The Carbon Budget

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Greenhouse gases (GHGs) such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) all contribute to global warming, but they do so at different rates and over different timescales. The
warming budget, therefore, refers to the total allowable emissions of all GHGs while still keeping global temperature rise within a specific limit, such as 1.5°C or 2°C above pre-industrial levels. Scientists estimate that to have a 50% chance of staying below 1.5°C, humanity can emit only about 250-300 gigatons of CO2 beyond 2023 levels. At current emissions rates (~40 gigatons per year),
this budget could be exhausted within a decade. According to the IPCC, “Historical cumulative net CO₂ emissions from 1850 to 2019 were 2400 ± 240 GtCO2, of which more than half (58%) occurred between 1850 and 1989, and about 42% occurred between 1990 and 2019.” In 2023, global fossil CO₂ emissions were 10.1 ± 0.5 GtC in 2023, 66% over 1990.

The Warming Budget: More than Just CO2

While CO₂ is the most well-known contributor to climate change, other greenhouse gases play a critical role in global warming. Two other main greenhouse gases are:

• Methane (CH₄): Has higher warming potential than CO₂— approximately 28 times greater over a 100-year period—with sources from agriculture, land use change, the energy industry, and landfill waste.
• Nitrous Oxide (N₂O): Has much higher warming potential than CO₂—about 265-298 times the warming potential of CO₂ over a 100-year period—with sources from agriculture, land use
  change, fuel combustion, industry, and waste.

CO₂ equivalent (CO₂e) is a metric that converts different greenhouse gases into a common scale based on their warming potential over a specific period, allowing for a more comprehensive assessment of climate impact.

Why Does the Warming Budget Matter for Solar Geoengineering?

The warming budget represents the total amount of heat-trapping gases that can theoretically be emitted while staying within a safe climate boundary. As this budget shrinks, the urgency of climate interventions increases. Solar geoengineering (also known as solar radiation modification or SRM) is being considered as a temporary measure to counteract warming, but it does not remove
greenhouse gases from the atmosphere. Instead, it reflects sunlight to reduce warming.
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Understanding the full warming budget, including the effects of methane and nitrous oxide, is essential because SRM primarily addresses temperature rise, but not the other impacts of emissions, such as ocean acidification and extreme weather intensification. If SRM is used without significant emissions reductions, the underlying problem of accumulating greenhouse gases remains unaddressed

Key Takeaways

The warming budget includes more than just CO₂—methane and nitrous oxide also significantlyimpact global temperatures. As the budget shrinks, the pressure to mitigate climate risks increases.While SRM may offer a temporary cooling effect, it does not address the root causes of climatechange: accumulated greenhouse gases and their broad environmental consequences. Effectiveclimate action requires a combination of deep emissions reductions, carbon dioxide removal, and acomprehensive understanding of all greenhouse gases contributing to warming.

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