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Study Finds Earth is Home to Three Trillion Trees

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There are just over three trillion trees in the world—well above previous estimates of around 400 billion—according to findings published yesterday in Nature.

The study was led by researchers at Yale University and is the most comprehensive enumeration of the planet’s forestation in history.

The tree density map was created using satellite imagery on climate, topography, and land use, as well as 429,775 ground-based measurements around the world. Together, the data helped researchers create models for predicting tree density. For any areas in which there were no field measurements, the team made estimates using information from areas with similar features.  

The 3.04 trillion total trees on Earth mean there are 422 trees for every person, and while that figure is larger than expected, the global population of trees is still in decline. The reports figure that vegetation has fallen by 45.8 percent since the arrival of humans, and continues to decrease by about 10 billion trees every year. About 15.3 billion trees are cut down annually, with regrowth and reforestation making up for some of that loss.

“We can now say that there’s less trees than at any point in human civilization,” lead author Thomas Crowther told The Washington Post. “Since the spread of human influence, we’ve reduced the number almost by half, which is an astronomical thing.”

The detailed look at the arbor population will be a useful guideline for future studies related to climate change, both in understanding how the trees affect shifts in climate, and in how environmental changes impact them.  


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13 Scientific Explanations for Everyday Life
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Science holds our lives together. It explains everything from why bread rises to why you need gas to power your car. In his book Atoms Under the Floorboards, author Chris Woodford lays out the abstract science that underlies the everyday world, from the big (how do skyscrapers stay up?) to the small (why does my laptop get hot when I’m watching Netflix?). Along the way, he also calculates the answers to whimsical questions like, “How many people would I have to gather together to keep my house warm without heat?” (A lot, but not as many as you'd think.) Here are 13 things we learned about the world through his eyes.

1. A POWER DRILL COULD SET YOUR HOUSE ON FIRE, IN THEORY.

Because of friction, electric drills generate heat. The motor, the drill bit, and the wall all get hot. It takes about 2000 joules of energy to heat one kilogram of wood just 1°C. Assuming a typical power drill uses 750 watts of electricity, and it puts out 750 joules of energy, Woodford calculates that it would take just four minutes to set fire to a wooden wall in a 68°F room.

2. STICKY NOTES COME OFF EASILY BECAUSE THEIR ADHESIVE IS UNEVEN.

Post-it Notes feature a plastic adhesive that is spread out in blobs across the paper. When you slap a Post-it onto your bulletin board, only some of these blobs (technically called micro-capsules) touch the surface to keep the note stuck there. Thus, you can unstick it, and when you go to attach it to something else, the unused blobs of glue can take over the adhesive role. Eventually, all the capsules of glue will get used up or clogged with dirt, and the sticky note won't stick anymore.

3. GUM IS CHEWY BECAUSE IT'S MADE OF RUBBER.

Early gums got their elastic texture from chicle, a natural type of latex rubber. Now, your bubble gum is made with synthetic rubbers like styrene butadiene (also used in car tires) or polyvinyl acetate (also used in Elmer’s glue) to mimic the effect of chicle.

4. OFFICE BUILDINGS ARE EVER-SO-SLIGHTLY TALLER AT NIGHT.

After all the employees go home, tall office buildings get just a little taller. A 1300-foot-tall skyscraper shrinks about 1.5 millimeters under the weight of 50,000 occupants (assuming they weigh about the human average).

5. A LEGO BRICK CAN SUPPORT 770 POUNDS OF FORCE.

LEGOs can support four to five times the weight of a human without collapsing. They are strong enough to support a tower 375,000 bricks tall, or around 2.2 miles high.

6. POLISHING SHOES IS LIKE FILLING IN A ROAD'S POTHOLES.

Regular leather appears dull to the eye because it’s covered in teeny-tiny scrapes and scratches that scatter whatever light hits the material. When you polish a leather shoe, you coat it in a fine layer of wax, filling in those crevices much like a road crew smoothes out a street by filling in its potholes. Because the surface is more uniform, rays of light bounce back toward your eye more evenly, making it look shiny.

7. YOU COULD HEAT YOUR HOUSE WITH JUST 70 PEOPLE.

People give off body heat, as anyone who has been trapped in a small crowded room knows. So how many people would it take to warm up your home with just body heat in the winter? About 70 people in motion, or 140 people still, figuring that humans radiate 100-200 watts of heat normally and that the house uses four electric storage heaters.

8. DENSITY EXPLAINS WHY COLD WATER FEELS COLDER THAN AIR AT THE SAME TEMPERATURE.

Because water is denser than air, your body loses heat 25 times more quickly while in water than it would in air at the same temperature. Water's density gives it a high specific heat capacity, meaning it takes a lot of heat to raise its temperature even a little, and it's very good at retaining heat or cold (the reason why hot soup stays hot for a long time, and why the ocean is much cooler than land). Water is a great conductor, so it's very effective at transferring that heat or cold to your body.

9. WATER CLEANS WELL BECAUSE IT HAS ASYMMETRICAL MOLECULES.

Because water molecules are triangular—made of two hydrogen atoms stuck to one oxygen atom—they have slightly different charges on their different sides, kind of like a magnet. The hydrogen end of the molecule is slightly positive, and the oxygen side is slightly negative. This makes water excellent at sticking to other molecules. When you wash away dirt, the water molecules stick to the dirt and pull it away from whatever surface it was on. This is also the reason water has surface tension: it’s great at sticking to itself.

10. THE "PULSE" SETTING ON A BLENDER WORKS BETTER BECAUSE OF TURBULENCE.

When your blender stops chopping up food and begins just spinning it around in circles, it’s because everything inside is spinning at the same rate. Instead of actually blending ingredients together, it’s experiencing laminar flow—all the layers of liquid are moving in the same direction with constant motion. The pulse function on the blender introduces turbulence, so instead of the fruit chunks rolling around the side of the blender, they fall into the center and get blended up into a smoothie.

11. BABIES' BODIES CONTAIN MORE WATER THAN ADULTS.'

Adults are around 60 percent water. By contrast, newborn babies are about 80 percent water. But that percentage quickly drops: A year after birth, kids' water content is down to around 65 percent, according to the USGS.

12. GLASS BREAKS EASILY BECAUSE ITS ATOMS ARE LOOSELY ARRANGED.

Unlike other solid materials, like metals, glass is made up of amorphous, loosely packed atoms arranged randomly. They can’t absorb or dissipate energy from something like a bullet. The atoms can’t rearrange themselves quickly to retain the glass’s structure, so it collapses, shattering fragments everywhere.

13. CALORIE COUNTS ARE CALCULATED BY INCINERATING FOOD.

Calorie values on nutritional labels estimate the energy contained in the food within the package. To figure out how much energy is in a specific food, scientists use a calorimeter. One type of calorimeter essentially burns up the food inside a device surrounded by water. By measuring how much the temperature of the water changes in the process, scientists can determine how much energy was contained in the food.

This story originally ran in 2015.

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How Does Blood Pressure Work?
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Your heart is the master pump for all the blood in your body. With every heartbeat, your heart pushes your blood to all the vital parts of your body, such as muscles and bones, through a network of arteries, capillaries, and veins. As blood flows through the tube-like arteries, it presses up against the walls of the blood vessels with varying degrees of strength. The strength or weakness of this pressure is called your blood pressure (BP).

Each time your heart squeezes, moving your blood to its various destinations, your blood pressure goes up—this number is referred to by a blood pressure reading as systolic. Then, as the heart relaxes after each contraction, your blood pressure goes down; that is called the diastolic reading. Together, these two numbers are presented as a score, systolic over diastolic: Your doctor might tell you that your BP is “120 over 80.”

According to the American Heart Association (AHA), normal blood pressure should reflect systolic pressure between 90 and 120, over a diastolic pressure between 60 and 80. Your doctor may take this measurement with a fancy-named instrument called a sphygmomanometer—an inflatable rubber cuff attached to a manual air pump. When the doctor inflates the cuff at your arm with air, it temporarily cuts off blood flow, and when it releases, the blood starts flowing again, revealing those two key numbers.

Nowadays, though, doctors are recommended to use an automatic blood pressure cuff, which relies on a different method and seems to be more accurate. While the manual cuff relies on auscultation, in which the doctor listens for the correct pressures using a stethoscope/microphone, automatic blood pressure cuffs are usually oscillometric. When blood passes under the cuff, the arm increases in circumference ever so slightly. And by measuring the amplitude of the oscillations (hence oscillometric) at a continuous interval of pressures, blood pressure can be calculated in much the same way.

If you have high blood pressure, a.k.a. hypertension—approximately 130/80 or higher in a person of average health—your heart is working too hard to pump the blood through your body, which becomes dangerous. According to the AHA [PDF], elevated blood pressure is 120–129/less than 80; hypertension stage 1 is 130–139 (systolic) or 80–89 (diastolic); and hypertension stage 2 is 140 or higher (systolic) or 90 or higher (diastolic). If your blood pressure hits 180/120, you're in hypertensive crisis, and you should get help.

If you fall into the above categories, your doctor will recommend changes to diet and exercise and probably medication. High blood pressure is often a precursor to heart disease or a heart attack and can be a side effect of other diseases, such as diabetes. However, your blood pressure can temporarily rise due to stress, pregnancy, and even some common medications, including over-the-counter pain relievers and antidepressants. One high reading will not necessarily mean you have hypertension—but it’s good to keep vigilant.

Editor's note: This story was updated in July 2018 to reflect new blood pressure guidelines from the AHA.

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