There is an old gag which asks: "Why don't polar bears eat penguins?" The answer is that polar bears inhabit arctic regions, whereas penguins inhabit antarctic regions, as shown here.

There is a saying that the poles are where weather is made. Weather is a heat-driven system, the temperature difference between the poles and the equator, amongst other things, drives our weather. Air is constantly being either warmed or cooled in a planet-wide system, but air is warmed much more in the equatorial regions and cooled more in the polar regions.  (Please note: this is a deliberate oversimplification for purposes of explanation.)

Current predictions, predictions which are not being widely reported, suggest that the Arctic Ocean may be ice-free as early as the summer of 2030.

"The scientific community has a range of predictions concerning when we could see an ice-free Arctic Ocean in summer. It could be as early as 2013 or as late as 2100. NSIDC’s projections generally fall somewhere in the lower half of this range."  Source: NSIDC

"... I was at a meeting of the Royal Society only yesterday in which a prediction was that the arctic might be free of ice in the summer of 2030 ..."  Prof John Beddington - 19-March-2009 towards the end of a BBC interview.  Source: BBC

Why is the ice melting?

A google search for "greenhouse gas" shows 10,200,000 results.

A search for "thermal forcing" shows 1,240,000 results.

A search for "anthropogenic thermal forcing" shows 129,000 results.

From these figures, if we believe in consensus truth, greenhouse gases must be the main cause of climate change because that is the focus of both the scientific and the unscientific communities.

I disagree. I am with the minority on this one. Here's why.

Every planet has a 'sun-side' and a 'space-side'. The sunward part gets heated to a greater or lesser degree by the planet's sun, and the spaceward side radiates heat away. In the absence of an atmosphere, the temperature peaks are extreme. An atmosphere tends to even out the temperatures so that the day-night peaks are less extreme.

In the case of our planet, this diurnal range is quite moderate. Now, despite the effects of greenhouse gases - and they are highly significant effects - our planet cools at night to a small degree.   The effect of greenhouse gases is to reduce the range of this diurnal thermal cycle.  One major problem of our era, in my opinion, is that at night, when our planet should be cooling, we in the most 'advanced' countries are pumping the heat from fossil fuels into our ecosphere at night almost as fast as we do in the daytime.

The thermal influence of our daily and weekly cycles of transportation, trade and commerce have been studied by relatively few researchers. I give only a few examples:

Studies correlate traffic with road surface temperatures - it seems that mere traffic density can keep a road frost free.  Here is one such study:
"In this study, consistent temperature differences of around 1.5 °C were found across the different lanes and carriageways of a multi-laned road. The thermal differences are significant and can be directly attributed to traffic heat fluxes."
Lee Chapman&John E Thornes;jsessionid=CE6FA14B...

Anthropogenic factors have been reported as factors in heat cycles and the 'heat island' phenomenon in many cities, and some studies show interesting correlations between midweek pollution patterns in one area with weekend pollution in others. It is tempting to speculate that commuters take their pollutants and heat outputs home with them.  Of course, more studies are needed.

Source: various authors Published in Atmos. Chem. Phys. Discuss.:16 November 2006
A  paper from Tokyo on heat island effects.

There is by no means a consensus about anthropogenic efffects on diurnal ranges, see e.g. this NCCR paper.  Nevertheless, the fact of anthropogenic climate change cannot be denied by any rational thinker.
As the American Geophysical Union recently concluded: "It is scientifically inconceivable that - after changing forest into cities, putting dust and soot into the atmosphere, putting millions of acres of desert into irrigated agriculture, and putting greenhouse gases into the atmosphere - humans have not altered the natural course of the climate system."
Source: WHOI

Yes, but what about the polar bears?

 It is difficult to predict extinction levels with any accuracy, but extinctions are inevitable as the outcome of current climate change scenarios.   Mark Schwarz of  UC Davis University of California has written on this topic.   The big problem is an ethical one:

Given that tens of millions of people live in areas that will likely be underwater in the near future, and given the ecologically disastrous effects of moving predators to new environments, should we even think about relocating polar bears to Antarctica?