Here, at last, is the much-delayed sixth instalment in my ongoing series of posts on the (mostly bad) arguments that contrarians make against global warming. (A list of earlier parts in the series can be found here.)
In this post, I want to clear out of the way one more factoid that contrarians hold up as evidence against greenhouse warming:
Claim: In the mid-20th century, CO2 levels rose but temperatures dropped.
Fact: Obviously, there are other influences on global climate as well as CO2. Since the 1970s, CO2 has clearly emerged as the dominant influence, and temperatures have risen by 0.17°C/decade. However, earlier in the 20th century when CO2 was rising more slowly, there were large variations in the temperature trend, most notably an accelerated warming up to the 1940s and a slight cooling from the 1940s to the 1960s.
Radiative forcing is a measurement of how much energy the planet is gaining or losing due to a particular influence on climate. To fully explain the observed temperature changes, we have to take into account all forcings, not just CO2. Other important forcings include solar activity (natural), other greenhouse gases (in this case anthropogenic), and aerosols (a mixture of natural and anthropogenic). The latter was probably the most important factor in mid-20th century climate change.
Aerosols are tiny particles like sulphur dioxide (SO2) that reflect sunlight and cool the Earth. Unlike CO2, aerosols are removed from the troposphere within days and the stratosphere within weeks (the troposphere and stratosphere are the lowest and second lowest layers of the atmosphere, respectively). Large volcanic eruptions naturally release large amounts of SO2 into the stratosphere and cause a temporary cooling.
Industrial activity has not only increased the amount of greenhouse gases in the atmosphere, but also the amount of aerosols, somewhat counteracting the greenhouse effect. The resulting increase in aerosol forcing appears to have been the main reason for the pause in the warming trend in the mid-20th century. (Because anthropogenic aerosols generally do not get into the stratosphere, they tend to remain at the latitude where they were emitted. This explains the fact that while the Northern Hemisphere saw three decades of dramatic cooling, the Southern Hemisphere experienced only a slight slowdown of warming.) However, the increasing influence of greenhouse gases has now won out over the more slowly increasing influence of aerosols.
Also vying for climatic dominance in the last century was the Sun. Solar activity rose constantly during the early 20th century as it came out of the 19th-century Dalton Minimum, adding fuel to the otherwise slow warming caused by CO2. But solar forcing flattened out around the 1950s, probably also contributing to the warming pause. Since then, solar forcing has remained relatively constant except for its usual 11-year cycle.
When all known influences (both natural and anthropogenic) are factored in to give the net forcing, it turns out that the result quite accurately matches global temperature changes over the last century.
The early-20th-century warming is only partly due to greenhouse gases, and partly due to solar activity and a relative lack of volcanic activity. The mid-20th-century cooling is explained by allowing for the effect of aerosols emitted by both humans and volcanoes. Since then, large volcanic eruptions have continued to cause temporary dips in global temperature, but without reversing the warming trend. Only the anomalous high temperatures of the early 1940s remain a mystery (though it has been suggested this is an artifact due to changing measurements during World War II).
So in conclusion, the temperature changes observed during the 20th century are explained by the interaction of multiple forcings, not just CO2. However, the latter has now emerged as the dominant forcing, and it’s set to continue growing as we keep adding CO2 to the atmosphere.
Update 19 July 2010: I recently came across an interesting post on Skeptical Science about mid-century cooling. It turns out that during that infamous mid-century period, although the global dimming effect of aerosols masked the global warming effect of greenhouse gases in the daytime, the minimum temperature during the night (when there is no sunlight to block) continued to rise. In the late 20th century, as greenhouse gases became a greater climatic force than aerosols, maximum (daytime) temperatures began rising at nearly the same rate as minimum (nighttime) temperatures. (However, we should not conclude from this that the subsequent warming was caused by global brightening. The dimming trend did not reverse until about 1990, when greenhouse warming was already well underway; and despite the recent brightening, there was still a net dimming trend from 1960 to 2010.)