The topic of how the US temperature record has been massively altered in recent years has been well covered by Tony Heller, myself, and others in the past.
Nevertheless, it is worth summarising again.
In 1999, James Hansen, Reto Ruedy, Jay Glascoe and Makiko Sato of GISS published a paper, “GISS analysis of surface temperature change”, which included this graph of the US temperature record at that time.
The drop in temperatures from the 1930s to the 1970s is absolutely clear.
The paper commented:
The U.S. temperature increased by about 0.8°C between the 1880s and the 1930s, but it then fell by about 0.7°C between 1930 and the 1970s and regained only about 0.3°C of this between the 1970s and the 1990s. The year 1998 was the warmest year of recent decades in the United States, but in general, U.S. temperatures have not recovered even to the level that existed in the 1930s. This contrasts with global temperatures, which have climbed far above the levels of the first half of this century.
This dichotomy with global temperatures clearly posed a problem for Hansen and co. So they wrote a brief, which attempted to explain why the US temperature trend was out of line:
What’s happening to our climate? Were the heat wave and drought in the Eastern United States in 1999 a sign of global warming?
Empirical evidence does not lend much support to the notion that climate is headed precipitately toward more extreme heat and drought. The drought of 1999 covered a smaller area than the 1988 drought when the Mississippi almost dried up. And 1988 was a temporary inconvenience as compared with repeated droughts during the 1930s “Dust Bowl” that caused an exodus from the prairies, as chronicled in Steinbeck’s Grapes of Wrath.
How can the absence of clear climate change in the United States be reconciled with continued reports of record global temperature? Part of the “answer” is that U.S. climate has been following a different course than global climate, at least so far. Figure 1 compares the temperature history in the U.S. and the world for the past 120 years. The U.S. has warmed during the past century, but the warming hardly exceeds year-to-year variability. Indeed, in the U.S. the warmest decade was the 1930s and the warmest year was 1934. Global temperature, in contrast, had passed 1930s values by 1980 and the world has warmed at a remarkable rate over the last 25 years.
Fig. 1: Annual and 5-year mean surface temperature for (a) the contiguous 48 United States and (b) the globe, relative to 1951-80, based on measurements at meteorological stations.
A picture of how U.S. climate change during the past half-century compared with the rest of the world is shown in Figure 2. This map shows that the trend has been toward warmer temperatures in most of the world. There has been nearly ubiquitous warming in the tropics, especially in the Eastern Pacific Ocean, where the largest warming coincides with the location of more frequent strong El Niños. The strongest warming has been in Alaska and northern Asia. Warming in Alaska is often associated with El Niños. A suspicion of many climatologists — as yet unproven — is that an increasing greenhouse effect may cause more frequent and intense El Niños. Asia has long been predicted to show the largest warming due to increasing greenhouse gases, especially in the winter, and observations are consistent with that.
Yet in the U.S. there has been little temperature change in the past 50 years, the time of rapidly increasing greenhouse gases — in fact, there was a slight cooling throughout much of the country (Figure 2). We caution that linear trends, as in Figure 2, can mask temporal detail. Indeed, Figure 1(b) indicates that the last 20 years have seen a slight warming in the U.S. Nevertheless, our analysis (Hansen et al., 1999a), summarized in Figures 1 and 2, makes clear that climate trends have been fundamentally different in the U.S. than in the world as a whole.
Is this a temporary fluke, a chaotic regional climate fluctuation? If so, as the regional fluctuation reverses and global warming continues, will the U.S. experience dramatic, perhaps “disastrous” climate change in the next few decades? Or is there an understandable and continuing reason that the U.S. is warming less than the rest of the world? In that case, will the U.S. be relatively immune to global warming in the next several decades?
In order to answer such questions and predict future climate change reliably, a prerequisite is an understanding of the cooling of the past half-century in the U.S. Figure 2 suggests that the U.S. cooling is associated, at least in part, with cooling in the North Atlantic Ocean. Climate model simulations tend to confirm this, yielding cooling in the U.S. during the past 50 years when driven by observed ocean temperatures (Hansen et al., 1999b).
But that only changes the question: what is the cause of the Atlantic cooling? In part, the Atlantic cooling is a natural fluctuation, the North Atlantic Oscillation, that occurs on decadal timescales. Observations of the past few years, summarized in Figure 17 of Hansen et al. (1999a), suggest that the North Atlantic Oscillation is now moving into its warmer phase.
However, North Atlantic cooling is also a predicted consequence of the transient growth of greenhouse gases. Climate models (Manabe and Stouffer, 1995; Russell and Rind, 1999) driven by increasing greenhouse gases yield increased precipitation at high latitudes, decreased ocean salinity in the North Atlantic, and thus a weakening of the ocean conveyor belt that transports heat to the North Atlantic.
Additional mechanisms may contribute to observed climate change. For example, in the decades after World War II, when the number of aerosols (fine particles) in the air grew most rapidly in the Eastern U.S., the pattern of cooling showed a clear resemblance to the distribution of aerosols. Also, changes in solar irradiance (the brightness of the sun) are difficult to dismiss as a mechanism of climate change because there are observed correlations of solar variability and climate change.
The upshot is that we will be able to understand climate change well only with the help of global climate models that are able to incorporate all of these mechanisms on an equal footing. We will be able to test our understanding during the era of satellite measurements when all of these forcing factors can be measured accurately.
But there was actually a much bigger problem for GISS. During that warm period of the 1920s to ’60s, most of the world’s temperature data came from the US, along with Europe and Australia.
Most of the rest of the world had very little in the way of reliable data.
Moreover, the US data was high-quality stuff from USHCN. This could not be so easily dismissed as it put into doubt the claimed increase in global temperatures.
NOAA, however, had the answer and adjusted the US data out of all recognition. Now it looks like this:
Take a close look at the data for 1934 and 1998.
The new chart tells us that the temperature anomalies (relative to the 1951-80 mean) were 1.19°C and 1.33° respectively. In other words, 1998 was 0.14°C warmer than 1934.
Yet on the original 1999 chart, 1934 was clearly a lot warmer than 1998. We don’t have figures, but eyeballing suggests that the 1934 anomaly was about 1.5°C, while 1998’s was about 0.9°C, that is 0.6° hotter.
Overall then, NOAA has reduced the 1934 temperature by 0.74°C, relative to 1998.
The hottest year on record, according to the current version of the US temperature history, is 2012, with an anomaly of 1.86°C. However, if you take off the adjustment of 0.74°C, it still leaves 1934 as the warmest year.
NOAA claims that these adjustments are mainly to allow for the time of observation changes (TOBS), yet their own analysis shows that this only account for around 0.5°F, or 0.3°C. (Also note that NOAA’s webpage explaining all of this is no longer available, but fortunately is still obtainable on Wayback).
So NOAA’s “explanation” accounts for less than half of their changes. Yet there are no specific adjustments for the effects of UHI, which must be substantial as well.
As Ronan Connolly has shown, there are very real concerns about whether the current homogenization methods used by USHCN adequately take account of UHI. Indeed there is a likelihood that rural stations are actually being adjusted to match urban trends.
And as Anthony Watts has also revealed, temperature trends at rural USHCN stations are lower than those at urban ones.
In summary, the adjustments to the US temperature record made by NOAA are more than double the ones they have admitted to. Furthermore, they have not been making proper allowance for UHI.
The US temperature record presented by NOAA and GISS is little more than a political construct.
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