There is a new study out in Nature that is liable to be misinterpreted, or that may be flawed in a way that lends itself to misuse, in the context of climate science driven policy.
The study is “Northern Hemisphere hydroclimate variability over the past twelve centuries” by Fredrik Charpentier Ljungqvist, Paul J. Krusic, Hanna S. Sundqvist, Eduardo Zorita, Gudrun Brattström & David Frank
I’ll make just a few comments here, but mainly, I want to point you to comments by climate scientist Michael Mann (author of The Hockey Stick and the Climate Wars: Dispatches from the Front Lines and Dire Predictions, 2nd Edition: Understanding Climate Change).
The main question related to policy is this. Do warmer conditions such as we are experiencing now as a result of human greenhouse gas pollution change the hydrology of the planet? The answer, based on various research projects, is yes. Two main things seem to pertain. First, there is more moisture in the air owing to the air being warmer and sea surface temperatures being higher. More moisture holding capacity and more evaporation (movement of water into the air as vapor) result in this effect. At the same time, changes in weather patterns can clump a good amount of this moisture up, so even a modest amount of increase in atmospheric moisture can (and does) result in major precipitation events, causing flooding and other untoward events. This clumping can also serve to deprive some areas of moisture for extended periods of time, and major droughts such as in the Middle East and California are attributed at least in part to this effect.
The study seems to show that this is not likely. The study looks at paleo data over thousands of years, testing and extending a model to apply to present and future climates. The result seems to show that the more extreme changes in hydrology, either wetter or dryer, are not likely. However, Mann makes the point that the kind of data used in this study, such as tree rings, do not reliably show extreme events. In other words, extreme events in the past likely happened without leaving much of a signal.
Mann’s comments are in a facebook post partly reproduced here:
…The study represents a laudable effort to document past changes in extreme rainfall and drought using paleoclimate proxy data, but there are some shortcoming with the study, and especially with the way it is being billed by some of the study’s authors and certain organizations.
A press release from the international paleoclimate organization ?#?PAGES? is accompanied by the rather bold headline “Climate models overestimate twentieth century wet and dry climate extremes”. The lead author Fredrik Ljungqvist is quoted in the press release stating that the discrepancy between the smaller hydroclimatic variations shown by their paleoclimate proxy reconstruction and the greater variations shown by climate models imply that “Climate models strongly overestimate the intensification of wet and dry extremes in the twentieth century”.
Does this study in fact meet the burden of establishing that models are overestimating extremes in rainfall and drought?
Almost certainly *not*.
The discrepancy could arise, of course, from the opposite problem: that the paleoclimate proxy data are *underestimating* hydroclimatic extremes. In my view, that is a far more likely explanation.
Our own extensive work analyzing paleoclimate proxy data has shown has demonstrated 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)
So, in conclusion, it would be rather dangerous to extrapolate from this one potentially flawed new paleoclimate study any sweeping conclusions about climate models and human-caused climate change. Such over-interpretations of paleoclimate data poorly serve the critical public discourse over the impacts of climate change, and can in fact do harm to the paleoclimate discipline by publicizing bold but unsubstantiated claims that are very likely to be refuted by further work.