Greenhouse Gases and Global Warming: Understanding CO₂, Methane, and Water Vapor

Carbon dioxide (CO₂) shows the strongest overall correlation with global warming among greenhouse gases, primarily because it accounts for the largest share of human-caused warming and accumulates in the atmosphere for centuries to millennia. While methane is more potent per molecule in the short term and water vapor is quantitatively the most abundant greenhouse gas, CO₂ remains the primary driver of anthropogenic (caused or produced by humans) climate change due to its dominant contribution to radiative forcing and extremely long atmospheric lifetime.climate+1

The Dominant Role of Carbon Dioxide

Carbon dioxide is responsible for approximately 64 percent of the total warming influence from human-produced greenhouse gases as of 2022. Since 1990, CO₂ has contributed 78 percent of the increase in the atmosphere’s heat-trapping capacity. This dominant position stems from several factors:climate

Eliminating all human caused CO2 from being released into the atmosphere would mean no beer. Beer brewing creates CO2.

The atmospheric concentration of CO₂ has risen dramatically due to fossil fuel combustion and land use changes, and once emitted, CO₂ persists in the atmosphere for an extraordinarily long time. After 100 years, around 40 percent of emitted CO₂ remains in the atmosphere, and after 1,000 years, approximately 20 percent still persists. This longevity means CO₂ accumulates continuously, creating a sustained and increasing warming effect.resources

According to many, Ice core records extending back 800,000 years demonstrate strong correlations between CO₂ levels and temperature reconstructions throughout Earth’s climate history. However, this isn’t readily apparent when plotting CO2 and temperature because proxies must be used for temperature going back that far. Daniel Gabriel Fahrenheit produced precise mercury-in-glass thermometers and a reliable temperature scale, enabling accurate, repeatable air and water temperature measurement and recording in the early 1700s. But world wide recording of temperature didn’t take place until much later. To avoid arguments regarding the temperature data it is best to concentrate on periods of time where temperature has been carefully recorded. Current CO₂ concentrations are higher than at any point in at least the past 800,000 years, providing compelling evidence of the gas’s close relationship with global temperature.blogs.edf But it is hard to argue otherwise when comparing CO2 to temperature measurements from 1980 to current.

BioScience, analyzes global data on Earth’s atmosphere, oceans, energy, ecosystems, food systems, and more. Researchers identified our planet’s so-called vital signs, including ocean temperature, surface temperature, sea ice extent, and carbon pollution. Of the 34 vital signs, 22 were at record levels, indicating a highly stressed Earth system.

2025 State of the Climate Report: Our Planet’s Vital Signs are Crashing By Grace van Deelen29 October 2025

Methane’s Potent but Shorter-Lived Impact

Methane (natural gas) is the second-largest contributor to global warming, accounting for 19 percent of the total warming influence from greenhouse gases. While methane represents a smaller share than CO₂, it is far more potent on a per-molecule basis.climate

One kilogram of methane has a global warming potential (GWP) approximately 27-30 times stronger than CO₂ over a 100-year timeframe. Over a shorter 20-year period, methane’s warming impact is even more dramatic—approximately 80-86 times stronger than CO₂. This higher short-term potency occurs because methane molecules are exceptionally effective at absorbing infrared radiation emitted by Earth’s surface.epa+4

Methane’s Atmospheric Lifetime

Methane remains in the atmosphere for approximately 9 to 12 years on average. More specifically:rifs-potsdam+4

The average time a physical methane molecule stays in the atmosphere is estimated at around 9.6 yearswikipedia
The “perturbation lifetime”—the time the atmosphere is affected by a methane emission before reaching equilibrium—is approximately 12 yearsrifs-potsdam+1

This relatively short atmospheric lifetime distinguishes methane fundamentally from CO₂. Methane is primarily removed from the atmosphere through chemical oxidation, particularly reactions with hydroxyl radicals. After breakdown, methane oxidizes to CO₂, contributing additional long-term warming.ghginstitute+1

The short lifetime creates an important asymmetry in climate policy: constant CO₂ emissions lead to continuously rising temperatures, while constant methane emissions eventually lead to a stable (though elevated) temperature plateau. This occurs because after several decades of sustained methane emissions, new emissions balance out with the chemical breakdown of existing methane in the atmosphere.resources

Water Vapor: The Feedback Amplifier

Water vapor is Earth’s most abundant greenhouse gas, responsible for about 41-67 percent of the overall greenhouse effect (with clouds accounting for roughly 25 percent). Despite its quantitative dominance, water vapor is not considered a primary driver of current global warming. Instead, it acts as a powerful positive feedback mechanism that amplifies warming caused by other greenhouse gases.science.nasa+4

The feedback loop works as follows: as greenhouse gases like CO₂ and methane increase temperatures, more water evaporates from oceans and land surfaces. Warmer air can hold approximately 7 percent more water vapor for every 1°C temperature increase. This additional water vapor then traps more heat, causing further warming and enabling the atmosphere to hold even more moisture.skepticalscience+1

Scientists estimate that water vapor feedback roughly doubles the warming that would occur from CO₂ alone. Atmospheric water vapor concentrations are increasing at a rate of 1 to 2 percent per decade as the climate warms.science.nasa+1

Unlike CO₂ and methane, water vapor has an extremely short atmospheric residence time of approximately nine days on average, as it constantly cycles through evaporation, condensation, and precipitation. This rapid cycling means water vapor concentrations respond to temperature changes rather than driving them independently. Human activities do not directly control atmospheric water vapor levels in the same way they control CO₂ and methane emissions.theconversation+2

Comparative Impact and Policy Implications

When evaluating which gas best correlates with global warming, context matters significantly:

For long-term temperature changes: CO₂ shows the strongest correlation because its multi-century to millennial persistence means every ton emitted contributes to near-permanent elevated temperatures. Stabilizing global temperature requires CO₂ emissions to reach net zero.resources+1

For near-term warming rates: Methane shows strong correlation with short-term temperature increases due to its high immediate radiative forcing, despite its shorter lifetime. Reducing methane emissions offers opportunities for rapid climate benefits within decades.ccacoalition+3

For total greenhouse effect: Water vapor correlates strongly with temperature but as a consequence rather than a cause of anthropogenic warming, amplifying the effects of the long-lived greenhouse gases that humans emit.skepticalscience+2

The most comprehensive measure of greenhouse gas warming influence—NOAA’s Annual Greenhouse Gas Index—shows that by 2022, the combined warming influence of human-produced greenhouse gases had risen 49 percent since 1990, with CO₂ as the dominant contributor, followed by methane. This index confirms that while multiple gases contribute to warming, carbon dioxide remains the primary gas whose atmospheric concentration correlates most strongly with the trajectory of global temperature increase.gml.noaa+1

Global and U.S. Methane Emissions Reduction Efforts (2015–2025)

Over the past decade, the world has implemented several major initiatives to limit methane releases from utilities and the fossil fuel industry:

U.S. Federal Actions:
The Inflation Reduction Act of 2022 created the Methane Emissions Reduction Program, providing $1.36 billion in financial and technical assistance to reduce methane from oil and gas operations through equipment upgrades, leak detection and repair (LDAR), and monitoring systems. A Waste Emissions Charge (WEC) was established, beginning at $900 per ton in 2024 and increasing to $1,200 per ton in 2025 and $1,500 per ton in 2026, though this has faced legal challenges. The EPA also strengthened the Greenhouse Gas Reporting Program (GHGRP) and issued regulations on methane from oil and gas infrastructure, including requirements to ban routine venting and flaring of natural gas.epa+2

International Efforts:
The European Union’s Methane Regulation (entered August 2024) requires mandatory measurement, reporting, and verification of methane emissions from oil and gas operators within the EU and imposes detection and repair requirements with strict timelines (5–15 working days). The regulation bans venting and routine flaring and applies to both domestic production and imported fossil fuels. The Oil and Gas Methane Partnership 2.0 (OGMP 2.0)framework, adopted by industry, now covers 42 percent of global oil and gas production and emphasizes direct measurement rather than estimates.energy.europa

Seattle’s Natural Gas Policies and the Electrification Effort

Building Emissions Performance Standard:
Seattle passed a Building Emissions Performance Standard (signed December 2023), requiring all existing commercial and multifamily residential buildings over 20,000 square feet to reach net-zero emissions by 2050 (2045 for commercial buildings). This effectively mandates replacement of oil and gas furnaces with heat pumps and electric systems, though it avoids an explicit natural gas ban to circumvent legal challenges faced by other cities.cascadepbs+1

State-Level Setback:
Washington voters approved Initiative 2066 in November 2024, which preempts cities from phasing out natural gas or enacting policies that discourage natural gas use. This significantly constrains Seattle’s ability to pursue aggressive electrification policies and contradicts some climate goals.wacities

Oil Heating Phase-Out:
Seattle has been more successful with heating oil elimination. The city estimated approximately 18,000 homes still used oil heating as of 2019. Mayor Jenny Durkan proposed a heating oil tax ($0.24/gallon) with rebates and grants for electric heat pump conversion, targeting to eliminate all oil heating by 2030. By 2024, the city expanded incentives with a $4,000 rebate for moderate-income households, stackable with a $2,000 federal tax credit and $2,000 city rebate for total incentives of up to $8,000.westseattleblog+1

Natural Gas Price Trends and Consumer Adoption

Price Concerns in Seattle:
Natural gas prices in the Seattle-Tacoma-Bellevue area have been volatile but not uniformly rising. In December 2024, piped natural gas cost $1.425 per therm, higher than the prior year’s $0.985 per therm—a 45 percent increase. However, NW Natural reported that after accounting for inflation, residential customers are paying less in 2024 than 20 years ago($83.70 average bill in 2024 vs. $85.79 in 2006). As of 2025, Washington natural gas rates are actually decreasing—residential customers using 56 therms monthly will see about a $4 monthly reduction starting November 2025. Puget Sound Energy (PSE) customers in the Seattle area faced a 10.6 percent natural gas rate increase in January 2025, followed by an additional 1.8 percent increase in 2026.bls+2

Not the Primary Driver of Heat Pump Adoption:
While price is a factor, natural gas affordability does not appear to be the dominant reason for lower natural gas hookups in new construction. Instead, the shift to heat pumps is driven by:

Heat pump cost savings: Converting from oil heat saves approximately $1,100 per year; switching from gas furnaces also yields substantial operating savingssmartcitiesdive+1
Climate policy incentives: Federal, state, and local rebates totaling up to $8,000 make heat pumps financially attractivesmartcitiesdive
Building codes and emissions standards: Seattle’s Building Emissions Performance Standard creates pressure to electrifycascadepbs
Environmental consciousness: Regional values in the Pacific Northwest favor decarbonizationkuow

Oil, Propane, and Heating Furnaces in Seattle and the Pacific Northwest

Current Usage:
The Pacific Northwest region uses oil and propane more extensively than the national average, though Seattle proper has lower usage than rural areas:

Nationally: Oil heating is used in approximately 5 million households (4 percent), and propane in another 5 million (4 percent)atlasbuildingshub
Seattle specifically: Approximately 18,000 homes use oil heat as of 2019westseattleblog
Washington State propane usage: Below 4 percent of households region-wide, with propane concentrated in agricultural and eastern Washington areascleanenergytransition

Future Trajectory:

Oil and propane furnaces will likely continue in use but increasingly paired with electric heat pump augmentationunder the following conditions:

Regulatory Pressure: Seattle’s Building Emissions Performance Standard creates a 27-year transition timeline (2023–2050) for multifamily buildings and 22 years (2023–2045) for commercial buildings, but these timelines are long enough that many existing systems will remain in operation for 10–20 more years as supplemental heating.seattletimes+1
Federal Tax Credits Expired: The 30 percent federal income tax credit (up to $2,000) for heat pump installation ended December 31, 2025. This removal eliminates a major financial incentive, likely slowing heat pump adoption rates for homeowners without state or local rebates.evergreenhomeheatingandenergy
Dual-System Adoption: Many Seattle-area homeowners are choosing hybrid or dual-fuel systems that combine high-efficiency heat pumps with existing gas or propane furnaces for backup heating during extreme cold. This approach leverages existing infrastructure while adding electric capacity.reddit+1
Initiative 2066 Constraint: Washington’s Initiative 2066, passed in November 2024, legally prevents cities from discouraging natural gas use, which will likely slow the phase-out trajectory for gas and propane systems. Utilities are required to continue providing service, limiting aggressive conversion campaigns.wacities
Remaining Oil Systems Accelerating Out: Oil heating is declining more rapidly than gas or propane due to high costs ($1,100+ annually per household), aging infrastructure (most tanks from 1920–1950), environmental liability risks (leaking steel tanks), and targeted incentives. These systems will largely be eliminated by 2030 if Mayor Harrell’s goal is met.westseattleblog+1

Summary: Oil furnaces in Seattle will likely disappear by 2030 due to targeted programs and cost advantages of heat pumps. Natural gas and propane systems will remain prevalent for 15–25 years, increasingly functioning as supplemental backup to heat pumps rather than primary heating, particularly as emissions standards tighten and the operating costs of electric heating advantage heat pumps. However, Initiative 2066 now legally protects gas utility service, meaning Seattle cannot mandate retirement of natural gas systems as aggressively as originally planned.wacities