By Tim Radford for Climate News Network
European and US scientists have cleared up a point that has been nagging away at climate science for decades: not only is the planet warming faster than at any time in the last 2,000 years, but this unique climate change really does have neither a historic precedent nor a natural cause.
Other historic changes — the so-called Medieval Warm Period and then the “Little Ice Age” that marked the 17th to the 19th centuries — were not global. The only period in which the world’s climate has changed, everywhere and at the same time, is right now.
And other shifts in the past, marked by advancing Alpine glaciers and sustained droughts in Africa, could be pinned down to a flurry of violent volcanic activity.
The present sustained, ubiquitous warming is unique in that it can be coupled directly with the Industrial Revolution, the clearing of the forests, population growth and profligate use of fossil fuels.
The finding is part of a sustained examination of global climate history, based not just on written and pictorial records but also studies of ancient lake sediments, ice cores, tree rings and other proxy evidence assembled by an international partnership called the Past Global Changes Consortium. It is reported in the journal Nature.
Research like this is a tidying-up operation. Climate scientists, conservationists, glaciologists, marine biologists, geologists and economists all know that climate change is happening, and that it is happening as a consequence of accelerated human activity over the last two centuries.
But from the start, there have always been gnawing questions: hasn’t the climate always changed? If global temperatures rose between 700 AD and 1400 AD, and then fell again, is what is happening now not part of some similar long-term cycle? And until now, that has remained without a confident, categorical answer.
So the latest study surprises nobody. But it matters, because the Nature study clarifies a point of possible confusion. There have been changes in modern human history, but none of them global and synchronous (happening at the same time). They were random fluctuations within the climate system, and even changes in solar activity or volcanic surges could not affect all of the planet at any one time.
“It’s true that during the Little Ice Age it was generally colder across the whole world,” says Raphel Neukom of the University of Bern in Switzerland, and first author, “but not everywhere at the same time. The peak periods of pre-industrial warm and cold periods occurred at different times in different places.”
The Little Ice Age began in Europe with no obvious trigger, but it was certainly reinforced and extended by more violent than usual volcanic activity in the tropics between 1808 and 1835. Mt Tambora in what is now Indonesia put so much ash into the stratosphere to screen sunlight and drop temperatures that 1816 became known as the Year without a Summer.
But there were also four other eruptions. Between 1820 and 1850, Alpine glaciers — now in alarming retreat — actually advanced. African and Indian monsoon systems weakened, and rain that should have fallen on hot soils dropped as more snow over Europe.
“Given the large climatic changes seen in the early 19th century, it is difficult to define a pre-industrial climate, a notion to which all our climate targets refer,” said Professor Brönnimann. “Frequent volcanic eruptions caused an actual gear shift in the global climate system.”
Commenting on the Nature finding, Mark Maslin, a climatologist at University College London, said: “Over the last 2000 years the only time the global climate has changed synchronically has been in the last 150 years when over 98 percent of the surface of the planet has warmed. This paper should finally stop climate change deniers claiming that the recent observed coherent global warming is part of a natural climate cycle.”
“This paper shows the truly stark difference between regional and localised changes in climates of the past and the truly global effect of anthropogenic greenhouse emissions.”
Image: NASA Goddard/Flickr CC BY 2.0