FactCheck Q&A: was it four degrees hotter 110,000 years ago?

20 February 2017

Associate Professor Nerilie Abram, The Australian National University

The Conversation fact-checks claims made on Q&A, broadcast Mondays on the ABC at 9:35pm. Thank you to everyone who sent us quotes for checking via Twitter using hashtags #FactCheck and #QandA, on Facebook or by email.

JACQUI LAMBIE: First of all, we've always had climate change - it's been much, much hotter and much, much colder. Even 110,000 years ago, it was four degrees hotter. Charging our pensioners and our businesses and families more for power...

TONY JONES: There'll be fact checkers on that one, Jacqui...- Tasmanian senator Jacqui Lambie, speaking on Q&A with host Tony Jones, February 13, 2017.

With renewable energy, heatwaves and climate change back in the headlines, Tasmanian senator Jacqui Lambie told Q&A that it was four degrees hotter 110,000 years ago.

Is that right?

Checking the source

When asked for sources to support her statement, a spokesman for Jacqui Lambie referred The Conversation to Al Gore's book An Inconvienient Truth and Tim Flannery's book The Weather Makers.

The spokesman confirmed that Lambie was referring to 4℃, not Fahrenheit, and added:

... most people think that the average world temperature has been constant for millions of years. The Gore and Flannery books prove it hasn't.

The detailed response from Jacqui Lambie's office, which is available here, also included a chart from Gore's book An Inconvenient Truth, which Lambie's office had annotated. That chart is based on data from Antarctic ice cores. A response that the The Conversation also sourced from Tim Flannery on Lambie's representation of his work can also be found here.

Let's check the scientific evidence.

Warmer, compared to what?

Most non-scientists probably think in terms of "warmer than today" or "cooler than today".

However, much of the science on this compares past and projected temperatures to a pre-industrial baseline, not to the temperature today in 2017. That's because temperatures now are rising too rapidly to serve as a useful baseline. (Industrialisation began in the late 18th Century, and the world has warmed by about 1℃ since then).

In this FactCheck, we will talk about both: comparing to both pre-industrial levels and comparing to today.

Was it 'much, much hotter' and 'much, much colder' in the past?

Jacqui Lambie was right to say that the Earth's climate has always changed and that, at different times, Earth has been hotter and colder than today.

The past 650,000 years of Earth's history (the interval shown in the annotated chart provided by Lambie's office) was characterised by large climate swings as Earth moved naturally in and out of "ice ages" triggered by changes in its orbit relative to the Sun.

Initial cooling, brought on by slow changes to the shape of the Earth's orbit and wobble of the Earth's axis, was amplified by natural effects, including the growing ice sheets and the drawing down of carbon dioxide into the deep oceans. Over tens of thousands of years these amplifying feedbacks caused Earth's climate to descend into an ice age.

At the peak of the last ice age (around 20,000 years ago), Earth's global average temperature is estimated by scientists to have been about 5-6℃ cooler than it was during the pre-industrial interval.

So yes, it is fair to describe the ice ages as much, much colder than now. But were the warm periods of the last 650,000 years "much, much hotter"?

No. The warm climates of the so-called "interglacials" - meaning the period between ice ages - were similar to today. A few of these periods were a little bit hotter; some were a little bit cooler.

None had a global average temperature that was 4℃ warmer than either today or pre-industrial times (we will return later to what the data say about local average temperatures).

How warm was it 110,000 or so years ago?

There was a warm interglacial period around 130,000 to 115,000 years ago, before the last ice age.

This last interglacial period was one of the warmest periods of the past 650,000 years. But it wasn't 4℃ hotter globally.

Extensive scientific evidence from across the globe shows that the global average temperature during this interglacial period was 1-2℃ warmer than pre-industrial times (or about as warm as it was in 2016).

This evidence comes from natural climate archives, including the tiny marine organisms that accumulate as sediment on the bottom of the oceans and whose chemical makeup fluctuates with surface ocean temperatures, and the water molecules in ice cores that reflect air temperatures over the polar regions.

The last time Earth's average temperature was 4℃ warmer than pre-industrial levels was around 5-10 million years ago. To put that in context, modern humans have existed for the last 200,000 years and civilised societies only began to form around 6,000 years ago.

Global average temperatures versus local warming

While the global average temperature during the last interglacial period was 1-2℃ warmer than pre-industrial times, there are some places like Antarctica and Greenland where local warming resulted in temperatures as high as, or even higher than, 4℃ warmer. These more extreme local temperature changes near the poles are referred to as polar amplification.

Scientists have used ice core data to calculate that during the last interglacial period, Antarctica was around 3-5℃ warmer than it was during pre-industrial times. But global average temperatures were not 4℃ warmer.

Why does it matter?

The fact that Earth has experienced natural climate changes in the past doesn't downplay the significance of how humans are changing the climate now.

The vast amounts of coal, oil and gas burned since the industrial revolution in 1750 has caused the levels of carbon dioxide in our atmosphere to rise very significantly.

Natural climate variations have continued to be a factor in Earth's climate since the industrial revolution, but the rapid rise in carbon dioxide (and other greenhouse gases) has been the dominant cause of climate warming during the industrial era.

In 2016, the planet's average surface temperature had risen to be about 1.1℃ warmer than in the late 19th century, when instrumental records began. This places our climate today at a similar global average temperature to the last interglacial.

When global average temperatures were 1-2℃ warmer than pre-industrial times between 115,000 and 130,000 years ago, this caused so much of the Antarctic and Greenland ice sheets to destabilise and melt that sea level rose by 6-9 metres.

It takes time to melt an ice sheet. But in some parts of Antarctica, climate warming since the industrial revolution has already triggered unstoppable changes in the ice sheets that will likely commit us to the higher end of the 28-98cm range of sea level rise predicted for the end of this century by the latest report from the Intergovernmental Panel on Climate Change.

Verdict

Senator Jacqui Lambie's description of past climate change on Q&A was not entirely correct.

She was right to say that Earth's climate has always changed. It always will - driven by a wide range of natural causes, and now dominated by the growing influence of human activities such as burning fossil fuels. And at different times, it has been hotter and colder than today.

But was it 4℃ hotter 110,000 years ago, as Lambie said? No, not globally.

The Antarctic was about 4℃ hotter during last interglacial period (around 130,000-115,000 years ago) than it was in pre-industrial times - but the global average temperature then was closer to 1-2℃ warmer than pre-industrial times.

Our climate today is at a similar global average temperature to the last interglacial period about 130,000-115,000 years ago.