The Truth About Minimum Temperature Increases

 The Seattle Times this week published an article, Seattle Nights Are Getting Hotter, that irresponsibly exaggerates the effects of global warming on minimum temperatures in our region.

This sponsored "journalism" by the infamous Seattle Times ClimateLab, is based on a report by a well-known climate change advocacy group, Climate Central.

Before I get a dozen strident emails from climate activists, let me note that global warming, forced by increasing CO2 and other greenhouse gases, is occurring.   The science is clear on this---and I have published many papers on this topic.

At this point, such greenhouse gas warming is on the order of 1.2°C--a little over 2°F.  

Not the end of the world.  

But that is not what is being communicated by the Seattle Times.  

They prominently display a figure from Climate Central that shows an EXPONENTIAL increase in the number of warm nights (see below) using the proprietary CLIMATE SHIFT INDEX.

As we will see, this is scary nonsense.

Why don't they show you changes in temperature?  Because those are unimpressive.

The Seattle Times starts by highlighting the "heat wave" of June 7-9 of this month as an "example" of what will happen in a warming world.  

Just silly.  This was a run-of-the-mill warm period--the kind of short-term warmth that has been observed frequently in the past.

Let me demonstrate this to you.

Below are the temperatures of the last month, with the observed highs and lows shown by the blue bars, and the extreme highs and lows by red and light blue shading.  The brown shading indicates the normal daily range.  During the past month, we have experienced both warmer and cooler than normal periods.  June 8-9 was warmer than normal, but no high temperature records were broken.  

Of the past 30 days, 22  days had low temperatures below the normal lows.    You note the Seattle Times did not point it out.

Of the 30-day warm periods in June, the three days of June 6-9, 2025, came in 47th. Stop the presses!

But it is worse than that.

SeaTac Airport is the last place you would want to use temperature to understand global warming.

Why?  Because of the profound warming influence of the construction of the third runway and massive development in and near the airport. A large (cooling) vegetated area was removed and replaced with a huge area of concrete.

You want proof of the impact of the development?  Here is the plot of summer nighttime mean minimum temperatures at SeaTac.  I have indicated (red bar) the period of construction.  Notice the shift upward?

The mean temperature was roughly 54°F before and 57°F after.   3F increase.

Now compare to the Olympic Airport, where no third runway was added and development was far less (below).

No big jump.  Summer minimum increased from about 48F to 50°F.  Yes, there is warming, but it is not abrupt or extreme.  You would hardly notice this.

Finally, the rapidly rising number of warm nights shown on the Climate Central plot is based on meteorological sleight of hand.  

They start by using an inappropriate station (SeaTac), in which development has caused substantial warming.  Then, they don't show temperature changes (which would not have impressed), but their "climate shift index", based on their artificial (and incorrect) assumptions of changes in probability distributions.   

This index exaggerates change, since only a very small temperature increase can radically increase the probabilities of exceeding some extreme value.  

Very deceptive and wrong.

The Northwest Dry Spell Will Soon End

This has been a relatively dry spring, although far from record-breaking.

For example, in Seattle, the precipitation total from March 22 through June 13 was below normal by roughly two inches.   However, the 2025 spring rainfall was not even close to the lowest precipitation total on record, with 34 years having less spring rainfall.

Below is the plot of spring precipitation (March 22-June 13) since the 1940s.  The trend is up, strongly suggesting that global warming is NOT associated with dry springs at Seattle.  

You can see that this year was low, but not even close to the driest year on record.

The models are insistent that rain is on the way after the next few dry days.

The reason is a deep upper-level trough will move over our region later this week, something shown by the European Center forecast for 11 AM on Friday (blue and purple indicate much lower heights than normal at 500 hPa, about 18,000 ft).

Such a trough brings cool, wet weather to our region.

Rain will move in on Thursday and Friday.  The 48-hour total ending Thursday at 5 PM is relatively light over Washington.

The 48h ending Saturday at 5 PM is considerably wetter in the mountains, with light rain over central Puget Sound.

More rain will follow (the 48 h ending 5 PM Tuesday is shown).

Putting it all together, the totals from this morning to 5 PM on June 26 are impressive for this time of the year.

Let me note that no heat waves are expected for the next two weeks, with the temperature being chilly next Friday and Saturday.

 
Finally, let me note that this spring has not brought serious cold waves like the previous years.  Cold waves can damage important crops such as cherries.   

The plot of temperatures at Wenatchee (below) illustrates this fact. No major cold excursions!

Without major spring cold waves, the WA State cherry farmers expect an excellent and sustained harvest.

Good news for all cherry lovers (like myself).