Thursday, December 19, 2013

A pause in global warming? What pause?!?

Much has been made of the pause in global surface temperatures since 1998.  Among the factors advanced to explain that pause include a change in ENSO, a decline in solar output, and an increase in aerosols (i.e. Foster and Rahmstorf 2011).  One of the previously neglected factors is artificial cooling in global surface temperature data sets.  Neither the HadCRUT4 nor the NOAA temperature data cover the polar regions.  As the polar regions are the fastest warming areas on Earth (UAH Arctic trend since November 1978: +0.44ºC per decade, Global trend: +0.14ºC per decade), excluding those regions leads to an artificial cooling bias in the data.

Anomaly map of HadCRUT4, showing the gaps in data coverage (in white)
Both NOAA and GISS are artificially cooled by an outdated ocean temperature data set (HadSST2) that doesn't account for the change from ship-based temperature measurements to buoy-based temperature measurements that occurred within the last 10 years (Kennedy et al. 2011).  Ship-based measurements are warmer than buoy-based measurements, meaning that the change from one to the other will create an artificial cooling trend that doesn't really exist unless the data is adjusted to account for the change.  While there's no sign as yet that GISS and NOAA will be updating their data to include the new HadSST3 that compensates for the change in how ocean temperatures are measured, a recent paper by Cowtan and Way (2013) proposes a method to correct the coverage gaps in HadCRUT4 data.

Cowtan and Way (2013) converted UAH satellite temperature data, which measures air temperature 1,000 meters above the Earth's surface, to surface temperature data (measured 2 meters above the surface) to fill in the blanks in the HadCRUT4 coverage map.  The way they did this was to take the difference between UAH data and existing surface temperature stations and interpolate the differences in surface data gaps via a method called "kriging."  Kriging is a well-established statistical technique and is used by NASA GISS as well as the Berkley Earth team for their global temperature data sets, although those teams interpolate directly from existing surface stations to cover the gaps.  Cowtan and Way's method should be more precise, as it uses satellite temperature data that covers the gaps to calculate the surface temperature in the gaps.

Covering the gaps in HadCRUT4 data has a large impact on temperature trends, especially the trends since 1998.  The trend in HadCRUT4 data since 1998 is +0.01976 ± 0.05555ºC per decade (trend ± 1σ standard error).  The trend since 1998 in the coverage-corrected version (I'll call it the "Cowtan-Way" data set) is +0.10508 ± 0.05458ºC per decade, 5.3x faster.  Once changes in ENSO, aerosols, and solar output are factored out of the Cowtan-Way data, the rate of rise since 1998 increases to +0.1880 ± 0.02765ºC per decade, 9.5x faster than the trend in HadCRUT4.


What does this mean for the "pause"?  Quite simply, there really is no pause.  The apparent "pause" is really just an artifact—the artificial effect of poor coverage of polar regions, combined with cooling from a shift in ENSO, an increase in aerosols, and a decrease in solar output.  While the rate of increase in the has been slower since 1998, an increase of +0.10508 ± 0.05458ºC per decade still an increase, not a pause.  Once the data is adjusted for ENSO, aerosols, and solar output, the rate of rise due to greenhouse gases for the Jan. 1998-Dec. 2012 is +0.1880 ± 0.02765ºC per decade—which is slightly faster than the overall rate of rise due to greenhouse gases since 1979 (+0.1841 ± 0.010282ºC per decade).  The other thing this shows is the over-sized effect small changes can have on short-term trends, as small changes in temperature can produce very different trends.  This effect is the consequence (and danger) of using short time periods.  Trends become far more stable over as the length of the time period increases.

The next time someone talks about a "pause" in global temperatures, the proper response is really "What pause?"  Global warming hasn't paused.  If anything, the rate of warming due to greenhouse gas concentrations has increased slightly since 1998.

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