This is not the first time in recent history that humanity has had to deal with a dramatic shift in the climate. To get a glimpse of the political and economic effects climate change might bring, we don't need climate-modeling systems to look to the future—we need to go back 200 years in the past. Donald Trump's ascent to the US presidency brings with it a renewed string of flagrant instances of global-warming denialism. But science has already spoken: A difference of just a few degrees in average global temperatures is associated with a number of far-reaching effects, such as food shortages, political unrest, mass migration, and a more rapid spread of diseases. How do we know? Well, that's what happened last time. In 1815, an Indonesian volcano called Tambora erupted, sending an astronomically sized ash cloud into the air. What followed was a dizzying series of catastrophes—from worldwide famine to the spread of cholera, the world's first pandemic—that paint an all-too-graphic picture