The climate-deniers notice that some places may NOT be getting hotter, or might only be hotter in winter (which they think is good), or that they have one extra cold winter and then the think the whole concept is bullshit. They don't get it. Global Warming means GLOBAL. "Global" refers to the temperature AVERAGED over the entire globe over the entire year, or years. What is happening on one day or season in one location is not the whole picture.
And then, they don't understand that there is uncertainty. Why they refuse to do so, I don't know. Most people will check the daily weather forecast and at least have some faith in it, even if they understand that the TV weatherman can miss the mark. The entire, global climate system is very complex and we don't understand all the factors. Thus our conclusions are bound to be "fuzzy".
There are many things besides CO2 concentration that can affect the climate. There are feedback loops: ocean currents, air currents, loss of glacial ice (which decreases Earth's albedo- ability to reflect heat back to space) , plant growth, etc. For example, if Earth gets warmer, growing seasons may be longer and plants may be able to grow in more locations (higher latitudes and altitudes). Since plants eat CO2, that can help reduce the amount of CO2 in the atmosphere, which is good. However, if plants (dark color) replace glacial ice (white), the reflectance of Earth will be lower and less heat will be reflected back to Earth. Which factor wins?? Depends on a lot of things and we do not know for sure.
There are numerous other feedback loops that can either mitigate or exacerbate global warming. We do not know how they all affect one another and to what extent. We're dealing with an entire complex planet.
About the warming in the North Atlantic, that is because of the weakening of the Gulf Stream. The Gulf Stream transports warm waters of the Caribbean up to the North Atlantic where it warms places like Cornwall in England to the point where coconut trees can actually be grown. If you take away the Gulf Stream, Britain would be a colder place. Why would the Gulf stream go away??? It's a loop. The warm Caribbean Gulf Stream water cools and sinks when it arrives in the cold North Atlantic because cooler water is denser than warm water. Once the Gulf water sinks, it travels back south towards the equator. If you warm the waters of the North Atlantic via general oceanic (global) warming and warming of the Arctic , the warm waters of the current will not cool as much when they arrive in the North Atlantic. They, will not sink as rapidly and the conveyer-belt system transporting Caribbean warmth to Britain breaks down. One consequence: The Caribbean can't dump its heat as efficiently so we get more hurricanes and more people suffering from heat in that area.
Anyway the point that some do not understand is that there is a worst-case scenario, we can avoid it, and the likelihood of a best-scenario at this time is not high. Probability and Certainty are not the same things. They gamble with long odds.
However, if plants (dark color) replace glacial ice (white), the reflectance of Earth will be lower and less heat will be reflected back to Earth.
Ouch-- I meant to say "less heat will be reflected back to SPACE"-- which obviously would exacerbate global warming. So additional plant growth could help or hurt, depending on how the see-saw tilts.