitive semi-arid regions, and this study is a significant contribution to support this record.”

And more comprehensive data allows scientists to refine climate models even further, adds Ljungqvist:

“[I]mproved reconstructions, with hopefully increased geographical coverage, are important for further testing the climate models.”

Ljungqvist, F. C. et al. (2016) Northern Hemisphere hydroclimatic variability over the past twelve centuries, Nature, doi:10.1038/nature17418.

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Comments

Comments 1 to 5:

  1. The idea that all models are wrong is both correct and misleading.  No model is 100% correct in any scientific or engineering discipline.  HOWEVER, this argument is used to IMPLY that the climate models are useless, when it is abundantly clear they are of significant use and more and more look in aggregate to be more correct than some would like to see.

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  2. I have a few questions.

    Why do tree rings smooth the signal for precipitation so strongly?
    I would have expected a tree ring not to smooth more than the one year it takes for a tree ring to grow.

    How does this paleo reconstruction for the twentieth century compare to direct measurements?

    Why do the reconstruction and the models stop agreeing for the twentieth century? Did the number of precipitation extremes increase significantly?

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  3. martin @2, Professor Sherwood's comment can be found here (along with two others).  In full, he states:

    "They did a lot of work and its a nice study and a valuable proxy dataset, but it has many complexities in the way it was done. They are heavily smoothing the data so as to look only at centennial-scale shifts, not what we usually think of as droughts or rainfall extremes (which would be scales of days to at most a decade or two).

    Previous studies, based on models, have shown that warming-induced trends in regional precipitation have not yet emerged from natural variability (“noise”). This seems inconsistent with the paper’s claim that the changes predicted by these same models are unrealistic, since it should not yet be possible to tell even according to the models themselves.

    They normalised all data series to unit variance over the 1200 years. If I understand correctly, that means if there is too little natural variability in the models (which has been reported in other studies) or noise in the proxy observations (which is inevitable), either of these would help create an apparent discrepancy such as they’ve noted. Thus, other interpretations of their results may be more consistent with past studies.

    In models, they looked at precipitation, but the observational proxies they have are probably measuring soil moisture or something similar. Many people think these two quantities are equivalent, but they are not, because as soon as you warm the whole planet they decouple (precipitation generally goes up while soil moisture may stay the same or go down). This will cause discrepancies during the last century, due to the global warming not seen in earlier periods. Future studies should compare these hydroclimate proxy results to a more appropriate model variable such as soil moisture.

    If this paper’s conclusion about model overprediction holds up to further scrutiny it will be extremely interesting; my own work focuses on model-data discrepancies so I am particularly interested. But due to the above aspects of the study, I am not convinced that this particular conclusion will hold up. We shall see, as I am sure this result will attract lots of attention."

    The claim appears to be that Llungqvist et al themselves smooth the data rather than that it is innately smoothed.  The reason the observational proxies "are probably measuring soil moisture" is that trees draw their moisture from the soil, not the air, and therefore respond to soil moiture rather than directly to precipitation. 

    Michael Mann' facebook page (link in main article) refers to several of his own peer reviewed papers to justify his comment regarding extremes:

    "Our own extensive work analyzing paleoclimate proxy data has shown that they are not well suited for reconstructing past climate *extremes*. Tree rings and many other chemical and biological climate proxy records, by their nature, tend not to record very large short-term fluctuations, and for this reason they are likely to show muted extremes, i.e. less extreme variation than actually exists in the climate record. We published several articles demonstrating this problem over the past several years:
    Schurer, A., Hegerl, G., Mann, M.E., Tett, S.F.B., Separating forced from chaotic climate variability over the past millennium, J. Climate, 26, 6954-6973, 2013.
    Mann, M.E., Rutherford, S., Schurer, A., Tett, S.F.B.,Fuentes, J.D., Discrepancies between the modeled and proxy-reconstructed response to volcanic forcing over the past millennium: Implications and possible mechanisms, J. Geophys. Res. 118, 7617-7627, doi:10.1002/jgrd.50609, 2013.
    Mann, M.E., Fuentes, J.D., Rutherford, S., Underestimation of Volcanic Cooling in Tree-Ring Based Reconstructions of Hemispheric Temperatures, Nature Geoscience, 5, 202-205, 2012.
    (all available here: http://www.meteo.psu.edu/…/publi…/Mann/articles/articles.php)."

    That being said, the relevant sections of all three articles are about responses of proxies volcanism, which is not an evident factor for the 20th century period in which the discrepancy lies.  On that basis, I cannot see the relevance of Dr Mann's remarks. 

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  4. For cripes sake a 'reconstruction' is in and of itself a Model ....

    That should be obvious

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  5. Our model of past climate proves your model of future climate is wrong

    Huh?

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