November 9, 2012
Climate Models With Higher Temps More Accurate Than Others
Alan McStravick for redOrbit.com — Your Universe Online
Regardless of which side of the issue you might stand on, regarding climate change and what is going to happen in the future, there is one thing we can all agree on: This summer was HOT! According to a new study funded by NASA, we may want to buckle in because apparently this ride is just beginning and its only going to get hotter.
According to analysis by scientists from the National Center for Atmospheric Research (NCAR), climate model projections that show a greater rise in global temperature are, more likely than not, more accurate than those models that show a lesser rise. Published in this week´s issue of Science, the findings could prove quite useful in narrowing the projected range of global warming over the next decades and even beyond.
The methods used by NCAR scientists John Fasullo and Kevin Trenberth, who co-authored the study, focused on observable data regarding the relative humidity in the tropics and sub-tropics and how well sophisticated climate models are able to reproduce that effect.
What Fasullo and Trenberth found was that the climate models that were most accurate at capturing the complex moisture processes and associated clouds were also the models that showed the greatest amounts of warming, as time progresses. These tropical and sub-tropical clouds and associated moisture have a major influence on our global climate.
"There is a striking relationship between how well climate models simulate relative humidity in key areas and how much warming they show in response to increasing carbon dioxide," Fasullo says. "Given how fundamental these processes are to clouds and the overall global climate, our findings indicate that warming is likely to be on the high side of current projections."
There is one important reason as to why one global climate model may differ from another. There are more than two dozen major global climate models, all of which are based on long-established physical laws known to guide the atmosphere. But, due to the difficulty of actually translating these relationships into software, each model differs slightly in its projection for our global climate. This is especially true regarding certain processes, like those that are associated with clouds, where the process itself is too small to be represented properly.
When scientists are comparing the different model projections, they look specifically for equilibrium climate sensitivity (ECS). ECS is the amount of warming that eventually occurs in a model when carbon dioxide is doubled over preindustrial values. We are seeing that the next doubling, at the current rate of global CO2 emission, will occur well before 2100.
What we do know is that for the better part of the last half-century, ECS in the leading models typically averages around 5 degrees Fahrenheit. When compared to the late 19th century values, this average provides the best estimate of global temperature increase that we can expect by the end of the current century. Of course, these models make the assumption that society is going to continue to emit significant amounts of carbon dioxide. The ECS average is derived from a pretty broad swing in projected temperatures from the individual models themselves. While some models have a projection of only 3 degrees Fahrenheit, others, on the higher end, show an increase in global temperature of 8 degrees Fahrenheit. This broad projected temperature range has created an air of uncertainty that has been difficult to bridge over the past three decades.
The most important reasoning that the differences must be brought closer together is because, as temperatures become higher, we will see greater impacts on the society at-large. These impacts would be most noticeable in the areas of sea level rise, heat waves, droughts and other threats. It is important to know just which model´s prediction of global temperature increase is most accurate to properly prepare for these possible eventualities.
The NCAR authors say that it is the clouds that are one of the main sticking points with resolving this issue. They point out that while satellites observe many types of clouds, it is the possibility of satellite failure, observing errors and other inconsistencies that make it most challenging to build a multi-year comprehensive global cloud census.
One thing satellites can do well, however, is measure water vapor. The overall estimation of the global distribution of relative humidity has become more reliable, as well. When global climate model projections have been put together, the relative humidity is taken into account for the generation and dissipation of clouds.
The researchers took our present climate and compared it, in relation to the relative humidity factors, against 16 of the leading climate models. They particularly focused on the subtropics. The subtropics are a global band on either side of the tropics that receive sinking air from the tropics. This produces very dry zones. Most of the global deserts are located in the subtropic bands.
"The dry subtropics are a critical element in our future climate," Fasullo says. "If we can better represent these regions in models, we can improve our predictions and provide society with a better sense of the impacts to expect in a warming world."
Observations have shown that the relative humidity in the dry zones has typically averaged between 15 and 25 percent. These observations contradict the projections of many of the models that believed the humidity would be at about 30 percent or higher for this period. Looking at the models that were more accurate at predicting the actual dryness, Fasullo and Trenberth saw their projections for global temperature rise for doubled carbon dioxide was actually more than 7 degrees Fahrenheit. Add to this the models that had the lowest ECS were also the least accurate in depicting the relative humidity in the dry zones.
"Because we have more reliable observations for humidity than for clouds, we can use the humidity patterns that change seasonally to evaluate climate models," says Trenberth. "When examining the impact of future increases in heat-trapping gases, we find that the simulations with the best fidelity come from models that produce more warming."
The authors focused on climate models used for the 2007 www.ucar.edu/atmosnews assessment by the Intergovernmental Panel on Climate Change. The next-generation models being used for the upcoming 2013 IPCC assessment were found to behave in a similar fashion, as described in a preliminary analysis by the authors in a supplement to their paper.
"In addition to providing a path forward and focus for improving models, results strongly suggest that the more sensitive models perform better, and indeed the less sensitive models are not adequate in replicating vital aspects of today's climate," write the authors in the paper.