Climate scientists writing in the journal Nature have found there is no evidence for "globally coherent warm and cold periods" over the past 2,000 years prior to industrialisation.

That's significant, because climate change deniers have sometimes pointed to epochs like the so-called "Little Ice Age" or "Medieval Warm Period" to argue that the current global warming is one among multiple similar global climate events.

But what the research actually shows is that other "peak warming and cooling events" over the past two millennia appear to have been localised, whereas the human-caused global warming observed over the past 150 years is unparalleled in its global scale (not to mention its absolute temperatures).

What were the Little Ice Age and Medieval Warm Period?

The Little Ice Age (LIA) has been used over the past few decades to refer to what has been seen as cold temperatures between roughly 1300-1850.

The Medieval Warm Period, also known as the Medieval Climate Anomaly (MCA), refers to what has been seen as warm temperatures between roughly 800-1200.

However, the international team of scientists writing in Nature, led by Raphael Neukom from the University of Bern, point out that there has been no consensus about the extents of these epochs.

As well, they say while these epochs have been painted as having been global events, the evidence does not support this.

So in the case of the Little Ice Age, research suggests the cold temperatures likely peaked at very different times around the world:

For central and eastern Pacific Ocean, in the 1400s

For central and eastern Pacific Ocean, in the 1400s For north-western Europe and south-eastern North America, in the 1600s

For north-western Europe and south-eastern North America, in the 1600s For the rest of the regions, in the mid-nineteenth century

As the scientists writing in Nature point out, it's only when the data is "smoothed over multidecadal timescales" that a loosely defined Little Ice Age can be seen.

So there were no anomalous globally averaged temperatures before industrialisation?

No, that's not what the scientists are arguing.

Instead, what they're saying is that anomalous globally averaged temperatures don't in and of themselves imply the existence of a global climate event.

"The interpretation of individual palaeoclimate time series should not be forced to fit into global narratives or epochs," they write.

Instead, they say the evidence suggests that over the past 2,000 years, prior to industrialisation, anomalous globally averaged temperatures have been driven by regional extremes, which in turn are being driven by "regionally specific mechanisms" (for instance, volcanic eruptions).

So what's different about the global warming we're currently experiencing?

Two things: the warming is being felt (almost) everywhere at the same time, and that warming is unprecedented over the past 2,000 years.

The evidence, as presented by the scientists in Nature, is clear:

"The warmest period of the past two millennia occurred during the twentieth century for more than 98 per cent of the globe."

The exception is Antarctica, where scientists say "contemporary warming has not yet been observed over the entire continent".

Fiona Armstrong, founder and executive director of the Climate and Health Alliance, called the research a "wake-up call".

"We are heating the planet and we will suffer increasing impacts if we fail to take action to reduce emissions very, very soon," she said.

Nerilie Abram, from the Research School of Earth Sciences at ANU, says the study shows just how unusual the current climate is.