50,000 Years

 It will take 50,000 years for the Earth to return to temperatures found in 2005. That is, if we do nothing such as to actively remove CO2, and we do not add more CO2.

From AI Gemini:

Without active carbon removal, 50,000 years is a scientifically realistic timeframe for the Earth to naturally cool back down to mid-2000s baseline temperatures purely through natural Earth systems.

While it sounds counterintuitive—given that carbon dioxide emissions accumulated in just two centuries—the natural removal mechanisms operate on wildly different timescales.

The Three-Phase Carbon Removal Timeline

Once fossil fuel emissions cease, atmospheric CO2 doesn't disappear all at once. It decays in distinct waves driven by different planetary processes:

1. Decades to Centuries (Fast Phase): Plants, soil, and surface ocean waters rapidly absorb roughly 50% to 60% of human emissions. This caps peak warming, but it leaves a massive blanket of carbon trapped in the atmosphere and upper ocean.
2. 1,000 to 10,000 Years (Ocean Equilibrium): Deep ocean circulation eventually mixes the excess carbon into deep water. Ocean acidification also dissolves carbonate sediments on the seafloor to neutralize atmospheric carbon dioxide. After several thousand years, about 15% to 25% of our historical carbon emissions remain stubbornly in the air.
3. 10,000 to 100,000+ Years (Silicate Weathering): The remaining tail of atmospheric CO2 can only be scrubbed by the slow chemical weathering of silicate rocks (rainwater reacting with exposed rock, washing carbon into the oceans to lock it into limestone). This process takes roughly 50,000 to 100,000 years to pull the atmosphere back to late 20th- or early 21st-century levels.

Other Long-Term Drivers

Thermal Inertia: The oceans have absorbed over 90% of global warming's excess heat. Even if atmospheric carbon drops, that oceanic heat bleeds back out into the surface atmosphere over multi-centennial scales, delaying surface cooling.
Loss of Ice Albedo: Melted polar ice exposes dark ocean water and land, absorbing more solar radiation. Re-growing major ice sheets to 2005 levels takes tens of thousands of years.

The main caveat to the 50,000-year timeline is human intervention. If future generations deploy large-scale, technological Carbon Dioxide Removal (CDR)—such as direct air capture or enhanced rock weathering—we could artificially shorten that curve down to centuries rather than geological epochs. But relying on the Earth's natural thermostat alone requires a 50,000-year wait.