Yes, It's Due to Human Activity: New Research 'Should Finally Stop Climate Change Deniers'

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By Tim Radford for Climate Newsย Network

European and US scientists have cleared up a point that has been nagging away at climate science for decades: not only is the planetย 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 naturalย cause.

Other historic changes โ€” the so-called Medieval Warm Period and then theย โ€œLittle Ice Ageโ€ย that marked the 17th to the 19th centuries โ€” were not global. The only period in which the worldโ€™s climate has changed, everywhere and at the same time, is rightย now.

And other shifts in the past, marked by advancing Alpine glaciers and sustained droughts in Africa, could be pinned down toย 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 fossilย 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 theย Past Global Changes Consortium. It is reported in the journalย 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 twoย centuries.

But from the start, there have always been gnawing questions: hasnโ€™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, categoricalย 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 evenย changes in solar activityย orย volcanic surgesย could not affect all of the planet at any oneย time.

โ€œItโ€™s true that during the Little Ice Age it was generally colder across the whole world,โ€ saysย Raphel Neukom of the University of Bernย in Switzerland, and first author, โ€œbut not everywhere at the same time. The peak periods of pre-industrial warm and cold periods occurred at different times in differentย places.โ€

Andย his Bern colleague Stefan Brรถnnimannย clears up another point in a related study in the pages ofย Nature Geoscience.

Volcanicย influence

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 asย the Year without a Summer.

But there were also four other eruptions. Between 1820 and 1850,ย Alpine glaciers โ€” now in alarming retreatย โ€” actually advanced. African and Indianย monsoon systems weakened, and rain that should have fallen on hot soils dropped as more snow overย Europe.

โ€œGiven 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,โ€ said Professor Brรถnnimann. โ€œFrequent volcanic eruptions caused an actual gear shift in the global climateย system.โ€

Commenting on the Nature finding,ย Mark Maslin, a climatologist at University College London, said: โ€œOver the last 2000 years the only time the global climate has changed synchronically has been in the last 150 years when over 98 percentย 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 climateย cycle.โ€

โ€œThis paper shows the truly stark difference between regional and localised changes in climates of the past and the truly global effect of anthropogenic greenhouseย emissions.โ€ย 

Image: NASA Goddard/Flickrย CC BYย 2.0

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