Whenever we talk about the consequences of climate change, we’re talking about probability. Scientists present a rainbow of possible outcomes for our little experiment in fossil fuel consumption, some more likely than others. We’re used to thinking of that uncertainty as being driven by the physics of the natural world. The more we learn about heat absorption, fluid dynamics and the behavior of clouds, the better our understanding of climate as a system becomes. The more we know, the less uncertainty.

But the release of Volume II of the federal National Climate Assessment late last week got me thinking about uncertainty in a different way. As I read through the report, I saw example after example of uncertainty driven not by physics — but by human society. It’s one thing to say that we can expect a global temperature increase of somewhere between 0.3 and 4.8 degrees Celsius, depending on emissions rates and natural processes we don’t fully understand. It’s another thing entirely to guess what happens when those global temperature changes meet human society. It could turn out to be harder to predict how humans will react to the climate crisis we’ve caused than it is to predict the details of the crisis itself.

Like the physical world, people are a system, and a messily complex one at that. Our numbers grow and shrink; our economic tides lift some regions while keeping others dry; we choose to welcome migrants, or don’t. Those shifts and struggles are happening constantly, everywhere on Earth. They affect how we respond to the consequences of climate change. And the changing climate affects these socioeconomic processes at the same time as it affects the weather.

Take coastal flooding, for instance, which last week’s report addressed in detail. The global sea level is rising about an eighth of an inch every year. In 2017, it was 3 inches higher than it was in 1993. And there is very little doubt that climate change is the major cause of that rise. That’s the physics. But what do rising seas mean for the people living next to them, and for those of us farther away?

This is where we edge away from physics and into sociology. The consequences of sea level rise are about more than just flooding. Politics, legal systems, social and cultural histories all play a role in determining humanity’s response. What infrastructure will get built and when? What laws will be written? Which communities will be forced to move? As the assessment released last week states, “The ability of adaptation to reduce severe climate impacts like these will ultimately depend less on scientific uncertainties and the ability to implement engineering solutions than on perceived loss of culture and identity, in particular identities associated with unique cultural heritage sites and a sense of place.”

That’s true for a number of ways that climate change will complicate our lives. Consider the way rising populations and deteriorating public infrastructure might alter the consequences of drought in California, for example, or whether displacement caused by climate change in other parts of the world may be creating national security risks for the U.S. The assessment makes it clear that we’re only beginning to understand these interconnections. To understand how climate change is changing the world, we’ll need more than just good models aimed at predicting temperature or storms. We’ll also need to figure out how to predict what people will do in response to climate scenarios — including whether people view a change in their climate as “good,” “bad” or something in between.

When scientists look to the future, they’re still spending much of their time grappling with natural variability of weather and how different parts of the climate system work together. But they know that human response to climate change is increasingly a big factor in what comes next. Last year, when Volume I of the National Climate Assessment came out, it included a chart showing different sources of uncertainty and how, over time, each plays a bigger or smaller role in muddying our vision of the climate future. Natural weather cycles, the complex system physics of climate, and human behavior are all important. But the further out you try to look, the more those human factors matter. “By about 2030,” the report concluded, “the human source of uncertainty becomes increasingly important in determining the magnitude and patterns of future global warming.”

We know humans are the cause of climate change. But we still don’t know how humans will respond to it.