Therefore, in many cases, they simply omit it from the model, assessment or discussion.  In economic assessments of climate change, some of the largest factors, like thresholds in the climate system , when a tiny change could tip the system catastrophically, and possible limits to the human capacity to adapt, are omitted for this reason.  In effect, economists have assigned them a value of zero, when the risks are decidedly not.  One example from the report: The melting of Himalayan glaciers and snow will both flood and profoundly affect the water supply of communities in which hundreds of millions of people live, yet this is absent from most economic assessments. 

A third and terrifying problem involves cascading effects.  One reason the harms of climate change are hard to fathom is that they will not occur in isolation, but will reinforce one another in damaging ways.  In some cases, they may produce a sequence of serious, and perhaps irreversible, damage. 

For example, a sudden rapid loss of Greenland or West Antarctic land ice could lead to much higher sea levels and storm surges, which would contaminate water supplies, destroy coastal cities, force out their residents, and cause turmoil and conflict. 

Another example: increased heat decreases food production, which leads to widespread malnutrition, which diminishes the capacity of people to withstand heat and disease and makes it effectively impossible for them to adapt to climate change.  Sustained extreme heat may also decrease industrial productivity, bringing about economic depressions. 

In a worst-case scenario, climate impacts could set off a feedback loop in which climate change leads to economic losses, which lead to social and political disruption, which undermines both democracy and our capacity to prevent further climate damage.  These sorts of cascading effects are rarely captured in economic models of climate impacts.  And this set of known omissions does not, of course, include additional risks that we may have failed to have identified. 

The urgency and potential irreversibility of climate effects mean we cannot wait for the results of research to deepen our understanding and reduce the uncertainty about these risks.  This is particularly so because the study suggests that if we are missing something in our assessments, it is likely something that makes the problem worse.