Deciding who has been the worst British prime minister of the 21st century (so far) is tough, but Theresa May (2016–19) strengthened her strong claim to this title shortly ahead of her ignominious departure from office. Desperate to secure a “legacy,” she saw to it that Britain became the first major country to legally bind itself to reaching net-zero greenhouse-gas (GHG) emissions by 2050. That is, the nation committed itself to releasing no more greenhouse gases into the atmosphere than it removes. Quite how this ambition could be fulfilled or what fulfilling it would cost was unclear, but no matter: This potentially enormous commitment passed into law with support across the political spectrum, astonishingly little scrutiny, and a great deal of self-congratulation. The 2050 target date reflected a widely held view that this is what it would take to contain the increase in the average global temperature since pre-industrial times to a more or less bearable 1.5 degrees Celsius.