The consequences of runaway global warming include changes in sea level with very profound effects upon the large percentage of humanity near coastal areas, decreases in large populations dependent upon fresh water rivers fed by glaciers, increased frequency of more intense “mega” storms, increased heat related deaths, the vast spreading of infectious diseases, tremendous increases in species extinctions, radical vegetation changes – including vegetation that has been, historically, fundamental food crops, degradation of physical and social infrastructure, increased within-state and between-state conflict, increased environmental and conflict migration and the increasing collapse of nation states.
Warming will increase sea level due to land ice melt (e.g. glaciers, the Greenland ice sheet, a significant percentage of the west Antarctic ice shelf, the east Antarctic ice sheet) and the thermal expansion of water molecules. If warming becomes large enough over time, so much ice will melt that sea level rise will be in the order of 225 or more feet. Sea level rise is already sufficient in some coastal areas to produce high tide flooding, e.g. Norfolk, VA and the abandonment of some of the Maldives and Marshall Islands. The 2014 IPCC report suggests a high estimate of sea level rise by 2100 of about 6 feet but the melting of the north polar ice sheet and the Greenland ice sheet have been more rapid than predicted. Hence, the IPCC high level sea level rise estimate is being challenged by current empirical evidence.
Since at least 1/3rd of the world’s population lives within 60 miles of the sea coast (53% in the U.S.), humanity will be widely affected as sea level rises. Sea level rise will affect coastal wetlands, river deltas, and many of the world’s largest coastal cities including Tokyo, Mumbai, Calcutta, New York, Los Angeles, Miami, Shanghai, Lagos, Buenos Aires, Karachi, Jakarta, Bangkok etc. A 5 foot rise in sea level will put 94% of Miami Beach and 20% of Miami as well as La Guardia, Kennedy, and Logan airports underwater. Also, the melting of glaciers in some areas, e.g. Himalayas and Andes, will reduce and, potentially, stop the flow of major rivers essential to large populations in China, India, southeast Asia as well as the cities of the Andes.
Increased extreme weather events as well as changing precipitation patterns are occurring all over the world. In fact, one of the consequences of the warming is an enlarging, i.e. increasing the height of the troposphere (the lower level of the atmosphere) , which in turn creates fuel for larger, more extreme storm systems. In particular, increased temperature increases the amount of water vapor in the atmosphere and the resulting heavier rainfall. For example, from 1980 to 1989 there were 15 category 4 hurricanes/typhoons/cyclones and 2 category 5 compared to 13 category 4 and 6 category 5 from 1990 to 1999, and 22 category 4 and 11 category 5 from 2000 to 2010. Cyclone Haiyan in 2013 had the strongest winds upon landfall in the Philippines and highest related death toll ever recorded. Note that increased water borne illnesses and deaths are associated with increased flooding. Extreme flooding is also changing where people can live safely in other ways. Aside from the direct risks of flooding from heavy rain, the risk of heavy rain makes safety questionable living in mud slide prone areas or sink hole prone terrain. In the U.S., weather related costs due to storm or drought events has been increasing each decade since 1980.
As temperature increases, increased directly heat related deaths, especially in the “heat island” cities (which retain more heat because of their heat absorbent materials and their design), has been consistently increasing for the oldest and youngest age groups. The European summer heat wave of 2003, Paris recorded 14,802 heat related deaths during the week with temperatures above 104 degrees. In July of 2010, Moscow recorded heat and smoke pollution (as a result of 550 forest fires) 14, 340 related deaths. In the U.S, there has been a large increase in directly heat related deaths from the 1980’s into the 21st century. Also, cases of water borne illnesses related to extreme weather events contribute to an increase in indirectly heat related deaths everywhere. Likewise, as temperature increases, cases of more severe asthma and bronchitis increased worldwide. According to the Centers for Disease Control (CDC), "...public health can be affected by disruptions of physical, biological, and ecological systems, including disturbances originating here and elsewhere." (Full CDC article) (Legacy Part 2 here.)