Friday, March 30, 2018

Why is climate change hard? - Data note

This is a post to accompany Why is climate change hard to solve?.

The GDP and population are from the Penn World Tables, while the energy use is from the 2017 edition of the Statistical Review of World Energy put out by British Petroleum.

The GDP is measured in what is known as “purchasing power parity.” In 1991 in Plzeň, Czechoslovakia, I was earning 3,000 Kčs a month. At an exchange rate of 30 Czechoslovak crowns for 1 US dollar, that meant I was earning $100 per month, which would obviously mean starvation. But I rented a very nice room in a good part of town for 1,000 Kčs (i.e., $33), and could buy a large loaf of good bread for 10 Kčs ($0.33) and a half-liter of excellent beer for 12 Kčs ($0.40).

When you measure in terms of purchasing power parity, you attempt to correct for this. Rather than saying, “3,000 Kčs can get you $100,” you say, “3,000 Kčs can get you decent lodgings, a fine diet, and the following amounts of clothing, magazines, entertainment, and (in-country) travel, which is roughly equivalent to earning $1,000.”

The idea is simply to have a measure of GDP that does a better job of comparing actual economic output across countries than if you just used the currency exchange rate you found at a bank.

(There are some serious criticisms of the PWT, such as here, but I’m not aware of those issues being correlated with the energy questions I’m looking at, and the pictures that emerge in the accompanying post are sufficiently robust that they would probably not be seriously altered by whatever problems are in the PWT.)

The energy use is given in terms of tonnes of oil equivalent.

A tonne is not a misspelling, but a metric ton, which is 1,000 kilograms, slightly more than our ton in the U.S. which is 2,000 pounds.

The energy we use comes from various sources: oil, coal, and natural gas make up the biggest part in rich countries, but significant amounts come from nuclear power, and increasingly from wind and solar. Geothermal plays a large role in a few places like Iceland, while biomass (wood, crop residues, etc.) is important in many poor countries.

In order to compare energy use across countries with very different mixes of sources, we need some common unit.

A tonne of oil has a certain energy content (or rather, an average tonne of oil has a certain energy content, since oil comes in different grades). A tonne of a particular type of coal has a different energy content, but we can compare that to the energy in a tonne of oil and convert it to measure the coal in tonnes of oil equivalent. Similarly, there’s a certain amount of energy in a million cubic feet of natural gas, and so that can be expressed as an equivalent amount of oil.

We can measure the electricity coming out of nuclear plants, solar fields, hydroelectric dams, or wind farms, and that too has an energy content which can be converted to tonnes of oil equivalent. That involves an unavoidable judgment call, however.

When we produce electricity by burning coal or gas, we already counted the energy use when we counted the fuel. If we look at the energy content of the resulting electricity, it will be half or less of the energy content of the fuel we burned, because a significant portion of the energy in the fuel was lost as waste heat.

So when we measure the electricity produced directly by nukes, hydro, etc., should we count the energy content in the electricity itself? Or the larger quantity of energy we would have used had we burned coal to produce it?

In the images in the main post I’m not making a judgment on those issues one way or another but simply accepting the aggregating that BP has done.

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