The report also characterized the known unknowns, as Mr. Rumsfeld might put it — those things we know at a fundamental level but about which we seek greater certainty. They include how much Earth will eventually warm, how rapidly oceans will rise, where and when weather extremes and water shortages might occur, and whether potential tipping points (like the collapse of Antarctic ice sheets) will, in fact, occur.

Unsurprisingly, the report carefully limited speculation about unknown unknowns: the many initially small environmental shifts that are potential consequences of the changing climate. What will actually emerge is largely unknowable because of the highly unpredictable nonlinear response to the warming of Earth’s complex and adaptive physical and ecological systems.

Yet credible speculation on climate’s unknown unknowns is sorely needed by policymakers. Future generations will be affected by today’s policy decisions, whether the underlying science is complete or not. The basics are simple: The more we warm our planet, the more likely it is that deeply surprising environmental changes will ensue.

Most of these smaller environmental changes should be manageable, readily addressed through adaptation. Inevitably, however, a rare few will most likely evolve and expand until they threaten our security, health or economy. We lack the ability to predict which are which. This is the curse of unknown unknowns. Nevertheless, things we can credibly imagine should accentuate our concern for what we are unable to imagine.

Perhaps a routinely ice-free Arctic summer, altering polar ocean life in subtle ways, sets off an unpredictable cascade of complex changes throughout the global ocean ecosystem, devastating fisheries. Maybe agricultural pests adapt to climate change stresses by evolving novel and frequently changing abilities to destroy crops, leaving farmers struggling to keep pace and feed populations. One unsettling risk is that mutant diseases — like Zika and Ebola today and the 1918 flu epidemic that killed 50 million people — could emerge more often because of altered evolutionary competition in a changing climate, each a greater medical challenge than the last.