Eric Wolff is a Royal Society Research Professor at the Department of Earth Sciences, University of Cambridge. Eric is one of the foremost investigators of Quaternary and recent climate records using ice cores. He was scientific leader of the European project that produced 800,000 year records of climate and greenhouse gases from an Antarctic ice core and his other prominent contributions include his re-interpretation of sea salt in ice cores as a proxy for sea ice, and pioneering work on how impurities affect the physical properties of ice.
We all want to know what’s going to happen in the future. But sometimes the discussion about the future of climate can seem a little theoretical. The problem is that, unless you have a time machine, you can’t observe the future. However, we can observe the past: this is the task of palaeoclimate scientists such as myself. I use ice cores (cylinders of ice drilled from the Antarctic and Greenland ice sheets). Those cores, and the other materials used by my colleagues, give us some solid facts on which to base our understanding of the situation we are in now.
The first thing to say is that humans are carrying out a unique experiment on the planet. Ice cores show that the amount of carbon dioxide (CO2) in the atmosphere is now over 30% higher than at any time from 800,000 years ago until the 20th century, and is still rising quickly. CO2 is a greenhouse gas, and greenhouse gases exert a strong control on climate. Indeed it’s almost impossible to explain major episodes in Earth’s history, such as the entry into and exit from snowball Earth, without invoking changes in the strength of the greenhouse effect. Similarly, the small changes in solar energy, due to Earth’s changing orbit, that occurred about 20,000 years ago could not have pulled us out of the last ice age without the help of rising greenhouse gas concentrations that amplified a modest regional signal into a large global one.
The past can also give us a feeling for the meaning of climate numbers. It is predicted that, if emissions of greenhouse gases are not curbed, the average temperature on Earth could rise by as much as 5°C from its preindustrial value by 2100. Maybe that doesn’t sound like a lot. But it turns out that the global average change estimated for the warming from the last ice age to the present is also about 5°C. That led to a wholesale change in the land surface and vegetation, and in the circulation of the ocean and atmosphere. If there had been human societies they would certainly have been disrupted.
While 5°C is at the top of the range of possible warmings at the end of the century, major consequences, such as significant sea level rise, are expected with smaller warmings. The past gives us a warning there. About 120,000 years ago, Earth was in an interglacial (the warm period between ice ages) similar to that of the last 10,000 years. It was actually slightly warmer than today for at least part of the time, particularly near the poles, probably because of a combination of a different orbit and a change in ocean circulation. Anyway, whatever the cause, it seems that sea level was at least 5 metres higher than today, implying substantial loss of either the Greenland or West Antarctic ice sheets, or both. This tells us that, if temperature in the polar regions is raised for thousands of years, we really should expect an irreversible rise in sea level, that people in the year 3000 may not thank us for (maps of how sea level may effect different regions can be found here).
Finally the past has something to say about precisely that question of the length of warming we can expect. Unfortunately while it is easy for us to put CO2 into the atmosphere rather fast by burning fossil fuels, it looks as if it takes a very long time for natural processes to take it all out again. The past provides us with one experiment where we can test this statement. About 55 million years ago, in what is known as the Palaeocene-Eocene Thermal Maximum (PETM), we find evidence that a huge amount of carbon was released into the atmosphere in at most a few thousand years. The exact cause is not known, but the amount released was comparable to that being released by humans in the 20th and 21st centuries. After the rapid release, the carbon did decay away but it took around 100,000 years to do so.
These are just a few examples of where the past gives us confidence that our understanding is good. Of course what we would really like is an analogue for the future. The PETM is the most obvious one, but unfortunately it’s too far back in time for us to have any detail about the effect that the large carbon release had on climate. So if we do want to know what will happen in the future, we have no choice but to use models of how Earth’s climate system works. However, it’s easy to think that they are theoretical and that somehow Earth will deal with whatever we throw at it. So it’s worth remembering that there are solid observations that show us that the climate system really does react, sometimes quite dramatically, to changing greenhouse gas concentrations.