PROVIDENCE — The state has met the first benchmark mandate for reducing greenhouse gas emissions set by the Act on Climate law.

The Rhode Island Department of Environmental Management on Friday released its Greenhouse Gas Emissions Inventory, which combines federal and state data for 2020 — the most recent data available — to give a snapshot of how much carbon the state is releasing into the atmosphere. The annual inventory is the primary tool for assessing progress toward the Act on Climate’s mandate of net-zero greenhouse gas (GHG) emissions by 2050.

The report shows GHG emissions decreased 20.1% below 1990 levels, more than twice the 10% reduction mandate for 2020 set by the climate law. Overall emissions decreased 6.5% between 2019 and 2020, according to the report. The data on emissions derived from federal agencies has a three-year lag.

Emissions from vehicles and heating saw decreases in 2020, according to DEM’s analysis. Transportation, the largest emissions contributor in the state at 38%, saw a reduction of 11.6% in emissions from 2019 levels, according to the report.

“Due to the reduced transportation activity in the first half of 2020 during the COVID-19 pandemic, the state’s emissions were significantly reduced beyond what would be typically expected,” according to DEM, which said the figure will likely rebound in the next inventory, as pandemic restrictions were lifted in 2021.

Emily Koo, senior policy advocate and Rhode Island program director at the Acadia Center, a regional nonprofit that works to transform energy use, pointed out that “2020 was an anomaly. There is certainly more work to do to reach the necessary emissions reductions.”

Koo, who is also president of ecoRI News’ board of directors, said, “We need a much faster rate of emissions reduction. The trends are in the right direction, but not at the rate required. We need a dramatic societal change to meet that rate of reduction.”

In May, Rhode Island became the eighth state to propose adoption of the Advanced Clean Cars II standard, which would require car manufacturers to slowly, over the next decade, ramp up the percentage of electric vehicles sold in Rhode Island until 100% of all new automobiles sold by 2035 were zero-emission vehicles.

Emissions from residential heating declined by 8.5% from 2019 levels. The report said 2020 was, on average, a warmer year in southern New England, and reduced demand for natural gas and fuel oil likely contributed to the decline in emissions from residential heating.

“Meeting the 2020 mandated benchmark is a sign that Rhode Island remains on the right path for complying with the Act on Climate,” DEM director Terry Gray said. “Both the heating and transportation sectors remain large sources of GHG emissions and primary drivers of climate change. It is critical that we remain focused on these sectors and continue our investments in renewable energy to meet our mandated emissions reductions for the future health and prosperity of the state.”

The emissions from consuming electricity increased by 16.7% in 2020, according to the report, which blamed the rise on “regional economic forces” and the closure of Massachusetts’ Pilgrim nuclear power plant in 2019.

“I think the [increase in emissions from] the electric sector really highlights the interconnectedness of the regional grid,” Koo said.

Results of the loss of open space and forestland in Rhode Island can be seen in the report: the state’s forests removed 15.7% less carbon dioxide from the atmosphere in 2020 than they did in 1990.

“The 2020 inventory underscores the continued work that is necessary through our collective climate action efforts to create a future of net-zero GHG emissions,” Gov. Dan McKee said.

This year, DEM’s Office of Air Resources updated the science used to compare the climate impact of methane and nitrous oxide to carbon dioxide, the most common GHGs. The way the state measures those gases compared to carbon dioxide in both its 1990 baseline data and all subsequent inventories was changed, bringing the state in line with requirements from the Environmental Protection Agency and the U.N. Framework Convention on Climate Change.

A second change added carbon capture rates from land use, land-use change, and forestry (LULUCF) sectors to all existing and future GHG inventories. The change will allow DEM to produce a net emissions estimate in the inventory. Last year was the first time DEM accounted for carbon sequestration from LULUCF sectors when crunching the numbers for the inventory. Prior to last year, the department had been relying on decades-old data estimates produced by a contractor for the first inventory back in the 1990s.

The inventory’s 1990 baseline, the critical data used to mark progress toward the Act on Climate, was also updated this year, after a two-week public comment period and a public listening session were held by DEM.

The changes follow “national and international practices [and] incorporates 21^st-century climate science into the inventory instead of using climate science from 1995,” DEM air quality specialist Joseph Poccia said. “It allows for a more apples-to-apples comparison across 30 years of data.”

But despite the changes, critics and advocates say the state is still undercounting the impact gases like methane have on the climate.

“It’s going to leave us in a worse position as the climate impacts are happening faster,” said Timmons Roberts, a climate professor at Brown University. “Methane has a huge impact in the short term, and people are saying it’s big part of the reason why our estimates of when climate impacts are going to happen have been underestimating how bad it’s going to be.”

While they all ultimately trap heat in the atmosphere and exacerbate climate change, not all greenhouse gases are created equal. Methane is a particular problem because of just how potent it is as a climate warmer. Unlike carbon dioxide, which can stick around in the atmosphere for centuries, methane only has a lifecycle of around 12 years. But despite its shorter lifespan, it traps 28 times more heat than carbon dioxide does in its lifetime.

Greenhouse gases are accounted for in inventories in two ways: by 20-year or 100-year global warming potential, each weighted toward how long a certain tranche of gases hangs around in the Earth’s atmosphere. Global warming potentials basically act like multipliers, that attempt to equate the shorter-term GHG emissions with carbon dioxide.

Critics like Roberts contend that undercounting the impact of methane hides the small number of emitters responsible for releasing it into the atmosphere, and prefer counting methane’s impacts across 20 years instead of 100. Unlike carbon dioxide, which can be emitted from hundreds of thousands of sources such as automobiles, methane emissions in the state primarily come from the natural gas — 90% of which is made up of methane — used in Rhode Island.

The next benchmark mandate is reducing emissions by 45% over 1990 levels by 2030.