Global warming potential

Based on Wikipedia: Global warming potential

Here is a number that should unsettle you: 81.2. That's how many times more powerful methane is than carbon dioxide at trapping heat in our atmosphere, at least over a twenty-year window. Release one tonne of methane into the sky, and it's as if you've released eighty-one tonnes of carbon dioxide. The math is that brutal.

But wait another eighty years, and methane's potency drops to less than a third of that figure. The same gas, the same molecule, the same basic chemistry—yet its climate impact varies wildly depending on when you're measuring. This is the strange world of global warming potential, a concept that sounds straightforward but hides surprising complexity beneath its surface.

The Yardstick Problem

Scientists needed a way to compare different greenhouse gases. The obvious choice was to measure them against carbon dioxide, the most famous and abundant of the heat-trapping gases we pump into the atmosphere. So they created global warming potential, or GWP—a number that tells you how much heat a gas traps compared to the same amount of carbon dioxide.

Carbon dioxide gets a GWP of one. By definition. It's the baseline, the reference point against which everything else is measured.

The concept seems simple enough. But here's where it gets interesting: a gas's global warming potential isn't a fixed number. It changes depending on how far into the future you're looking. And this isn't some minor statistical quirk. The differences can be enormous.

Take methane again. Over twenty years, its GWP is about 81. Over a hundred years, it drops to around 28. Over five hundred years? Just 8. Same gas, same heat-trapping properties, wildly different numbers.

The Disappearing Act

What's happening here? The answer lies in how long different gases stick around in the atmosphere.

Carbon dioxide is stubborn. When we release it, a portion lingers for centuries, even millennia. The carbon cycle eventually reabsorbs it—into oceans, forests, soils—but the process is slow. Very slow.

Methane, by contrast, is a sprinter. It enters the atmosphere, wreaks havoc, then breaks down. Chemical reactions in the air convert it into water vapor and, ironically, carbon dioxide. The whole process takes about twelve years. So while methane is far more potent at trapping heat than carbon dioxide molecule for molecule, it doesn't last nearly as long.

This creates a fascinating temporal problem. If you're worried about what the climate will look like in twenty years, methane matters enormously. If you're thinking in terms of centuries, carbon dioxide dominates because it's still there, accumulating, year after year after year.

The Rogues Gallery

Methane isn't even the most potent greenhouse gas. Not by a long shot.

Consider sulfur hexafluoride, a synthetic gas used mainly in electrical equipment as an insulator. Its hundred-year GWP is 22,800. That means releasing one kilogram of sulfur hexafluoride is equivalent to releasing nearly twenty-three tonnes of carbon dioxide. The gas is extraordinarily stable—it simply doesn't break down. Molecules released today will still be trapping heat three thousand years from now.

Then there are the hydrofluorocarbons, or HFCs. These chemicals were created as replacements for the chlorofluorocarbons that were destroying the ozone layer. In that sense, they were a success—HFCs don't harm ozone. But nobody paid much attention to their climate impact. Some HFCs have global warming potentials in the thousands.

Nitrous oxide, better known as laughing gas, falls somewhere in between. It has a GWP of 273 over a hundred years and persists in the atmosphere for about 109 years. Most of it comes from agricultural practices—fertilizers break down in soil and release nitrous oxide as a byproduct. We've been inadvertently pumping this gas into the atmosphere ever since the invention of modern farming.

The Hundred-Year Convention

Given that global warming potential changes so dramatically depending on the timeframe, which number should we use? Twenty years? A hundred? Five hundred?

The world settled on a hundred years. Not because a century is scientifically special, but because it offered a reasonable compromise. It's long enough to capture the effects of longer-lived gases while still giving meaningful weight to shorter-lived ones like methane.

This choice has enormous practical consequences. International climate agreements, carbon taxes, corporate emissions reporting—almost everything uses the hundred-year GWP. When a company claims to be carbon neutral, they're typically using hundred-year values to calculate their equivalent carbon dioxide emissions.

The Kigali Amendment, a landmark international agreement to phase down those troublesome hydrofluorocarbons, explicitly requires countries to use hundred-year GWP values from the Intergovernmental Panel on Climate Change's Fourth Assessment Report. Not the latest report—the 2007 one. This might seem odd, but there's logic to it: everyone needs to use the same numbers for the system to work, and constantly updating the baseline would create chaos in tracking progress.

New York State, characteristically, decided to go its own way. Its Climate Leadership and Community Protection Act uses twenty-year GWP values instead. This makes methane look three times worse than under the international standard. Whether this represents better science or just different policy priorities depends on whom you ask.

Carbon Dioxide Equivalent: The Common Currency

Global warming potential gives us a conversion rate. But to actually compare emissions, we need a common currency. Enter carbon dioxide equivalent, written variously as CO2e, CO2eq, or CO2-e (scientists love their notation).

The math is straightforward: multiply the mass of any greenhouse gas by its GWP, and you get the equivalent mass of carbon dioxide that would cause the same warming. If methane has a GWP of 28 over a hundred years, then ten tonnes of methane equals 280 tonnes of CO2 equivalent.

This allows for meaningful comparisons across vastly different activities. A factory releasing HFCs, a cattle farm belching methane, a power plant burning coal—all can be expressed in the same units. It's like converting different currencies to dollars so you can compare prices.

The UN's climate panel talks in billions of tonnes of CO2 equivalent. Industry prefers million metric tonnes. Automakers express vehicle emissions in grams per kilometer. Different scales for different purposes, but all fundamentally measuring the same thing: how much warming are we causing?

What Makes a Gas Powerful?

Two factors determine a gas's global warming potential: how effectively it absorbs infrared radiation, and how long it survives in the atmosphere.

The first part is pure physics. Greenhouse gases work by absorbing thermal radiation—the heat that Earth emits back toward space—and re-emitting it in all directions, including back down toward the surface. Different molecules absorb different wavelengths of this radiation, depending on their structure and how their atoms vibrate.

But there's a subtlety here. The atmosphere isn't a blank slate. Carbon dioxide already absorbs strongly at certain wavelengths, and we've added so much of it that those wavelengths are essentially saturated—nearly all the radiation at those frequencies gets absorbed regardless of whether we add more carbon dioxide. Adding another CO2 molecule doesn't do much; the radiation it would have absorbed is already being absorbed by existing molecules.

This is why some gases have such extraordinary global warming potentials. They absorb at wavelengths where the atmosphere is still relatively transparent—in the "windows" between the absorption bands of carbon dioxide and water vapor. A gas that operates in one of these windows has an outsized impact because it's blocking radiation that would otherwise escape to space unimpeded.

The Curious Case of Water Vapor

Water vapor is the elephant in the room. It's actually the most important greenhouse gas in Earth's atmosphere, responsible for more warming than carbon dioxide. Yet its global warming potential, by the standard definition, is essentially zero. How can this be?

The answer reveals something important about how GWP is defined. The concept specifically measures the impact of emissions—gas we add to the atmosphere through human activity. And while we do emit water vapor (cooling towers, irrigation, jet contrails), it doesn't accumulate. Precipitation removes it within days or weeks. Any water vapor we add is gone almost immediately.

But water vapor does matter for climate change, just through a different mechanism. As the planet warms from other greenhouse gases, the atmosphere can hold more water vapor. This additional water vapor traps more heat, causing more warming, allowing the atmosphere to hold still more water vapor. It's a positive feedback loop—one of several that amplify the initial warming caused by our carbon dioxide and methane emissions.

The GWP calculation deliberately excludes these indirect effects. It only measures the direct heat-trapping from the gas itself, not the chain reactions it might trigger. This keeps the math tractable but misses some important dynamics.

The Saturation Problem

Here's a twist that makes the whole system more complicated: carbon dioxide's heat-trapping ability isn't linear.

At low concentrations, adding carbon dioxide to the atmosphere has a big effect. But as concentrations rise, each additional molecule does less and less. The absorption bands get saturated. It's like adding more sponges to a bucket that's already soaking up all the water—at some point, more sponges don't help much.

Methane and nitrous oxide behave differently. Their absorption happens at wavelengths that aren't as saturated, so adding more of these gases still has a relatively strong effect. This means that as we continue pumping carbon dioxide into the atmosphere, the relative importance of other greenhouse gases actually increases.

Since all GWP calculations use carbon dioxide as the baseline, and since carbon dioxide's effect is becoming less linear, the whole system of comparison gets skewed. The numbers in those official tables aren't quite as solid as they appear.

The Methane Arithmetic

Here's a practical example that illustrates both the power and the limitations of these concepts.

Natural gas leaks are a major source of methane emissions. But when natural gas burns, the methane converts to carbon dioxide. This might seem like trading one problem for another—and it is—but the math works out in favor of burning.

Start with one tonne of methane. Using the hundred-year GWP of 25 (from older IPCC reports), that's equivalent to 25 tonnes of carbon dioxide. Now burn it. The combustion produces 2.74 tonnes of actual carbon dioxide (methane is lighter than CO2, but the reaction produces more mass of product than reactant because it incorporates oxygen from the air). Those 2.74 tonnes of carbon dioxide have a GWP of, well, 2.74—they're just carbon dioxide.

Net result: you've reduced the climate impact from 25 tonnes of CO2 equivalent to 2.74 tonnes. A reduction of about 89 percent.

This is why capturing and burning methane from landfills and oil wells makes sense from a climate perspective, even though it releases carbon dioxide. The carbon dioxide is bad, but the methane would have been much worse.

The Policy Tightrope

All of this matters because global warming potential isn't just a scientific curiosity—it's the foundation of climate policy. When governments set emissions targets, when companies calculate their carbon footprints, when carbon offset programs verify their impact, they're all relying on these GWP values.

The choice of timeframe—twenty years versus a hundred—isn't neutral. Using shorter timeframes emphasizes the impact of methane and other short-lived gases. Using longer timeframes prioritizes carbon dioxide, which persists for centuries.

Those who argue for focusing on methane point out that we're already seeing climate impacts today, and reducing methane could deliver quick wins. Cut methane emissions in half, and you'd see measurable cooling effects within a decade or two. Carbon dioxide reductions, by contrast, take much longer to show results.

Those who argue for focusing on carbon dioxide counter that it's the long-term game that matters. Methane reductions only buy us time; if we don't also slash carbon dioxide emissions, we're just delaying the inevitable. And there's a risk that focusing on methane could become an excuse for not tackling the harder problem of fossil fuel combustion.

Both sides have a point. The climate system doesn't care about our policy frameworks; it just responds to whatever gases are in the atmosphere. The hundred-year GWP is a useful simplification, but it's not the truth—it's a convention, a choice we made because we needed some number to use.

The Numbers Keep Changing

Scientists have recalculated global warming potentials several times as our understanding improves. The IPCC's Sixth Assessment Report from 2021 lists different values than the Fifth Assessment Report, which listed different values than the Fourth. Methane's GWP has bounced around particularly dramatically as researchers refined their understanding of its atmospheric chemistry.

This creates headaches for policymakers. If you set regulations based on one set of numbers and then the science updates, do you redo all the calculations? The Kigali Amendment's solution—freezing the GWP values at 2007 levels—trades accuracy for consistency. Everyone uses the same numbers even if they're slightly outdated.

The values also vary by source. Different scientific papers, using different assumptions about atmospheric chemistry and background conditions, produce different estimates. The numbers in official IPCC reports represent a kind of consensus, but they come with uncertainty ranges that the policy world tends to ignore.

Beyond the Numbers

Perhaps the most important thing about global warming potential is what it reveals about the challenge of managing climate change. We're trying to compare fundamentally different phenomena—gases with different lifetimes, different absorption properties, different sources—using a single index. It's like trying to compare the danger of different diseases using one number. Useful, but inevitably incomplete.

The concept has also shaped how we think about the problem. By expressing everything in terms of carbon dioxide equivalent, we've made carbon dioxide the default frame of reference. This has benefits—it gives us a common language—but it also obscures the distinct characteristics of other greenhouse gases. Methane isn't just "like 28 carbon dioxides"; it's a different substance with different dynamics.

Still, for all its limitations, global warming potential remains essential. It lets us add up emissions across different sectors and gases. It enables international agreements and corporate reporting. It provides a basis for comparing policy options. Without some way to compare different greenhouse gases, coordinated climate action would be nearly impossible.

The number 81.2 that opened this essay—methane's twenty-year global warming potential—is both precise and approximate. It emerges from careful scientific calculation, yet it depends on choices about timeframes and assumptions about atmospheric chemistry. It tells us something real about how methane affects the climate, yet it doesn't capture everything we need to know.

That's the nature of these tools. They're imperfect instruments for understanding an enormously complex system. But they're the best instruments we have, and the climate won't wait for us to develop perfect ones.