Global warming is accelerating

Figure 1 shows how the global surface temperature has increased since pre-industrial time.

The blue dots are the monthly temperature anomalies, the blue curve is the five years moving average, and the red line is the trend calculated over the last 30 years. The reference period is from January 1880 till December 1909. The temperature in that period is often considered as the temperature in pre-industrial time, mainly because we do not have reliable global temperature measurements before that. Consequently, the figure shows the warming since pre-industrial time. 

The trend line ends at 1.19°C in August 2020. It is reasonable to say that the global surface temperature has increased by that value since pre-industrial time.

Figure 1: The average of four global surface temperature series. The reference period is from January 1880 till December 1909.

The temperatures in Figure 1 are the average of the four series NASA GISTEMP, NOAA Global, Berkeley BEST and HadCRUT4 kriging. These series all have global coverage. They were downloaded from their sources in the beginning of October 2020.

Climate is usually defined as the "average weather" over a period of time. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). It is therefore common to calculate the temperature trend over 30 years.

The red line in the figure shows that the trend over the the last 30 years is increasing with 0.21°C per decade. The next figure shows that this is a record high increase.

The blue curve in Figure 2 shows how the 30 years temperature trend has evolved. The curve consists of 1329 points, each showing the result of a trend calculation over 30 years. The first point is drawn at the end of 1909, showing the trend from January 1880 till December 1909. The last point is drawn at August 2020, showing the trend from September 1990 till August 2020.

When the blue curve is above the black horizontal line, i.e. positive, the trend up till that month is warming. When the line in addition is increasing, the warming is accelerating. The figure shows that there has been an accelerating warming since the mid 1970s, and that the warming now is larger than ever before. This applies to both the temperature measured in degree Celsius and the warming measured in degree Celsius per decade.

Figure 2: 30 years temperature trend as a function of end date in the trend calculation

The four temperature series are calculated by different teams at different sites. The team at Berkeley started to calculate the BEST temperatures ten years ago mainly because many, themselves included, were critical of the methodologies applied by the other teams. Despite this, their results are quite similar.

Figure 3: The differences between each of the four global surface temperatures and the average of them

Figure 3 shows the differences between each of the temperature series and the average of them. The root mean square (RMS) of the differences is close to 0.05°C. That is reassuring in terms of the quality of the temperature series.