CLIMATE
/ 2026 — 2100

The 1.5°C target was always tight. With ~1.3°C already realized and emissions still climbing, the remaining carbon budget for a 50% chance is approximately 200 GtCO₂ — roughly five years at current rates.

• Annual emissions: ~40 GtCO₂. Budget exhausts ~2030.
• Most realistic pathways involve overshoot — peaking above 1.5°C, returning by 2100 via large-scale CDR.
• Overshoot is not symmetric: ice loss, coral, species do not come back.
• Politically: 1.5°C is now an aspirational floor, not a forecast.

Holding warming to 2°C requires emissions to peak before 2030 and decline ~25% by then, reaching net-zero around 2070. It is harder than it looks and easier than 1.5°C — a meaningful difference.

• Carbon budget at 67% probability: ~900 GtCO₂. About 22 years at current rates.
• Coral reefs largely lost (~99%) but Arctic summer ice may recur in some years.
• Sea-level rise commitment: ~0.4–0.6m by 2100, multi-meter over centuries.
• Tipping-point risk rises sharply between 1.5°C and 2°C — the difference is non-linear.

Existing policies, if fully implemented, point to roughly 2.6–3.1°C by 2100. This is where we are heading without further commitment. It is not the end of civilization. It is, however, a profoundly different planet.

• Heat-humidity stress makes parts of South Asia and the Gulf seasonally lethal.
• Major staple crop yields decline 10–25%; food prices structurally higher.
• Hundreds of millions exposed to coastal flooding annually.
• Wildfire seasons extend 4–8 weeks across temperate biomes.
• Adaptation possible for wealthy nations; brutal for the rest.

Above ~4°C, the question is not adaptation cost — it is whether organized global civilization persists in its current form. The honest answer is that we don't know, because we have no historical reference and the feedback loops are not fully understood.

• Permafrost methane and CO₂ release: estimated 50–250 GtC by 2100 — a budget unto itself.
• Amazon dieback flips a sink to a source; further 50–80 GtC committed.
• Multiple breadbasket failures in same year become statistically common.
• Wet-bulb 35°C events expand into India, Pakistan, the Gulf, the Sahel.
• Migration pressure unprecedented in scale; political systems strain.

By 2100: 0.3–1.0m of rise is likely; the upper tail extends to ~2m if ice-sheet processes accelerate. The harder truth is what happens after — sea-level rise responds to warming over centuries, not decades.

• Thermal expansion + glacier loss: well-constrained, slow, inevitable.
• Greenland ice sheet: ~7m equivalent. Threshold for irreversible loss possibly 1.5–2.5°C.
• West Antarctic ice sheet: ~3.3m equivalent. May already be committed.
• 2300 commitment at 2°C: ~2–6m. At 4°C: ~5–15m.
• Coastal cities are deciding now what to defend, retreat, or sacrifice.

• Cheapness is the danger. Within reach of single states or wealthy individuals.
• It is not a substitute for decarbonization. Ocean acidification continues regardless.
• Termination shock — if SAI stops abruptly, decades of suppressed warming arrive in years.
• Governance is essentially absent. No treaty regime; weak research norms.
• Likely outcome: not whether but when limited deployment occurs — possibly within 10–20 years.

"An imperfect solution to a problem we caused, deployed by humans we don't trust, governed by institutions we haven't built."