Causes of Global Warming

How much of global warming is attributed to humans versus the natural cycle of heating and cooling we experience over time on earth are there seasons for climates on earth such as ice age and drought that are ever recurring and cud this be in effect part of the problem we know the sun has warming and cooling periods and also if the sun is on the downside of it's life wont it continue to get hotter and hotter regardless of the courses we take to reduce our footprints
- Preston Markham (age 35)
lake city fl us

Many things have caused changes in the average temperature of the Earth over its history. The increase in the last century, however, was essentially all caused by human emissions of greenhouse gases. There haven't been big enough changes in solar activity, volcanoes, etc. to have much of any effect during this period. It's clear that as our emissions of greenhouse gases continue the warming will continue and become a very big problem. The other major factor in recent changes is increasing reflection ("albedo") also caused by human emissions, mostly of sulfur compounds. That factor has somewhat reduced the warming caused by our greenhouse gas emissions. 

As you point out, there have been some rather dramatic changes in the past. Most of the last 12,000 years have been rather stable. That's the period in which human beings have done particularly well. The last big change, the Younger Dryas () had severe consequences for people. Extreme climate changes tend to cause mass extinctions, particularly if they happen quickly. The global warming which we are causing is very quick by these standards.

You mention very long term changes in the Sun. Those will be on the billion-year time scale, not an immediate concern.

On a slightly more abstract side of the same question, it's not true that the changes caused by any one factor (us, the Sun, volcanoes,..) happen regardless of the other changes. The changes from the different causes more or less just add up.  We happen to be the only important cause now.

Mike W.

(published on 05/01/2014)