The price of bitcoin has been rising so fast that it has become almost impossible to write about it. Any lede mentioning a new record high must be amended within a matter of hours to account for a new record, or, alternatively, for a sudden plummet. The publicity also caused a different sort of analysis of bitcoin, namely its environmental impact. Are there real world costs to using this purely digital currency? Bitcoin may not have physical weight, nor require iron bars to lock it away, but some argue that the bitcoin mining, storing, and eventual transaction requires an increasing amount of processing power and in turn, increasing amounts of energy. Others, however, say that these fears are far-fetched.

Bitcoin is created by computers “mining” or solving complex mathematical equations. These transactions are recorded in “blocks,” files that also record transactions between bitcoin users. Each of these blocks is linked to a mathematical problem. Bitcoin miners race against each other to complete these problems in order to win newly minted bitcoin. Over time, the math behind bitcoin mining becomes increasingly complex, requiring more and more computing power to unlock a single coin.

This shift increases the input costs of mining, in terms of hardware, time, and also energy. The digital world may be made largely of silicon, but it runs on electricity. As a result, even the most mundane digital task–such as a Google search–has real carbon costs. Back in 2009, when Apple had sold a measly 34 million iPhones, a Harvard physicist calculated the carbon cost of web browsing. Dr. Alex Wissner-Gross estimated then that browsing a simple website results in about 0.02g of CO2 emissions for every second it is viewed. A website playing a video can require 10 times as much CO2.

These all use far less computing power than bitcoin mining. As prices rise and more and more people are incentivized to mine bitcoin, the amount of electricity used by this digital currency continues to increase. It is difficult to make an exact comparison between bitcoin mining and other energy uses. However, analysts from Digiconomist estimate that each bitcoin translation, or the solving of the math problem, requires more energy than it would take to power eight American homes for a day. According to their numbers, as of late 2017, bitcoin mining is estimated to have required as much energy as the country of Serbia.

Even though the currency itself is digital, its effects are not. Generating this much power results in the annual emission of 15,952 kilotons of CO2 and energy consumption for mining has increased sharply in the last year. In Britain, bitcoin energy consumption increased by 30 percent in the last month. It is this growth rate that has some observers concerned.

“In just a few months from now, at bitcoin’s current growth rate, the electricity demanded by the cryptocurrency network will start to outstrip what’s available, requiring new energy-generating plants. And with the climate conscious racing to replace fossil fuel-based plants with renewable energy sources, new stress on the grid means more facilities using dirty technologies,” writes Eric Holthaus, a contributing writer for Grist. “By July 2019, the bitcoin network will require more electricity than the entire United States currently uses. By February 2020, it will use as much electricity as the entire world does today.”

His prediction is certainly flamboyant and has already attracted the attention of other industry observers, who point out that the energy consumption statistics he references are shaky at best. At heart, the problem stems from the impossibility of concretely determining how much power is dedicated to bitcoin mining worldwide. The statistics claim that bitcoin mining uses a staggering 32.56 terawatt-hours per year. This figure is based off of a series of assumptions, however.

The first is that a fixed “60%” of mining revenues are spent on electricity. Both the origin and accuracy of this statistic are uncertain. Working backwards from this number, the researchers estimated an average price per kilowatt-hour in different countries, drawing off of reported data by a handful of bitcoin miners.

However, other argue that these calculations are based on estimates that are wildly off base. Different white papers have suggested a range of different figures for what percentage of bitcoin mining costs are tied to electricity. Marc Bevand, an entrepreneur who wrote one of the first , performed his own calculations, basing them off of a range of different hardware configurations with varying energy usage, and dividing the history of bitcoin mining into different phases. These phases reflect the bitcoin hash rate and the points at which the digital mining became mathematically more complex.

Bevand found that even under the assumption that all of the hardware being used was the least efficient model while still remaining profitable, bitcoin mining probably used less than 6.78 terawatt-hours annually, nearly one-fifth of the Digiconomist estimate. However, if bitcoin miners were using the newest and most efficient technology, their electricity costs could be much lower, around 2.85 terawatt-hours annually.

“This may sound like a lot of electricity but when we considering the big picture, I believe Bitcoin mining is not wasteful,” Bevand concludes. “Also an interesting comparison to make is that according to a 2008 study from the United States Energy Department’s Energy Information Administration (EIA) these figures are comparable to or less than the annual electricity consumption of decorative Christmas lights in the country (6.63 TWh/year.)”

Critics of his analysis argue that profit margins for bitcoin operators are much lower and that Bevand presents a rosy picture of the economic conditions that miners are operating under. The debate is complex and technical, resting to a large degree on the power and efficiency of the hardware being used to mine. Few bitcoin operators are willing to be forthcoming with these details.

Determining the global carbon footprint of the industry is even more difficult. So far, bitcoin miners have been migrating to countries with cheap energy prices, though many remain in the U.S. The benefit of a digital currency is that these entrepreneurs can locate their servers wherever they have access to low-cost electricity and the internet. These countries use a variety of different means to generate their electricity, including green options like hydropower. Trying to estimate carbon emissions from estimates of average electricity use and the location of various bitcoin mining pools requires an increasing number of presumptions.

Bitcoin has a carbon footprint of some sort, the same as any computer-based task. The size of that footprint, though, is a mystery. At its heart, the question of how environmentally-friendly bitcoin is becomes a discussion of resource allotment. Miners are pouring large amounts of electricity into bitcoin mining because they believe it to be a worthwhile investment versus the cost of energy that’s being expended.

As first envisioned, bitcoin was a digital currency, one untied to any government. If you went to the right store, you could buy a pizza or t-shirt using bitcoin. Today, each bitcoin transaction requires the same energy as 4,000 Visa card swipes. Investors may be able to justify the increasingly high energy costs. However, bitcoin’s recent surge in popularity has exposed its very real carbon footprint. Bitcoin is soaring, but its electricity bills may not be far behind.

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