This week, the US Energy Information Agency (EIA) released a report on the carbon dioxide (CO 2 ) emissions of each state between 2000 and 2015. The good news? CO 2 emissions dropped in 41 states, with Maine taking home the prize for the greatest percentage decrease in emissions (by 25 percent). Ohio, meanwhile, showed the greatest absolute decrease, using 51.7 million fewer metric tons of CO 2 emissions in 2015 than it did in 2000.

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The bad news? Nine states saw increases in COemissions over the same period. Nebraska was one of the worst offenders, with a 22-percent increase in carbon emissions between 2000 and 2015. Though Nebraska's population grew by roughly 200,000 in those 15 years, Kansas also welcomed about 200,000 people into its state between 2000 and 2015, and it cut emissions by 17.2 percent. (Kansas' success is probably in part due to the state's embrace of wind, where "wind energy has grown from less than 1 percent of net electricity generation in 2005 to 24 percent in 2015, making wind the state's second largest power provider, after coal," the EIA writes .)

The EIA cautions against evaluating these emissions numbers as a direct reflection of how green a state is, though, because the agency only counted energy-related emissions in the state where they were created. That doesn't account for exporting energy across state lines. If one state uses only renewables but buys a considerable supply of electricity from a coal plant in a neighboring state, the neighboring state bears the burden for all those emissions.

Energy Information Agency

Energy Information Agency

Energy Information Agency

Weather, driving, resources

Though the end-period data is now three years old (unbelievably, we're now living in 2018), looking at 15 years of data can help tease out interesting trends and help industry and policymakers target problem areas. Transportation and heating are often intractable problems for policymakers trying to decrease CO 2 emissions, especially in states where commutes are long (and thus a lot of gas is used) and in cold, rural states (where heating is crucial, but economies of scale from heating population-dense apartment buildings don't exist).

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Per-capita emissions data can sometimes reflect this. The EIA divided each state's total COemissions by the number of people in the state. In 2015, Wyoming, North Dakota, and West Virginia filled the top three slots. Although those three states have relatively small populations, they're also highly dependent on fossil fuels. But another factor is surely at work: the states are also in colder climates, and driving is a primary means of transportation, which pushes up per-capita emissions further.

Though Texas emitted the most CO 2 in 2015 by far, it also had a low per-capita emissions number because it's home to millions of people, and its mild winters mean burning fuel for heating is not imperative. The EIA also noted that New York had the lowest per-capita carbon emissions of the 50 states.

Although New York is also in a cold climate, its dense population center means that less heat is needed per person in the winter (apartments offer economies of scale in terms of heating). Less energy is needed for transportation because a dense cluster of the population can rely on a public transportation system and avoid driving altogether. In addition, "The New York economy is oriented toward low energy consuming activities such as financial markets," the EIA wrote. "For example, New York accounted for about 6 percent of the US population in 2015, but consumed only 1 percent of the country's industrial energy."

Renewable energy

Much of the work of reducing carbon emissions per state happened toward the end of the 15-year period. (You can check out a year-by-year breakdown of emissions estimates in Table 1 of the EIA's state analysis report here (PDF). Year 2000 and years 2005-2015 are represented). In many states, carbon emissions increased between 2000 and 2005.

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In the last decade, falling prices of wind and solar installations, along with state and federal policies to encourage renewable adoption, have helped grow low- and zero-carbon technologies. The EIA makes no attempt to correlate policies with COemissions reduction (though Ars wrote about a recent study that did just that here ). But the agency does break out the change in renewable energy mix for some of the most populous states with lower per-capita emissions.

Back in 2000, hydropower and nuclear power were the two most important sources of carbon-free electricity, but the EIA notes that almost no new hydro or nuclear power has come online since then. Instead, we see an astounding amount of wind generation come online in Texas by 2015, with Illinois, Pennsylvania, and Washington also adding significant wind. California actually saw its total carbon-free electricity generation drop between 2000 and 2015, largely due to a drought that diminished hydroelectric power in 2015 as well as the retirement of nuclear power capacity. Wind and solar took up much of the slack in 2015, but not enough to show a net gain in renewable generation.

That should tell us that the gains the US saw in reducing carbon dioxide emissions between 2000 and 2015 aren't guaranteed to continue in the future. As older nuclear power plants retire around the country, a concerted effort must be made to replace that carbon-free electricity with new carbon-free electricity. Heating systems must become more efficient, and fuel economy has to be a priority for automakers.