Because the US is the only country that defected from the Paris agreement. The US is the only country led by climate change deniers. Tons of countries are led by malevolent and selfish leaders, but none are as incompetent and unpredictable as the US.
Climate change mitigation is a collective action problem in the form of a prisoner's dilemma or a tragedy of the commons. If every agent (i.e. country) refuses to cooperate, every agent will suffer major damage from environmental disasters. If all agents cooperate, they only suffer minor damage from economic policies designed to reduce greenhouse gas emissions.
At first sight, this doesn't seem like much of a problem. The solution seems self-evident, before one considers countries adopting different strategies:
If one country defects, they benefit massively from hosting the world's carbon intensive processes, yet all countries will equally share in environmental catastrophe. Thus, the optimal strategy for any single self-interested agent is always to defect, no matter what the others do. Paradoxically, the optimal strategy for each agent in isolation leads to a catastrophically bad outcome for all agents if they all choose that strategy. Everyone wants to be the parasite, but if no one is the host, we all die.
It wouldn't matter if the US were a tiny island nation, but the US has the largest carbon footprint, the largest economy, and the most capable military. The US led the democratic world. They could have solved the prisoner's dilemma by enforcing global cooperation. If the US and its allies would threaten to sanction those countries who don't cooperate, the payout matrix would shift towards cooperation being a stable Nash-equilibrium. It would no longer be in a country's interest to screw everyone else over, so they'd stop. The US and the entire world would be better off.
Climate change mitigation is a collective action problem in the form of a prisoner's dilemma or a tragedy of the commons. If every agent (i.e. country) refuses to cooperate, every agent will suffer major damage from environmental disasters. If all agents cooperate, they only suffer minor damage from economic policies designed to reduce greenhouse gas emissions.
At first sight, this doesn't seem like much of a problem. The solution seems self-evident, before one considers countries adopting different strategies:
If one country defects, they benefit massively from hosting the world's carbon intensive processes, yet all countries will equally share in environmental catastrophe. Thus, the optimal strategy for any single self-interested agent is always to defect, no matter what the others do. Paradoxically, the optimal strategy for each agent in isolation leads to a catastrophically bad outcome for all agents if they all choose that strategy. Everyone wants to be the parasite, but if no one is the host, we all die.
It wouldn't matter if the US were a tiny island nation, but the US has the largest carbon footprint, the largest economy, and the most capable military. The US led the democratic world. They could have solved the prisoner's dilemma by enforcing global cooperation. If the US and its allies would threaten to sanction those countries who don't cooperate, the payout matrix would shift towards cooperation being a stable Nash-equilibrium. It would no longer be in a country's interest to screw everyone else over, so they'd stop. The US and the entire world would be better off.