Probably. I would not live in the Northeast without it. The weather here is greatly improved when my parents allegedly walked several miles uphill every day in 3 foot high snow to get to school and back when they were kids.
have humans contributed?
I'm thinking a tiny % of it might be. Something less than 1% of it maybe; I don't know for sure. The rest is cause by things like the sun (as in Mars is warming up) and our gradual exit from the Little Ice Age.
On thing I do know is that after the 9/11 attacks and airplanes were grounded, we had a way warmer summer due to no airplane trails in the sky.
Airplane contrails and their effect on temperatures
Then Sept. 11, 2001 presented a unique opportunity to study what the sky looked like without airplanes and contrails. In the wake of the 9-11 terrorist attacks, the FAA prohibited commercial aviation over the United States for three days. That's when David Travis, an atmospheric scientist at the University of Wisconsin, Whitewater, thought to look at how temperatures might differ at temperature stations around the country.
He found that [PDF], for those three days, the average range between highs and lows at more than 4,000 weather stations across the US was 1 degree C wider than normal. In other words, contrails seemed to raise nighttime temperatures and lower daytimes ones.
But the real effect was in daytime highs, which were much higher. That would seem to indicate that, contrary to prevailing thinking, contrails might have a net cooling effect.
So, the only real proof we have of anything is of a human activity that is cooling the planet.
I think it's real, but I don't think it is necessarily one way (ie global warming). The weather seems more extreme to me.
I think it's possible we contribute, but I think other factors are either the main cause or are more influential (the sun for example).
In general I think this is a very difficult topic to analyze that is used politically to further certain means or ends. I just remember Al Gore flying all around the country in a private jet, driving SUVs, living in a huge house, etc. To me that says that some of the main proponents don't necessarily believe in it. I also remember Arizona State (I believe this is correct) falsifying a huge climate dataset that was used for all of the arguments for global warming. I might have the university wrong, maybe it was U of A.
So I guess I would be labeled a skeptic. I still think it's great to recycle, use electric cars, etc. To me resource limitations are much more of an issue than climate change.
However, according to Drew Shindell, a climate researcher from NASA's Goddard Institute for Space Studies in New York, NY, the most recent studies have confirmed that changing levels of energy from the Sun are not significant enough to be a major cause of global warming: "...the solar increases do not have the ability to cause large global temperature increases...greenhouse gases are indeed playing the dominant role..." The Sun is once again less bright as we approach solar minimum, yet global warming continues."