Trends in global mean temperature are not static through time. Changes due to radiative forcings are influenced by internal climate variability. A recent paper by Karl et al. concluded that:
the central estimate for the rate of warming during the first 15 years of the 21st century is at least as great as the last half of the 20th century. These results do not support the notion of a “slowdown” in the increase of global surface temperature.
Are these conclusions, based on comparing a few periods, correct?
The figure below shows rolling 15- and 30-year linear trends in both HadCRUT4.3 (top) and the new Karl et al. (bottom) global temperature estimates, along with trends in radiative forcing (including solar, but without volcanic eruptions).
Karl et al.’s conclusion that the warming during the most recent 15 years compared to the second half of the 20th century (grey dashed line) is broadly correct, although I would suggest that ‘similar to’ is more appropriate than ‘at least as great as’.
But, there has clearly been a slowdown in the rate of warming when compared to other periods, e.g. those centred on the 1990s.
What causes these variations in rate of global temperature change? The response to radiative forcings (black) and volcanic eruptions (black triangles) is superimposed on changes due to modes of climate variability, primarily the Inter-decadal Pacific Oscillation (IPO) but maybe the Atlantic Multi-decadal Oscillation (AMO) as well.
We do not expect changes in temperature to be uniform in space or time. We need to move away from the binary comparison of comparing individual short periods of temperature change and improve our understanding and communication of the whole spectrum of global temperature changes.
The efforts of Karl et al., and the other similar international groups working on improving estimates of global temperatures, are to be applauded. There are still uncertainties in historical global temperature trends, but the big picture of a warming planet due to anthropogenic factors is certainly clear to see.
These anthropogenic changes have always occurred, and will continue to occur, against a background of natural climate variability.