The Icy Nenana River

Guest Post by Willis Eschenbach

The last time I was in Alaska, I had the good fortune to stop by the town of Nenana, home of the Nenana River Ice Classic. Nenana sits at the junction of the Tanana and Nenana Rivers. The dates of the ice breakup at Nenana form one of the longest-term modern temperature proxies in the area, extending back unbroken to 1917. Figure 1 shows my photo of the tripod which is set out on the ice to determine the exact tim of the breakup.

Figure 1. The tower and the tripod. The tripod is placed out on the ice before the breakup. When the ice breaks up, the tripod falls, tripping the clock. These days the tripod is actually a quadripod, or perhaps a quadruped.

Every year people pay money to bet on the exact time of the breakup, with the winner taking the pot. At present, the pot is $318,500 …

There’s a recent WUWT post by Psalmon about the Ice Classic here. Although I’d written about it previously, there were a couple of things I didn’t understand about Nenana until I’d visited the place.The first was the reason the ice breakup was so important. It was critical because both then and now, the river is navigable, and becomes a main highway for people and supplies during the summer. Until the breakup, little villages and cabins and camps along the river can’t get their supplies or travel by water. Although this is less important now with the advent of highways, there still are many places along the river that can only be reached by traveling along the river. That made the breakup a huge event in the old days.

The second thing I didn’t understand was the reason why the breakup was so sudden and complete. The map shows the Tanana River and surroundings:

Figure 2. The Tanana River and its tributaries flow north to the Yukon River. The Yukon flows from the top center to the upper left of the figure, with a portion appearing dark blue.

The reason the breakup comes suddenly is that unlike most US rivers, the rivers around Nenana are flowing north. As a result, the more southerly upriver parts of the drainage would tend to melt earlier. At some point this increasing upstream meltwater will put pressure on the downriver ice, and as the ice at Nenana rots and melts, the whole thing will break and collapse at once.

Now, you’d think that the river breakup dates would be a perfect temperature proxy. After all, urban warming surely won’t be an issue. However, nature always sides with the hidden flaw, so of course there is a confounding factor—rain. Rain can hasten the breakup significantly by melting the ice from the top. In addition to starting out warmer than the ice, rainwater pools have less albedo than ice, so they warm more for a given amount of sunlight. Rain also increases the volume of water flowing in the river, so it puts additional pressure on the ice. As a result, the breakup dates form the usual imperfect proxy for temperature.

Given all of that, here are the inverse dates of ice breakup since the Classic was first run in 1917.

Figure 3. Nenana ice breakup dates since 1917. The result for 2013 will be equal to or greater than today, May 15th. Blue dots show the standard error of the Gaussian average at the endpoint in 2013.

For me, it is clear that what we are seeing are the effects of the Pacific Decadal Oscillation. This is a slow, decades long cycle in the heat distribution in the Pacific Ocean. In about 1945 the PDO shifted to the cool phase, then went back to the warm phase in about 1975, and has recently switched again to the cool phase.

Overall? I’d say there’s not a whole lot to see in the ice records. Temperatures went up a bit, down a bit, up a bit, and appear to be on their way back down again … be still, my beating heart, it’s all too exciting.

I leave you with the current photo from the Nenana Icecam, at 8:47 Alaska Daylight Time May 15, still no breakup, but goodness, it’s a lovely spring day in Alaska … makes my heart leap just to look at it.