In a surprise development, Governor Kathy Hochul has recommended changing the way the state accounts for emissions of methane, a greenhouse gas. The proposal has big implications for New York’s climate policy, with one critic describing it as a “grenade.” 

Although the Governor seems to have retreated from pressing this change during budget negotiations, it remains a live issue.

Currently, New York accounts for the global warming potential (GWP) of greenhouse gases over a 20-year time frame (called GWP20), an approach mandated by the state’s Climate Leadership and Community Protection Act. Hochul’s proposal would shift New York’s approach to the much more commonly used model of accounting for methane emissions over a 100-year period (GWP100), used by 49 other states, the U.S. and most other countries.

In this explainer, I will show how the GWP20 and GWP100 approaches differ, and why that difference matters for public policy.

But while the choice matters, there is no definitive answer as to which we should choose. Politically, the choice depends on how we weigh the inputs of science, economics, and practical politics.

Understanding GWP20 and GWP100

Each greenhouse gas has a unique global warming potential—how much heat it traps in the atmosphere. To make comparisons easier, all are compared to the heat-trapping ability of one ton of carbon dioxide.

Methane, for example, escapes into the atmosphere from the production and transport of fossil fuels, from agriculture, landfills, and wastewater treatment. It traps more heat than carbon dioxide (CO2), so it takes multiple tons of carbon to equal the heat-trapping potential of one ton of methane. Another way to say the same thing is that one ton of methane is equal to multiple tons of carbon.

But determining how many tons of CO2 that ton of methane equals is not a straightforward matter, because methane emitted today will dissipate in an average of about 14 years. So it will trap a significant amount of heat for that 14 or so years, but then no more.

Meanwhile, CO2 emitted today will persist in the atmosphere for an average of about 300 years, so it will keep trapping heat long after that methane has dissipated. 

When we compare methane and CO2 on a 20-year basis, we’re comparing how much heat one ton of methane traps in its 14 years to how much heat one ton of CO2 traps in 20 years.

When we compare them on a 100-year basis, we’re still looking at how much heat methane traps in 14 years—because it dissipates and stops trapping heat after that—but now we’re comparing it to how much heat CO2 traps over 100 years. That measurement increases the effect of CO2 compared to methane, because it measures 80 more years of CO2 warming.

The difference is huge. Scientists estimate that over a 20-year period one ton of methane captures as much heat as around 80-100 tons of CO2, while over 100 years one ton of methane captures only as much heat as about 28 tons of CO2.

So which measurement is best? That’s a judgement call.

If we were to compare one ton of methane’s total global warming potential to one ton of CO2’s total global warming potential, we might use a 300-year time frame, or even, as scientists have at times done, a 500-year time frame, to make sure we capture the total warming effect of carbon.

But nobody does that because we humans are more concerned with the relatively near future than the very distant future. The question is how near a future should we be focused on

If one is more concerned with the 100-year outlook, then the 100-year timeframe makes sense. Because some greenhouse gases, like CO2 last for hundreds of years, GWP100 strikes a balance between the short-lived and long-lived greenhouse gases. Because methane drops out of the atmosphere so quickly, its warming effects can be reversed more quickly—whenever we do get around to reducing it. Advocates of this approach argue that we should not sacrifice long-term gains for short-term ones.

But many scientists now argue that GWP100 is just an “accident of history,” selected as the standard for the 1992 Kyoto Protocol when scientific understanding of methane’s global warming potential was less developed. What has been learned over the past 30 years about how significant methane’s short-term impacts lead many to favor the GWP20 standard. The idea is that warming is happening quickly enough that we need to make big gains now, within the next two decades, to prevent substantial harm. Reducing methane is the fastest way to do that.

The common use of the GWP100 framework has led the Hochul administration to call it “international best practices.” But being the more common practice does not automatically make it the best one. 

And while climate scientists probably mostly lean toward the 20-year time frame now, science does not definitively tell us whether we ought to focus on the short-term or long-term, as both are problematic.

In short, it’s very much a judgement call on which timeframe to use.

The Choice of Framework Determines How Much Greenhouse Gases Are Reduced, and How Much Emissions Remain 

Even if GWP20 wins the debate from a cutting-edge science perspective, the choice has many political consequences that also must be considered.

New York’s Climate Act requires the state to reduce its 2050 greenhouse gas emissions by 85 percent from a 1990 baseline. But the calculation of that baseline itself depends on whether we calculate it using GWP20 or GWP100.

The state’s published numbers from 2019 demonstrate how big the difference is. Table ES-2 of the Department of Environmental Conservation’s 2022 Statewide GHG Emissions Report shows that using the GWP20 method, New York’s total greenhouse gas emissions in 2019 was 376 million tons of CO2 equivalency, while using the GWP100 method, New York emitted only 194 million tons, about 49 percent less!

Obviously it would be easier to cut 85 percent of a lower amount than of a larger baseline, but cutting 85 percent of the larger baseline would produce a greater amount of emissions reductions.

Now for the mind-bender—even though the state would reduce more emissions under GWP20, we would calculate it as still emitting more than if we used GWP100.

Some simple math with made-up numbers can make this clear.

Assume that under the 100-year model New York is estimated to emit 100 tons of greenhouse gases, while under the 20-year model we estimate its emissions at 200 tons. 85% of 100 is 85 tons, while 85 percent of 200 tons is 170 tons. So using the 20-year model leads to the state making greater absolute reductions in greenhouse gas emissions.

But now let’s look at the remaining 15% of emissions. 15 percent of 100 is only 15 tons, while 15 percent of 200 is 30 tons. So, paradoxically, under GWP20, we both reduce more emissions and count ourselves as still emitting more than under GWP100.

That’s a nice mathematical paradox, but it doesn’t get us any closer to a definitive answer on whether GWP20 or GWP100 is the right approach. 

For climate activists, this makes GWP20 more attractive. It not only leads to more absolute emissions reductions, but by leaving the calculated amount of continuing emissions higher, it gives them political ammunition to continue pressing for even greater cuts.

For those concerned about the costs and challenges of meeting mandated emissions reductions, GWP100 is attractive because it’s easier, and so presumably less costly, to reduce a smaller absolute amount of emissions.

Considering Costs and Benefits of GWP20 vs. GWP100

GWP20 clearly has the highest total benefits, but also the highest total costs. GWP100 has lower total costs, but also lower total benefits. From an economic perspective, we should choose the one that maximizes the net value when we subtract costs from benefits.

People naturally do this in their own personal spending decisions. They don’t always buy best available car or rent the best available apartment. As nice as those things are, sometimes the price is just too high.

But in politics, because people aren’t spending their own money, they lose that sense of caution and often want to maximize total benefits, regardless of cost. Some will even come to believe that the goal they’re supporting is so valuable that it’s impossible to overpay for it.

But it’s always possible to overpay for something, even mitigation of greenhouse gases.

Unfortunately, nobody seems to have yet done the analysis that tells us whether GWP20 or GWP100 maximizes our net benefits. The state’s Department of Environmental Conservation is in a position to do so, but if they have, they have not made the result public.

It’s worth noting that when we do overpay for benefits—like spending more for an apartment than it’s worth—not only do we end up worse off overall, but we are sacrificing opportunities to use our financial resources for more valuable purposes. In this case, one could either think there are other public policy goals that could use that funding, or one could think the money is best left in consumers’ pockets so they can better meet their own wants and needs. Either way, paying more than necessary for greenhouse gas emissions does harm to other good ends.

The Role of Practical Political Considerations 

Surveys show that while a majority of the public wants to combat climate change, their willingness to pay for climate policies is very low. Maybe their willingness should be higher, but in politics you have to work with the people you have, not the people you wish you had.

Reducing greenhouse gas emissions faces a very challenging problem of costs and benefits. The costs will be felt immediately, and they’ll be felt strongly, as they hit people right in the wallet. The benefits will come years in the future, and to the extent they largely consist of things that don’t happen, they won’t be felt very strongly. Humans just don’t like to pay high upfront costs for abstract future benefits.

This is true even if those benefits ultimately outweigh the costs. So even if GWP20 does maximize net benefits, it could ultimately prove politically unacceptable and cause a backlash against the state’s climate policy, causing less overall reduction in greenhouse gas emissions.

In a recent op-ed arguing for GWP100, two administration officials correctly noted that, “Fighting climate change won’t work if people and businesses can’t afford it.”

Conclusion: Choosing a Time Frame Is Political, not Scientific

Neither climate science nor economics wholly answers the question of whether to use GWP20 or GWP100. The knowledge we get from them should not be ignored, but they are not the only considerations. Nor should the voices of the loudest minority necessarily set our policies.

Practical political considerations remind us that we need to choose policies that most of the public can—even if grudgingly—go along with. Perhaps the public can be persuaded to bear the higher costs of GWP20 in order to reap the greater benefits. But it will be no surprise if, when they start feeling the costs, they react by demanding an approach they can better afford.

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