Indeed it has, and the causes are well-documented - the Milankovitch cycles is probably the best summary. Not keen on links as you know, but this one will do as (correctly in all scientific papers in my view) it also points out some of the flaws as well.
http://pmt.physicsandmathstutor.com.../OCR/Climate-Change/Climate Change Causes.pdf
BUT all of this is completely irrelevant to the current topic of man-made AGW because it is on a completely different timescale. The climate in the past has always changed, has been much hotter, and much colder, BUT on cycles of tens of thousands of years.
What we are talking about is levels of CO2 that have changed in the last 50-100 years, and that are now 30% higher than they've been in the last 800,00 years.
So this is different - and carbon isotope measurement of the Co2 shows it is from a predominantly organic carbon source (like burning fossil fuels) not inorganic (from volcanic irruptions) - which have caused historic peaks.
So it is correct, but irrelevant, to say the Earth's climate has always changed. It in no way impacts the veracity or seriousness of current AGW and ocean acidification. An inconvenient truth indeed.
Hope this helps Chosun...