Watch this video to learn more about science behind a blue ocean event. It doesn't spell immediate disaster. However it does indicate we've crossed an irreversible tipping point. Once that threshold is passed it will be next to impossible to reverse the melting.
The artic is like the planet's refridgerator. It reflects the suns rays and bounces it back into space. As the artic melt less heat is reflected thus leading to more melting. this what is called a positive feedback loop. A warmer artic leads to more sporadic & extreme weather, droughts, sea level rise etc.
A common arguement denialist make is that the climate is always changing. This is true. However natural climate change happens over the course of thousands of years. This is happening within the span of decades. And its happening faster than anything we can adapt to.
Think of our civilization as a heat engine. We are pumping out more Co2 than what is sustainable for our planet to handle. Vegetation sucks up co2 but we're emitting more co2 than what these "carbin sinks" can handle leading to an overall warmer planet, thus melting the artic. In essense the more the planet warms the more the planet warms.