Monday, May 30, 2011

Will the warmer North Dakota be drier or wetter?

Like most areas across the globe North Dakota is currently in the midst of a long-term warming trend. It is well-established that warming will continue because of human activities. Over the 21st century even if we seriously curb our warming greenhouse gas emissions we face noticeable additional warming. Continuing with the carefree business-as-usual is playing a game with no winning results.

What about local effects? Global average temperature numbers like linked above have use, but the driving reason behind using the term "climate change" rather than "global warming" is that the changes are not simply that the temperature is however many degrees higher everywhere. The surface temperature change is and will be variable (such as land areas warming much more than ocean and the Arctic warming most extreme of all) and temperature is not the only thing that changes.

With the last couple decades of the rising Devils Lake and and many historically high flooding levels, in this region we obviously know how impactful long-term shifts in precipitation are. Is the current wet period a symptom of climate change and something that will continue and worsen?

Though obviously more relevant, these questions about much smaller scale are more difficult to answer. These are different beasts, but similar to how weather forecasts can readily say there will be a system passing through the area but typically not for certain if it will bring precipitation where you are much less exactly how much, we can say with great certainty there will be changes to climate but not exactly what they will be for, say, North Dakota. Regional impacts is an area of much interest these days because of the importance and how much we do not know on it at this point.

Many people, thinking of how deserts are hot and dry, may initially assume that warming means it simply will be drier. With warming indeed you expect more evaporation. Yet, moisture that evaporates from somewhere will return to the surface as precipitation somewhere. Maybe at a given area you get more evaporation, but maybe you get even more precipitation. Another factor to consider is changes in the general flow of weather patterns (e.g, through shifts in the jet stream). Maybe you get more storm systems and resulting precipitation, but maybe you get less.

Yet another issue to consider besides how much precipitation occurs is when it occurs. There could be no change in the annual average, but if the distribution through the year changes significantly that can have major impacts. Average annual precipitation for the Grand Forks area is about 20 inches (counting the snow amount in liquid equivalent). Almost half the amount falls in June-August. Suppose we kept the same annual average but lost half the amount during summer, balancing it by increasing the winter amount. Agriculture would likely suffer during those warmer and drier months, and the greater cold season precipitation would mean worse flooding in spring. Or suppose that the summer rain amount remains the same but is compiled over much fewer days with precipitation but having much heavier amounts.

Climate models, though often maligned by people who do not understand them, allow us to investigate what is likely to happen.

The Interior Department Bureau of Reclamation produced a recent report analyzing impacts to water supplies in the western US (pdf) under the expected changing conditions of the 21st century. The work separately examined various western river basins and found the warming of 5-7 degrees Fahrenheit area-wide but found that farther southwest drying and decreased runoff were likely while up like in the Missouri River basin the evidence suggests increases in the annual average precipitation.

A central facet of this analysis was compiling the results from 112 comparable climate model simulations and surveying the results. Below are figures 2 and 3 from the report which show the median change in temperature (in deg F) and precipitation (in % of annual average) comparing projected 2070-2099 to historical 1950-1979. The median value is likely very similar to the average, but by taking the median value - the middle value when all the values are ranked in order - extreme outlier values can have much less effect on the result than they could by computing the average.

Yet even with more precipitation on average per year, overall conditions could be such that they would not be "wetter". A recent review by Dai focused on drought over the 21st century with continued climate change. Cutting to the chase, the breadbasket of North America, including North Dakota, could face almost unimaginable drought conditions by within about 50 years.

There may be difficulty using the Palmer Drought Severity Index or variations on it for such projections as Dai does. The point still applies that even with more precipitation in a year, it can still be drier with increased evaporation because of warming and if the distribution of that precipitation changes such that more ends up as runoff.

Rather than have a dismissive attitude because of the uncertainty about how North Dakota and this region will be affected, that should be cause for even greater concern. We do not have the issue of mountain snowpack building during winter and releasing at the right rate during spring and summer, but we do have worries here when it comes to flooding and agriculture. There are a lot of changes that can mean negative results. Seems like something to try to avoid rather than speed toward faster and faster.

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