A common complaint made by so-called climate sceptics is the fact that global temperatures haven’t precisely tracked the trend of rising greenhouse gas levels over the twentieth century. The argument seems to be that if carbon dioxide emissions are a significant driver of climate change then temperatures should have inexorably risen over the recent past. The assumption behind all this is that natural processes don’t have any major effect on hemispheric or global temperatures. Recent published work shows this just isn’t the case.
When we look at data collected by weather stations since the nineteenth century, we find the world’s temperature has increased by around 0.8 °C. This may not sound much but the average for the planet hides a complex mix of regional warming and cooling in different places over different times. The sceptics have picked up on the fact that in spite of the long-term trend, there hasn’t been a constant upward drift.
The early increase in temperature up to the 1940s was interrupted by a decade-long cooling that began in the 1960s, followed by the steep rise in warming we’re so familiar with today. Importantly, year-on-year, the physical world adds noise to the warming effect caused by our seemingly insatiable desire to increase the amount of greenhouse gases in the atmosphere. A natural change in the circulation of our planet or the eruption of one or more volcanoes can all have a global impact on temperature. We can see this when we look at individual events.
The exceptionally strong El Niño event of 1998 led to yet more heat being pumped into atmosphere from the Pacific Ocean, adding to the effect from accumulating greenhouse gas levels and making it the joint warmest year on record alongside 2010. In contrast, during 1991, the eruption of the Philippine’s volcano Mount Pinatubo released minute droplets of sulphate into the stratosphere, reflecting incoming heat from the sun and disrupting the amount of moisture in the air, temporarily cooling the world by up to 0.5˚C.
But when the world cools over several years, one-off events are rarely the explanation. In particular, why was it comparatively cold in the latter part of the 1960s? It was originally thought that most of the downward trend in temperatures at this time was caused by sunlight-reflecting liquid particles of sulphate produced by an increased burning of fossil fuels and a spate of volcanic eruptions. New research, however, suggests there may be more to this cooling pattern than first thought.
David Thompson at Colorado State University and colleagues have just published a paper in the scientific journal Nature called ‘An abrupt drop in Northern Hemisphere sea surface temperature around 1970’. In it, they compare twentieth century temperature trends between the hemispheres. In contrast to the north, southern hemisphere ocean temperatures rose steadily over the second half of the twentieth century. At first glance this supports the idea that warming in the north was held back by sulphate released into the air from industrial activity in North America, Europe and Asia. Significantly, and this is the key point, when 1968 to 1972 was looked at more closely, the temperature in the north was found to drop very suddenly relative to the south.
This abrupt shift can’t have been due to the gradual build up of sulphate countering warming in the north; it was too fast for that. It had to have been down to something else. Intriguingly, when these differences were compared to sea records collected by shipping around the world, the cooling was found to coincide with a 0.3˚C temperature change in the North Atlantic. These bracing conditions were associated with a rapid freshening in the ocean known as the ‘Great Salinity Anomaly’ which originated as a large pocket of water off the east coast of Greenland, before dissipating in the 1970s.
Crucially, once the Great Salinity Anomaly had passed on, the enhanced warming from greenhouse gases kicked back in, resuming the temperature rise we’ve seen since the 1970s. And because the North Atlantic is a large source of heat exchange between the ocean and atmosphere, what sounds like a relatively a small change in temperature had a hemispheric (and ultimately global) climate punch. It’s quite a revelation. Although this new finding goes a long way to explaining a major kink in the temperature trend of the twentieth century, the implication is there will almost certainly be other cooling periods in the future. The take home message is even with long-term warming, abrupt changes can still happen...in either direction.
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