The planet’s bad days
Ice at the equator, a mass extinction, the rhythm of the ice ages, two thresholds that may already be behind us, and the layer we are leaving.
6 stopsSurface to deep
The order, and why it is this one
Join anywhere. Each stop carries the route with it, so the foot of every question here offers the next one and the way back to this page.
- Was the whole planet once frozen?Twice in the Cryogenian period, glaciers reached sea level in the tropics. Whether the ocean froze over completely, or kept a belt of open water, is still argued, and the difference matters because the first version is hard to escape from and hard to survive in.Deepdepth, meaning: Major unanswered questions.Uncertain. Meaningful scientific, historical or philosophical disagreement remains.Read
- What killed the dinosaurs?The impact at Chicxulub is established, dated to the boundary layer, and a 2010 review of the evidence concluded it triggered the extinction. The Deccan eruptions were also underway. The remaining argument is about how much the volcanism contributed and whether the impact changed its behaviour.Surfacedepth, meaning: Things we understand reasonably well.Known. Strongly supported by evidence and broadly established.Read
- What causes ice ages?Variations in Earth’s orbit and tilt change how sunlight is distributed, and the record of past ice ages carries those periods clearly. What the orbit cannot explain is why the dominant rhythm for the last million years is one hundred thousand years, when the orbital forcing at that period is weak.Deepdepth, meaning: Major unanswered questions.Uncertain. Meaningful scientific, historical or philosophical disagreement remains.Read
- Where are Earth’s tipping points?The physics of warming is settled. Whether specific components of the Earth system have thresholds beyond which change becomes self-sustaining (and where those thresholds sit) is not.Deepdepth, meaning: Major unanswered questions.Uncertain. Meaningful scientific, historical or philosophical disagreement remains.Read
- How high will the sea rise?Thermal expansion and melting glaciers are understood well enough to project. Antarctica is not: whether its marine-based ice can collapse rapidly is an open question whose answer separates a manageable rise from an unmanageable one, and it cannot currently be ruled out.Shallowsdepth, meaning: Interesting questions with substantial evidence and real uncertainty remaining.Uncertain. Meaningful scientific, historical or philosophical disagreement remains.Read
- Are we in a new epoch?Human activity has left a distinct and worldwide layer in sediments and ice, and the case that it is stratigraphically distinguishable from the Holocene has been made in detail. A formal proposal to define an Anthropocene epoch was rejected in 2024, and the timescale still ends with the Holocene.Shallowsdepth, meaning: Interesting questions with substantial evidence and real uncertainty remaining.Uncertain. Meaningful scientific, historical or philosophical disagreement remains.Read