Thursday, March 12, 2015

Climate Models and Drought

From NASA Video

NASA has just begun collecting data from the SMAP (Soil Moisture Active Passive) mission satellite. The SMAP mission will map the entire globe every two to three days for at least three years and provide the most accurate and highest-resolution maps of soil moisture ever obtained. The spacecraft will orbit Earth once every 98.5 minutes and repeat the same ground track every eight days for at least three years.

This data will be used for weather forecasting, more accurate modeling and forecast of climate variability and change, planning and predicting agricultural productivity, effective water resources management, drought prediction, flood area mapping, and ecosystem health monitoring all require information on the status of soil moisture. Soil moisture affects plant growth and agricultural productivity, especially during times of drought or water shortages. This can improve our ability to monitor and forecast agricultural productivity and allow for a famine early warning in the most food insecure regions of the earth. Also it is hoped that the SMAP mission will provide missing data on the carbon cycle that has impacted the recent period accuracy of the climate models.

In the meantime Dr. Jason Smerdon from Columbia University’s Lamont-Doherty Earth Observatory and Dr. Benjamin Cook from NASA Goddard Institute for Space Studies in New York City and others have just published an article in “Science Advances” that used the existing climate models and tree ring data to predict drought in the next century. The study is based on projections from several climate models, including the model sponsored by NASA. How global warming will affect specific regions of the country is not really known but is essential for planning to meet the future.

Climate scientists have in the past predicted that in a warming world existing weather patterns will intensify, causing wet regions of North America to get wetter and dry regions to become drier. With the Southwest experiencing a drought that has lasted more than four years this is a “hot” topic and seemingly can’t wait for the data from the SMAP mission. Instead the scientists used a drought reconstruction that had been created using tree rings over the past 1,000 years. Using a metric called the Palmer Drought Severity Index the scientists reconstructed the history of droughts in North America.

While the Palmer Index is widely used because it can assess and calculate droughts from precipitation and temperature measurement alone, the simplicity of the Palmer Index has raised doubts in its accuracy. The use of the Palmer Index has been hotly disputed among hydrologists and climatologists. Drs. Cook and Smerdon back tested the Palmer Index on the period 1931-1990 for which the tree ring data and drought data are most complete. They found that the Palmer Index was a more reliable measure of soil moisture than many believed. However, the Palmer Index was developed in 1965, so this may be a case of data fitting-at least during that period when it was developed.

The scientists used 17 different climate models and created two projections for each model using different levels of atmospheric carbon dioxide (CO2) one at moderate emissions levels and one at high emission levels. The high emissions scenario projects the equivalent of an atmospheric carbon dioxide concentration of 1,370 parts per million (ppm) by 2100, while the moderate emissions scenario projects the equivalent of 650 ppm by 2100. Currently, the atmosphere contains 400 ppm of CO2.

In nearly every climate model, the Southwest and Midwest are predicted to have severe drying. The models all predict that continued increases in CO2 concentrations in the atmosphere drives up the risk of severe droughts in these regions. At the higher CO2 levels the droughts were forecast to last up to thirty years. The last period of mega droughts in North America was about 1,000 years ago and was caused by natural changes in climate.

As you will hear in the NASA video above, the models no doubt have weaknesses, but the fact that the models all yielded similar trends is worry some to California in the throes of a drought with no end in sight. The scientists say that there is an 80% likelihood that at least on multi-decade drought will hit the southwest between 2050 and 2100. Such severe droughts combined with poor management of limited water resources may end up driving people from their homes. Santa Barbara, California with its reservoirs falling below critical level is reopening their desalination plant. That was closed after the last drought passed. Desalination is expensive.

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