Route 1 Paris (2015) = rare voluntary consensus, bottom-up NDCs, <2°C → US exit a real setback (rests on collective commitment, not enforcement)

Route 2 US withdrawal (Trump 2017 → finalised 2020) = retreat from multilateralism → tests whether a consensus regime survives one major emitter leaving

Falkner: exit weakens the collective commitment the accord depends on → Ban Ki-moon: US as 2nd-largest polluter burdens the carbon budget + makes finance harder → domino effect: developing states may relax pledges (decarbonisation cost + energy security) → onus shifts to EU / China / India (none experienced in a multilateral regime) + unclear Katowice Rulebook → dilution risk → IPCC: 1.5°C budget gone by 2030 even if Paris followed → bottom-up design absorbs the exit, US rejoins under Biden → Stern: link climate action to development for equity

one emitter’s exit can unravel a voluntary consensus (contagion, diluted finance) vs a bottom-up architecture is resilient — survives exits and re-entries, held together by remaining emitters, finance and equity

Route 1 prospects turn on sustained finance + CBDR + leadership of the remaining emitters, not any single state

Route 2 setback real but not permanent → future secured by integrating climate with equitable development, not the presence of the US