“What keeps you up at night?” was the question we were asked at the Frontiers of Resilience Conference yesterday, sponsored by Sandia National Laboratories, with a pretty powerful array of scientists and Department of Homeland Security reps.

I take the long view. First, let’s pull all the way back: When you see a time lapse image of the globe from above the poles, you will see a curious pentagon of cloud swirls. This is the geometry of our current phase of weather. The jet stream, the gulf stream—all the familiar patterns of flows of wind and water that have persisted long enough that we have named them—bob around this pentagon in an orderly form of variation described by the mathematics of chaos theory. They are in a state of equilibrium, more or less, for the current amount of energy in our climate system.

Even an amateur mathematician knows what happens when the energy in the system increases above a certain—yet unknowable to us—threshold. It’s called phase change. We are perhaps approaching such a threshold. The IPCC has long talked about 1.5 degree Celsius as a threshold. Now they have commissioned a report for what might come after. I look forward to seeing the report and its details. We don’t know what temperature change will mark the border between phases. But here is what we do know.

The global weather system will exhibit high volatility as it crosses the border between phases. Astounding storms and ruthless droughts, unprecedented highs and lows all in quick succession. The good news is the weather system will find a new balance for its new phase. The bad news for those who have invested human and financial capital in existing cities is that the new weather balance will have a new geometry. Some regions that today are virtually uninhabitable will become balmy; others might wither. There will be winners and there will be losers among cities and their climate.

The population of those cities where the new normal is so different that it is beyond their ability to adapt, will have to move. To accommodate these people we will have to build new cities in better locations at a pace faster than we ever have before. I worry less about our ability to design or even finance these cities. I worry about our inability to facilitate migration in a world of national boundaries. This is a political problem, a question of immigration, on which we are already failing miserably.

Our current wan hope is sustainability and resilience. Let us mitigate our carbon emissions if it is not already too late, and let us adapt as we can in the cities we have today, and let us hope that climate does not pass that violent boundary and enter into the unpredictable geometry of its next phase. If it does, however, and the world’s habitable regions shift, the most important concept in global sustainability and resilience will not be mitigation or adaptation, but brotherhood.

This essay was part of a speech delivered at the 2017 Frontiers in Resilience Symposium, sponsored by George Mason University and the Sandia National Laboratories. Featured image via howstuffworks.com.