Ответ:

But the most difficult problems the humanity faces are global problems. - Present Simple Active

But the most difficult problems the humanity faces aren't global problems.

Are the most difficult problems the humanity faces global problems?

But the most difficult problems the humanity faces are global problems.

Регулярное действие, значит это PS, активный, т.е Present Simple Active.

Вопросительная форма с be в начале:

Are the most difficult problems the humanity faces global problems?

Отрицательная форма с are not:

But the most difficult problems the humanity faces aren't global problems.