"Six inches of snow - global warming my ***!" is no different from, "The average temperature has gone up 0.3K over the past ten years!" Both statements use the same spurious logic to reach a flawed conclusion.
As was pointed out, we have a hundred and fifty years of temperature data. The last fifty of those years were more precise by orders of magnitude than the first fifty. A hundred and fifty years ago today, J.C. Maxwell's unified equations for electricity and magnetism had not yet been published. Radio had not yet been invented. Transatlantic telegraphy had not yet been successful.
Now let's talk about the word about the word average. Take a look at
this table. As you can see, there are fourteen different definitions for the word average. Even if we assume average to be some sort of mean, there are nine of those. Which one is being used? Which one is best for this purpose? Let's say I use a quadratic mean. What period am I integrating over? If I use a weighted mean, how do I weight the values? Can you see how it could be both problematic and appropriate to use a truncated mean?
Back to that temperature data. Where are these observatories that I'm recording temperatures at? Are they near populated areas (heat islands)? How many of them are at the poles? When I go back to average my data, how do I account for the fact that the poles are the coldest places on Earth while still using the data obtained there? How am I recording temperatures for the 70.8% of the Earth's surface that's covered in water?
Earth's temperature and climate are both periodic. That means they change in cycles. One of these cycles is well known to us, that is to say: the seasons. There are other cycles, however. The Earth's magnetic poles flip every so often. Precession of the Earth's orbit causes our seasons to flip. We have had multiple ice ages. Some of these things have a period much greater than a hundred and fifty years. We don't have nearly enough data to show that any kind of observable fluctuations in our climate are not part of a naturally occurring cycle.