Is California getting Hotter?

Today, as I am suffering through yet another extreme heat wave in California with attendant rampant wildfires, it is but natural to wonder if this is the result of a warming planet? The climate change deniers would likely say there is nothing to see here, heat waves or heat spells are perfectly normal and happen from time to time. But, as locals we know in our gut these heat waves are becoming more common than we remember in the past. What evidence can lay people procure to try and bolster the gut feeling that it is generally getting hotter here in California.

Fortunately, it is rather simple to confirm this gut feeling. The National Oceanic and Atmospheric Administration (NOAA) makes climate datasets available for download here: https://www.ncdc.noaa.gov/cdo-web/. The datasets of interest are historical records of daily temperature. Some of the stations have records going back to the 1800s.

I thought it would be interesting to go through these daily temperature records to see what we can learn from daily minimum and maximum temperatures records from year 1900 to year 2019. I exclude 2020 since 2020 is still not over.

I was hoping the data would help me to answer the following questions:

1. Do we have many more warm/hot days in the recent years as compared to years past?
2. Has the warmer period of the year expanded? Meaning are we seeing more months during the year with hot days?
3. Do we have fewer cold days in the winter in recent years than in the years past?
4. Do we have shorter winters? i.e. have the cold days been restricted to fewer months?

In order to answer these questions for Northern California, I needed to select at least 2 weather stations that have daily temperature summary records going back to at least year 1900. In addition I wanted to select locations that are bit inland, where they are less affected by the vagaries of the coastal air. The coastal air in California is greatly affected by the cold Pacific Ocean water and locations such as San Francisco tend to be greatly affected by the Ocean air. So I selected San Jose, a location somewhat further than San Francisco is from the Ocean air, as well as Sacramento, a city much further inland. Also, to qualify as a hot day I used a threshold max temperature of 100F and to qualify as a cold day I used a threshold min temperature of 32F, these I believe are reasonable cutoffs for Northern California.

I developed a quick tool to massage the raw data into something suitable for Microsoft Excel to be able to plot 3D charts that would hopefully help us answer the questions above.

So without further ado, here is the data.

The Data

Note: The charts below display the years on the X axis, months on the Y axis and the number of days in the month where the temperature was above or below the threshold value on the Z axis.

Hot Days in San Jose and Sacramento

San Jose Hot Days over past 120 years

Sacramento Hot Days over past 120 years

Observations

The first decade to 15 years of the 1900s appear to have very few days above 100F in both San Jose and Sacramento, apart from the year 1905 where there is ostensibly an anomalous streak of many 100+F days in San Jose. Then it appears that since roughly 1915 the frequency and length of the heat waves has gradually increased, becoming far more frequent in the 1990s and 2000s. We can also notice that the frequency of Hot days in May and September has increased since about the 1970s. In fact, we see a few 100F days starting to show up in October since the 1980s in both San Jose and Sacramento.

Cold Days in San Jose and Sacramento

San Jose Cold Days over past 120 years

Sacramento Cold Days over past 120 years

Observations

In almost a mirror image of the Hot days, it appears that San Jose had very frequent and long duration Cold waves (freezing or below days) in December and January, until about the beginning of World War II (mid 1930s). Roughly the same period that saw very few Hot days. In that period San Jose would also see below freezing days in March and November. Since about the 1980s San Jose rarely experiences below freezing temperatures and have completely stopped seeing such temperatures in March.

Sacramento picture looks somewhat similar, the only visible difference being that Sacramento appears to have experienced colder January temperatures between about 1918 to 1970, and since about 1970 Sacramento too has experienced a pause in the number of cold or below freezing days.

Conclusion

It appears that the data very much confirms the gut feeling. Northern California is getting hotter.

1. Yes, we do have more Hot days than in the years past. Of course, we don’t have reliable data from beyond 120 years back, but over the period experienced by two or three human generations we can say with confidence it is hotter than we can remember.
2. Yes, the warmer period in the year has indeed expanded. We are seeing hot days in months such as May and October that rarely were ever hot in the past.
3. Yes, we do have fewer Cold days in the winter than we had in the past.
4. Yes, we are indeed experiencing shorter and warmer winters. Spring and Fall are not as cold as they used to be.

Is this the result of Climate change? I’m not a climate scientist, but I would say yes, the climate we experience in terms of heat and cold, has most certainly changed.

Is this anthropomorphic? Is this reversible? This data has no answers to those questions unfortunately, but if you wanted a confirmation for your gut feelings, here it is.