Has It Already Gotten Hotter?

A reasonable question to ask is, "We've been pumping greenhouse gases into the atmosphere for a while now...is it getting noticeably hotter yet?" I've attempted to answer that with a few different metrics.

Data Source

To create these plots, I'm using the daily temperatures from Berkeley Earth. I settled on comparing two centuries, so I took 1900 - 1910 and compared it with 2000 - 2010.

Do We Have More Hot Days?

To try to answer this question, I arbitrarily picked 90 degrees (F) as the threshold for a 'hot day'. I know that means nothing in Phoenix but I still can't believe people actually survive there (this from a guy who lives in Austin). To determine this, I simply counted the number of 90 degree days in the period from January 1, 1900 to December 31, 1909, did the same for the period from January 1, 2000 to December 31, 2009, subtracted the first from the second, and divided by 10. The results are below:

Note how the color scheme makes it look like it's on fire

As you can see pretty clearly, the number of 90 degree days increased almost everywhere and did so to a larger degree in the southwest. I don't know how many of these it takes to be 'noticeable', but many places here saw an increase of more than 20 days per year which I think most people would notice. 

Are The Hot Days Hotter?

To try to answer this question, I arbitrarily picked the hottest 10 days per year. Using the same time periods as above, I compared the hottest average 10 days per year from the two decades, and the difference is below:

Again, I do not know what increase is 'noticeable', but in Denver, that number went from 87.6 degrees F to 91.5 degrees F which I think most people would notice.

Are The Summers Longer?

To try to answer this question, I arbitrarily picked 90 days to represent summer. Then, I took the hottest 90 days each year from the 1900 - 1909 time period, found the coldest of those days, then found the number of days each year in the 2000 - 2009 time period that were hotter than that day. The results are below:

This one is a bit harder to read to me, but it's clear if you compare with the colorbar that the majority of the areas effectively have a longer summer now, and the difference is really large in the southwest.

Are The Cold Days Hotter?

Similar to the 'Are The Hot Days Hotter' section above, I ran the max temperatures of the 10 coldest days per year. The results are below.

Like above, the majority of areas are much warmer now than they were a century ago.

Summary

For the most part...summers are effectively longer and hotter than they were a century ago, and the difference in most places crosses the threshold that I would consider 'noticeable'. If you can think of any other ways to check this, just let me know in the comments. As a note, I did this for other decades (e.g., 1890's vs 1990's) and the results were very similar. Including images for all of those will destroy data usage though so I've kept only the most recent.