Were the 1930s the Warmest Decade?

(c) 2009 by Barton Paul Levenson

The idea is floating through the blogosphere that the 1930s were actually the warmest decade on record, and therefore that global warming can't really be much of a problem. Is this idea correct?

The story came from a 1999 NASA press release by James Hansen and colleagues which concluded that "Indeed, in the U.S. the warmest decade was the 1930s and the warmest year was 1934." A revision of the temperature station figures from updated data and a new algorithm showed that 1934 was 0.02 K hotter than 1998, the year previously held to be the hottest in US history.

In fact, this is irrelevant to global warming. Here's why:

1. The figures are for the continental United States only, the "lower 48." This is only 1.54% of the Earth's surface. Global warming refers to the average.
2. Hansen et al.'s statement is from 1999. The 2000s have been hotter than the 1930s even in the US.

Let's look at the "temperature anomalies" (deviations from the mean temperature in a base period). Two time series are available for this that go back to the 1930s: the NASA GISS series (GISS = Goddard Institute for Space Sciences), and the Hadley Centre CRU series (CRU = Climate Research Centre). The first is from land stations only and is compiled in the United States. The second combines land and sea readings and is compiled in the United Kingdom. Here are the figures for the 1930s, 1990s, and 2000s, for the entire globe:

Year	NASA	Hadley
1930	-0.04	-0.165
1931	0.02	-0.124
1932	0.04	-0.155
1933	-0.11	-0.297
1934	0.05	-0.159
1935	-0.08	-0.184
1936	0.02	-0.152
1937	0.12	-0.034
1938	0.15	0.009
1939	-0.02	-0.001
1990	0.48	0.254
1991	0.44	0.212
1992	0.15	0.061
1993	0.19	0.105
1994	0.32	0.171
1995	0.46	0.275
1996	0.39	0.137
1997	0.41	0.351
1998	0.72	0.546
1999	0.46	0.296
2000	0.42	0.270
2001	0.57	0.409
2002	0.68	0.464
2003	0.67	0.473
2004	0.60	0.447
2005	0.76	0.482
2006	0.66	0.422
2007	0.74	0.405
2008	0.55	0.328

Here are the means for each decade (for the 2000s, nine years, since we don't have complete 2009 data yet):

Decade	NASA	Hadley
1930s:	0.015	-0.1262
1990s:	0.402	0.2408
2000s:	0.628	0.4111

It's quite clear that for both series, the 1990s were warmer than the 1930s, and the 2000s were warmer still. Are the differences significant? Here are the between-means t-tests for each series and each pair of decades:

Decades	NASA	Hadley
1990s:1930s	-6.81	-6.43
2000s:1930s	-14.25	-13.98
2000s:1990s	-3.58	-3.28

Every t statistic is significant at the 99% confidence level or better for a sample size of nine or more. So on a global scale, the 1930s were NOT the warmest decade. Not even close.

How about in the continental US? Here are the statistics (NASA GISS land surface temperature anomalies):

Year	NASA
1930	0.15
1931	1.08
1932	-0.01
1933	0.67
1934	1.24
1935	0.04
1936	0.20
1937	-0.14
1938	0.86
1939	0.84
1990	0.88
1991	0.69
1992	0.31
1993	-0.43
1994	0.47
1995	0.36
1996	-0.16
1997	0.04
1998	1.24
1999	0.94
2000	0.55
2001	0.78
2002	0.56
2003	0.53
2004	0.46
2005	0.71
2006	1.08
2007	0.77
2008	0.10

The means are:

Decade	NASA
1930s:	0.493
1990s:	0.435
2000s:	0.616

Looks like the 1930s were warmer than the 1990s in the US, but the 2000s were warmer than either of them. Are the differences significant? Here are the between-means t-tests:

Decades	NASA
1990s:1930s	0.26
2000s:1930s	-0.65
2000s:1990s	-0.94

None of the differences are significant. The 1930s were not significantly warmer than the 1990s and were not warmer than the 2000s at all.

Looks like there hasn't been any significant global warming in the lower 48, which constitutes 1.54% of the Earth's surface. Or has there? Turns out if you use all the anomalies for the lower 48, from 1880 to 2008 (N = 129), you get a regression accounting for 18% of the variance going by elapsed time alone. Not a great fit, but the slope is positive and has a t-statistic on the year term coefficient of 5.2, significant at the 99.9% level. So yes, even in the continental United States, which is warming more slowly than the rest of the globe, there is still significant warming. The argument that "the 1930s were warmer, so there's no global warming" is spurious.