A common argument against global warming is that Earth’s climate has always varied. Skeptics often say that temperatures on Earth rise and fall sometimes, and this is perfectly natural. To some extent, that is perfectly true.
However, Svante Björck, a climate researcher at Lund University in Sweden, has now shown that global warming – that is, simultaneous warming in both the northern and southern hemispheres – has not occurred in the past 20 000 years, since the end of the last ice age. That is as far back as it is possible to analyze with sufficient precision to compare with modern developments, he said, adding:
What is happening today is unique from a historical geological perspective.
Svante Björck’s study goes 14,000 years further back in time than previous studies have done. He reviewed the global climate archives, which are presented in a large number of research publications, looking for evidence that any of the climate events that have occurred since the end of the last Ice Age (20,000 years ago) could have generated similar effects on both the northern and southern hemispheres simultaneously.
He could not verify that warming took place on both hemispheres simultaneously, as is happening today. Instead, Björck found that – historically – when the temperature rose in one hemisphere, it fell or remained unchanged in the other. He said:
My study shows that, apart from the larger-scale developments, such as the general change into warm periods and ice ages, climate change has previously only produced similar effects on local or regional level.
The so-called Little Ice Age is an oft-cited example of climate change. It took place between the years 1600 and 1900, when Europe experienced some of its coldest centuries. While the extreme cold had serious consequences for European agriculture, state economies and transport, there is no evidence of corresponding simultaneous temperature changes and effects in the southern hemisphere.
The climate archives, in the form of core samples taken from marine and lake sediments and glacier ice, serve as a record of how temperature, precipitation and concentration of atmospheric gases and particles have varied over the course of history, and are full of similar examples, according to Dr. Björck.
Instead it is during ‘calmer’ climatic periods, when the climate system is influenced by external processes, that the researchers can see that the climate signals in the archives show similar trends in both the northern and southern hemispheres. He said:
This could be, for example, at the time of a meteorite crash, when an asteroid hits the earth or after a violent volcanic eruption when ash is spread across the globe. In these cases we can see similar effects around the world simultaneously.
Professor Björck draws parallels to today’s situation. The levels of greenhouse gases in the atmosphere are currently changing very rapidly. At the same time, global warming is occurring. He said:
As long as we don’t find any evidence for earlier climate changes leading to similar simultaneous effects on a global scale, we must see today’s global warming as an exception caused by human influence on the Earth’s carbon cycle. This is a good example of how geological knowledge can be used to understand our world. It offers perspectives on how the Earth functions without our direct influence and thus how and to what extent human activity affects the system.