How much do different sectors of the U.S. economy contribute to greenhouse gas emissions, and how much does this vary by region? That's a complicated question, but you can see the answer at a glance through a nifty, interactive map on the New York Times Web site.

A bar across the top gives the overview by sector – electric, transportation, industrial, residential, and commercial. Click on a bar to see the breakdown by state, shown on a map of the U.S. via proportionally-sized circles. When you hover your mouse on a circle, you see text with the state name and million metric tons of emissions.

If you'd like to dig into the numbers in full, gory detail, check out the latest U.S. Greenhouse Gas Inventory Report.

For example, most deforestation occurs in poor tropical countries (in fact, in many of these countries deforestation is a much larger source of CO₂ than fossil fuel use). In contrast, most CO₂ emissions from fossil fuel use come from the developed countries.

As I described in an earlier post, the U.S. is the largest emitter in the world, both today and historically. Next let's take a closer look at the U.S. greenhouse gas pie.

As one might expect in a developed country, virtually all our CO₂ emissions come from burning fossil fuels. We use relatively more natural gas than the rest of the world – which is good because natural gas is the cleanest of the fossil fuels. The percentages are: about 40 percent coal, about 40 percent petroleum, and about 20 percent natural gas. Worldwide, deforestation is a net source of CO₂, but in the U.S., forests and urban trees are growing. They are taking CO₂ out of the atmosphere and storing it. That growth cancels out about 11 percent of U.S. fossil fuel emissions.

Forty percent of our country's CO₂ emissions come from electricity generation. Half of our electricity comes from coal-fired power plants, which produce 32 percent of America's CO₂ emissions. Because of that, utilities will play a huge role in our fight against global warming, but everyone will need to cut emissions because every sector contributes to greenhouse gas pollution:

U S. greenhouse gas pie: Sliced by economic sector

It is interesting to note that residential and commercial emissions – coming from our homes and offices – together make up almost 40 percent of our emissions. What the chart doesn't show is that commercial and residential emissions are growing about twice as fast as the overall average.

These statistics suggest that there are multiple sources to worry about in our fight against global warming. Clearly, coal is a major challenge – almost one-third of our emissions come from coal-fired power plants. But the good news is that there are clean coal technologies that would allow us to use our huge coal reserves without harming the climate.

Transportation is another biggie – also about one-third of our emissions. Here again there are solutions – more fuel efficient cars come to mind. And for commercial and residential sectors, building green buildings and improving the efficiency of our existing ones will help stem the flow of greenhouse emissions.

As I've said before, it's not going to be one silver bullet; the solution is going to be more like silver buckshot.

One way is to slice the pie up by the type of gases we're talking about. There are dozens of human-produced greenhouse gases, three of them get special attention: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). Here's why.

CO₂ is the most important because it causes about 67 percent more warming than all the other greenhouse gases combined. Methane causes less than a third as much warming as CO₂, but it is responsible for about three times more warming than nitrous oxide.

Greenhouse gas pie: Sliced by the amount of warming contributed by each human-produced greenhouse gas.

That's somewhat interesting, but doesn't do much to show us opportunities to reduce emissions of these gases. What are the principal man-made sources of each gas? That requires us to look at another set of pies.

Let's first look at CO₂ in more detail, since it's the biggest slice of the pie above. Globally, about 80 percent of CO₂ comes from burning fossil fuels like coal, oil and gas. (If you've ever wondered why they're called "fossil" fuels, it's because they are made from the remains of plants and animals that died hundreds of thousands or millions of years ago – in other words, they are fossils.) The biggest sources of fossil fuel CO₂ are coal and petroleum (about 40 to 45 percent each), with natural gas contributing the rest. The other 20 percent of global CO₂ is released through deforestation.

Carbon dioxide pie: Sliced by human-produced sources, which are burning fossil fuels and deforestation.

Turning next to methane — worldwide, the major source is agriculture, primarily rice farming and livestock. Methane is a normal digestive byproduct in domesticated animals like cows, sheep and goats. As the EPA diplomatically describes it, the animals "exhale" the gas after digesting their food (it comes out the other end, too). Animals also produce manure, which releases methane as it decays.

Other important methane sources are deforestation, coal mining and the production and distribution of natural gas and oil. In the United States, landfills are the largest source of methane emissions. The good news is that it is relatively easy to capture methane from manure and landfills. Not only does this prevent emission of methane into the atmosphere, the captured methane can also be burned to generate electricity.

Finally, the other major greenhouse gas, nitrous oxide, comes primarily from agricultural soils. The opportunity to reduce emissions here is in more efficient use of fertilizers. Applying fertilizer in a different form, at a different time, or in a different amount can mean more is used by the crops instead of being transformed into nitrous oxide.

Coming up shortly, another pie showing another view of where the greenhouse gases come from.

1. How Warm is Too Warm?
2. Worldwide Emissions Target
3. U.S. Emissions Target
4. Technologies to Get Us There

In Part 1 of this series, I defined the global tipping point as the melting of the Greenland ice sheet, which could cause sea levels to rise 20 feet. In Part 2, I showed by how much global emissions of carbon dioxide (CO₂) must drop to avoid this tipping point. They must start to decline around 2020, drop 50 percent by 2050, and drop at least 75 percent by the end of the century.

Meeting these global emissions targets will require a global effort. Even if the U.S. and other developed countries were to cut their emissions to zero, global emissions would likely exceed the targets by mid-century. This is because of the rapid rise in emissions from China and other developing countries.

So both developed and developing countries must participate, but how much of the burden should each bear? We could require all countries to make the same emission cuts at the same time, but many believe that would be neither realistic nor fair.

Although the U.S. can't solve the problem alone, its participation is crucial because the U.S. is the single largest CO₂ emitter. China is a close second and rapidly catching up – but only if we consider current emissions.

If we look at historical or accumulated emissions, the dominant role of the U.S. is even more apparent. The U.S. is responsible for almost 30 percent of accumulated emissions in the atmosphere today; China is responsible for only 8 percent. If we look at emissions per person, the distribution is even more lopsided: the U.S. emits about six times more than China.

Graph adapted from J. Hansen presentation.

Developed countries have the resources to cut emissions immediately without damaging their economies. It's therefore reasonable to expect them to take the lead. Some of the bills currently before Congress recognize this, and would have the U.S. cut emissions almost immediately, so that total emissions are capped at 10 to 30 percent below current emissions in 2020, and 60 to 80 percent by 2050.

Note that the cut for the U.S. of 60 to 80 percent by 2050 is higher than the global requirement of 50 percent by 2050, and the U.S. peak in 2008 is earlier than the global requirement of 2020. This allows developing countries like China to take more time to drop their emissions so their vulnerable economies aren't over-burdened. But it doesn’t give them a free ride. The growth rate of emissions in developing countries would have to slow almost immediately, then around 2025 emissions would begin to decrease at a rate comparable to that of developed countries. The challenge for the global community in the "son of Kyoto" negotiations will be to get both developed and developing countries to commit to this scenario.

Next week, in the last part of this series, I’ll talk about the technologies we can use to meet these emissions targets.

1. How Warm is Too Warm?
2. Worldwide Emissions Target
3. U.S. Emissions Target
4. Technologies to Get Us There

In Part 1 of this series, we described why scientists and policy-makers have identified the melting of the Greenland ice sheet, which would lead to a 20-foot rise in sea level, as the tipping point that must not be crossed. To stay below the tipping point, average global temperatures must not rise more than 3.6^oF above pre-industrial temperatures, or 2.3^oF above current temperatures.Today we consider how global emissions of greenhouse gases must change over the coming century to stay below that tipping point.

To answer this question, scientists have developed models – mathematical descriptions of the relationship between greenhouse gas concentrations and global warming. To verify the validity of the models, scientists check how well they do in predicting what's happened in the past. They do a good job. Since the models can accurately describe past climate patterns, they should be able to do a reasonable job describing future climate patterns.

The most important of the greenhouse gases is carbon dioxide (CO₂), because its rising concentration – caused primarily by the burning of fossil fuels – is having the largest single impact on climate change. Model calculations indicate that to avoid a temperature increase of 3.6^oF, we must stabilize CO₂ concentrations at about 450 parts per million (ppm) or less.

Source for graph: IPCC Third Assessment Report (2001).

This turns out to be a tall order. Today, CO₂ concentration is 380 ppm. The rate of increase is about 2 ppm per year, and is expected to accelerate. If we follow a "business as usual" course, we could cross the 450 ppm tipping point well before 2050.

Naturally, the more CO₂ we produce, the higher CO₂ concentration in the atmosphere. Four gigatons of carbon emitted into the atmosphere will raise CO₂ concentrations by 1 ppm (see CO₂ Arithmetic, Science Magazine). Worldwide, 7 to 8 gigatons of carbon are emitted into the atmosphere each year.

To avoid the tipping point, global CO₂ emissions should peak no later than 15 years from now, and then begin to decrease. By 2050, emissions must be about 50 percent less than today, and by the end of the century 75 percent less. (Note that this is a reduction in total emissions, not the reduction relative to projected business-as-usual emissions that President Bush referred to in his 2007 State of the Union address.)

Source for Graph: Confronting Climate Change, United Nations Foundation

Reducing CO₂ emissions by 75 percent will require a profound change in the way we produce and use energy, but there is no need for panic or despair. If we get started now, we can make this transition slowly, a percent or two each year. It's a job that our children and grandchildren will continue to work on through the end of the century, but we can start today. As we will see in Part 4 of this series, the technologies we need are already in hand.