Disaster
Part of speech: noun
Definition: An unexpected natural or man-made catastrophe of substantial extent causing significant physical damage or destruction, loss of life or sometimes permanent change to the natural environment. An unforeseen event causing great loss, upset or unpleasantness of whatever kind.
Example sentence: Know your enemy and know yourself and you can fight a hundred battles without disaster.
Calamity
Part of speech: noun
Definition: An event resulting in great lossThe distress that results from some disaster
Example sentence: Economic growth is the aggregate effect of the quest to accumulate capital and extract profit. Capitalism collapses without growth, yet perpetual growth on a finite planet leads inexorably to environmental calamity.