Global warming has two components.
One is due to changes in the earth itself; shifting of the earth's axis, continental drift and piling up on continental margins causing changes in oceanic and atmospheric currents, amount of volcanic activity, etc.
Ice ages occurred in many periods, we are most familiar with those of the last million or so years because of the physical evidence preserved in the rocks and in the shaping of our landscape, and in remaining vestiges of glaciation.

The second is the result of man's activity, beginning with the Industrial Revolution and increasing with time, as we pollute the atmosphere and destroy the forests, disrupting the ecosphere.

Denial of the latter effect, or quibbling about the contributions of different possible causes, is resorted to by those who have a vested interest in particular industries which contribute to atmospheric pollution, and well as by those of us whose livelihoods depend on those industries in some way.

Scientific evidence is found in the gases and pollutants preserved in ice cores taken from ice sheets that have accumulated over the past 1000 years, and in the disruption of atmospheric layers, not just in the retreat of glacial sources of water, increases in oceanic temperatures or changes in the distribution of life forms.

Regardless of the causes- and this is important- steps must be taken to diminish the effects and provide alternatives.

As noted, the average Bangladeshi or shoreline dweller is severely affected by a rise in sea level, regardless of whether it is caused by long term earth cycles or man's activities, the Indian farmer is starved by the decrease in water as rivers dry up because of less ice in the Himalayas, and temperature changes cause shifts in location and extent of arable lands.

Mankind, however, tends to muddle on until disaster strikes.