Recent climate change trends 'unprecedented' in the last 2,000 years

The science is clear: Humans are changing the Earth's climate at an unparalleled rate. 

Using nearly 700 proxy temperature records, a collaboration of international scientists have reconstructed the Earth's climate history from 1 AD to 2000. The extensive study paints a picture of Earth's climate as far back as when Augustus ruled the Roman Empire, finding current warming trends, caused by human greenhouse gas emissions, are unprecedented.

Two studies, authored by Raphael Neukom and published in academic journals Nature and Nature Geoscience on Wednesday, focus on surface temperature data over the last 2,000 years, homing in on the variability of Earth's climate over that time period. A third study, published in Nature Geoscience Wednesday, analyzed global climate during the early 19th century, demonstrating the end of the Earth's last cooling period (the so-called "Little Ice Age") was due to a recovery from five major volcanic eruptions before transitioning to a period of human-induced warming. 

All three studies feature comprehensive analyses of climate trends over Earth's recent history and some of their potential causes. As the northern hemisphere swelters through a heatwave and off the back of the hottest June temperatures ever recorded last month, the new research casts the climate emergency in an ever-startling light, highlighting the unusual nature of warming events in recent decades and demonstrating how natural variation in the Earth's climate can not be responsible for the current extreme increase in warming.

There are three major takeaways:

1. Warming of the Earth over the last 2,000 years mostly occurred regionally, rather than globally, in the pre-industrial era. 
2. The global average temperatures in the pre-industrial era fluctuated but rates of warming were fastest in the late 20th century.
3. Volcanic eruptions had a powerful effect on climate in the early 19th century.

Let's dive into these points a little more.

Climate variability

The Earth has experienced extended periods of warming and cooling in the past, with previous research suggesting these periods were synchronized across the entire planet. A research team led by Neukom set out to test whether or not this was true, using a database of temperature recordings that extends back 2,000 years. 

Thermometer recordings of Earth's temperature only extend back 150 years, which makes obtaining historical climate records a little difficult. However, the PAGES 2k consortium offers proxy recordings, using data obtained from tree rings, glacier ice, lake sediment, coral skeletons, mollusks and other biological and geological archives, which keep an accurate record of previous temperatures.

The full database contains 692 records from 648 locations across the globe, allowing Neukom and colleagues to assess how the climate changed annually across the entire planet.

The researchers focused on several time periods, two epochs in particular: a warmer period known as the Medieval Climatic Anomaly, occurring between 950 and 1250, and the Little Ice Age, a cooler period that followed between 1400 and 1800. The team discovered the rate of warming and cooling in these periods didn't occur simultaneously across the globe, with only 40% of the Earth's surface reaching maximum temperatures at the same time.

When they looked at the entire period, from 1 AD to 2000, they discovered the warmest period on record for 98% of the globe (only leaving out a slither of Antarctica) was the second half of the 20th century.

"It is significant that this study has shown that such prolonged warming, over 98 percent of the globe, has never occurred previously," said Fiona Armstrong, executive director of the Climate and Health Alliance. "This research is a wake-up call that we are heating the planet, and we will suffer increasing impacts if we fail to take action to reduce emissions very, very soon."

The final line in the paper is perhaps, the most telling. 

"Against this regional framing, perhaps our most striking result is the exceptional spatiotemporal coherence during the warming of the twentieth century. This result provides further evidence of the unprecedented nature of anthropogenic global warming in the context of the past 2,000 years."

Basically, we're turning the heat up on the planet in a way we've never seen before. 

"There is absolutely no doubt that humans are fundamentally changing Earth's climate," said Nerilie Abram, an Earth sciences researcher at the Australian National University.

There are some drawbacks to the research using the PAGES 2k proxy temperature data which can't be overcome, making it harder to draw direct comparisons across the entire two millennia. 

However, in a companion paper, Neukom and his team detail seven different methods to perform statistical analyses of global average temperatures over the 2,000-year period, pulling data from different datasets. The methods all lined up with very little variation, lending more credence to the warming trends the team has seen. The work, which includes comparing the new climate reconstructions with existing climate simulations, reveals that volcanic eruptions played a major role in the swings in temperature we see before 1800. After 1850, greenhouse gas emissions dominate. 

Eruptions 

The remaining study, conducted by Stefan Brönnimann and his team, drilled deeper on large volcanic eruptions occurring over 30 years in the early 19th century, towards the end of the Little Ice Age. Using climate reconstructions and simulations, the team confirmed the series of eruptions saw temperatures drop across the northern hemisphere during summer. This resulted in droughts across Africa, enhanced glacier growth and a weakening of monsoonal activity across the Earth. 

The findings show the impact of volcanic eruptions on the global climate during the pre-industrial period (before 1850), syncing up with the previously detailed research by Neukom and his team. It helps provide a clearer picture of the way climate might have varied across the Earth.

Climate change skeptics often fall back on arguments suggesting the Earth has experienced these wild fluctuations in temperature before. But those arguments are dead. The alarming increase in modern-day temperatures shows just how damaging greenhouse gas emissions have been and heralds a future where the extreme weather events we have been experiencing over the last few years aren't aberrations, but the norm. 

Climates do change. The data certainly supports that view. But in two millennia they've never changed like this.