Listen to part of a lecture in an environmental science class. [

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So since we're on the topic of global climate change and its effects,?

in alaska in the northern arctic part of alaska over the last oh,?

30 years or so.?

Temperatures have increased about half a degree C per decade.?

And scientists have noticed that there's been a change in surface vegetation during this time.?

Shrubs are increasing in the tundra. [

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Hunter is flatland with very little vegetation.?

Just a few species of plants grow there because the temperature is very cold and there is not much precipitation.?

And because of the cold temperatures, the tundra has two layers.?

Top layer, which is called the active layer.?

Is frozen in the winter and spring but thaws in the summer.?

Beneath this active layer is a second layer called permafrost, which is frozen all year round and is impermeable to water. [

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So because the permafrost, none of the plants that grow there can have deep roots can they know.?

And that's one of the reasons that trump survive in the arctic.?

Shrubs are little bushes, they're not tall and being low to the ground, protect them from the cold and wind.?

And their roots don't go very deep, so the permafrost doesn't interfere with their growth, okay??

Now since the temperatures have been increasing in arctic alaska,?

the growth of shrubs has increased.?

And this is presented climate scientists with a puzzle.?

Um, i'm sorry, when you see the growth of shrubs is increased, um, do you mean that the ships are bigger or that there are more shrubs??

Good question. And the answer is both.?

The size of the shrub has increased and shrub cover has spread to what was previously shrub free tundra. [

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Okay, so what's the puzzle??

Warmer temperatures should lead to increased vegetation growth, right??

Well, the connection is not so simple.?

The temperature increase has occurred during the winter and spring.?

Not during the summer.?

But the increase in troubles has occurred in the summer.?

So how can increase temperatures in the winter and spring result in increased shrub growth??

In the summer??

Well,?

it may be biological processes that occur in the soil in the winter because increased shrub growth in the summer.?

And here's how

there are microbes, microscopic organisms that live in the soil. [

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These microbes enable the soil to have more nitrogen which plants need to live and they remain quite active during the winter.?

There are two reasons for this.?

First,?

they live in the active layer which remember contains water that doesn't penetrate the permafrost.?

Second,?

most of the precipitation in the arctic

it is in the form of snow,?

and snow, which blankets the ground in the winter actually has an insulating effect on the soil beneath it.?

And it allows the temperature of the soil to remain warm enough for microbes to remain active. [

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So there's been increased nutrient production in the winter.?

And that's what's responsible for the growth of shrubs in the summer and their spread to new areas of the tundra.?

Areas with more nutrients are the areas with the largest increase in shrubs. But what about run off in the spring??

When the snow finally melts,?

won't the nutrients get washed away??

Spring thought always washes away soil, doesn't it??

Well, much of the soil is usually still frozen during peak runoff and the nutrients are deep down in the active layer anyway.?

Not high up near the surface, which is the part of the active layer most affected by runoff.?

But as I was about to say there is more to the story.?

The tundra is windy and snow is blown across, the tundra is caught by shrubs.?

And deep snow drifts often form around shrubs.?

And we've already mentioned the insulating effect of snow

so that extra warmth means even more microbial activity.?

Which means even more food for the shrubs.?

Which means even more shrubs and more snow around them, etc, it's a circle, a loop,?

and because of this loop, which is promoted by warmer temperatures in the winter and spring,?

well, it looks like the tundra may be turning into trouble and

but will it be long term??

I mean, maybe the shrubs will be abundant for a few years, and then it'll change back to tundra.?

Well, shrub expansion has occurred in other environments like semiarid grassland and tallgrass prairies.?

And tropic spanish and in these environments.?

Does seem to persist almost to the.of causing a shift.?

Once it's established shrub thrives,?

particularly in the arctic,?

because arctic shrubs are good at taking advantage of increased nutrients in the soil.?

Better than other arctic plants.?

Listen to part of a lecture in an environmental science class. [