Global warming during the 20th century was uneven, to say the least. In fact it occurred in two episodes, a warming from about 1915 to 1940, and another from about 1975 to the present. This is evident from a graph of global temperature anomaly over the last 130 years:
Naturally many wonder, why was warming interrupted from about 1940 to 1975, only to resume with a vengeance about 1975? There are two answers.
First, the causes of warming 1915-1940 and 1975-present are different. The present warming is almost entirely due to increases in man-made greenhouse gases, mainly CO2 but also methane and other trace gases. The 1915-1940 warming was a combination of factors, including a lull in volcanic activity (therefore the absence of its cooling influence), a slight increase in solar output, and yes, an increase in greenhouse gases too (although not nearly so much as during more recent times)
Second: the 1940-1975 time period experienced anthropogenic global cooling. This cooling was from the same root cause as volcanic cooling, namely aerosols (mostly sulfate aerosols) in the atmosphere. Whereas volcanic eruptions are natural, and often inject aerosols into the stratosphere (the upper layer of earth’s atmosphere), aerosols from industrial activity are man-made and almost entirely in the troposphere (the lower layer of earth’s atmosphere, where most of our weather takes place).
Because volcanic aerosols can reach the stratosphere, they hang around much longer than man-made aerosols. It takes a couple of years for most of the aerosols from a large volcanic eruption to settle out of the air, so their cooling effect likewise lasts a couple of years. This has been observed with great precision, especially during the most recent eruption with major climate impact, the explosion of the Mt. Pinatubo volcano in 1992.
Man-made aerosol emissions don’t last nearly so long. Confined to the troposphere, they settle out of the atmosphere in a matter of weeks to months, so in order to have a sustained climate impact there must be sustained emissions. But, that’s one of the hallmarks of human industrial activity: sustained emissions. Therefore anthropogenic tropospheric aerosols are a consistent feature of earth’s 20th- and 21st-century atmosphere.
Those who want to deny the reality of global warming will use any excuse to create doubt about our understanding of recent climate change. The mid-20th-century warming pause is one of their favorite excuses, and even when its cause is explained to them they still dispute it. The question is valid: were sulfate aerosols really that much higher during the 20th century than before the industrial revolution? Where’s the data?
There are a number of estimates of sulfate emissions during the industrial era, including this review from Pacific Northwest National Laboratories. They find (as do previous studies) rapid increase in sulfate emissions from mid-century until about 1975-1980, followed by a decline in emissions. A decline of sulfate emissions is soon followed by a decline in atmospheric concentration because they settle out of the air so quickly — unlike carbon dioxide, for which an increase tends to last hundreds of years even after emissions cease.
The cooling effect of man-made sulfates also helps explain the hemispheric asymmetry in temperature history. Most industrial activity is in the northern hemisphere, so most of the anthropogenic sulfate cooling should be there too. The northern hemisphere has warmed faster than the southern because there’s more land in the north than the south, and land has far less thermal inertia than ocean. But if sulfates are mostly in the northern hemisphere, that means that there should have been a stronger mid-century cooling effect in the north than in the south — and that’s exactly what we observe:
Estimates of emissions are fine, but they’re not actual measurements of concentration. There’s not much sulfate concentration data out there, but there is some. In particular, there’s sulfate chemistry data from Greenland ice cores. The GISP2 data can be found here, and here’s the measured sulfate concentration over the last two thousand years:
The many spikes are volcanic sulfate aerosols, markers of past volcanic eruptions. But there’s also a more persistent rise in the last century. We can, to some degree, remove the influence of volcaninc aerosols just by computing 10-point moving averages. Another way is to rely on the fact that volcanic aerosols only persist in the atmosphere for a couple of years, so we can get a good estimate of the non-volcanic background level by computing 10-point moving averages which leave out the two highest values. This method isn’t perfect, but it’s a good way to smooth the data while removing almost all the impact of volcanic eruptions — and that’ll tell us the background sulfate level in the GISP2 ice core. And here it is:
The graph of the atmospheric concentration of sulfates over the last two thousand years resembles — what’s that shape like? — oh yeah! A hockey stick.
We can even compare the sulfate concentration from the GISP2 ice core to the estimates of emissions:
The agreement is pretty good.
To the question: were sulfate aerosols really that much higher during the 20th century than before the industrial revolution? The answer is: definitely yes. To the question: did sulfate emissions really level off, even decline, around 1975? The answer is: definitely yes.