Population growth equals more greenhouse gas emissions. Economic decline sends emissions downward.

Those realities emerged from two studies - one local and one national - showing the state and Pima County's galloping population growth over the past 10 to 20 years has boosted emissions of carbon dioxide and other greenhouse gases faster than the national rate. At the same time, Arizona and Tucson's worse-than-average economic bust since 2007, when population started stagnating, dropped emission levels faster than national levels.

Greenhouse gas emissions have been linked by most climate scientists in this country to generally warming temperatures since the 1990s, if not earlier. One study, from the U.S. Energy Information Administration, looked only at carbon dioxide emission trends. The local study from the Pima Association of Governments looked at emissions of CO2, methane and nitrogen oxide.

Here are some questions and answers about the studies from Perry Lindstrom, an economist for the Energy Information Administration, and Leslie Ethen, director of the city of Tucson's Office of Conservation and Sustainable Development.

Q. Why does more growth mean more emissions?

A. With Arizona typically the country's fastest- or second-fastest growing state, "that will be a biggie right there: a lot of people getting in cars, driving them and putting gasoline in their cars," Lindstrom said.

Q. There's been plenty of talk about energy conservation, through more efficient appliances and home insulation and more fuel-efficient vehicles. Has that talk translated into action?

A. Maybe, Lindstrom said, particularly in new national emission data the energy administration has received, which show continued declines in total emissions nationally in 2011 and 2012 compared with 2010. No state-by-state figures have been released yet for post-2010 data.

The growth of mandatory state renewable energy standards around the country and of federal gas mileage standards could be having an impact, he said.

Emission reductions in Tucson and Pima County since 2006 were probably enhanced - above and beyond the recession's effects - by more energy efficiency at home and on the road, Ethen added. Even before the recession started, per-person greenhouse gas emissions in this region dropped since 2000, and in 2010 they were lower in the entire Tucson metro area, although not within the more densely populated Tucson city limits, than in 1990, the PAG study showed.

Q. Are these statistics good or bad news?

A. Overall, "there's nothing that made me feel pessimistic, but there's nothing much to make me feel optimistic either," Ethen said. "I feel like people, on average, they have their lifestyle, and we haven't seen a whole lot of change in that."

As a sign of potential obstacles to energy conservation, Ethen pointed to a recent study from the Proceedings of the National Academy of Sciences that showed when conservatives and liberals are given the choice of buying a regular or compact fluorescent light bulb for the same cost, both groups were equally inclined to buy the more energy-efficient CFL bulb unless that bulb has a label saying "Protect the Environment." Then, conservatives and moderates are less likely to buy that bulb than the conventional one.

The study's authors said the results show how the political polarization around environmental issues can affect real-world consumer energy choices.

Q. What about the future, with the economy improving?

A. Ethen predicts that when the economy improves, many people will go back to their pre-recession, energy-using ways.

"That's the problem, especially when you are first starting out being conscious about conservation," she said. "It requires thinking about it. It takes a little effort. You have a busy life. It's easier to do your normal patterns. I don't think it's a willful thing: It's just that when we have extra money, we spend it on extra energy."

That's a concern, she said, because the concentration of greenhouse gas emissions just hit 400 parts per million in the Mauna Loa Observatory in Hawaii, which many climate scientists have said is a major threshold pointing to future problems.

Nationally, however, Lindstrom's agency predicts declines in total greenhouse gases until 2040. Such emissions won't even reach 2007 levels by then, he said. For that, he credits federal tightening of gas mileage standards, California's new "cap and trade" carbon trading program and a growth in natural gas production, since it produces fewer emissions than coal.

Greenhouse gas: Fast facts

• From 1990 to 2010, greenhouse gas emissions in the Tucson metropolitan area rose 41 percent.

• 77 percent of the increase in greenhouse gas emissions since 1990 was from home energy use, 63 percent from commercial energy use, 31 percent from transportation and 13 percent from industry.

• Per person, greenhouse gas emissions dropped 3.7 percent in the Tucson metro area from 1990 to 2010, while rising 0.7 percent in the city of Tucson.

• In 2010, transportation caused 32 percent of all metro Tucson greenhouse gas emissions, home energy use was 39 percent, industrial energy was 20 percent, commercial energy was 18 percent and garbage and other wastes accounted for about 1 percent.

• In 2010, Arizona ranked 23rd nationally in total carbon dioxide emissions and 33rd in per capita CO2 emissions.

• Arizona's total emissions grew 11.6 percent from 2000 to 2010. Arizona's per-person emissions shrunk 13.7 percent in that period.

• Coal furnished 45.1 percent of Arizona's CO2 emissions between 2000-10, petroleum was 36.3 percent and natural gas was 18.6 percent.

• Over this period, Arizona's economy ranked as the 24th most carbon intensive nationally.

Sources: "Regional Greenhouse gas inventory, 1990-2010," by Pima Association of Governments, and "State-level, energy-related carbon dioxide emissions, 2000-2010," by the U.S. Energy Information Administration.

Contact reporter Tony Davis at tdavis@azstarnet.com or 806-7746.