Skip to main content

The last time Earth was this hot hippos lived in Britain (that's 130,000 years ago)

Image taken from Wikimedia Commons. Credit Paul Maritz.
It’s official: 2015 was the warmest year on record. But those global temperature records only date back to 1850 and become increasingly uncertain the further back you go. Beyond then, we’re reliant on signs left behind in tree rings, ice cores or rocks. So when was the Earth last warmer than the present?

The Medieval Warm Period is often cited as the answer. This spell, beginning in roughly 950AD and lasting for three centuries, saw major changes to population centres across the globe. This included the collapse of the Tiwanaku civilisation in South America due to increased aridity, and the colonisation of Greenland by the Vikings.

But that doesn’t tell the whole story. Yes, some regions were warmer than in recent years, but others were substantially colder. Across the globe, averaged temperatures then were in fact cooler than today.

To reach a point when the Earth was significantly warmer than today we’d need to go back 130,000 years, to a time known as the Eemian.

For about 1.8m years the planet had fluctuated between a series of ice ages and warmer periods known as “interglacials”. The Eemian, which lasted around 15,000 years, was the most recent of these interglacials (before the one we’re currently in).

Although global annual average temperatures were approximately 1 to 2˚C warmer than preindustrial levels, high latitude regions were several degrees warmer still. This meant ice caps melted, Greenland’s ice sheet was reduced and the West Antarctic ice sheet may have collapsed. The sea level was at least 6m higher than today.

Across Asia and North America forests extended much further north than today and straight-tusked elephants (now extinct) and hippopotamuses were living as far north as the British Isles.

How do we know all this? Well, scientists can estimate the temperature changes at this time by looking at chemicals found in ice cores and marine sediment cores and studying pollen buried in layers deep underground. Certain isotopes of oxygen and hydrogen in ice cores can determine the temperature in the past while pollen tells us which plant species were present and therefore gives us an indication of climatic conditions suitable for that species.

We know from air bubbles in ice cores drilled on Antarctica that greenhouse gas concentrations in the Eemian were not dissimilar to preindustrial levels. However orbital conditions were very different – essentially there were much larger latitudinal and seasonal variations in the amount of solar energy received by the Earth.

So although the Eemian was warmer than today the driving mechanism for this warmth was fundamentally different to present-day climate change, which is down to greenhouses gases. To find a warm period caused predominantly by conditions more similar to today, we need to go even further back in time.


The past 540 million years. Note the Eemian spike and the Miocene Optimum. Glen Fergus / wiki, CC BY-SA

As climate scientists, we’re particularly interested in the Miocene (around 23 to 5.3 million years ago), and in particular a spell known as the Miocene-Climate Optimum (11-17 million years ago). Around this time CO2 values (350-400ppm) were similar to today and it therefore potentially serves as an appropriate analogue for the future.

During the Optimum, those carbon dioxide concentrations were the predominant driver of climate change. Global average temperatures were 2 to 4˚C warmer than preindustrial values, sea level was around 20m higher and there was an expansion of tropical vegetation.

However, during the later Miocene period CO2 declined to below preindustrial levels, but global temperatures remained significantly warmer. What kept things warm, if not CO2? We still don’t know exactly – it may have been orbital shifts, the development of modern ocean circulation or even big geographical changes such as the Isthmus of Panama narrowing and eventually closing off – but it does mean direct comparison with the present day is problematic.

Currently orbital conditions are suitable to trigger the next glacial inception. We’re due another ice age. However, as pointed out in a recent study in Nature, there’s now so much carbon in the atmosphere the likelihood of this occurring is massively reduced over the next 100,000 years.
------------------------------
This blog is written by Cabot Institute members Emma Stone, Research Associate in Climatology, University of Bristol and Alex Farnsworth, Postdoctoral Researcher in Climatology, University of Bristol.

Emma Stone

Alex Farnsworth
This article was originally published on The Conversation. Read the original article.

Popular posts from this blog

Are you a journalist looking for climate experts? We've got you covered

We've got lots of media trained climate change experts. If you need an expert for an interview, here is a list of Caboteers you can approach. All media enquiries should be made via  Victoria Tagg , our dedicated Media and PR Manager at the University of Bristol. Email victoria.tagg@bristol.ac.uk or call +44 (0)117 428 2489. Climate change / climate emergency / climate science / climate-induced disasters Dr Eunice Lo - expert in changes in extreme weather events such as heatwaves and cold spells , and how these changes translate to negative health outcomes including illnesses and deaths. Follow on Twitter @EuniceLoClimate . Professor Daniela Schmidt - expert in the causes and effects of climate change on marine systems . Dani is also a Lead Author on the IPCC reports. Dani will be at COP26. Dr Katerina Michalides - expert in drylands, drought and desertification and helping East African rural communities to adapt to droughts and future climate change. Follow on Twitter @_k

Urban gardens are crucial food sources for pollinators - here’s what to plant for every season

A bumblebee visits a blooming honeysuckle plant. Sidorova Mariya | Shutterstock Pollinators are struggling to survive in the countryside, where flower-rich meadows, hedges and fields have been replaced by green monocultures , the result of modern industrialised farming. Yet an unlikely refuge could come in the form of city gardens. Research has shown how the havens that urban gardeners create provide plentiful nectar , the energy-rich sugar solution that pollinators harvest from flowers to keep themselves flying. In a city, flying insects like bees, butterflies and hoverflies, can flit from one garden to the next and by doing so ensure they find food whenever they need it. These urban gardens produce some 85% of the nectar found in a city. Countryside nectar supplies, by contrast, have declined by one-third in Britain since the 1930s. Our new research has found that this urban food supply for pollinators is also more diverse and continuous

#CabotNext10 Spotlight on City Futures

In conversation with Dr Katharina Burger, theme lead at the Cabot Institute for the Environment. Dr Katharina Burger Why did you choose to become a theme leader at Cabot Institute ? I applied to become a Theme Leader at Cabot, a voluntary role, to bring together scientists from different faculties to help us jointly develop proposals to address some of the major challenges facing our urban environments. My educational background is in Civil Engineering at Bristol and I am now in the School of Management, I felt that this combination would allow me to build links and communicate across different ways of thinking about socio-technical challenges and systems. In your opinion, what is one of the biggest global challenges associated with your theme? (Feel free to name others if there is more than one) The biggest challenge is to evolve environmentally sustainable, resilient, socially inclusive, safe and violence-free and economically productive cities. The following areas are part of this c