More Raindrops Are Falling on Our Headby Bill Chameides | February 23rd, 2011
posted by Erica Rowell (Editor)
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Rainfall has become more intense in the Northern Hemisphere. Why is much of the discussion swirling around models instead of data?
Data show extreme precipitation in the Northern Hemisphere is on the rise.
We all know that without the showers we get no flowers. But too much showers — not so great. A little more than average rainfall can be a bother, and a lot more can be life-threatening. (Some anecdotal evidence here, here, here and here.) And so the prospect that global warming will lead to more intense rainfall and snowfall events probably should not be taken lightly.*
All well and good (or not so good), but should we do anything about it? Scientists’ warning of a potential outcome is one thing; proof that heavier rainfall is actually occurring is another. Now into the breach comes a new paper published in the journal Nature by Seung-Ki Min of Environment Canada and colleagues that purports to do exactly that.
Synopsis of the Study
The study consisted of two components. First the authors used observations of precipitation (i.e., rain and snow) between 1951 and 1999 across 6,000 ground-based stations to determine if the frequency of extreme precipitation events has changed. They then used eight different climate models with close to 40 separate simulations to assess if (in the model world) emissions of greenhouse gases from human activities change the frequency of these same types of events. Both the observations and the model simulations consistently showed an increasing trend in intense precipitation events over the study period. The correspondence between observations and model simulations led the authors to conclude that:
“human-induced increases in greenhouse gases have contributed to the observed intensification of heavy precipitation events found over approximately two-thirds of date-covered parts of Northern Hemisphere land areas.”
The Debate: What’s It All About
Not surprisingly, the publication of the Min et al paper was greeted with a good deal of media attention (some examples here and here). Much of it and the discussions that followed focused on the second part of the study — the authors’ use of computer models to establish a linkage between the positive trend in storm intensity in the Northern Hemisphere and anthropogenic releases of greenhouse gases.
Questions have been raised in some places about the robustness of the Min et al’s claim that they “show” a cause-and-effect relationship between increased precipitation intensity and greenhouse gases. Even the authors put a caveat on their findings: that the modeling simulations used to establish the linkage systematically “underestimate the observed increase in heavy precipitation with warming.” This is actually a two-edged sword: on the one hand it may mean that there is another (perhaps natural) driver of intensity not accounted for by the models; on the other, it may mean that the models underestimate the link between precipitation and greenhouse gases and therefore underestimate the consequences of future emissions.
Not Seeing the Data for the Model
Establishing the role of human activities in climate change is clearly an important issue, and so the focus on whether Min et al actually established such a link is germane, especially in the scientific realm. But from a policy point of view, you’ve gotta keep in mind that using a model simulation to “prove” the role of human activities and climate change is and will always be problematic. If you have any doubts, take a look at some of the musings of Kurt Gödel. Or, if you’re looking for a somewhat less esoteric take on the subject, try this article [pdf] by Naomi Oreskes and co-authors from 1994 (“Verification, Validation, and Confirmation of Numerical Models in the Earth Sciences,” Science, Vol. 263, No. 5147, Feb. 1994, pp. 641-646).
In short: if we’re going to insist on slam-dunk proof that what we see happening with the climate is due to greenhouse gases, we are going to have to wait a very long time — especially if we’re not going to depend on climate model simulations.
So Weary All the Time
I find all the debate over whether a link between human activities and precipitation trends has been scientifically established by Min et al to be scientifically interesting, but it takes the focus away from another really important part of their study. And that part is their finding that there has been a significant increase in the probability of extreme precipitation over the Northern Hemisphere over the last half of the 20th century. The trend, on average of a few tenths of a percent per year, has been especially pronounced over the United States and was especially large in the 1990s — an increase on the order of perhaps four percent over the decade.
This trend is not from a model simulation — it’s from data, real-world, empirical data. It’s really happening, folks.
While much of the world seems content to argue about climate models and twiddle their collective thumbs waiting for proof positive, the climate is changing and changing, more or less, in the ways we would expect it to as a result of greenhouse gas emissions. We can wait, the climate won’t.
* As the climate warms, more evaporation occurs transferring more moisture to the atmosphere. That increased atmospheric moisture means that when it does snow or rain, those events tend to be heavier. More details here and here.
Correction: March 9, 2011, This post has been corrected to fix the misspelling of the surname of Naomi Oreskes.filed under: climate change, faculty, global warming, Planetary Watch, rainfall, science, weather
and: climate, climate disruption, climate models, climate science, greenhouse gas emissions, greenhouse gases, Kurt Gödel, research, snow, snowstorms, storms