A lesson from past global warming

When looking to the future, it’s only natural to scrutinise yesterday. Be it studying the form of a favoured racehorse, probing twentieth century economic trade figures or analyzing international diplomatic relations over the centuries, history offers an insight into what might lie ahead. But written records aren’t our only source of information when we wish to learn from the past. Geology can serve a similar purpose when looking at climate change. Although the timescale may seem interminably long, records of our planet’s environment, preserved within rock, mud and ice (often referred to as ‘natural archives’ within scientific journals), can extend historical testimony and provide unique insights into the climate of our planet and where things might be heading in times to come. The question is how? To look at one example from the past, let’s ignore virtually all of the 4.5 billion years of our planet’s history and concentrate on only the most recent sliver of time. 

Over the last century, scientists delving into ocean sediments, ice and mounds of rubble have learnt that our planet has experienced massive swings in climate, lurching from ice ages to interglacials – warm periods similar to today – and back again. Indeed, over the last 2.5 million years, there’s been some 50 ice age cycles; creating, melting and rebuilding vast ice sheets around the world, with some in North America and Eurasia attaining thousands of metres in thickness. The practical upshot of all this was not only the creation of gloriously crinkly edges along some coastlines but also big upheavals in the state of the world’s climate and its environment: average global temperatures varied by some 6˚C while sea levels swung up and down by more than 120 metres. The origin of these immense changes are relatively well understood and appear to have been driven by small adjustments in the earth’s orbit, subtly influencing the amount, timing and location of the sun’s heat falling on the surface. Intriguingly, however, when kilometre-long cores have been drilled into today’s Greenland and Antarctic ice sheets, it is clear from their climate records that some interglacials were significantly warmer than now. Although the origin of these so-called ‘super-interglacials’ is different to recent warming caused by human emissions of greenhouse gases, they can provide a valuable insight into what we might expect in the future.  

The most recent super-interglacial took place between 130,000 and 116,000 years ago and has a string of monickers around the world. A common title is the Last Interglacial but if you’re north European, you may have heard this period labelled the ‘Eemian’, in North America, the ‘Sangamonian’, in Russia, the ‘Mikulino’ ... you get the idea. Regardless of its name, the crucial point is at this time, the amount of sun’s heat falling on the northern hemisphere was marginally higher at summertime than it is today and this appears to have triggered a cascade of changes that drove the planet to significantly warmer temperatures, with several major knock-on effects. We know, for instance, that at this time there was a dramatic decrease in polar sea ice coverage while large parts of the Antarctic and Greenland ice sheets melted. Critically, the warmer temperatures appear to have helped global sea levels become some 6.6 to 9.4 metres higher than today, with a rate of rise of between 60 to 90 millimetres per decade; more than double that recently observed.  The question is by how much warmer was it during the Last Interglacial?

Greenland ice from the Last Interglacial

Since the 1970s, a number of efforts have been made in gauging the temperature of the Last Interglacial, some more comprehensive than others. When the often handful of numbers have been crunched, estimates have ranged from no significant difference to today, to global annual temperatures being warmer by more than 2˚C. This is a huge uncertainty, given the rise in sea level that we know took place at this time. To try and resolve this conundrum, Dr Richard Jones (a friend and colleague at the University of Exeter) and I looked at more than 450 reports on the Last Interglacial. We were particularly interested in attempts to reconstruct temperature at individual sites using known relationships between the makeup of natural archives and modern temperatures. From this analysis, we were fortunate to find 263 estimates of the conditions when sediments and ice were laid down during the Last Interglacial, allowing Richard and I to reconstruct past temperatures around the globe. To compare the reconstructed estimates with today, we took the Last Interglacial values away from modern temperatures averaged over the period 1961 to 1990. The results were startling and our findings have just been published in the Journal of Quaternary Science under the title ‘Does the Agulhas Current amplify global temperatures during super-interglacials?’. Temperatures appear to have been more than 5˚C warmer in polar regions while the tropics only warmed marginally; strikingly similar to recent trends. Not only this, but taken together, the world appears to have been some 1.5˚C warmer when compared to the 1961 to 1990 average. If we take into account the rise in temperature that has happened since industrialization, we find the Last Interglacial was around 1.9˚C warmer. Furthermore, this period also shows the warming in the Indian and Southern oceans took place before that of the northern hemisphere, suggesting these regions may cause further global warming beyond that directly forced by increasing greenhouse gas levels. 

Worryingly, the observed temperature difference between the Last Interglacial and today is comparable to projections for the end of this century under the low emission scenarios contained within the recent Fourth Assessment Report of the Intergovernmental Panel on Climate Change (the IPCC). As 2010 looks set to become tied with 1998 as the warmest year on record, international negotiators are working towards agreeing binding targets on the emissions of greenhouse gases, with the next scheduled meeting being COP16 in Mexico, to be held during December 2010. Importantly, the results from the Last Interglacial suggest sea levels will rise significantly higher than anticipated and that stabilizing global average temperatures at 2˚C above pre-industrial levels may not be considered a ‘safe’ target as envisaged by the European Union and others. The inevitable conclusion is emission targets will have to be lowered further still. Not a popular message.

Crucially, the scientific and policy implications of the Last Interglacial demonstrate it pays to look back to yesteryear. As the great poet and playwright Thomas Eliot once wrote, ‘Time present and time past, are both perhaps present in time future, and time future contained in time past.’ Fingers crossed these words are heeded.