In New York City, today’s temperature was forecast to be hotter than any yet this summer (though it hasn’t got there yet, there is still time). This is a little unusual since it is already September. I got an inquiry from a reporter today, containing the following questions:
“This summer’s seemingly mild weather was actually warmer than average. Has climate change caused us to have a new normal? And while a 92 degree day on September 2nd is not unheard of, is there a larger case of climate change going on?”
Here is the answer I sent, edited and expanded a bit for this post:
Yes, there is certainly a new normal as far as temperature is concerned. It was a cool summer compared to recent decades, but those have been warm compared to the longer-term historical record, due in large part to human-induced global warming. Thus this summer was still warm compared to the long-term average (which means, since the mid-19th
It’s worth also mentioning that the coolness this summer was limited to our half of the country; the west was extremely hot even compared to recent years, with bad forest fires in the Pacific NW, the drought in California etc.
One can’t assign the hot weather today to global warming though – at least not for the most part. It’s never a good idea to attribute a single day of weather to long-term trends, because the natural day-to-day (and even month-to-month or year-to-year) fluctuations are large. On the other hand, it is safe to say that global warming will mean more 92-degree days after Labor Day in years to come, compared to the past.
Because of natural variability, some summers are a little warmer, some are a little cooler. But global warming keeps continuously pushing them all warmer. So by sometime in mid-century, the coolest summer in any given decade is still very likely to be hotter than even the hottest summer that anyone alive now (or their parents, grandparents etc.) has yet experienced.
Climate Central did a nice piece recently which includes an online form allowing you to see how this summer stacked up against the historical record for many US cities. Here is the image I got by selecting San Francisco, where this summer broke the all-time record according to the graph – I’m not sure why the title is just “Near Record Warmth”. The bars are the different years ranked by their average temperature.