Climate model predicts 40 percent rise in US Southwest and Great Plains rainfall

However, if the build-up of carbon dioxide concentrations over the next century is reduced by one half in the model, the extra rain and snow disappears and the global temperature rises by only 1.5 degrees C.

The NCAR model simulated the earth’s climate from 1870 to 1990 and then continued the simulation to 2100 under two different scenarios. The first was a “business-as-usual” increase in greenhouse gases in which atmospheric carbon dioxide doubles over the next century. In the second, carbon dioxide increases are stabilized at 50% above today’s concentrations.

In the first projection, changes in precipitation vary markedly by region and by season. Within the US, the greatest increases occur in the Southwest and Great Plains in winter and substantially exceed the range of natural variability. Precipitation changes are reduced when carbon dioxide emissions are limited, according to the model.

Global average temperature climbs by 2 degrees C for “business as usual” and 1.5 degrees C when carbon dioxide emissions are limited. These changes are three to four times larger than the warming that has occurred since 1900. On the continental scale, carbon dioxide stabilization reduces climate warming over Eurasia more than over North America.

NCAR scientist Tom Wigley says, “These results show that we will experience not only future climate change, but also the results of policies to reduce these changes, in ways that are not simply related to changes in the global mean temperature. Policy decisions about reducing greenhouse emissions should not, therefore, be dictated by projected changes in global mean temperature alone.”

The model shows no clear separation between the business-as-usual and the stabilization cases until around 2060, even though the carbon dioxide concentrations begin to diverge in 2010. The half-century lag until the changes in greenhouse emissions begin to affect the climate noticeably is the result of large thermal inertia in the earth’s climate system, especially in the oceans, say the scientists.

The NCAR model is one of the world’s first global models not to require special corrections to keep the simulated climate from drifting to an unrealistic state. It is also one of only a handful of models in the world capable of realistically simulating the chemistry and transport of individual greenhouse gases and sulphur compounds.

NCAR claims the model employs a more realistic scenario for future emissions of sulphur dioxide, thought to have a cooling effect on the climate. Assuming that societies will take steps to reduce sulphur dioxide emissions over the next century, the scientists incorporated this decline into the model. The sulphur dioxide cooling effect gradually diminishes, allowing the simultaneous greenhouse warming to emerge more clearly.