bbguy, #27
Yes the planet's climate had changed in the past. Scientists are busy ferreting out the reasons for those past changes. That's what paleoclimatology is about. At least part of the reason for changes in the past million years or so is Milankovitch Cycles and the feedbacks they generate.
The obvious reason for the changes we are seeing now is the change in greenhouse gas levels in the atmosphere, particularly CO2. None of the other known climate changing factors are changing in magnitude fast enough to explain the current climate change. The change in CO2 levels we are seeing now is pretty obviously from human activities, primarily from the burning of fossil fuels that have sequestered the carbon in them from the carbon cycle for hundreds of millions of years.
In the past there have been massive climate changes due to changing GHG levels, notably the PETM (
http://en.wikipedia.org/wiki/Petm).
It's not impossible that we may discover in the future some major factor in climate that we don't know about. Science it always open to new discoveries. But given the extreme scrutiny the subject has received in the past 50 years or so it doesn't seem likely that we will find such a revolutionary factor. The basics of climate are pretty mechanical in nature. There is a level of energy input from the sun, primarily in the visual light range, that gets captured by the Earth's atmosphere and surface that is balanced by energy re-radiation, primarily in the infrared range. The greenhouse gases and aerosols (clouds, dust, SO2, etc.) in the atmosphere modify that infrared energy output producing the temperature levels we see on the planet. The rest is just details.
