The maps represent predictions by the latest climate models for what the United States will look like in 2100 under different emissions scenarios. In the top left corner is RCP 2.6, a world in which everyone has taken very aggressive action to cut greenhouse-gas emissions. Parts of the United States get a few degrees hotter, thanks to the carbon we've already loaded into the atmosphere, but temperatures stabilize after that.
The bottom right-hand corner, meanwhile, shows RCP 8.5, a world in which we continue to burn fossil fuels at our current rate with no effort to tackle emissions. In this scenario, the report notes, average temperatures in the United States rise somewhere between 5°F and 10°F by century's end (or 2.8°C to 5.5°C). A few parts of the country get up to 15°F hotter. Needless to say, that's significant.
Of course, that's over the longer term. What about in our lifetimes? The report points out that average U.S. temperatures have already risen 1.5°F since 1895 and are expected to rise another 2°F to 4°F in most areas within "the next few decades."
There's a whole lot more in the draft report, which offers up this summary: “Climate change threatens human health and well-being in many ways, including impacts from increased extreme weather events, wildfire, decreased air quality, diseases transmitted by insects, food and water and threats to mental health." (Yes, even mental health: The report compiles a bunch of studies on the subject on p. 349.)
If you're looking for good news in the report, there is a tidbit about how U.S. agriculture is expected to remain "relatively resilient" in the face of unchecked climate change for the next 25 years or so. But after that, crops and livestock don't fare so well and productivity starts declining thanks to heat and drought. So it's not exactly great news.