Greenhouse gasses, especially CO2, have gotten into the atmosphere because of our burning fossil fuels and they are trapping heat. This trapped heat is not only in the atmosphere but also in the oceans and other places.
Part of what makes weather is a relationship between hot air and cold air. For instance, tornadoes are caused by hot air rising when cold air blows in on top of it. Since water evaporates more at higher temperatures the warm air is moister than the cool air above it. As the warm air rises it gets cooler and as it cools it releases its moisture. This is what causes the rain.
One of the things that can warm the air near the surface of the Earth is oceans. The warm water heats the air above it and makes the air more moist. The oceans are not still but have currents that move the water around. Water that is warm can wind up at the North Pole or the South Pole where it will cool down. Sometimes that water circulates around in one place for a while and if it is a warm place the water will get warm. Where the hot and cold water will be at any given time changes so where storms are or how powerful they will be changes as well.
Over a long period of time the places the hot and cold water are and the hot and cold air above it change in a pattern that almost repeats itself. That means that over a long period of time the storms in any one place will average out. Any one place gets about the same amount of hot water and hot air and same amount of cold water and cold air over time. Some years there will be lots of big storms and some years there won’t be but if you add up all the big storms over time on average there will be about the same amount in any one place. That is why we think of some places we could live as rainy places and some as dry.
Over time another thing will happen. Every once in a while an area will get weird weather. It will have a much bigger storm than usual or no storms at all. It is a lot like shaking the dice. Every once and a while you will get two sixes or two ones but not that often. We call that extreme weather or an extreme weather event. Just like the weather in general if you look at any one area for a long time there will be about the same number of extreme weather events. This is just like if you roll the dice many times it will come up snake eyes (two ones) or boxcars (two sixes) a predictable number of times.
Since for any one place the average weather is predictable that means that if you look at everyplace on Earth all the weather on Earth is, on average, predictable. We know that some places are rainy places and some places are dry places. We also know that every once and a while anyplace might get unusual weather. This long term predictability is called climate. The curve in the drawing below is the climate, the distribution of weather events over time.
What is happening now is that the distribution of these events is changing. That is because the patterns of warm and cold water and air are changing. Extreme weather events are becoming more common and more extreme relative to what we know. The climate looks more like the following drawing.
Notice that more extreme events happen more often. It is easier to see in the following drawing.
What used to be extreme is happening more often. It is getting closer to the middle. Overall, the pattern has shifted. That is what climate change is.
It is difficult and in many cases impossible to attribute any one weather event to global warming but global warming is changing the overall pattern of weather events. Climate change is a consequence of global warming.