BioEd Online

So, how much have US emissions gone up?

Overall, they are 16% above 1990 levels.

The report looks at four main categories of greenhouse gases pumped out by human activities in the United States. It lists all emissions in 'carbon dioxide equivalents', which reflect the amount of a gas that would cause the same amount of warming in the atmosphere as carbon dioxide. Methane and nitrous oxide emissions have declined, but carbon dioxide is up so much it completely swamps these gains (see ' Changes in US annual emissions between 1990 and 2004').

Total emissions topped 7 billion tonnes CO₂ equivalent in 2004.

How does that compare with other countries?

The United States is the biggest national emitter of greenhouse gases worldwide, and these figures reinforce that position.

Many other countries have committed to reducing their emissions to below 1990 levels, although it is questionable how well they are doing (see ' European greenhouse emissions climb again').

UK emissions, for example, came to about 656 million tonnes CO₂ equivalent in 2004 (less than a tenth of US emissions), which is 14% below its 1990 levels. The British government says it is on track to hit targets set by the Kyoto Protocol, an international agreement to reduce emissions of greenhouse gases.

Why are US carbon dioxide emissions increasing so fast?

It's mainly down to the obvious reasons: lots of cars driving around, lots of people needing lots of energy and burning carbon-based fuels to get it.

But there are some quirks. Between 1990 and 2004 there has been a 78% rise in carbon dioxide emissions from garbage, because plastics and other carbon-containing wastes are being burned. And emissions from cement production have gone up a third, reflecting a boom in construction.

Haven't there been any cuts in carbon dioxide?

In the report's list of 19 main carbon dioxide sources, only 6 have gone down: producers of iron and steel, ammonia, aluminum, phosphoric acid, iron alloys and zinc are now emitting less carbon dioxide. But this isn't necessarily thanks to cleaner industry: much of the reduction is due to the closing of plants or changes in import and export.

Isn't the United States trying to cut emissions?

The United States has not ratified the Kyoto Protocol. But President George W. Bush has a goal of reducing the 'intensity' of the country's greenhouse-gas emissions. Intensity is the ratio of emissions to economic output, and Bush wants to cut this by 18% between 2002 and 2012.

"This is not a formula for reducing emissions," points out Bob Ward of Britain's Royal Society. If the US economy does well then emissions will go up.

Some states have made more stringent individual commitments. California, for example, has set its sights on bringing pollution in line with 1990 levels by 2020.

Why have methane and nitrous oxide emissions gone down?

They have only dropped a little. For methane, the drop is mainly thanks to programmes that capture the methane gas seeping from rotting landfill sites and burn it, sometimes making usable energy (and carbon dioxide). Fertilizers and agricultural processes are the largest source of nitrous oxides, but there hasn't been a clear long-term trend either way in these emissions. One reason for the recent dip has been introducing technologies to limit nitrous oxide emissions from cars and trucks.

Anything else?

The final category of greehouse gases is the family of chemicals, including HFCs and PFCs, that are replacements for nastier chemicals that were found to be eating a hole in the ozone layer. The replacements are better for ozone, but bad for global warming. Along with sulphur hexafluoride, emissions of these have rocketed by more than 50%, nearly countering the cuts in methane emissions.

So... is all this good news, or bad?

"Even with a dramatic increase in economic activity, the United States is making significant progress toward the president's greenhouse-gas reduction goals," said the EPA administrator, Stephen Johnson, at the time of the report's release.

Others are not so optimistic. "Emissions are actually growing," laments Ward. "The United States is a leading scientific country, but the government position flies in the face of everything everyone is advising them."

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