July 6, 2004

Precipitate Modeling

Filed under: Precipitation

Global warming shows no significant influence on precipitation over land, a new study finds.

Despite human-caused alterations to the natural chemical composition of the earth’s atmosphere, researchers are unable to identify an anthropogenic impact on global precipitation rates—one of the fundamental measures of climate.

While it would certainly be convenient for carbon dioxide emissions-control lobbyists if an unusual occurrence of floods and droughts were plaguing the globe, unfortunately (for them), this is simply not the case. While it is true that at any given time in multiple places on earth some level of drought (or precipitation surplus) is occurring, but these reflect normal patterns inherent in the earth’s climate variability. That these are highlighted in the press simply plays into the media’s fascination with catastrophe.

Their fascination with one particular “catastrophe,” climate change, leads them to make connections where connections don’t really exist. Such is the case with greenhouse warming and global precipitation, as demonstrated in a June 2004 paper appearing in Geophysical Research Letters by Nathan Gillet and three colleagues.

Global climate models have always had a difficult time simulating precipitation amounts. And for good reason: Scientists don’t fully understand the process, and linking up what’s happening within a cloud to the very coarse scale of a climate model is beyond our computational and, more important, scientific capabilities. Nevertheless, climate models do make projections of future precipitation patterns under input scenarios depicting various levels of human activity.

Gillett ‘s research team attempted to find out if the observed patterns of global precipitation over the last 50 or so years matched climate model expectations.

To examine this question, they used the National Center for Atmospheric Research Parallel Climate Model that was run with three different climate forcings: volcanic aerosols, solar radiation, and the combined effects of greenhouse gases, sulfate aerosols, and ozone depletion. They also ran the model for a case in which all three factors were entered simultaneously. All those forcings were allowed to change monthly based on historical observations, and the resulting modeled precipitation was compared with observed precipitation over land areas across the globe (precipitation observations of the world’s oceans are quite sparse).

Figure 1 compares the observed precipitation changes with the model results. Before being plotted, the data were first smoothed to remove shorter-term variation, in this case to remove impacts of El Nino and other non-external components of natural precipitation variability. (In many cases, data are smoothed when scientists are trying to hide bad results, but in some cases, the procedure can be justified.)

Precipitation

Figure 1. Modeled (broken lines) and observed (solid line) global land average daily precipitation.

The quality of this simulation is in the eyes of the beholder. It’s obviously not a perfect match—the model misses the precipitation peaks in the mid 1950s and the early 1970s, but some of the low precipitation periods are picked up pretty well. The model also misses the average rainfall rates because it tends to vary less than reality. Overall, the correlation between the models and observations is a statistically significant 0.45, which means that only 20%, or one-fifth, of the variation in global land precipitation can be accounted for by greenhouse gases, volcanic aerosols, and solar variations.

Nevertheless, it is reasonable to ask to what extent greenhouse gases are responsible for the changes observed over the period of record. So the authors compared the observed patterns with each of the forcing responses separately. They found that only the volcanic signal was detectable. In other words, monthly changes in greenhouse gas levels from the late 1940s to 1998 had no demonstrable impact on global precipitation rates over land. According to the authors, “Overall, we thus find evidence of volcanic influence on terrestrial precipitation over the past 50 years, but no evidence of anthropogenic or solar influence using our methodology.” Other modeling work has suggested that the rainfall minimum in the early 1990s was linked to the eruption of Mount Pinatubo in 1991. And the previous minimum in the mid-1980s followed on the heels of the eruption of El Chichon in 1982.

The theory here is that changes in shorter wavelengths of radiation (the heat coming from the sun) are more important than variations in long-wave radiation (the radiation emitted by the earth that is absorbed by greenhouse gases). There is also evidence that planet cools following volcanic eruptions, so a cool planet is apparently less rainy than a warm planet.

So the rainfall problem continues. It’s hard to blame human activities on precipitation changes unless by burning fossil fuels we are provoking the wrath of the volcano gods.

Reference:

Gillett, N.P, A. et al., 2004. Detection of volcanic influence on global precipitation. Geophysical Research Letters, 31, L12217, doi: 10.1029/2004GL020044, 2004.




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