Say you said to yourself, “Gee, I wish we could prevent global warming.” Your next thought might be, “Gosh, where do greenhouse emissions come from?” Well, I asked myself just that question a while back. So I decided to jump into the IPCC Working Group III Assessment Report, and I’ve posted a Google workbook, called “GreenhouseGasEmissions,” which should let you know just about everything you always wanted to know about the global sources of carbon dioxide and other greenhouse gases.

The biggest surprise to me was the sheer number of major sources. I don’t know whether it would be easier to slay a few big greenhouse gas monsters or a bunch of medium-sized ones, but we’re basically stuck with the latter.

Speaking of monsters, according to my calculations, all coal-fired power plants together are responsible for 18 percent of global greenhouse gases (all of these figures are for 2004, in CO2 equivalent megatonnes, from IPCC Working Group III reports, and any errors are mine). Shutting down all coal-fired power plants would decrease greenhouse gas emissions by 18 percent — but that would still leave 82 percent, and I’m assuming we want to get as close to zero human-made greenhouse gas emissions as possible.

Amazingly, the fossil fuels (coal, oil, and natural gas) used to provide heat for buildings and industry are responsible for 21 percent of greenhouse gas emissions — more than all the coal-fired power plants. In a way, that statistic understates the importance of using carbon-free sources like wind, solar, and geothermal for electricity generation, because if we want to switch transportation from oil to electricity, we will have to replace transportation’s oil, responsible for 14 percent of emissions, with electricity sources that do not include the use of fossil fuels. And if we want to eliminate the emissions from heating, we will have to use carbon-free electricity and also redesign/retrofit buildings.

Forests might be some of the cheapest of the “lowest hanging fruit” to save, since they account for almost 16 percent of emissions. But I’m worried about what to do about belching livestock — how do we get rid of their 4 percent? It might be easier to prevent the 5 percent of all emissions caused by the overuse of nitrogen-based fertilizers.

Before we get into details, however, let’s take a stroll through the basics of greenhouse gas accounting.

There are various ways to look at the sources of greenhouse gas emissions, depending on the problem in which you are interested. The variation comes from the fact that fossil fuels are used to generate electricity, which is used for buildings and industry, and fossil fuels are also used directly in buildings, industry, and transportation. So is it important to know: the amount of greenhouse gas (GHG) emissions that fossil fuels emit? What the generation of electricity emits? Or what buildings or industry emit? It depends on what you’re interested in at the time.

Let’s start with my recalculation of a pie chart that the IPCC uses ([PDF], p.105), which separates electricity as a source of GHGs from each sector (you can see the following three tables at the top of the “Summary and Energy” tab in my workbook):

Table 1. Electricity separated
Source CO2 eq Mt % of total
Electricity 11,215 22.84
Buildings 5,515 11.23
Industry + ff processing 10,851 22.09
Transportation 6468 13.17
Agriculture 6,100 12.42
Deforestation 5,800 11.81
Other forests 18.62 3.79
Waste Mgmt 1,300 2.65

What this table means is that buildings emit 11 percent of GHGs before considering the electricity they use; industry, 22 percent. If we want to take the full account of GHG emissions for buildings, for industry, and the tiny bit for transportation, then we have to present a table that allocates electricity among buildings and industry (and the tiny bit for transport):

Table 2. Electricity allocated to sectors
Source CO2 eq Mt % of total
Buildings 13,215 26.97
Industry 13,893 28.35
Transportation 6,829 13.94
Agriculture 6,100 12.42
Deforestation 5,800 11.81
Other forests 18.62 3.79
Waste Mgmt 1,300 2.65

Notice that the last four entries didn’t change at all, because they don’t use electricity. These four I’ll call land use sources of emissions. Now, let’s say we want to know the responsibility of carbon dioxide from fossil fuels in our oncoming climate nightmare:

Table 3. Fossil fuels separated
Source CO2 eq Mt % of total
Coal CO2 10,600 21.63
Oil CO2 10,200 20.82
Natural gas CO2 5,300 10.82
Buildings 2,000 4.08
Industry 2,130 4.35
Agriculture 6,100 12.42
Deforestation 5,800 11.81
Other forests 18.62 3.79
Waste Mgmt 1,300 2.65
Methane from fossil fuel processing 3,071 6.27
Transportation nonCO2 636 1.30

Notice how buildings can be held responsible for 11 percent of total GHG emissions if electricity is separated out, up to 27 percent if electricity emissions are allotted to the final destination, and down to 4 percent if all fossil fuels are taken out (that last 4 percent is mostly gases from air conditioning, by the way). The decision about which to use depends on the question: What if we wanted to make all buildings zero emissions, that is, totally self-reliant for energy plus nonpolluting? Then use the 27 percent figure. What would we be left with if all fossil fuels were replaced with carbon-free sources? Use the 4 percent figure for buildings. And so on, for all sectors.

In part two, I’ll go through more details concerning the various main sectors. In the meantime, if you are so inclined, you can look at the “Details” tab of my workbook.

A note about sources: There are basically four sources of data from the IPCC for GHG emissions. First, there is a wonderful flowchart ([PDF], p. 259) from the International Energy Administration that shows the sources of energy and the final categories of use. Second, there are a few pie charts in the Introduction chapter of the Working Group III report ([PDF], p.103-5). Third, spread throughout the various chapters of the WG III assessment report are various pieces of data. Finally, the Netherlands Environmental Assessment Agency maintains a database called Edgar that is used throughout the
IPCC reports.

However, these sources do not seem to match up exactly. Much of this is inherent to one problem: It is very difficult to estimate, over the entire globe, what humans are doing to emit greenhouse gases, and as the "Forestry" chapter explains, this is particularly a problem with forests (and to a lesser extent agriculture — energy use seems to be much easier to keep track of). However, the exact numbers are not important; what is important is to get some sense of the relative importance of various sectors, to know what sectors are involved, and ultimately, to solve the problem by eliminating virtually all GHG emissions.