What does "epistemological" and "epistemological ignorance" mean in the context stated here?

Epistemology relates to more than limits to knowledge. It relates to the reasons for and the validity of our knowledge.

The word embraces a large area of philosophical endeavour, but one brief description is:

= of, relating to, or based on epistemology : relating to the study of the nature, origin, and limits of human knowledge

Merriam Webster

As such it necessarily covers the perceptions on which we base knowledge, the beliefs, the logic, the relations between these things.

Wallace-Wells is discussing many of the threats to our existence, among them sea level rise. Our knowledge of sea level rise depends on measurements that vary with time and place, are subtly connected to many other phenomena such as tectonic movements, astronomic dynamics (sun and moon), meteorological phenomena and the complexities of ice-water phase transition. To these we may add variously contrived accounts of human activity releasing greenhouse gases.

The interpretation of these measurements and the validity that we attribute to any predictive models are therefore complex and beset with uncertainty. This is not a simple issue like applying heat to water in a saucepan and measuring a temperature rise!

The response to this threat is therefore complicated not only by the illogicalities, special interests and traditions of human behaviour (what Wallace-Wells terms a mystery), but also by the uncertainties in the evidence, validity and predictive models of sea level rise.

{Edited to answer a reasonable comment: These uncertainties of fact, perception and modelling comprise the epistemological ignorance that he mentions. This goes beyond simple ignorance because we are not entirely ignorant of relevant perceptions such as sea-level measurements, astronomical determinisms, meteorological phenomena or carbon budgets. We have some knowledge, some information, and some understanding (as in the following definition) but we do not know their uncertainties or the validity of the intellectual concepts by which we knit them together to make predictive models.}

ignorance = lack of knowledge, understanding, or information about something

Cambridge Dictionary

He is probably correct to conclude with the comment about cloud formation, because that turns out to be even more complex, explicitly involving turbulence (a notoriously difficult theme in physical science), more phase transitions (ice-water-vapour-gas) and many other phenomena.