PROVIDENCE — With just weeks left before Rhode Island releases its latest greenhouse gas (GHG) inventory, the state Department of Environmental Management (DEM) is proposing to change some of the math behind it.

There are two main changes under consideration by DEM’s Office of Air Resources for the GHG inventory, which is the state’s primary tool to assess progress toward the emission reduction mandates in the Act on Climate. The first converts the way the state measures methane and nitrous oxide compared to carbon dioxide in both its 1990 baseline data and all subsequent inventories, bringing the state in line with requirements from the Environmental Protection Agency and the United Nations Framework Convention on Climate Change.

The second proposal adds 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 way back in the 1990s.

In a virtual listening session Monday, DEM air quality specialist Joseph Poccia said adopting the changes would align the department’s calculations with the best science possible ahead of the state’s first benchmark goal year under the Act on Climate.

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

Rhode Island’s first major test under the Act on Climate is coming up this year. The law, passed in 2021, requires the state to meet a series of bigger and bigger emission reduction goals every 10 years before reaching net-zero by 2050. Failing to do so opens the state to legal challenges from the general public.

The Act on Climate requires the state to have reduced emissions by 10% below 1990 baseline levels by 2020 and, ahead of that inventory getting released later this month (because of a delay in available data, inventories are released on a three-year delay), the state is looking pretty good. According to the inventory released last year, as of 2019 the state has reduced emissions by nearly 20% under the 1990 baseline.

But despite the proposed changes in the way DEM crunches the numbers for its GHG inventories, 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 child 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 gasses 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 Rhode Island primarily come from the natural gas — 90% of which is made up of methane — used in Rhode Island.

DEM typically estimates a 1% leakage rate for natural gas leaks, but studies have long suggested the actual number is much higher. A 2019 study from researchers at the University of Michigan and National Oceanic and Atmospheric Administration found methane leaks in Boston and New York City were twice as much as originally projected. A 2016 study published in the peer-reviewed journal Environmental Pollution showed that just 7% of methane leaks in Boston accounted for up to 50% of all emissions in the city.

“There’s a fairness issue,” said Larry Chretien, executive director of Green Energy Consumers Alliance. “You’re saying to some sectors, we’re going to make them work harder than necessary because we’re just ignoring this issue about methane for over 20 years.”

He added, “It essentially would send a signal to all of us that we ought to be doing whatever we can to reduce gas leaks and gas consumption and whatever else we can do.”

Members of the public can comment on the changes to the 1990 baseline here until 4 p.m. Oct 6.

The 2020 GHG inventory will be released the week of Oct. 16.