What evidence would convince you it is a large problem? Massive dieoff of coral reefs worldwide? A cruise ship from Alaska to New York?
Coral reef recovery in the heat damaged areas since 1998 "massive die off"? Similar arctic melt and temperatures in early 20th century? Significantly warmer Greenland temperatures in the last 1000 years?
There is a lot of evidence on both sides that activists actively ignore or rationalize.
Arctic melt: "there is no precedent as far back as 1850 for the 21st century's minimum ice extent of sea ice on the pan-Arctic scale" http://onlinelibrary.wiley.com/doi/10.1111/j.1931-0846.2016....
Greenland: I haven't seen temperature numbers specifically for Greenland but it probably has to do with the medieval warm period, and we're warmer then that now. http://www.ncdc.noaa.gov/paleo/globalwarming/medieval.html