This doesn't strictly have to be after 1900, but I am putting it here because average temperatures are higher now than pre-1900.
What would the impact be of a year or two where temperatures remain more or less at summer levels for the entire duration of the calendar year? We all know about the "Year Without A Summer" that took place in the 1800s IOTL that was caused by volcanic activity in which the New England states experienced heavy snow and frost in every single month, but has the reverse ever happened IOTL or at least explored in AH?
I'm not sure that any human PoD could produce this, but let's say that the green movement in the west to cut greenhouse gases doesn't get off the ground and the U.S./western Europe continue to burn fossil fuels (especially coal) at the exact same rate as China and India today. But essentially, it would just have to be a whim/fluke of nature.
It probably wouldn't happen across the whole, entire world--much of the world would experience nothing more than a milder-than-normal winter. It would mostly impact equatorial, Mediterranean, and desert-type climates. I see it being especially forceful in the American Southwest where such scenarios already happen to a certain extent in some localities. In some years, southern California in January has the same average temperature as June (2009 was such a year). 90+ degrees have been recorded in early January in SoCal. The Big Bend of extreme southwest Texas is arguably, on an elevational basis, the hottest part of the United States, with January temps well into the seventies being extremely normal.
Some impacts:
--Substantially lower fuel use in the winter for cooling that will have quite a bit of impact on the world oil market. (But air conditioner use is also presumably much higher.)
--Warm-climate areas lose quite a bit of tourism revenue from "snowbirds" because their regions back home are much milder this year.
--Longer and more productive crop growing seasons that quickly become disastrous for next year due to lack of fall/winter/spring rain.
--Water crisis accelerates in urban areas, to a very desperate situation of water rationing and perhaps the need for desalinization by the next summer (since snowpack in mountains will presumably be much lower or nonexistent).
--Southern desert regions that rely on summer monsoons for most or all of their moisture (Arizona, the southern Sahara, northern India, parts of Australia, etc.) either get stiffed or are constantly deluged by flash floods.
Does anyone have any more ideas?
ArmchairPhilosopher