What caused the decline in average global temperature starting in 1940 and ending in 1975?

Question

I've heard about this from a number of sources but no one has provided any sort of explanation. In the 1960's there were scares of an impending ice age. I'm just wondering if anyone knows why.

I'm a geologist.I understand the whole global warming/ ice age theory. I'm looking for physical causes. Such as, C02, natural disaster, sun cycles ect.

Answer 1

The phenomenum that you're referring to is called global dimming and is a consequence of atmospheric pollution.

Back in the middle of the 20th century there were vast amounts of certain pollutants being pumped into the atmosphere through power generation, industry, manufacturing processes and the burning of gas (back then it was town-gas or coal-gas, not the natural gas that we use today). The most critical of these pollutants was sulphur dioxide and as a geologist you'll be well aware of this gas in respect of volcanoes.

Sulphur dioxide has a reflective property which reflects sunlight (solar radiation) straight back out into space before it's even had chance to reach the Earth's surface. Levels of SO2 were continually rising and more and more sunlight was being reflected back into space, this masked the underlying warming trend and led to a period of global cooling.

In December 1952 a blanket of fog and pollution descended over London and remained there for some time. The consequence of this was that thousands of people died each day and the government of the time were forced to act. They introduced the first of the Clean Air Acts which led to a massive reduction in the amount of pollution, over the next couple of decades other countries around the world followed suit. The result being less pollution in the atmosphere and as the SO2 dissipated the solar radiation began reaching earth's surface again and the warming recommenced.

It's somewhat ironic that by cleaning up the atmosphere we've exacerbated the effects of global warming.

A similar thing happens following major volcanic eruptions. For example, following the 1991 eruption of Mount Pinatubo there was so much SO2 injected into the atmospere that globally temperatures fell by about 0.3°C over the next couple of years. Similarly, following the 1815 eruption of Mount Tambora global temperatures fell and ever since 1816 has been known as the year without a summer.

Answer 2

One of the theoretical results of Global Warming is iroincally enough, a new ice age. The idea is that the change in temperture and melting water dumped into the oceans changes weather patterns and caused ocean currents to shift which would lead to tempertures dramatically dropping again below the levels they were at before the warming started. THe earth has gone through natural heating and cooling cycles before which were always precipated by rises in Carbon Dioxide levels in the atmosphere. A new ice age is still held as a possibilty in the future, but since the new warming trend is manmade, and the carbon levels are much higher than the levels of the natural cycles there is no real way to know exactly what the effects of this current warming cycle will be.

Answer 3

The same thing that caused the warming that lasted until the mid 1990's.... sun spot cycles.