There's been no simultaneous warming of the northern and southern hemispheres in the last 20,000 years, scientists say, indicating that climate change can't be ascribed to natural variation.
Svante Björck, a climate researcher at Lund University in Sweden, examined global archives 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 had similar effects on both the northern and southern hemispheres simultaneously.
Instead, he found that when the temperature rises in one hemisphere, it falls or remains unchanged in the other.
"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," he says.
During the last clear period of climate change - the Little Ice Age between 1600 and 1900 - Europe experienced some of its coldest centuries. However, says Björck, there's no evidence of corresponding simultaneous temperature changes and effects in the southern hemisphere.
Simultaneous change is only found when the climate system is influenced by external processes, such as a meteorite crash or after a violent volcanic eruption.
"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," says Björck.
"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."