Fight or flight: can a tiny bird that breeds in greenland win the battle against climate change? Teens at a California school get the scoop from their adventurous science teacher

In the meantime, Mara and her classmates use what they've learned from their teacher's trip to carry out experiments in a small stream two blocks from their school. There, they measure the health of the stream by identifying macroinvertebrates (animals without backbones that can be seen without a microscope). Pella-Donnelly is quick to show the students how their work relates to Harding's. "The [zooplankton] that [Harding] is looking for are an indicator of the ocean's temperature," Pella-Donnelly says. "In our stream, the macroinvertebrates are an indicator of the pollution in the water." You don't need to travel to Greenland to see how humans are affecting the environment, she says.

GO WITH THE FLOW

Differences in water density drive a system of currents that winds around the globe, in this so-called great ocean conveyor belt, dense, cold, salty water sinks and flows south along the ocean floor toward the equator and is relaced by warm, less-salty surface waters flowing north. Climate change could alter the density of these currents and shut down the conveyor.

HANDS-ON SCIENCE (No Lab Required)

(PREDICT)

Salinity (saltiness) and temperature determine water's density, or mass per unit volume. Seawater either sinks or moves up toward the ocean's surface depending on its density. The movement of seawater with different densities powers the great ocean conveyor belt. Which is denser: salt water or fresh water?

MATERIALS

large pan * 2 baby food jars * yellow food coloring * blue food coloring * water * permanent marker * salt * tablespoon * index card

DIRECTIONS

1> Fill two baby food jars almost to the top with water. Leave just enough room so that you can stir the liquid without spilling it.

2> Add three drops of yellow food coloring to one jar and label it "fresh water." Add three drops of blue food coloring to the other jar and label it "salt water." (Caution: Some food coloring can stain clothing.)

3> Set the salt water jar in the center of your pan.

4> Put one tablespoon of salt into the salt water jar and stir.

5> Place the index card over the opening of the fresh water jar.

6> with the index card firmly pressed against the fresh water jar's rim, turn the jar directly on top of the salt water jar, with the index card in between. (Note: Do this over the pan to catch any water that may leak out.)

7> Have your lab partner hold the salt water jar while you carefully slide out the index card. Keep the rims of the two jars tightly aligned so that water does not leak out once the card is removed.

8> Observe what happens to the blue water and yellow water.

9> Keeping the jars aligned, have one person pick up the jars and simultaneously flip them both over. The fresh water jar should now sit in the pan with the salt water jar on top of it.

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10> Observe what happens to the water.

CONCLUSIONS

1> What happened when you removed the index card from between the two jars?