Shrubby Tundra – Why Should We Care?

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President and Senior Scientist Dr. Eric A. Davidson

Shrubs are growing better in the tundra, thanks to climate change.  News that something is doing “better” sounds, well, “better” than the news we’ve been getting lately.  The shrubs are responding to a longer frost-free growing season and maybe more nutrients being released from the soil in a less harsh climate.  The shrubs also are gradually accumulating some carbon in their woody stems as they grow more abundant and taller.  In some respects, this is good news.

Unfortunately, deeper understanding from recent research points to undesirable implications that outweigh the good.  The shrubs are the first to stick up above the snow cover as snowmelt begins in the spring, and they become islands around which the snow melts more rapidly than over the short, sedge-dominated vegetation covering most of the tundra.  A tundra landscape without shrubs keeps a cover of snow much longer into the spring than does one with shrubs.  For those of us who grow tired of late winter snow, this may sound pretty good, but alas, it has a very important downside for the planet’s climate.

The huge expanse of tundra across Alaska, Canada, Scandinavia, and Russia is a highly reflective surface when it is covered with snow throughout most of the autumn, winter, and spring.  The bright, white, snowy surface reflects much of the sun’s energy back out to space.  When the snow melts early, this reflective surface is lost prematurely, so more of the sun’s energy is absorbed by the Earth.  If you are a shrub, that is a good thing, but it also means that the frozen permafrost soil will thaw more rapidly.  And that means that microbes living in the once-frozen soil will eat away the soil organic matter and convert the carbon that was stored there into carbon dioxide that goes back up into the air.  Loss of carbon from the permafrost exceeds gains of carbon in the stems of the shrubs.  The net effect is a double whammy for the Earth’s climate – it gets warmer because the snowy reflective surface is gone and it gets warmer because more heat-trapping carbon dioxide is going back into the atmosphere as permafrost thaws.  In other words, the warming that favors shrubs also feeds more warming.

Woods Hole Research Center scientists are doing cutting-edge research to measure how widespread this shrub expansion is, and what impacts it is having on permafrost.  Drs. Pieter Beck, Michael Loranty, Scott Goetz, and their collaborators have just published a paper in the scientific journal Arctic, Antarctic, and Alpine Research.  They show how shrubs can be mapped across northern Alaska using a combination of different types of data collected by satellites orbiting the Earth.  First, they use one kind of satellite image that is almost like a photo, zooming in for detailed analysis over relatively small regions, which allows the scientists to estimate shrub cover and height with pretty good accuracy.  They then use results from that detailed approach to “train” a computer to use another kind of satellite image that has less detail, but covers a larger area.  The result is a reliable map of shrub cover over most of northern Alaska.  Not only are they measuring area of shrub cover, but they are also identifying tall shrubs, which is very important for snowmelt.  Next steps would be to do the same for the entire arctic region.

We know from historical photos and studies conducted over decades at specific locations that shrub cover is expanding in the arctic due to climate change.  Our team’s novel work with satellite data will allow us to measure this expansion over much larger areas as the satellite analyses are repeated over time.  At the same time, we have other studies designed to measure changes in permafrost and how some of the carbon from thawing permafrost ends up in rivers and in the Arctic Ocean.

This is exciting science, but I still haven’t answered why we should care about shrubby tundra.  I’m writing this blog using battery power on my computer, because the winds of hurricane Irene have caused a power outage.  Others in Irene’s track have lost lives and property, including over a dozen deaths and millions of dollars of damage.  The hurricane started in the warm waters of the tropical Atlantic.  That is a long way from the arctic, but nevertheless, when the arctic absorbs more of the sun’s energy due to loss of reflective ice and snow cover during the spring and summer, and when carbon lost from thawing permafrost adds heat-trapping carbon dioxide to the atmosphere, then there is more energy trapped in the atmosphere to warm the tropical Atlantic during hurricane season.  To be sure, arctic shrubs didn’t cause a hurricane, but they are part of an interconnected world of multiple sources of energy to warm the oceans and to feed storms.  The arctic affects the tropics, and vice versa, just as humans affect the climate, and vice versa.

Scientists may never be able to blame any specific storm – Irene, Katrina, or any other – on human-caused climate change, but we can say unequivocally that climate change is loading the dice with respect to how frequently severe storms of various kinds occur.  Normal dice will roll snake eyes (a pair of ones) every once in a while, but loaded dice enable a cheater to roll snake eyes more often than chance alone would allow.  The devastating hurricane in New England in 1938 is like the occasional roll of snake eyes with normal dice.  In contrast, the unusually active and severe hurricane seasons of the last two decades are like living with loaded dice.  While the cheater benefits from loaded dice, someone else gets cheated.  Changing climate, due primarily to burning gas, oil, and coal, is cheating the many who lie in the paths of extreme weather events, which are occurring with greater frequency than chance alone could explain.  Today I write of hurricanes in the eastern US, but I could have also written of this summer’s record breaking and costly drought in Texas or last winter’s extreme snow storms.  The distant shrubby tundra is simply another example of how loading the climate dice has unexpected consequences, which come back to bite us in surprising ways and in distant places on this small planet.

Below is a map of shrub cover in northern Alaska derived from analysis of satellite data by Pieter Beck and colleagues, published in Arctic, Antarctic, and Alpine Research. Shrub cover on a background of elevation. Panels show the entire mapped area, which includes the North Slope and part of the Brooks Range.

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