Since you've arbitrarily picked a point in time to begin your reasoning, I'll do the same. I chose the turn of the 1940s. The climate temperature was declining into the 1970s, some even thought we were coming into another ice age. The average temperature does begin to warm from there out, so what's changed? Our use of fossil fuels? Not really. How about our level of pollution? Sorry, that's been in decline too. With this in mind I blame unleaded gasoline and catalytic converters for global warming.