Connect with us

Science

Americans have Texas-sized carbon footprints—here’s why

Published

on

Enlarge / Fairgoers gather at Big Tex at the State Fair of Texas in 2018. Based on American data, presumably some not-insignificant portion of fairgoers traveled to Dallas in not-the-most-fuel-efficient of vehicles.

Greenhouse gas emissions are most commonly reported at the national level, which tends to make us compare nations to other nations. This makes some sense, as national policy can significantly influence emissions trends. But it’s easy to forget that borders are just lines on a map, and some lines have considerably more people inside them than others. The citizens of Luxembourg don’t ensure their country’s low carbon emissions because they’re lightyears ahead of the people of China in terms of efficiency—there are just a whole lot fewer of them.

In order to make more meaningful comparisons, you obviously have to calculate emissions per person. And when you do that, the United States really sticks out. (As does Luxembourg, by the way.) It’s not surprising that per capita emissions in the United States are much greater than in India, where millions of people still lack electricity. But why are they also much greater than in the wealthier Western nations in Europe?

To answer that question, we need to do more than divide a national total by population. We need to break down the contributions to a person’s carbon footprint—the emissions behind the things we buy and do. Doing that in a detailed way is a challenge, and researchers haven’t been at it that long. “A lot of the research that’s been done has been done quite quickly [with] available data and resources,” UC Berkeley’s Chris Jones told Ars, “And there really is a lot of work to do.”

Some of that research breaks national or (regional) economies into sectors to look at what comes in and goes out in each category. That also helps account for things like moving your manufacturing to another country. Other studies try to work from the bottom up, getting people to fill out surveys or even log diaries of their purchasing habits—an interesting but labor-intensive (and expensive) source of data.

A third approach combines more types of data into an econometric model, applying demographic information to the big economy-scale stuff. “And then once you’ve done that, kind of the cool thing is you can now scale emissions to any location within countries,” Jones said. With data on things like home size, power plants, climate, and vehicle ownership, a more recognizable carbon footprint can be calculated for the community around you.

But any way you estimate it, the average American carbon footprint is in the ballpark of double that of the average European. (For the purposes of this article, “Europe” will refer to the pre-Brexit EU group of nations.) This huge difference is sometimes brushed off as a simple consequence of Europe’s greater population density. It’s not that simple, though—even the efficient high-rise cores of the biggest US cities can barely dip into the range of European national averages. So what exactly is it about American homes, communities, and behavior that adds up to explain this gap?

Vertical axis shows emissions per person, while the horizontal axis shows population for that country or group of countries.
Enlarge / Vertical axis shows emissions per person, while the horizontal axis shows population for that country or group of countries.

Excuse me?

In 2014, Chris Jones and his colleague Daniel Kammen published a map of US household carbon footprints by ZIP code based on that econometric method—the most detailed estimate out there. The US is pretty varied in a number of ways, which produces some regional patterns.

Emissions from electricity use are higher in areas that still had coal-dominated grids at the time, while home heating is a much larger factor in Northern states with colder winters. But most interesting is the pattern within and around major cities.

The map estimates the average household carbon footprint for every ZIP code in the Lower 48.
Enlarge / The map estimates the average household carbon footprint for every ZIP code in the Lower 48.

The average American household footprint is a little below 50 tons of CO2 per year (actually “tons of CO2-equivalent” to include other greenhouse gases), but that number can drop to around 30 in city centers—closer to the average of a country like Germany. American cities are surrounded by sprawling suburbs, though, which swing very much in the opposite direction, going as high as 80 tons.

There are a number of reasons for that, including much larger homes and lengthy commutes. But even if European cities tend to trade suburban sprawl for a more concentrated area of medium density, it’s not like Germany is a continuous megacity. Even US ZIP copes with considerably higher average population densities than European nations have higher household carbon footprints. What’s going on here?

Is it the weather? Maybe Americans have to contend with colder winters and hotter summers? Not so. Heating and cooling demand is calculated in terms of “heating degree days” and “cooling degree days”—the number of days your thermostat kicks on combined with the difference in temperature inside and outside. On average, the United States does have more cooling degree days in the summer than Europe’s average. But Europe has a larger lead in heating degree days. So if anything, it’s European homes that have the larger heating/cooling demand, on balance.

(As an interesting sidenote, heating degree days are decreasing and cooling degree days are increasing as Earth’s climate warms—and by more than you might think. Europe’s average heating degree days have dropped about 15 percent since 1980. California’s data, as an example, looks similar. And because home cooling runs on cleaner energy than heating in California, the net effect of climate change has reduced the average footprint a bit.)

What about industry? If the EU outsourced more of its manufacturing and materials industry, that might make it look cleaner than it really is. In reality, the US and Europe are relatively similar in this regard. If you go back to the national emissions chart above, the dashed lines show the effect of adjusting for imports. That boosts US and EU per capita emissions by about the same amount (while decreasing China’s). And carbon-footprint estimates largely capture this, anyway, by assigning you the emissions associated with your consumption of goods and services.

Perhaps it’s the energy mix? After all, a cleaner grid would shrink your footprint even with no change in behavior. But that doesn’t explain much here, either—at least when comparing the averages for these two regions.

There are exceptions at smaller scales, like France’s nuclear-dominated grid or Switzerland’s grid, which is powered almost entirely by hydroelectric dams and nuclear power. And in the United Kingdom, the rapid replacement of coal with renewables over the last decade has reduced per capita emissions considerably. UK households used to have an above-average carbon footprint for the EU; that’s no longer true.

Continue Reading

Science

What happens if a space elevator breaks

Published

on

TCD | Prod.DB | Apple TV+/ | lamy

In the first episode of the Foundation series on Apple TV, we see a terrorist try to destroy the space elevator used by the Galactic Empire. This seems like a great chance to talk about the physics of space elevators and to consider what would happen if one exploded. (Hint: It wouldn’t be good.)

People like to put stuff beyond the Earth’s atmosphere: It allows us to have weather satellites, a space station, GPS satellites, and even the James Webb Space Telescope. But right now, our only option for getting stuff into space is to strap it to a controlled chemical explosion that we usually call “a rocket.”

Don’t get me wrong, rockets are cool, but they are also expensive and inefficient. Let’s consider what it takes to get a 1-kilogram object into low Earth orbit (LEO). This is around 400 kilometers above the surface of the Earth, about where the International Space Station is. In order to get this object into orbit, you need to accomplish two things. First, you need to lift it up 400 kilometers. But if you only increased the object’s altitude, it wouldn’t be in space for long. It would just fall back to Earth. So, second, in order to keep this thing in LEO, it has to move—really fast.

Just a quick refresher on energy: It turns out that the amount of energy we put into a system (we call it work) is equal to the change in energy in that system. We can mathematically model different types of energy. Kinetic energy is the energy an object has due to its velocity. So if you increase an object’s velocity, it will increase in kinetic energy. Gravitational potential energy depends on the distance between the object and the Earth. This means that increasing an object’s altitude increases the gravitational potential energy.

So let’s say you want to use a rocket to increase the object’s gravitational potential energy (to raise it to the right altitude) and also increase its kinetic energy (to get it up to speed). Getting into orbit is more about speed than height. Only 11 percent of the energy would be in the gravitational potential energy. The rest would be kinetic.

The total energy to get just that 1-kilogram object into orbit would be about 33 million joules. For comparison, if you pick up a textbook from the floor and put it on a table, that takes about 10 joules. It would take a lot more energy to get into orbit.

But the problem is actually even more difficult than that. With chemical rockets, they don’t just need energy to get that 1-kilogram object into orbit—the rockets also need to carry their fuel for the journey to LEO. Until they burn this fuel, it’s essentially just extra mass for the payload, which means they need to launch with even more fuel. For many real-life rockets, up to 85 percent of the total mass can just be fuel. That’s super inefficient.

So what if, instead of launching atop a chemical rocket, your object could just ride up on a cable that reaches all the way into space? That’s what would happen with a space elevator.

Space elevator basics

Suppose you built a giant tower that is 400 kilometers tall. You could ride an elevator up to the top and then you would be in space. Simple, right? No, actually it’s not.

First, you couldn’t easily build a structure like this out of steel; the weight would likely compress and collapse the lower parts of the tower. Also, it would require massive amounts of material.

But that’s not the biggest problem—there’s still the issue with speed. (Remember, you need to move really fast to get into orbit.) If you were standing on the top of a 400-kilometer tower with the base somewhere on the Earth’s equator, you would indeed be moving, because the planet is rotating—this is just like the motion of a person on the outside of a spinning merry-go-round. Since the Earth rotates about once a day (there’s a difference between sidereal and synodic rotations), it has an angular velocity of 7.29 x 10-5 radians per second.

Angular velocity is different than linear velocity. It’s a measure of rotational speed instead of what we normally think of as velocity—movement in a straight line. (Radians are a unit of measurement to use with rotations, instead of degrees.)

If two people are standing on a merry-go-round as it spins, they will both have the same angular velocity. (Let’s say it’s 1 radian per second.) However, the person that is farther from the center of rotation will be moving faster. Let’s say one person is 1 meter from the center and the other person is 3 meters from the center. Their speeds will be 1 m/s and 3 m/s respectively. This same thing works with a rotating Earth. It’s possible to get far enough away such that the Earth’s rotation gives you the required orbital velocity to stay in orbit around the planet.

So let’s go back to our example of a person standing on the top of a 400-kilometer tower. Are they far enough away from Earth that they can stay in orbit? For one complete rotation of the Earth, their angular velocity would be 2π radians per day. That might not seem very fast, but at the equator this rotation gives you a speed of 465 meters per second. That’s over 1,000 miles per hour. However, it’s still not enough. The orbital velocity (the velocity needed to stay in orbit) at that altitude is 7.7 kilometers per second, or over 17,000 miles per hour.

Actually, there’s another factor: As you increase your distance from the Earth, the orbital velocity also decreases. If you go from an altitude of 400 to 800 kilometers above the surface of the Earth, the orbital speed decreases from 7.7 km/s to 7.5 km/s. That doesn’t seem like a large difference, but remember, it’s really the orbital radius that matters and not just the height above the surface of the Earth. Theoretically, you could build a magical tower that was high enough that you could just step off of it and be in orbit—but it would have to be 36,000 kilometers tall. That’s not going to happen.

Continue Reading

Science

Study: Leidenfrost effect occurs in all three water phases: Solid, liquid, and vapor

Published

on

Slow-motion video of boiling ice, a research project of the Nature-Inspired Fluids and Interfaces Lab at Virginia Tech.

Dash a few drops of water onto a very hot, sizzling skillet and they’ll levitate, sliding around the pan with wild abandon. Physicists at Virginia Tech have discovered that this can also be achieved by placing a thin, flat disk of ice on a heated aluminum surface, according to a new paper published in the journal Physical Review Fluids. The catch: there’s a much higher critical temperature that must be achieved before the ice disk will levitate.

As we’ve reported previously, in 1756, a German scientist named Johann Gottlob Leidenfrost reported his observation of the unusual phenomenon. Normally, he noted, water splashed onto a very hot pan sizzles and evaporates very quickly. But if the pan’s temperature is well above water’s boiling point, “gleaming drops resembling quicksilver” will form and will skitter across the surface. It’s called the “Leidenfrost effect” in his honor.

In the ensuing 250 years, physicists came up with a viable explanation for why this occurs. If the surface is at least 400 degrees Fahrenheit (well above the boiling point of water), cushions of water vapor, or steam, form underneath them, keeping them levitated. The Leidenfrost effect also works with other liquids, including oils and alcohol, but the temperature at which it manifests will be different. 

The phenomenon continues to fascinate physicists. For instance, in 2018, French physicists discovered that the drops aren’t just riding along on a cushion of steam; as long as they are not too big, they also propel themselves. That’s because of an imbalance in the fluid flow inside the Leidenfrost drops, acting like a small internal motor. Large drops showed a balanced flow, but as the drops evaporated, becoming smaller (about half a millimeter in diameter) and more spherical, an imbalance of forces developed. This caused the drops to roll like a wheel, helped along by a kind of “ratchet” effect from a downward tilt in the same direction the fluid in the droplet flowed. The French physicists dubbed their discovery a “Leidenfrost wheel.”

In 2019, an international team of scientists finally identified the source of the accompanying cracking sound Leidenfrost reported. The scientists found that it depends on the size of the droplet. Smaller drops will skitter off the surface and evaporate, while larger drops explode with that telltale crack. The culprit is particle contaminants, present in almost any liquid. Larger drops will start out with a higher concentration of contaminants, and that concentration increases as the droplets shrink. They end up with such a high concentration that the particles slowly form a kind of shell around the droplet. That shell interferes with the vapor cushion holding the drop aloft, and it explodes when it hits the surface.

And last year, MIT scientists determined why the droplets are propelled across a heated oily surface 100 times faster than on bare metal. Under the right conditions, a thin coating formed outside each droplet, like a cloak. As the droplet got hotter, minuscule bubbles of water vapor began to form between the droplet and the oil, then moved away. Subsequent bubbles typically formed near the same spots, forming a single vapor trail that served to push the droplet in a preferred direction. 

But can you achieve the Leidenfrost effect with ice? That’s what the Virginia Tech team set out to discover. “There are so many papers out there about levitating liquid, we wanted to ask the question about levitating ice,” said co-author Jonathan Boreyko. “It started as a curiosity project. What drove our research was the question of whether or not it was possible to have a three-phase Leidenfrost effect with solid, liquid, and vapor.”

Continue Reading

Science

Two cannabinoids have opposing effects on SARS-CoV-2 in culture

Published

on

Enlarge / Don’t try this at home. Seriously. We mean it.

Over the course of the COVID-19 pandemic, researchers have tested a wide range of drugs to see if they inhibit the virus. Most of these tests didn’t end up going anywhere; even the few drugs that did work typically required concentrations that would be impossible to achieve inside human cells. And a few (looking at you, ivermectin and chloroquine) took off with the public despite iffy evidence for effectiveness, seemingly causing nearly as many problems as they would have solved if they actually worked.

Nevertheless, two years on, word of yet another one of these drug experiments caused a bit of a stir, as the drug in question was a cannabinoid. Now, the full data has gone through peer review, and it looks better than you might expect. But the number of caveats is pretty staggering: the effect is small, it hasn’t been tested in patients, the quality assurance of commercial cannabidiol (CBD) products is nearly nonexistent, and—probably most importantly—another cannabinoid blocks the effect entirely.

With that out of the way, on to the data.

Why test cannabinoids?

One of the big focuses of the drug testing was to look for chemicals that were already approved for use in humans, which would simplify their use as treatments for a separate disorder since all the safety data should be available already. And CBD is approved for use in people with seizure disorders, although the biochemical basis of its effectiveness is unclear.

In any case, the researchers behind the new work (primarily at the University of Chicago) started with lung cancer cells that produce the protein that SARS-CoV-2 uses to infect cells and dumped both the virus and CBD on the cells. And it worked. At non-toxic doses, the reproduction of the virus was strongly inhibited by CBD. The team went on to confirm the result in other lung cell lines. They also demonstrated that a partly metabolized derivative had a similar effect, but a range of additional cannabinoids did not.

And this is where we get to one of the downsides. THC, the most potent mind-altering substance in cannabis, did not have an effect on its own. But when given at the same time as CBD, it reversed CBD’s inhibition of viral growth. So simply trying to use cannabis for viral protection will fail pretty miserably.

In any case, this is where the work starts to move beyond the hundreds of similar “let’s throw drugs on some cells” studies that have been done: the researchers do their best to figure out how CBD works. They checked whether it stopped human cells from producing the protein that the virus latches onto when infecting them, but that wasn’t the cause. And they confirmed that viruses could still get inside cells by using the SARS-CoV-2 spike protein.

But once the virus gets inside, not a lot seems to happen. Very little of the spike protein gets made in infected cells treated with CBD, and levels stay low for up to 15 hours after infection.

Continue Reading

Trending