We can take a look at the past history of yearly world temperatures and look at how much the temperature has change and how fast.
We can also look at how the rising heat has affected the globe. Glaciers are melting and pieces break off, floating away. Similarly, warmer temperatures mean that when the sun hits the ocean, more water is evaporating into the air, which in turn is what is helping hurricanes to become more frequent and more powerful. Additionally, Antarctica is getting more snowfall. You might be surprised to hear that, but you have to understand that warm air holds more moisture--The Antarctic has been considered a desert because of how little snow falls--it's just been too cold for precipitation to form much. With the warmer weather, more moisture can gather, and thus when it cools down again, snow falls.
Then you can look at animal breeding and migrations: fish schools moving to new places, mosquito breeding seasons lasting longer and birthing more.... The list goes on.