Global temperatures have seemingly plateaued in the past 10 years. Those dubious about climate science or wary of the social implications of carbon regulations have seized on this point to argue that fears of global warming have been overblown.
However, a careful analysis of the data reveals that this decade has in fact been anomalously warm – the warmest in the history of recorded global temperatures by a fair margin – and the rate of warming is consistent with that over the prior few decades. The real question at hand is not whether warming is occurring, but rather whether the rate of warming is faster or slower than expected by climate scientists.
The world has, in fact, continued to warm over the past decade in all five available temperature series (including both satellite and surface records), though the trend for some series is statistically indistinguishable from zero. The only way to obtain a cooling trend over the period is to cherry-pick an earlier start date to include the 1997-1998 El Niño event or to look at a time span of eight years or fewer.
However, such discussions of short-term trends obscure the fact that temperatures over the entire decade have been high relative to all other decades on record.
According to both NASA’s Goddard Institute of Space Studies (GISS) and the United Kingdom’s Hadley Centre, since global temperature records began in the late 1800s, most of the 10 warmest years recorded have occurred in the past decade. For both temperature records, eight of the 10 hottest years on record were in the decade spanning 1999-2008, with only 1999 and 2000 not making the cut.
The satellite records, which cover a shorter period but are often used as the series of choice by those skeptical of the integrity of surface temperature measurements, largely bear out the results of the surface records. In the University of Alabama, Huntsville, (UAH) and Remote Sensing Systems (RSS) interpretations of Microwave Sounding Unit (MSU) satellite data, for instance, seven of the 10 hottest years on record occur in the current decade. 2008 is the odd year out, with satellite records showing cooler temperatures that exclude it from the top 10 lists by a fair margin, while it just barely makes the cut in the surface temperature records.
|Figure One: Based on monthly data from 1979 through June 2009 from GISS, HadCRU, UAH, RSS, and NCDC. Decadal averages calculated using cross-series 10-year averages.|
The monthly temperature data for all four series (with an additional surface temperature series from the National Climate Data Center – NCDC – included) are shown in Figure One, with the decadal averages across the four series shown as black squares.
Not only was the current decade the warmest on record, as shown in Figure Two, but the rate of temperature increase between the 1990s (1989-1998) to the current decade (1999-2008) was considerably higher than that between the 1980s and 1990s, and higher than that between any other decade since global surface temperature records began in the late 1800s.
Figure Two: Decadal averages from 1880 to present calculated using Hadley Centre (HadCRU) monthly anomaly data.
Perhaps the best way to show that the current decade is not cooler than expected based on past trends is to project what we would expect 1999-2008 temperatures to be using data from the prior three decades, 1969-1998.
The Hadley Centre surface temperature record is used for this analysis because satellite records do not extend back far enough, though a similar analysis using UAH or RSS from 1979-1998 would yield similar results.
Figure Three: HadCRU monthly anomalies 1969 to June 2009. The black OLS trend line is based on all points between 1969 and 1999; the purple OLS trend line is based on all data.
Figure Three shows that, once data from the prior decade are included, the trend in temperatures over the entire period (the purple line) is higher then when it is not included (the black line). This means that, overall, temperatures in the past decade are higher than scientists would expect given the past trend in temperatures.
The other question that often comes up is about the dip at the end of the temperature series: is it unusual? Scientists check to see how anomalous the dip is by plotting the uncertainty interval. Figure Four shows the Hadley Centre data with both the trend uncertainty (in green) and the weather noise (in yellow), correcting for autocorrelation in the data. The chart demonstrates that the 1998 large El Niño event was outside the 95 percent confidence interval for “weather” noise, and the anomalously cold January 2008 is close; but all other months fall well within the bounds associated with underlying variability (as a result of El Niño Southern Oscillation – ENSO – and volcanoes and other factors).
Figure Four: HadCRU monthly anomalies 1969 to June 2009. The orange confidence intervals represent the expected range of variability in the series. The green lines represent the uncertainty in the mean linear trend and intercepts. Statistical analysis presented herein was done by Lucia Liljegren.
Such analyses demonstrate that arguments of global cooling over the past decade are misleading at best, and temperatures for the most part have been higher than expected given the past trend. In many ways, the real question is not whether or not global warming has stopped, but rather whether Earth is warming as fast as models predict it should?
This is a much trickier question, in part because there is a limited period of time since projections were created to test those projections against observations. More insights into addressing this question are likely to emege over time as scientists continue their research.