Quick navigation

1. What the greenhouse-gas past says

For the recent ice-core era, we have direct measurements of past air trapped in ancient ice. For deeper time, we do not have neat little atmospheric jars from fifty million years ago, because geology refuses to organize itself for our convenience. Instead, scientists reconstruct ancient CO₂ levels from multiple proxies and then compare those greenhouse-gas estimates with temperature, sea-level, and ice-sheet evidence.

The result is not a single perfect curve. It is a strong pattern: when long-lived greenhouse gases remain elevated long enough, Earth usually becomes much warmer, ice sheets shrink, and sea level rises over long timescales. The estimated temperature you see in the chart below is the final or committed temperature that was eventually reached due to that level of the various greenhouse gases listed in the four columns after the climate description column.

Fuck

Benchmark table: greenhouse gas levels and temperatures through time

2. Why does the same CO₂ level not cause the same immediate temperature

This is where people get lost, and climate communication often gets lazy. The phrase “we were at this CO₂ level in the Pliocene” is useful, but it can be misleading unless we include the missing context.

Reason 1: The climate system has not fully caught up yet

CO₂ changes the energy balance fast, but oceans, ice sheets, ecosystems, and carbon-cycle feedbacks respond much more slowly. That means today’s temperature is not the final temperature implied by today’s greenhouse-gas levels. There is a lag between forcing and full Earth-system response.

Reason 2: The ancient Earth had different boundary conditions

Past warm periods had different ice sheets, coastlines, vegetation patterns, and ocean circulation. Those differences matter. They can amplify or reshape warming. So the same CO₂ number does not always give the same exact temperature right away, because the rest of the climate machine is not identical.

Reason 3: Today’s aerosol pollution masks part of the greenhouse warming

Human-produced aerosols partly cool the climate by reflecting sunlight and changing clouds. That means today’s measured warming is somewhat suppressed relative to the full greenhouse forcing. This is one reason modern temperatures can sit below the long-run temperatures suggested by comparable CO₂ or CO₂e states.

Reason 4: Today’s CO₂e matters, not just CO₂ alone

Modern civilization has not only raised CO₂. It has also raised methane, nitrous oxide, and several industrial gases. That means a straight “today’s CO₂ versus ancient CO₂” comparison understates the total long-lived greenhouse forcing now active in the atmosphere.

3. Why the past still warns us anyway

Even after all those qualifications, the paleoclimate warning stands. In fact, it gets sharper.

• • Today’s CO₂ is already in the rough neighborhood of the Mid-Pliocene. The past climate was several degrees warmer than preindustrial and had a much higher long-run sea level.
  • Today’s long-lived greenhouse-gas forcing is even stronger when expressed as CO₂e. That means a simple “CO₂-only” analogy probably understates the forcing problem!
  • Slow feedbacks do not rescue us. They are the reason the warning is bigger, not smaller. They mean more warming and sea-level rise can keep unfolding long after the initial forcing increase.

So the correct takeaway is not, “The past was different, therefore no problem.” The correct takeaway is, “The past was different, and even with those differences, it shows where sustained greenhouse forcing tends to take the planet.”

4. Why was ancient warming often slower, and modern warming is much faster

Why were many ancient warmings slower?

• • Many ancient CO₂ increases were driven by tectonics, long volcanic episodes, and slower carbon-cycle changes.
  • Orbital cycles paced glacial changes over thousands of years, with greenhouse gases acting as amplifiers.
  • Oceans and ice sheets had time, at least compared with today, to adjust over long intervals.

Why is modern warming faster?

• • Humans are moving carbon from fossil reservoirs to the atmosphere extremely fast.
  • The modern CO₂ rise rate is roughly 100 to 200 times faster than the natural increase at the end of the last ice age.
  • Methane, nitrous oxide, and industrial gases add extra forcing on top of CO₂.
  • Aerosol declines can reveal more of the greenhouse warming that had been partly masked.

One of the best ancient cautionary episodes is the Paleocene–Eocene Thermal Maximum, or PETM. It was a major ancient warming event, but today’s human carbon release rate is still roughly an order of magnitude faster than the PETM estimate. That is not comforting. That is the opposite of comforting.

How long did warming take in the past after greenhouse gases rose?

There is no single answer, because the climate system has fast and slow parts.

• • Ice-age to interglacial changes: global temperature changed by about 5°C over roughly 5,000 years, with greenhouse gases rising as part of that transition.
  • Pliocene-like states: thousands of years were available for oceans and ice sheets to respond.
  • Deep-time warm climates: the full response often reflects very long Earth-system adjustment, not a quick 50-year jump.
  • Today: some warming shows up quickly, but the full response to current forcing will continue to unfold over decades, centuries, and longer.

5. Greenhouse-gas sources then and now

Ancient greenhouse-gas sources were mostly natural. Modern ones are increasingly industrial and land-use driven. Many natural sources still exist, but the dominant modern forcing comes from human activity.

The short version is this: most of the old natural source categories still exist, but several of the biggest modern drivers are new or massively amplified by civilization. What no longer exists as a modern driver, at least thankfully, is an active flood-basalt apocalypse on the scale of events linked to some past mass extinctions. Humanity decided to improvise its own carbon pulse instead. Bold choice.

6. What current CO₂ and CO₂e levels imply

Current measurements are brutal enough without dramatic music. In 2024, global average atmospheric concentrations reached about 423.9 ppm CO₂, 1942 ppb methane, and 338 ppb nitrous oxide. NOAA’s long-lived greenhouse-gas index translates the total forcing of these long-lived gases to about 539 ppm CO₂-equivalent.

Below is an illustrative committed warming table using standard equilibrium climate sensitivity assumptions. This is not a precise year-by-year forecast. It is a physically grounded way to show the long-run warming that would result if the climate system were allowed to catch up.

Why a CO₂e chart matters

A CO₂-only comparison tells you part of the story. A CO₂e comparison gets closer to the total long-lived forcing burden in the atmosphere right now. That makes today look less like a neat “423 ppm” talking point and more like a broader forcing state with a larger long-run temperature commitment.

7. Specific danger levels by CO₂ and CO₂e

Around 350 ppm CO₂

Often discussed as a safer long-run target because it sits below current levels and below the rough Pliocene danger zone. We are already past it.

Around 386 ppm CO₂

This is a threshold some analysts treat as the entrance into a more clearly irreversible warming phase, especially once slow feedbacks are considered. It is not a magical cliff, but it is a useful warning marker because it sits well into the range where the long-run climate story stops looking comfortably Holocene-like.

Around 400–425 ppm CO₂

This overlaps the upper range of the Mid-Pliocene Warm Period. Paleoclimate evidence suggests that sustained conditions in this zone are compatible with global temperatures roughly 2.5–4°C above preindustrial and substantially higher sea level over time.

Around 450 ppm CO₂

This pushes beyond today’s measured CO₂ into a zone that many long-run analyses associate with roughly 3°C or more eventual warming under central assumptions, and potentially more if Earth-system sensitivity is higher.

Around 500–550 ppm CO₂e

This is where the modern atmosphere already becomes especially alarming in forcing terms. It does not mean we jump overnight to a 3°C or 4°C world. It means the long-lived forcing burden is high enough that a much hotter climate becomes physically credible unless concentrations are brought down and cooling masks are not mistaken for safety.

What humanity should actually be worried about

We should be worried about both the slow-moving lessons of the past and the fast-moving forcing of the present. But if forced to choose which one deserves more fear, the answer is today’s rapid rise. The ancient record tells us where sustained greenhouse forcing can take Earth. The modern rate tells us we are running toward that destination with the brakes removed and then congratulating ourselves for not being there yet.

Three clean conclusions

1. 1. The distant past is a warning, not a comfort. Similar or lower long-run greenhouse conditions often ended up much warmer than today.
   2. Today’s temperature is not the end of the story. The climate system has not fully equilibrated with current CO₂ and CO₂e. There is significantly more heat to come from today's levels.
   3. The modern danger is amplified by speed. Human forcing is arriving much faster than the climate system usually gets pushed, which raises the risk of overshoot, disruption, and feedback activation before society has time to adapt.

Three unusual ancient episodes worth footnoting

1. 1. PETM, around 56 million years ago: a geologically abrupt carbon release drove about 5–6°C of warming and lasted roughly 200,000 years. It is one of the best ancient analogues for fast carbon release, and modern emissions are still faster.
   2. End-Permian crisis, around 252 million years ago: massive volcanic carbon release from the Siberian Traps helped drive extreme warming, ocean acidification, deoxygenation, and mass extinction.
   3. End-Triassic crisis, around 201 million years ago: huge volcanic pulses linked to the Central Atlantic Magmatic Province added rapid carbon emissions and major ecological upheaval.

8. FAQ

Does the same CO₂ always produce the same temperature?

No. Not immediately. The answer depends on how long the level stays elevated, what the ice sheets and oceans are doing, what aerosols are doing, and what other greenhouse gases are present.

So, is the Pliocene a good analog or not?

It is a useful analog, not a perfect copy. It is good for showing what long-run climate and sea level can look like around similar CO₂. It is not a one-to-one forecast of what will happen next Tuesday.

Why use CO₂e at all?

Because CO₂ is not acting alone. Methane, nitrous oxide, and industrial gases also matter. CO₂e helps express the total long-lived forcing in a more honest way.

Why are today’s temperatures not already as warm as the past analogues suggest?

Because the climate system is still catching up, and because atmospheric soot aerosols partly mask warming. The full Earth-system response takes much longer than a few decades.

Can natural cycles still be causing today’s warming?

They still exist, but they do not explain the speed and size of modern warming. Orbital cycles are too slow, solar changes are too small, and the greenhouse-gas fingerprints line up far better with human activity.

9. Glossary

CO₂

Carbon dioxide, the most important long-lived greenhouse gas added by human activity.

CO₂e

Carbon dioxide equivalent. A way of expressing the warming effect of several greenhouse gases, carbon, methane, and nitrous oxide, as if they were all CO₂.

Committed warming

Warming that has not fully appeared yet but is already implied by the greenhouse gases in the system.

Equilibrium climate sensitivity

The long-run warming expected after atmospheric CO₂ doubles and the fast parts of the climate system adjust.

Earth-system sensitivity

A broader long-run sensitivity that also includes slower feedbacks such as ice-sheet, vegetation, and carbon-cycle changes.

Aerosols

Tiny soot-like particles in the air that can cool the planet by reflecting sunlight or changing clouds.

Boundary conditions

The larger background setup of the planet, including ice sheets, geography, vegetation, and ocean circulation.

10. Bibliography

• • WMO. Greenhouse Gas Bulletin 2025 update for 2024 concentrations.
  • NOAA Global Monitoring Laboratory. Annual Greenhouse Gas Index (AGGI).
  • NOAA Climate.gov. Climate change: atmospheric carbon dioxide.
  • NASA Science. Carbon Dioxide Earth Indicator.
  • Judd et al. 2023. Toward a Cenozoic history of atmospheric CO₂. Science.
  • IPCC AR6 WG1, Chapter 9 and related FAQs on past warm climates and glacial climates.
  • USGS. Pliocene climate.
  • Forster et al. 2025. Indicators of Global Climate Change 2024.
  • WMO. 2024 warmest year on record press release.
  • Nature Communications and related PETM literature on rapid ancient carbon release and warming.
  • IPCC SROCC Technical Summary on long response times and committed change.
  • NOAA, NASA, EPA, and global methane-budget sources on modern greenhouse-gas origins.