There are many impacts, both predicted and observed, from increased carbon concentrations in the atmosphere, including more extreme weather (precipitation and drought), wildfires, shifting of arable land locations, lower global food production, ocean acidification (and the eventual death of marine life), and rising sea levels. Accompanying all of these will be severe social unrest as people no longer can find enough food and fresh water to survive. While all of these are serious problems, sea level rise is perhaps the biggest threat. Unlike changes in arable land, where some people will benefit, there are no winners with sea level rise. Currently, there are about 100 cities worldwide with populations of 1 million or more in coastal areas, including NY City, Boston, Shanghai and Calcutta. Along with small island nations and low-lying countries such as Bangladesh, approximately 15% of the world's population (over 1 billion people) live in low-lying areas. So why is sea level rise such a threat? Up until a few years ago, projections for global sea level rise were for about 1 to 4 ft by the year 2100. That is a lot, but an amount that might be manageable in many locations. However, the rate of global ice melting (which leads to sea level rise) has accelerated well beyond the predictions scientists made just 20 years ago, exceeding the worst-case scenario by 60%, and now melting at a rate in Greenland that is 5 times the rate of just 20 years ago. So while the only honest answer is that we don't know how fast sea level will rise, there is a very high probability that we could see sea levels rise in excess of 10 ft by the end of the century. Sea level rise is primarily driven by the melting of land-based ice, essentially all of which is located in Greenland (10% of the world's ice) and the Antarctic (90%). Further dividing the Antarctic, the Western Antarctic is about 10% of the ice volume, and the East Antarctic is about 90%. If Greenland melts entirely, and as of today it appears it will, it will cause global sea levels to rise 21 feet. If the West Antarctic melts, which appears to be happening in parallel with Greenland, it will add another 26 feet to global sea levels. And if the East Antarctic (which is currently stable) melts, it will add an additional 213 feet to global sea levels.
We have had 5 mass extinctions since life arrived on earth, the last being the extinction of the dinosaurs about 66 million years ago. A typical extinct involves the loss of about 85% of the species on the planet. We are undoubtedly heading toward a 6th extinction of our own making if we don't take relatively drastic action soon to curb carbon emissions. The number that the majority of respected climate scientists say we can live with is 350 ppm, which we have already exceeded, and we are now over 400 ppm. Although CO2 stays in the atmosphere for several thousand years, methane breaks down after about 20 years. If we globally make a concerted effort to reduce both CO2 and methane emissions, we have our best chance of preserving the planet for future generations. However, the situation is dire and we have to act now. We should have started addressing this problem decades ago.
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