Depending on which global temperature data one looks at, temperatures have not increased in the last 18 or so years. The reasons proposed have been various, ranging from natural cycles to increased aerosols, to heat escaping to space or the deep ocean.
Perhaps there are some other reasons that have not been considered. The following is a simple list, with illustrations. The list is divided into two sub-lists. Things that are natural and things that are anthropogenic or man-made.
SOME NATURAL REASONS
1. It’s The Atlantic Multidecadal Oscillation (AMO)
The AMO has been at the top of it’s warm phase since 1998. The index doesn’t get much higher than it is now. It can only go down from here. It was at a similar peak during the warm 1930s through the 1960s. It was negative during the cool 1970s. The peaks of the AMO tend to be flat for a couple of decades before flipping cool. We don’t know what drives the AMO. Data here.
2. It’s The Pacific Decadal Oscillation (PDO)
The PDO has been trending down since the early 1980s. It also was up during the 1930s and negative during the 1970s. The AMO and the PDO are the natural ocean cycles that climate scientists talk about. The PDO reached a peak in the 1980s and has been declining since. This index is volatile. The PDO has a huge effect on weather on the Pacific Coast of North America. Data here.
3. It’s The AMO and PDO together
They are sometimes roughly added together. (Even though they are not measuring the same thing.) If one adds them together, it can be seen why the late 1930s were warm and the 1970s cool. The sum (green trace) reached a peak in 2000 and is now declining because of the declining PDO. (Computed by author.)
4. It’s the sun
The sunspot number (SSN) average has declined since the mid-1990s. One can see a cause for the 1970s cooling in the SSN, but not for the 1930s warming. The early 20th century cooling may have been caused by the low SSN around the turn of the century. The sun is excused for the recent pause because the total solar index (TSI) changes only by a fraction of a Watt/m2 over large changes in SSN. But other factors may be in play. (Source: WDC-SILSO, Royal Observatory of Belgium, Brussels.)