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twentieth century trends in global temperature


The three central factors which climate models use to explain trends are solar variability, aerosols and greenhouse gases. In what follows, I will briefly introduce each, after which we will take a look at twentieth century trends in temperature and we will see how well they can be understood in terms of these factors.

The Main Drivers of Climate Change

The more solar radiation the earth receives, all things being equal, the earth's temperature will tend to rise. As such, solar radiation is regarded as a positive forcing. It is probably one of the most obvious reasons for climate change although climatologists generally believe it has not been the dominant factor for some time.

Of course, given our understanding of what will be driving climate change for some time to come, what we are focusing on are greenhouse gases. They lower the rate at which thermal energy is able to leave the climate system, and assuming thermal energy is entering the system at a roughly constant rate, this implies that the temperature of the climate must rise in order to radiate thermal energy at the same rate at which thermal energy is entering the system. Now the two largest anthropogenic greenhouse gases are carbon dioxide and methane, but methane is significantly less important than carbon dioxide, particularly towards the end of the twentieth century.

For the purpose of what follows, I will focus simply upon carbon dioxide. In addition making the trends more easily understood, this has the benefit of assigning solar variability more prominence. If one were to group all anthropogenic gases together, it is quite possible that solar variability was never the dominant driver of twentieth century climate change. However, I will give it the benefit of a doubt.

Aerosols are in some ways a little more complicated. Principally they will tend to increase the amount of light which gets reflected before it gets the chance to be absorbed at the surface where it becomes thermal radiation and raises the global temperature, particularly the sulfates. Likewise they tend to encourage cloud formation. To the extent that aerosols do this, they are generally regarded as a negative forcing.

But then there is black carbon. The effects of black carbon won't be especially significant except when it comes to snow and ice. In fact, it is believed that in the Arctic sea, black carbon darkens the ice enough that its current local effect is roughly comparable to that of greenhouse gases. As such we say that it lowers the albedo at the surface and refer to it as a positive forcing. But in the long-run, carbon dioxide will come to be the dominate forcing even in the arctic. For the purpose of what follows, I will set aside black carbon as its role is major only in the arctic and glacial regions, but otherwise plays a minor role in terms of the global climate.

Finally there is the complexity of the climate system itself. For example, during El Ninos the global temperature will tend to be higher than during La Ninas. Even if the changes in each of the above forcings were smooth curves, we would expect some ups and downs from one year to the next. This is why we will tend to look at five, ten or fifteen year moving averages. And likewise, there are the uncertainties associated with the climate system. Strictly speaking and given the data available, the period of cooling which would appear to have taken place between 1945 and 1975 is probably not statistically significant. However, its lack of statistical significance is largely due to the swings during this period which given the data available would seem to be the result of solar variability. As such, it would appear to have taken place and I will assume that it is real.

Now lets look at the temperature record…

temperature anomaly over the 20th century, from

Understanding the Temperature Record

As late as 1930, the human population was only 2 billion. By 1975 it was 4 billion, many of whom were enjoying living standards made possible by a relatively advanced economy. Likewise, the sun has been cooling - by some indices, apparently as far back as 1950.

Since the sun was the dominant driver in the earlier part of the 20th century, it would make sense that sulfates could overwhelm solar variability as far back as 1944. By 1960 the sun was clearly cooling off, and if solar variability were the only driver, there is no reason to expect temperatures to rise after 1970.

During the Great Depression the world economy was largely flat. This would have depressed the production of both aerosols and carbon dioxide. But by 1939, economic output was rising, at the time in preparation for a world war which was only just beginning. As such, it should come as no surprise that the effects of aerosols should come to dominate the effects of both solar variability and the cummulative effects of carbon dioxide. Then pollution laws came into play in the advanced economies around 1970 which started reducing the levels of anthropogenic aerosols relative to the rate at which greenhouse gases were entering the atmosphere.

Sulfates don’t stay in the atmosphere for very long. They reside in the lower atmosphere and tend to get washed out in the rain. But carbon dioxide stays in the atmosphere for a very long time. Its cummulative.

As such it makes sense that for a while the sprinter of aerosols will do better than the marathon runner of carbon dioxide early on, but we have every reason to expect carbon dioxide to win in the end. Even if pollution laws had not reduced the production of aerosols, carbon dioxide would have eventually become the dominant forcing. As it was, temperatures only began to take off by 1975 and they have been accelerating since.

Given solar variability, one would not expect temperatures to be accelerating upwards but to be trending downwards for the past several decades. By itself, with a leveling off of aerosols one would expect temperature trends to be flat or trending downwards with the continued growth of the world economy, not accelerating upwards. Clearly greenhouse gases are now the dominant forcing.

Towards the end, the big exception to the acceleration due to greenhouse gases was the sudden rise in temperature in 1998 followed by a roughly equal drop during the following year. However, this was during a particularly intense El Nino. Since then (up to 2007) it would appear that the rise in temperature has continued to accelerate. With methane levels having plateaued over the past one or two decades, this would seem to suggest that carbon dioxide is now the dominant forcing for the climate system as a whole.


There you have it: solar variability, aerosols and carbon dioxide.

To the best of our knowledge, these were the main drivers responsible for climate change in the twentieth century - in their chronological order. Solar variability dominated until about 1945, aerosols dominated from about 1945 to 1975, and carbon dioxide have dominated ever since. Along the way, the climate system has meandered a bit from one year to the next given its complexity, but the trends and climate regimes are clear.

Ba-da bip, ba-da bang, ba-da boom.

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