No warming since 1979?!

This title represents some people's opinions of the temperature records since 1979 when satellite measurements began. The data from the horse's mouth [UAH, Spencer and Christy] are shown in Figure 1.

 

Figure 1 Lower troposphere temperatures since 1979 [from www.ddroyspencer.com]

Roy has chosen to present the trend of the data by a sine curve or a polynomial curve with the first half of the data having a general temperature lower than the second half, divided by the extraordinary El Niño of 1998. Over a limited period of 32 years since the satellite records have been available there have been many changes in temperature trends. It seems imprudent to attempt to avoid or hide the obvious general positive trend which itself might or might not represent the trend over a longer period.

            The proponents of the 'no warming' assertion express doubts about the probity of the terrestrial temperature records such as the HADCRUT3 from the UK Met Office. Figure 2 shows a comparison of the UAH and HADCRUT3 records since 1979.

 

Figure 2 UAH and HADCRUT3 records compared

            The two sets of data correlate very closely with a correlation coefficient of 0.82. The equation of the best straight line drawn through the UAH data is: ΔT = 0.00115x - 0.228, where ΔT is the temperature anomaly and x represents the number of months since January 1979. The positive trend equates to 0.00115 × 12 = 0.014°C per annum or 1.4°C per century for a linear extrapolation.

The best line drawn through the HADCRUT3 data is: ΔT = 0.0013x + 0.001, and the positive trend is 0.00126 × 12 = 0.015°C per annum or 1.5°C per century for a linear extrapolation. The trends of the two sets of data are positive and almost identical and the difference between the intercepts is due to the different bases that they have, HADCRUT3 being 0.229°C higher than UAH.

A proper conclusion is that both sets of data are measurements of the same phenomenon; that of the rising temperature with time over the relatively small range of 32 years and that their trends might or might not be representative of future changes. The attribution of the positive temperature trends to the rising atmospheric concentration of CO₂ is dealt with extensively on other pages of this site and in the scientific literature.  

Is the world cooling down?

Much is being made of the observed cooling of the globe since 1998 with arguments such as: ‘If the world is cooling and the atmospheric concentration of CO₂ is rising, there must be something wrong with the physics’...

A statistical study by Liebmann, Dole, Jones, Bladé, and Allured in Bull. Am. Met. Soc., 91, 1485 (2010) is entitled ‘Influence of Choice of Time Period on Global Surface Temperature Trend Estimates’. They take the HADCRUT3 and CRUTEM3 data sets for mean global and mean global land temperatures respectively from 1850 to 2009 and compute all the temperature trends from 2-year periods, 3-year periods, etc., to the overall 159-year period representing the maximum period in the data. The <159-year periods were computed for every possible starting year. No stone was unturned in the process!

A clear upward trend is evident in these series, both for the entire record and for many shorter segments. Large interannual-to-decadal variability is also apparent. These substantial short-term variations can lead to marked differences in trend estimates for time intervals whose starting and ending dates differ by only a few years. It is evident that time segments of a few decades or shorter can exhibit either warming or cooling trends, while trends for longer segments are mostly positive, though quite weak compared to those present in shorter segments.

Clearly, positive changes dominate the longer segments. The largest changes are positive and occur for segments longer than 30 years ending in recent years. The maximum change (0.82°C) is observed for the 108-yr period ending in 2009. There is also a secondary peak in warming that took place from the early twentieth century to the mid-1940s, culminating in a 0.62°C rise for the 39-yr period ending in 1945. For the sub-period 1945–2009 every trend longer than 22 years is positive, while for the entire record all segments longer than 82 years exhibit a positive trend. The overall warming trend, however, is interrupted by brief periods of cooling. From 1900 onwards, these cooling periods have not lasted more than 19 years, with two exceptions. The most pronounced episode of extended cooling took place following the warm interlude of 1937–45. When these warm years occur near the beginning of a segment, their influence on the trends extends all the way into the 1980s. There is some question, however, as to the accuracy of sea surface temperatures in the middle 1940s. A second, shorter period of cooling took place after the high temperatures of the late 1950s and early 1960s. The recent cooling that has been the subject of much popular media attention is presently of 9 years duration and amounts to a change of −0.07°C (the 4–8-yr changes ending in 2009 are also all negative). In total, there are 98 positive and 54 negative 9-yr segments. The segment ending in 2009 is only the 44th most negative of the record, well within the range of historical variability. Short-term trends of such magnitude (of either sign), therefore, are far from unusual.

 

Every possible trend (longer than two years) and its associated linear temperature change are calculated for the available record. Changes for segments longer than 82 years have all been positive. Within the constraints of the statistical significance tests, the positive changes of long duration (several decades and longer) ending in recent years are determined to be extremely unlikely to have occurred by chance. A secondary transient peak reflecting warming over a roughly 40-yr period ending in the 1940s is also unlikely to have occurred by chance. Since 1945, all periods longer than 22 years indicate warming, although only those segments ending recently stand out significantly from the noise. The land surface exhibits larger long-term changes than the entire globe, but the transient warming ending in the 1940s is less pronounced. In contrast, changes shorter than a few decades can be either positive or negative. The recent cooling trend is evident in the global record beginning in 2001. Such changes, however, are not statistically significant and are in fact relatively common in the historical record.

 

The paper makes it quite clear that there has been significant warming over the 1850-2009 period and opposite conclusions based upon any one set of ten consecutive years of data have a 35% chance of being wrong as representative of the general trend. In any case, the accepted definition of climate as the mean of thirty years of weather indicates the folly of attempting to come to conclusions about temperature trends using less than thirty consecutive years of data.