How did we go from melting out of the last ice age, to present day global warming?

John O'Quinn in Ottawa wants to know: "How did we go from melting out of the last ice age, to present day global warming?"

To answer, Dr. Derek Mueller, an assistant professor in the School of Geography and Environmental Studies at Carleton University, takes us back about 21,000 years ago to the last glacial maximum — the coldest temperatures and greatest extent of the glaciers were at that time. 

Back then, Ottawa was under about one to two kilometres of ice. The city was six to seven degrees C cooler than now.

Mueller says areas further north, for instance about 65 degrees latitude such as around Yellowknife received extra sunlight in summer. It was subtle, but enough to tip the scales towards a warming climate. The ice sheets really started to melt rapidly about 15,000 years later and were starting to shrink polewards as temperature warmed.

This continued until about 6,000 years ago when the ice sheets were at the approximate extent that they are today. 

Mueller says this very gradual cooling trend would probably have continued, but since 1880 there's been a warming of 0.85 degrees C, overwhelmingly caused by humans adding vast quantities of carbon dioxide and other greenhouse gases into the atmosphere, which alters the Earth's energy budget.