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

Powering the economy through the engine of Smart Local Energy Systems

How can the Government best retain key skills and re-skill and up-skill the UK workforce to support the recovery and sustainable growth? This summer the UK’s Department for Business, Energy and Industrial Strategy (BEIS) requested submission of inputs on Post-Pandemic Economic Growth. The below thoughts were submitted to the BEIS inquiry as part of input under the EnergyREV project . However, there are points raised here that, in the editing and summing up process of the submission, were cut out, hence, this blog on how the UK could power economic recovery through Smart Local Energy Systems (SLES). 1. Introduction: Factors, principles, and implications In order to transition to a sustainable and flourishing economy from our (post-)COVID reality, we must acknowledge and address the factors that shape the current economic conditions. I suggest to state the impact of such factors through a set of driving principles for the UK’s post-COVID strategy. These factors are briefly explained belo

Farming in the Páramos of Boyacá: industrialisation and delimitation in Aquitania

Labourers harvest ‘cebolla larga’ onion in Aquitania. Image credit: Lauren Blake. In October and November 2019 Caboteer  Dr Lauren Blake  spent time in Boyacá, Colombia, on a six-week fieldtrip to find out about key socio-environmental conflicts and the impacts on the inhabitants of the páramos, as part of the historical and cultural component of her research project, POR EL Páramo . Background information about the research can be found in the earlier blog on the project website . Descending down the hill in the bus from El Crucero, the pungent smell of cebolla larga onion begins to invade my nose. The surrounding land transforms into plots of uniform rows of onion tops at various stages of growth, some mostly brown soil with shoots poking out along the ridges, others long, bushy and green. Sandwiched between the cloud settled atop the mountainous páramos and the vast, dark blue-green Lake Tota, all I can see and all I can smell is onion production. Sprinklers are scattered around, dr

IncrEdible! How to save money and reduce waste

The new academic year is a chance to get to grips with managing your student loan and kitchen cupboards. Over lockdown the UK wasted a third less food than we usually would. This is brilliant, as normally over 4.5 million tonnes of edible food is wasted from UK homes every year. For students, it’s even higher. The average cost of food waste per student per week is approximately £5.25 - that's about £273 per year !  It’s not just our bank accounts that are affected by food waste – it’s our planet too. The process of growing, making, distributing, storing and cooking our food uses masses of energy, fuel and water. It generates 30% of the world’s CO₂ greenhouse gas emissions. The same amount of CO₂ as 4.6 million return flights from London to Perth, Australia! So it makes sense to keep as much food out of the bin as possible, start wasting less and saving more.  Start the new term with some food waste busting, budget cutting, environment loving habits! Here’s five easy ways to reduce