Here are some other things that nature alone can’t be blamed for:
Lightning has caused forest fires throughout the ages. Can reckless campers or arsonists not also be causing forest fires?
Over eons, animals have gone extinct due to natural causes ranging from asteroids to ice ages. Does that mean Man can’t be causing modern extinctions by destroying species habitat, or via hunting with efficient weapons?
Volcanoes have created random bouts of air and water pollution throughout the centuries. Does this mean people can’t generate pollution from oil, coal, atomic bombs and synthesized chemicals?
Hurricanes have toppled trees throughout history. Does that mean people cannot fell trees with axes and chain saws in greater numbers?
If a mammoth knocked a boulder off a cliff 740,000 years ago, is it impossible for dynamite and bulldozers to level entire mountaintops for coal mining in 2010?
This is an attempt to get people to stop changing the subject to “natural cycles” and deal with the here-and-now of human impact on Earth.
It doesn’t matter if CO2 levels were much higher in ancient times when the Earth was younger. Back then, volcanic activity, plant growth & decay was more extensive and many climate factors have changed over time. Most people would never know about those factors were it not for scientists, whom they have the luxury of second-guessing with superficial knowledge.
The cause of modern CO2-increase has been shown to be mostly man-made (anthropogenic). We are releasing ancient stored carbon through systematic mass-burning that nature has never seen before. Man is causing many changes that were previously the sole domain of nature. It’s a fact. Deal with it.
Assuming that you don’t deny the anthropogenic origin of MODERN CO2 increases, and you don’t deny that CO2 traps heat, how can you deny that man-made CO2 levels are causing warming in MODERN times?
Try to answer that basic question without changing the subject to the usual canards, or trying to make the fundamentals more complex than they need to be, i.e. challenging the whole concept of the greenhouse effect.
To put it another way: If you asked a contractor whether dual-pane windows would raise the interior temperature of a remodeled house, he (if he had any integrity) would say “yes, they would” and wouldn’t try to change the subject to Al Gore’s monthly heating bill, or the tax rate in West Virginia.
In other words, stay on topic or don’t bother answering. It’s a small thing to ask.