The main reason stated by them is an increase in greenhouse gases and aerosol. Alaska, western Canada and eastern Russia are warming at twice the rate as the rest of the world ("Global warming science"). The reason for this is because air temperatures increase faster over the land than over the sea, and most of the northern hemisphere is covered by land mass. I feel that if you compare day-to-day temperature changes to a change of 2-4 degree Celsius (over a span of years), it may seem infinitesimal. However, when this temperature change is compared to the temperature change from that of the ice age, it seems significant. What I mean to say is that, it is evident that the global temperature have been rising more rapidly than the past, and this rise is causing several changes like melting snow caps which in turn has its own effects. Hence, it is extremely necessary to address this issue.
Rising temperatures leads to melting of snow and ice caps, which in turn cause a rise in the sea levels. This is another impact of global warming that only adds to the urgency to address it. As the temperature increases, snowcaps and ice begin to melt. The run-off trickles to the nearby seas and oceans, and the sea level rises. Also, since snow reflects back the sun's heat, reduction in snowcaps further increases the rate at which the bare ground absorbs the sun's heat and raises temperatures. This is a cycle that goes on and on. In his book 'Global Warming', Michael Anderson states that the IPCC has projected that the artic could have a complete ice-free summer by the year 2070. Another recent study projects that this could happen decades earlier. An example of melting glaciers is Montana glaciers national park. The 150 glaciers in 1910 have now reduced to a mere 27 ("Global Warming Fast Facts"). The melting of the Greenland ice sheet due to rise in temperatures by only 2 degree Celsius could increase the sea level by 20 feet! (Anderson 2012).
