Unmasking Sulfates' Effectiveness

Global warming theorists who believe climate models can accurately portray global temperature variations have a favored storyline. It goes like this. Earth’s climate began to warm when the atmospheric carbon dioxide concentration began to grow in the mid- to late-19th Century. During the 1940s and up to the mid-1970s, the warming was offset completely by the cooling effect of sulfate aerosol emissions. But then CO2’s warming effect began to dominate the sulfate cooling and we’ve rapidly warmed ever since (although at a lesser rate because of the atmosphere’s increasing sulfate aerosol burden). This is a useful storyline for two reasons.
     First, if you choose to accept it, this storyline confirms that much of the last century’s temperature change is of human origin. It also consigns solar variation to a small role in the warming before 1940. Secondly, it makes sulfate aerosols a major player when it comes to earth’s climate.
     Somewhere down the road, when sulfate emissions come under tighter control, earth’s temperature therefore can be expected to shoot upward. It is this notion that is embraced by the Intergovernmental Panel on Climate Change in its 2001 Third Assessment Report (TAR). It’s what leads to the middle and upper boundary temperature increases in the IPCC’s projection of a 1.4ºC to 5.8ºC increase in temperature by 2100.
    Introducing sulfate aerosols into the projected temperature equation almost reads as a carefully constructed plan to produce precisely those results. The IPCC’s First Assessment Report in 1990 made very little mention of sulfate aerosols and their impact on climate. But by the time of the IPCC’s Second Assessment Report in 1995, it was evident that climate models accounting only for the effects of increasing greenhouse gas concentrations greatly overestimated the amount of warming that had been observed during the 20th century. The IPCC rationalized:
     

When increases in greenhouse gases only are taken into account…most [climate models] produce a greater mean warming than has been observed to date, unless a lower climate sensitivity [to the greenhouse effect] is used… There is growing evidence that increases in sulfate aerosols are partially counteracting the [warming] due to increases in greenhouse gases.

     This possible explanation grew in popularity and rapidly spread across the modeling community because it meant their models weren’t wrong after all. They had simply failed to include the effects of another anthropogenic emission! After carefully concocting a sulfate emissions history to feed their models, the modelers could closely reproduce the observed temperature variation—even the absence of warming between 1940 and 1976. The genius of this solution wasn’t fully revealed in the second assessment. Rather, it planted a seed, gave time for everyone to get comfortable with the supposed role of sulfates in climate change, and saved the Big Surprise for the next report. What a surprise it was!
     In unveiling of their Third Assessment Report in 2001, the IPCC revealed that the global temperature projections for the year 2100 had been changed from the previous report’s range of 1.0ºC to 3.5ºC to the now famous range of 1.4ºC to 5.8ºC. Climate disaster loomed. Things were getting worse the more we learned!
     But what was the primary reason for the increased warming? It wasn’t carbon dioxide. The third assessment’s scenarios increase CO2 at a rate very similar to that of the second assessment. The increased warming came about from the removal of sulfate aerosols. Mirabile dictu, the sulfate aerosols inserted five years before to explain why climate models couldn’t come closer to accurately depicting reality get removed, with the result of a forecast for still greater temperature increases! You’d think if there were any truth in this assertion that some sort of observation would exist to support it other than, “How else can you explain the cooling between 1940 and 1975 when greenhouse gases were rising?”
     We’ve already noted (www.co2andclimate.org/Articles/2003/vca10.htm) that the pattern of temperature change since 1975 is nearly opposite that of the pattern that models project as a result of sulfate aerosols. We don’t find any evidence that industrialized and more populous areas of the globe (where sulfate emissions begin or are transported) are warming more slowly than are surrounding less populous areas. Instead, observations show how the industrialized/populous regions are warming at a greater rate than are their less populous neighbors. Strike one against the models.
     Recent research by Vladimir Semenov and Lennart Bengtsson pitches strike two. Their recent paper in Geophysical Research Letters looks at modes of wintertime temperature variability in the Arctic. This is a subject investigated in the past, but Semenov and Bengtsson were able to push their analysis back to the late 19th Century. By doing so, they were able to examine the patterns of temperature behavior for more than a hundred years (1892-1999).
     These two researchers found that two patterns of temperature variability explain most of the long-term temperature trends in the Northern Hemisphere’s high latitudes (Figure 1). The temporal evolution of the first pattern explains a large majority of the wintertime temperature increase since the 1970s. The second pattern’s behavior explains nearly all of the rise in Arctic wintertime temperatures between 1920 and 1940 and it explains the subsequent decline in temperatures from 1940 to 1970. This second pattern has a role in the temperature rise since 1970, but not so strong as the first pattern.
     This 20th Century’s pattern of temperature evolution (warming, cooling, and warming again) is characteristic not only of the Arctic, but of the Northern Hemisphere and the entire planet. That these two independent patterns of temperature change explain different aspects of the temperature evolution of the Arctic winter during the past century means that a simple sulfate explanation just won’t cut it. Earth’s climate didn’t simply begin warming as a result of enhanced greenhouse gas concentrations, then pause as sulfates exactly cancel them out, only to resume as greenhouse gases re-exert their dominance. Instead, it appears that the way the world (okay, at least the Northern Hemisphere Arctic) warmed in the latter part of the 20th century was different from the manner in which it warmed in the early part of the 20th century. This indicates different root causes.
     Because the cooling interval seems to be a relaxation of the early century warming pattern, it has the characteristic of natural fluctuation rather than human tinkering. To state it as succinctly as is possible, the warming experience since the 1970s is of a different nature than the warming (and cooling) that took place in fifty years earlier. But that’s not how climate models project the tandem effect of greenhouse gases and sulfates.
     How about “two strikes and you’re out?” Oh, not in this game.

Reference:
Semenov, V.A., Bengtsson, L., 2003. Modes if the wintertime Arctic temperature variability. Geophysical Research Letters, doi:10.1029/2003GL017112.

Figure 1. The two primary patterns of temperature variability as determined by Semenov and Bengtsson (2003) to be responsible for most of the long-term temperature fluctuation in Arctic wintertime temperatures since 1892. Areas of like coloration vary in tandem (yellows and reds act opposite to that of greens and blues). Darker coloration indicates a greater level of association. (Left): This pattern of temperature change is associated with most of the warming since the 1970s. It includes enhanced warming in Siberia and northwestern North America with a region of cooling in the western North Atlantic. (Right): This pattern of temperature change is associated with the rapid warming between 1920 and 1940, and the subsequent cooling from 1940 to the early 1970s. It is dominated by warming (cooling) in the high latitudes around the North Pole and to a lesser extent cooling (warming) in central Eurasia.   Back to Top