It turns out that, when it comes to the warming of the Arctic Ocean, it’s not just how much ice that matters, but also how thick it is. A new study published last week in the Annals of Glaciology suggests that the way we’ve been thinking about the relationship between the ocean surface waters, the ice and the sun’s radiation doesn’t tell the whole story. The researchers from Japan, the U.S. and Canada, examined observed ice cover, thickness, water temperature and salinity in the Arctic Ocean during the summers of 2006 and 2007.

Sea ice in the High Arctic. The ice reflects the sun, but its thickness makes a difference.

Here’s the context:

Most of these kinds of concepts are developed first by making observations and measurements of all the pertinent conditions, then trying to create mathematical models based on specific assumptions of what the mechanisms are. The assumptions are then tested by seeing whether the model gives results that are consistent with what’s seen in the real world. When the models do a good job of generating results like the ones you measure in the real world, then you can begin talk about what might happen under various conditions—if factor X (e.g., ice thickness) changes, what will happen to factor Y (e.g., sea surface temperature)?

It’s important to note that just because a model accurately “predicts” what we’re observing in the field that doesn’t mean we’ve discovered “how it all works”. Researchers will continue to test models against new observed data and make changes to the way the models work, usually making them more complex with more variables and better precision. For example, while we all like to complain about the accuracy of our local weather reports, ask your parents whether they are more or less accurate now than 30 years ago. Our weather prediction models have become quite refined, more accurate and reliable because they are consistently tested and tweaked as we compare observations to predictions and our understanding of the mechanisms involved grows.

Which brings us back to the current study.

There has been a long-observed concept known as the ice-albedo feedback. The basic idea is that the sun warms the surface of the ocean, but where sea ice exists, the sun’s radiation bounces off the ice back into the atmosphere the way a mirror reflects light. The more ice, the more reflection; the more reflection, the less heat reaching the ocean’s surface, and the slower the ocean warms.

The corollary is that as the extent of sea ice begins to decrease, more water is exposed; more exposed water means less reflection and more heating; more heating causes faster melting ice exposing more water, causing more heating and so on. That’s the feedback mechanism (albedo refers to the reflection of the heat off the ice). There are various mathematical models that are used to try and predict the outcome under various potential ice cover situations.

The current study picks up where standard ice-albedo feedback models leave off by considering the thickness of the ice, not just how much of it there is. Through a series of observations and measurements, the researchers determined that thinner ice, which tends to develop ponds on top (see below) actually appears to transmit a significant amount of the sun’s radiation down to the sea surface. They then took a standard ice-albedo feedback model and modified it to account for ice thickness as well as amount of open water in order to test assumptions about how this works.

The scientific take-home message from their work is that we should probably be paying more attention to the thickness of the ice when considering how the sun’s radiation will affect ice melt. We’re pretty confident that the ice-albedo feedback accelerates the melting of the ice but it’s probably not enough to simply look at how much ocean is covered with ice.

Thinner ice does appear to also accelerate melting. That’s not necessarily good news, as the average age (and therefore thickness) of sea ice in the Arctic has been decreasing rapidly. The authors suggest that even if the total amount of ice cover does not change, ice melt in the Arctic will continue to accelerate due to the decreasing percentage of older, thicker ice.

The take-home messages for the rest of us:

While so much discussion in the news focuses on how much ice there is at any point in time as a key indicator of the current and predicted state of the Arctic Ocean, it’s much more complex than that. The scientific community is actively working to build and refine the knowledge base about just how complex it all is. One thing we can say for sure, though, is there is no single factor—ice extent, ice thickness, average temperature, amount of snowfall or anything else—that on its own tells the whole story. And that’s always worth keeping in mind while surfing the sea of soundbites, headlines and opinions associated with global warming, climate change and the Arctic.

For a previous post on the many scientific descriptions of sea ice click here.

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