Extreme weather changes in front of us

Researchers from Purdue University have used their supercomputers to run the largest simulation of what could become the weather in the U.S. between 2071 and 2095. And their model, which was validated by using data from last century, predicts more extreme temperatures throughout the country.

Researchers from Purdue University have used their supercomputers to run the largest simulation of what could become the weather in the U.S. between 2071 and 2095. And their model, which was validated by using data from last century, predicts "more extreme temperatures throughout the country and more extreme precipitation along the Gulf Coast, in the Pacific Northwest and east of the Mississippi." The computer model used a grid with an 25-km horizontal resolution and required five months of computing time. Still, the researchers think they need a computer at least 100 times as powerful as the clusters they used to really improve the accuracy of their predictions.

Here are two short excerpts of the Purdue University news release.

The climate model, run on supercomputers at Purdue University, takes into account a large number of factors that have been incompletely incorporated in past studies, such as the effects of snow reflecting solar energy back into space and of high mountain ranges blocking weather fronts from traveling across them, said Noah S. Diffenbaugh, the team's lead scientist and a member of the Purdue Climate Change Research Center.
Diffenbaugh said a better understanding of these factors -- coupled with a more powerful computer system on which to run the analysis -- allowed the team to generate a far more coherent image of what weather we can expect to encounter in the continental United States for the next century. Those expectations, he said, paint a very different climate picture for most parts of the country.

The research work has been published by the Proceedings of the National Academy of Sciences under the title "Fine-scale processes regulate the response of extreme events to global climate change" (published online before print, October 19, 2005).

Here are two links to the abstract and to the full paper (PDF format, 5 pages, 740 KB), from where the details and quotes below have been extracted.

Here are the characteristics of the model used for this computer simulation.

The grid was centered at 39.00°N and 100.00°W and consisted of 145 points in the latitude direction and 220 points in the longitude direction. Grid points were separated at 25-km horizontal resolution with 18 levels in the vertical. We have performed two model integrations.

The researchers performed two reference integrations (RF) on the model, first for the period 1961 through 1985 to validate the model, then for the period covering the years 2071 to 2095.

Below is an image showing one of the results obtained for this second period. It shows the anomalies in mean heat-wave length. This T95 index value on this map was "calculated as the mean of the 95th percentile daily temperature maxima at each grid point in each of the 25 years of the RF integration" (Credit: Noah Diffenbaugh).

Additional heat-wave days in 2095

As you can see, anomalies were mostly positive, indicating higher temperatures than today.

Peak values of up to 15 days per event (up to 550%) stretched from northern Mexico into the northern Great Basin, with substantial spatial variability linked to the rugged terrain of the Great Basin. The northeast Atlantic coast showed little response in mean heat-wave length, whereas eastern Texas showed a strong response.

Here is a short summary of the results obtained from this simulation.

Our results indicate that, should atmospheric greenhouse gas concentrations continue to increase over the next century, changes in the frequency and magnitude of extreme events are likely to be dictated not only by changes in large-scale climate dynamics but also by a suite of climate-system modifiers operating at very fine scales.
We project substantial, spatially heterogeneous increases in both hot and wet events over the contiguous United States by the end of the next century, suggesting that consideration of fine-scale processes is critical for accurate assessment of local- and regional-scale vulnerability to climate change.

And it seems that all human activities will be affected.

The changes in frequency and magnitude of extreme temperature and precipitation events projected here could have dramatic impacts on human and natural systems. For instance, agricultural production, water storage, seasonal energy demands, catastrophic flood loss, and human mortality could all be substantially affected. Further, natural ecosystems could be severely impacted through changes in plant community composition and biogeography and increases in risks of extinction, invasion, and exotic disease.

After these alarming predictions, let's return to the Purdue news release which points out to some reasons for hope.

The model, Diffenbaugh said, assumes that greenhouse gases will attain a concentration more than twice their current levels, but he said he is confident that the model's performance gives as accurate a picture of the future as we can hope for at the moment.
Diffenbaugh emphasized that, while the model was in no way designed to return an alarmist image of our climate's future, the picture it painted should be considered.

Now, we can just hope that enough countries and industries will limit their greenhouse gases emissions and that the concentration of CO2 in the atmosphere will not be multiplied by two before the end of the century.

Sources: Purdue University news release, October 17, 2005; and various web sites

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