To answer your question directly...We don't know for sure that the current TREND towards Global Warming is a natural factor or influenced by humankind. Ice cores can state one thing but that begs many questions (is our sampling methodology based off of current data? is that reasonable?).

There has not been any scientific study that has proven that human factors are leading to Global Warming. There are many studies that have been released that claim differently but check the sources and the data. Global Warming is something to be taken seriously as it helps to ensure we are moving towards having a environmentally sustainable culture.

I do not believe that we need to spend billions of dollars a year in tax money to make drastic changes to society for meager results. My example on this is the Kyoto Protocol. This did nothing to help the environment except to help those countries that fall below the "emissions requirements" to make money off of those countries that exceed their respective requirements.

Here's a few questions to consider:
Are the measures we have been taking over the last 20 years having any effect (lower vehicle emissions, The Clean Air Act, etc), positive or negative? Are buying energy credits actually helping the environment or just making other people rich? Is Al Gore a hypocrite because he uses more energy than the average household (18x the national average) even though he "offsets" this with energy credits?

Global warming IS supposed to happen. When the sun appears on the horizen every morning, it's supposed to warm the atmosphere. When it goes beyond the horizon every night, you should observe a cooling effect.

Furthermore, as the axis of the earth tilts more toward or more away from the sun during its long orbit, the northern and southern hemispheres experience sometimes quite dramatic changes in temperature, known as seasons.

The question of whether warming is natural or to be expected is a question of range and causes. There's plenty of evidence to show that solar activity does have a measurable and direct effect on climate. There's also evidence to suggest that volcanic activity below the earth's crust has some effect on surface temperature. Ocean currents, jet streams, and other naturally occurring factors can converge and diverge to produce dramatic changes in climate.

Currently we are experiencing a cooler than average year (of late). You'll notice that the people who were screaming in panic about global warming until recently will now, when challenged, just say something obtuse like "Oh yeah, it's colder than average! See what global warming can do to mess up the atmosphere?" The problem with this viewpoint is that it offers no distinction between the claims of a sophisticated scam artist who just is looking for an excuse for you to conform to his money raking scheme, versus a scientist who can model what's happening based on predictive mechanisms that can be tested again and again with equal results.

Furthermore, if the earth's temperature does heat up significantly, who's to say that the status quo or current claimants have an absolute right to dictate worldwide policy? There's ample reason to believe that the massive landscapes of Canada and Siberia would be tremendous beneficiaries of a warming global climate. Most models also suggest that rainfall should increase with a rising average temperature, and crop production could increase in the most fertile lands available even if it were to drop in some places.

In short the public debate is highly charged and pseudo-scientific in many respects, based on prior assumptions and unproven theories. You should put that in your hat and don't be afraid to ask simple questions of science and proof and reason when people throw out the cliche arguments that are repeated without question day to day to day as a lever for policy goals.

Let's say for a minute that it is a normal cycle. The problem is that species will go extinct and never come back if we don't interfere with the normal cycle. We are too concerned with whales and polar bears disappearing as the icecaps vanish that it doesn't matter if this is supposed to happen we still want to avoid it.

I think we (humans) are a huge cause of global warming, but it will be decades before people and not polar bears feel the consequences.

Global Warming, I feel, is overhyped and fueled by ignorance in a way. You can't possibly have facts that prove that we are having an effect on how the earth's climate is changing, because we have no direct comparisons from 200, 300+ years ago. When people have looked back at Ice Ages in the past they noted that the temperatures actually rose beforehand. In that light Global Warming is actually the onset of an Ice Age. People focus too much on the polar bears and the melting ice caps and some unusual weather between now and the last few decades, yet they ignore the fact that the earth has been around for millions, even billions of years. In the grand scheme of things our actions are doing little to affect the natural progression of this world we live in.

Oh and as a side note, polar bears may be listed as extinct - but did you ever consider that such animals have been deliberately miscounted just to bring them to the forefront of attention?

Another side note, centuries before we became more technologically advanced (or even walked this earth) the surface was littered with volcanoes spewing sulfur and carbon dioxide into the air. You can't possibly suggest that our cars and factories in the cities are doing as much damage as thousands of volcanoes on the earth's surface?

We have records of fluctuations in the Earth's climate over many many decades. Ice cores hold excellent evidence of warming and cooling of the earth. The human factor was never quite so big in the equation of climate change, but as technology advanced and environmental regulations were never followed, global warming has really gotten out of hand. There is more and more record-breaking temperatures with every season, which is why the UN has decided to take a more active role in reducing emissions.

Global warming is an elaborate hoax perpetuated by ignorant masses. There is overwhelming evidence that these cycles have occurred throught history, more dramatic than the cycle we are experiencing now and there weren't even any humans on earth at the time. Also, with the evdence that there is warming on some of the other planets in our system it seems that the human factor is negligible to the extent of being non-existant. It is very arrogant of us to think that we have that much influence as to influence climate.

I found it quite interesting while watching a show on the Discovery channel about Noah' flood that they were talking about the climate being much warmer and wetter than it is today. There were still humans and animals then, I'm sure to include the polar bear.

Speaking of polar bears, did you know that there are more now than 50 years ago when it was colder? Ask an Alaskan.

Okay, so we do know it is a part of the Earth's natural cycle. We have recorded temperatures for many many centuries. The temperatures rise and fall every hundred years. The Earth keeps this natural cycle to prevent ice ages and the loss of some of our natural resources. The Earth in orbit is not a perfect pattern or perfect circle for that matter, so every century or so the Earth moves a little bit closer to the sun, which causes a rise in temperatures by a few degrees. Then after a century has past the orbit moves out a little ways dropping the temperature a few degrees. It's a natural cycle of the Earth, you can't stop it.
Since Al Gore wasn't alive 200 years ago, he made his proposal on Global Warming without knowing the facts, and then he got everyone worried about it. ;)
Haha.

Good question, Matt! That's actually exactly what the debate is about. No one doubts that the earth's climate has become warmer, the debate is whether or not humans have caused it.

The climate does in fact oscillate, and now that we're going through the Pacific Decadal Oscillation, which is a periodic fluctuation in the temperature of the Pacific ocean, we might have a cooling trend for the next 5-15 years. Looked at over thousands of years, the relatively little ups and downs don't really matter, though.

Cycle changes in climate do happen. It's not a bad thing in and of itself.

The problem is that human actions are INTENSIFYING that natural cycle, to an extent where it now threatens our well-being. As my favorite astronomer, Phil Plait, once wrote (paraphrasing from memory):

Some global warming is okay. Too much is Venus. (despite being a lot farther away from the sun, Venus's surface temperature is HIGHER than Mercury's because of the high level of global warming there)