Are September and October Getting Warmer?

Recent changes in temperature patterns have caught the attention of environmental scientists and the general public alike. Particularly, there has been a growing concern about the warming trend in September and October. Let's explore the evidence behind this claim and its implications.

Temperature Trends in the Northern Hemisphere

In the Northern Hemisphere, including regions such as Canada and the northern United States, there has been a notable shift in seasonal temperatures. While July and August are typically the warmest months of the year, September has surpassed May in terms of temperature, and October has outpaced April. This trend highlights a potential shift in the historical temperature patterns.

Personal Experiences and Observations

For instance, in Reno, Nevada, a consistent upward trend in temperature has been observed since the 1970s. According to local records, the average temperature has risen by 11 degrees Fahrenheit (6 degrees Celsius) compared to the same period in the 1970s. This represents a significant increase, especially when considering that this rise has occurred over a span of four decades.

Additionally, gardeners in the area have noticed that they can harvest tomatoes well into October, which was historically the time when the first killing frost would typically occur around mid-September. This shift suggests that the region is experiencing a delay in the onset of colder weather, making the fall months much warmer than they used to be.

Global Temperature Trends

On a larger scale, the phenomenon of global warming has been a subject of extensive research and debate. According to satellite temperature records, the period from 1998-1999 marked the peak of the "Modern Warm" trend. Since then, there has been a gradual decline in global mean temperatures by approximately 0.2 degrees Celsius. Roy Spencer, an atmospheric scientist, compiled satellite temperature data that shows a significant cooling trend after the late 1990s.

The chart below compares satellite readings (RSS) with "surface" temperature data adjusted by the National Oceanic and Atmospheric Administration (NOAA). While both data sets show warming in the 1990s, the satellite data reveals a cooling trend since then, while surface data continues to show warming. This discrepancy highlights the importance of considering multiple sources of temperature data when assessing global climate trends.

Figure 1: Comparison of Satellite (RSS) and Surface (Gistemp) Temperature Data

According to Spencer's data, the global mean temperature has been declining since the late 1990s, contradicting the idea of continuous global warming. This cooling trend is supported by satellite temperature records, which have not been subject to the same degree of adjustment as surface data.

Regional Variability and Trends

While the general trend on a global scale is cooling, regional variability is significant. In Middle Tennessee, for example, there has been a noticeable warming trend. Historically, the fall season was characterized by cool, crisp temperatures. However, recent years have seen a shift towards warmer autumns, with July often being one of the hottest months on record. This suggests that regional climate trends can differ significantly from the overall global pattern.

In Montana, a different trend is observed. Over the last dozen years, September, October, and November have typically been colder than the climatic average. This cooling has been exacerbated by late frosts, shortening the growing season by 35 days. The first hard frost came in August last year, which is several weeks earlier than usual.

Fluctuations and Long-Term Trends

While it might be tempting to draw conclusions from a few months of data, it is important to recognize that temperature fluctuations can be significant from year to year. Natural variability and localized weather patterns can cause considerable deviations from long-term trends. For example, December and January in Montana were slightly above average, while February, March, and April were well below average, leading to record snowfall.

Additionally, the spring of 2023 saw warmer temperatures compared to previous years, emphasizing the need for a long-term perspective. The fact that there was 8 inches of snow on June 8 in a previous decade underscores the unpredictability of regional weather patterns.

In conclusion, while there is evidence of warming in September and October in certain regions, the overall global trend suggests a decline in mean temperatures. Regional variations and natural climate variability underscore the complexity of long-term climate trends.

For further reading, explore the following links:

Satellite Temperature Data NOAA Surface Temperature Adjustments Historical Climate Records