The Climate Calculation that Started a Political Firestorm

A fight broke out in New York this week over a small calculation with big implications for the state’s climate policy.

The trouble started when a group of lawmakers, including Democratic Governor Kathy Hochul, proposed a change to the way the state accounts for the greenhouse gas methane. Her administration argued the adjustment would result in lower costs for consumers as the state transitions from fossil fuels, but a broad coalition of environmental groups were incensed, claiming it would “sabotage” New York’s climate law for the benefit of natural gas companies. Under pressure, Hochul backed down, announcing Wednesday that she wouldn’t be holding the state’s budget hostage before sorting it out.

The debate — which isn’t yet over — is an enlightening example of the high stakes of climate math, particularly the formulas used to calculate greenhouse gas emissions. And before you yawn and close this tab over having to read a story about math, give me a chance to show you how these calculations can translate into real differences in how we approach the energy transition, and our odds for avoiding the worst climate outcomes.

It may seem strange that there's more than one way to calculate greenhouse gas emissions. The dispute stems from the fact that methane is a very different beast than carbon dioxide, but when it comes to creating climate policy, we often elide those differences for the sake of simplicity.

When carbon dioxide emissions hit the atmosphere, they’re essentially permanent. The more carbon accumulates, the more it heats up the planet. Methane, on the other hand, is short-lived — it breaks down in a little over a decade. But while it’s around, it traps a lot more heat than carbon dioxide, like wrapping the quilt of CO2 in a second, goose-down duvet. As the methane in the atmosphere decays, it’s like that duvet is constantly losing feathers. If emissions decline, the blanket will thin out and eventually go away.

So while the warming power of carbon dioxide depends entirely on the total amount released, the effects of methane depend on the rate at which it is emitted.

Despite these key differences, the two pollutants often get lumped together into a common metric, like the Environmental Protection Agency’s estimate that the U.S. emitted almost 6 billion metric tons of “CO2 equivalent” in 2020, or the Biden administration’s goal of a 50-52% reduction in greenhouse gas emissions by 2030. To arrive at those numbers, scientists convert methane into carbon dioxide using a formula called global warming potential, or GWP, which basically asks how many tons of carbon dioxide it would take to warm the planet as much as one ton of methane.

The problem is, there’s not one answer. Because of methane’s short but powerful life span, there’s another variable at work in the calculation: time. Over 20 years, one metric ton of methane has a similar effect to about 80 metric tons of carbon, but over 100 years, it’s more akin to 25 metric tons of carbon. The federal government, as well as most U.S. states and much of the rest of the world, take the long view, accounting for methane emissions over 100 years. But scientists say it’s an arbitrary choice.

“There's no single timescale that's clearly most appropriate,” said Drew Shindell, an earth science professor at Duke University and an expert on methane.

Scientists have been arguing about the problems with using global warming potential formulas for years, so it’s somewhat surprising that so many governments have taken this consequential choice for granted.

New York’s climate law, which passed in 2019, is unique in that it explicitly requires a 20-year accounting of methane. When the law was being crafted, the state’s environmental community argued this would more accurately capture the consequences of emissions in the near term. Since methane can cook the planet quickly, it could push the climate past 1.5 degrees of warming, risking irreversible impacts. But encouraging steeper cuts to methane over the next few decades could actually cool the planet, buying slightly more time to reduce carbon emissions.

The Hochul administration has proposed aligning New York with the status quo and switching the state to 100-year accounting. This would have tangible repercussions for New York’s climate planning. For one, it would make the state look like it’s further along in achieving its climate goals, when in reality nothing has changed. The nonprofit outlet New York Focus estimates that under the current methane accounting method, the state must cut emissions by about 134 million metric tons this decade, but the 100-year method would change that number to 86 million.

It could also shift the state’s priorities. Under the current system, the largest source of emissions in New York is its buildings, most of which rely on natural gas and other fossil fuels for heating and cooking. Methane is the primary component of natural gas, and it leaks out of wells and pipelines, all the way to homes, where it can also leak out of boilers and stoves. That fact has dominated New York’s climate discussion over the past two years. It led the Climate Action Council, an appointed group of government officials, scientists, advocates, and industry leaders tasked with drawing up a roadmap to achieve the state’s emission targets, to conclude that one to two million homes should be electrified by 2030, followed by the large majority of buildings statewide by 2050.

But under a 100-year accounting system, buildings wouldn’t look like such an urgent problem. Methane emissions related to the residential use of natural gas equaled about 200,000 metric tons of carbon dioxide equivalent in 2020 under the 20-year scheme, according to state data. Using the 100-year formula, that number would drop to 60,000. Suddenly, transportation would look like New York’s number one climate culprit, which could lead regulators to turn more of their attention to boosting electric vehicles, improving public transit, and decarbonizing trucking.

Is that really the worst outcome? If it truly did result in steeper cuts to transportation emissions, that could mean lower temperatures in the long term, because less carbon would get lodged in the atmosphere. “Any reductions of methane we make today only really affects people who are alive today,” said Zeke Hausfather, a climate scientist at Berkeley Earth. “Whereas any emissions of CO2 we have today affect, you know, dozens of generations to come. And so trading off between those two is sort of a thorny, ethical question.”

But that result is by no means guaranteed. When I ran the idea by Liz Moran, a New York policy advocate for Earthjustice, she argued that the transportation sector is harder to address so those emission reductions may not materialize. Moran added that giving more weight to methane has enabled the state to make strides in environmental justice, for example by leading to decisions to reject polluting power plants. “This would have some very tangible and immediate impacts to communities where we're already starting to make some progress,” she said.

While some New York environmental groups have claimed that using GWP 100 is “outdated science,” Shindell disagreed with that characterization. “I think we've learned more about the limitations of GWP 100,” he said. “I think you can make an argument that it's not very aligned with what most countries are talking about now: net-zero by 2050.”

The use of 100-year accounting doesn’t have to mean that policymakers ignore methane’s near-term impacts. While California uses this metric for its overall emissions goals, it also has a separate, specific target to reduce methane. Though in the case of New York, with the Hochul administration’s explicit intention to cut costs, that's not yet on the table and it would surely prompt another political fight.

Who said climate math had to be boring?