Reckoning

Copilot:

We seem to be facing a choice between an Ice Age and an Earth-flooding warming. Is there no middle ground? What is the Goldilocks point??

You’re asking the right question, Louise — and it’s one climate scientists wrestle with constantly. It feels like the planet is offering us two extremes:

A future Ice Age if CO₂ drops too low

A flooded, Pliocene‑like world if CO₂ stays too high

But the truth is more nuanced. There is a middle ground — a “Goldilocks zone” — but it’s narrower than most people realize, and it depends on how the Earth system responds over thousands of years, not decades.

Let’s map it out clearly.

🌍 1. Earth’s climate has a natural Goldilocks zone

For the last 2.6 million years (the Pleistocene), Earth has oscillated between:

Glacial states (ice ages)

Interglacial states (warm periods like today)

These swings happen because the climate system is sensitive to small orbital nudges. But the interglacial periods — the warm, stable ones — cluster around a surprisingly tight CO₂ range:

~260–300 ppm

That’s the natural Goldilocks band for the Pleistocene climate mode.

Inside that range:

Ice sheets remain stable

Sea level stays within a few meters of modern

The climate doesn’t tip into runaway cooling or warming

Outside that range, the system shifts.

🔥 2. Why we’re no longer in that zone

Today’s CO₂ is well above 420 ppm.

That pushes us out of the Pleistocene “ice‑age engine” and toward a climate mode more like the Pliocene, where:

Greenland melts

West Antarctica becomes unstable

Sea level eventually rises 20–25 meters

The Arctic becomes warm and wet

This is why the system feels like it’s losing its middle ground — we’ve moved outside the range where the old equilibrium works.

❄️ 3. Why we’re not at risk of an Ice Age anymore

Even if emissions stopped today, CO₂ would remain high enough to:

Prevent the next glacial inception

Keep Northern Hemisphere ice sheets from regrowing

Hold the planet in a warm interglacial‑like state for tens of thousands of years

So the “Ice Age” side of the dilemma is already off the table for the foreseeable future.

🌡️ 4. So what is the Goldilocks point now?

If we define “Goldilocks” as:

No new ice age

No catastrophic long‑term sea‑level rise

A stable climate system

A world recognizable to modern civilization

Then the best estimate is:

~300–350 ppm CO₂

Why this range?

It’s high enough to suppress glacial inception

It’s low enough to avoid triggering Greenland’s instability threshold

It keeps long‑term sea‑level rise to manageable levels

It preserves the basic structure of the Holocene climate

This is the “new” Goldilocks zone — the one compatible with both avoiding ice ages and avoiding Pliocene‑level flooding.

🧭 5. The uncomfortable truth

We’re already above that zone.

But that doesn’t mean all is lost. It means:

The long‑term equilibrium (thousands of years) is shifting

The sooner CO₂ stabilizes or declines, the more of the old equilibrium we preserve

The difference between 420 ppm and 500 ppm is enormous in terms of ice‑sheet commitment

In other words, the Goldilocks zone still exists — we’ve just overshot it, and the question becomes how far we go before stabilizing.

🌱 6. The real choice isn’t Ice Age vs Flooded Earth

It’s:

**Do we stabilize the climate near the edge of the Goldilocks zone,

or do we push the system into a new, hotter equilibrium?**

The first path preserves coastlines, ecosystems, and the climate rhythms that shaped human civilization.

The second path leads toward a Pliocene‑like world — still habitable, but profoundly different.

If you want, we can explore:

What a 350 ppm world looks like

Whether CO₂ removal could bring us back into the Goldilocks zone

How much warming is “locked in” vs reversible

What the long‑term climate looks like at 420, 450, or 500 ppm

Just tell me where you want to go next.