Researchers studying state-level climate policy in the US confirm what high school teachers already know: if you make an assignment voluntary and offer no incentives for completing it, no one’s gonna do it.

In an assessment of 17 climate and energy policies enacted by US states between 1990 and 2014, researchers from Emory University found that mandatory policies usually had a positive effect on emissions reduction while voluntary policies always had negligible or no effect.

What may be more interesting, however, is to look at which policies worked best. Such an analysis has growing practical implications. This year, the Trump administration reversed many of the Obama administration’s federal emissions-reducing guidelines, rules, and regulations, meaning states that want to curb emissions are left to their own devices. Legislators who are serious about crafting good environmental policy would do well to look at what has worked for others before making proposals.

What worked

Pulling apart what worked and what didn’t was a challenge, the authors write, because a handful of states have adopted several policies applicable to power sector emissions—some mandatory and some voluntary. It should be no surprise that California’s wide-net climate change legislation AB 32 was associated with a 25-percent reduction in emissions from the power sector since it came into force in 2006. The state established a cap-and-trade program, as well as a Climate Action Plan (CAP), mandatory greenhouse gas reporting, binding renewable portfolio standards, and emissions performance standards. California also joined the largely voluntary Western Climate Initiative. Were any of these more effective than others?

When the researchers used data from additional states to tease out combination effects and policy start-dates, they were able to find three policies that offered the most dramatic reductions in emissions overall. Those policies included:

Electric decoupling: Utilities are re-organized so that they don’t make money on the amount of energy sold or lose money based on year-over-year reductions in electricity use. Customers are charged the same no matter how much or little energy they use. In Colorado, Xcel recently dropped a request for rate increases and agreed to move forward with electric decoupling in a compromise with solar advocates in the state. Mandatory greenhouse gas registry/reporting: This requires companies and other organizations to report their greenhouse gas emissions at regular intervals. Mandatory reporting makes emissions data more reliable and helps governments target problem areas in future legislation. It also helps the companies identify major sources. Public benefit funds: Often collected through small charges on customer utility bills or through mandatory utility contributions, these funds are used for renewable energy or energy efficiency programs. PBFs create revenue streams for projects (PDF) that reduce emissions and also ideally reduce the cost of energy for customers long-term.

The second and third policies were “also associated with a large reduction in emissions intensity,” the authors write, meaning that the average amount of emissions from a state's power sector fell.

Emissions performance standards were associated with the largest reduction in CO 2 emissions for the power-sector in that state, “reducing emissions by approximately 3.9 MMTCO2 [million metric tons of CO 2 ] per year.” But only six states had adopted emissions performance standards in 2014. The authors write that these limits are effective but aren’t politically feasible in many states. In lieu of those standards, “market-based mechanisms, such as a tax or cap-and-trade program, can achieve the same result as regulation at lower costs."

Many states also have renewable portfolio standards (RPS), but when analyzed against 2014 data, RPS had mixed results. In some instances, the power sector increased renewables but didn't increase the percentage of power coming from renewables. In other words, you can build a utility-scale solar facility, but if you keep adding fossil fuel plants, too, it won’t do much good. The authors noted:

“Ultimately what matters from a climate lens is not whether policies increase renewable energy, but rather whether they are successful in curbing GHG emissions. Increased investment in renewables, greater renewable capacity or even higher renewable generation levels cannot serve as a suitable proxy for the decarbonization of the power sector, as simply increasing renewable capacity without decreasing fossil fuel electricity generation is not a solution to climate change.”

It's important to think of renewable energy as a tool to reduce greenhouse gases. Renewables are a means to an end, not the end in itself.

What didn’t work?

Voluntary greenhouse gas emission reporting seemed to have no effect on greenhouse gas emissions. The authors admit, however, that there may be some selection bias working against “voluntary emissions reporting” as a policy. If a state has no intention of controlling greenhouse gas emissions or if it knows that its emissions will increase in the coming years (due to population influx or a booming oil market, for example), then it will be more likely to adopt voluntary greenhouse gas emission reporting. The data doesn’t tell us how well voluntary emissions reporting works in a state that is committed to making changes or in a state that's less tied to emissions-producing industries.

The Western Climate Initiative (WCI) has also not driven a significant reduction in emissions. The initiative created a "weak alliance" of western states that "established an economy-wide [greenhouse gas] emissions target of 15 percent below 2005 levels by 2020." But before 2014, only California instituted market-based reforms to achieve that goal. "For the other six states within the WCI, the signing of the agreement in 2007 did not send a strong enough market signal to see a noticeable decrease in emissions," the authors wrote.

Finally, Climate Action Plans (CAPs) didn't show a significant decrease in emissions. But the researchers argue that that doesn't necessarily mean CAPs don't work—many CAPs address sources outside of the power sector, or they focus on "adaptation" to a changing climate rather than "mitigation" of the causes of that changing climate.

What we're left with is that mandatory policies work to reduce emissions in the power sector, and voluntary policies show less promise. This is helpful information for states, and it has implications for a future in which the federal government will eventually have to respond to a changing climate. As the researchers note, "If and when the federal government takes leadership in climate mitigation efforts, it can learn from the successes of state-level actions analyzed here."

Nature Climate Change, 2017. DOI: 10.1038/s41558-017-0001-0 About DOIs.