When I mentioned to my Gran that we were in an ice age she was immediately worried. “Should I get my boiler serviced?” she wanted to know. With varying reports in the media of global warming having ‘stopped’ and extreme weather events dominating headlines this worry is hardly surprising. But is there a genuine need for concern?
We are currently living through the Pleistocene Ice Age, although thankfully we’re experiencing an interglacial period – a respite from the type of cold periods that spring to mind when talking about an ‘ice age’. But how long will this warm period last?
The historical record suggests that within the Pleistocene (roughly the last 2.5 million years) the onset of warm eras such as the one we’re currently in have been controlled by the Earth’s orbit. On scales of 100,000’s of years the shape of the Earth’s orbit changes, becoming more or less elliptical (or less or more circular), which changes the amount of energy we receive from the Sun at different times of year.
The direction in which the earth is tilted also changes with time, which alters the distribution of incoming energy over the earth’s surface. This change can alter the chances of an ice age. Land absorbs more energy than ocean so whether or not the northern hemisphere (which contains a lot more land than the southern hemisphere) is tilted towards the sun will determine how much solar energy is absorbed or reflected and therefore how warm things become.
To further complicate things the angle at which the earth rotates on its axis (or its precession), changes too. Currently the earth’s is rotating so that the southern hemisphere is more prone to severe seasons but this was once (and will be again) the case for the northern hemisphere; maybe our Winters aren’t so bad after all…
These three changes, known collectively as Croll-Milankovitch cycles, happen on different time scales and a combination of all three being in the right phase for increased cooling gives the optimum conditions for an ice age. The way they have fluctuated in the past is shown below:
However, it’s not as straightforward (if you can call it straightforward) as calculating when the cycles will next send us into a cool period. The change in absorbed solar energy caused by the changes in the earth’s orbit is not enough to cause an ice age alone. Various other factors have been suggested as being important, including the position of the earth’s tectonic plates, the concentration of greenhouse gases which cause warming of the planet through the greenhouse effect and feedback mechanisms which increase or decrease the energy we receive from the Sun.
A key feedback is the ice-albedo feedback and this is the part where my work comes in. Albedo is just a measurement of how much of the Sun’s energy a surface reflects. Ice, being white, reflects quite a large proportion. However, if ice melts it exposes the darker ocean/ground beneath it, more energy is absorbed, things get warmer and more ice melts. This can also work in reverse: more ice, things get colder, more ice can form and so on. Factors like this have the potential to influence the Earth’s climate a great deal and may well play an important role in the onset of ice ages.
So where does this leave us today? We can calculate the solar cycle of the earth and can estimate the solar energy we will receive, and these suggest we may have as long as 60-70,000 years to wait for the next cold period. What we are less able to predict is the effect of the greenhouse gas concentration in the atmosphere. The concentration of carbon dioxide (CO2) alone has risen by around a third over the past few hundred years as a result of human activities adding it to the atmosphere faster than natural processes can remove it.
CO2 levels during the last interglacial period reached a maximum level similar to that observed today, the difference then was concentrations subsequently began to decrease as the earth began to cool and enter a new ice age. CO2 concentrations today are still increasing.
This leaves us with a problem. What are the consequences of this increasing CO2 for future ice ages? The simple answer is that we don’t know. For climate scientists the Earth is their laboratory and when you’re dealing with a test subject that’s over 12 million metres in diameter accurate predictions are not straightforward. Despite climate models constantly improving we’re still not at a stage where we can say with certainty what the long term future of the earth will be.
One possibility is that fossil fuels will run out and/or we will stop adding CO2 to the atmosphere, so even if a return to an ice age is delayed it will still come about at some stage. Alternatively, we may first increase the CO2 concentration so much we reach a tipping point: one of the potential “nasty surprises” that an increased global temperature could lead to. Scenarios such as the release of methane currently locked up on the land and in the oceans could have a ‘domino-effect’ as this will lead to further warming (since methane is a greenhouse gas), which will cause even more methane to be released, and so on. Potentially we could push the earth so far that we avert another ice age altogether.
Based on this, an ‘ice age’ in our lifetimes and those of many generations after us seems extremely unlikely. Although given the latest wait we’ve had to get our boiler serviced I’d probably still book yours in to be on the safe side.
Frozen earth: the once and future story of ice ages by Doug MacDougall gives more details on the above while still being an easy read. It also gives a lot of history on the people who discovered these things – it’s hard to believe that 200 years ago the concept of an ice age was completely unheard of. It’s also the reason that I’ve gone with ‘Croll-Milankovich’ cycles rather than the common Milankovich cycles – Milankovich credited Croll’s contribution so we should too!
http://www.ncdc.noaa.gov/paleo/abrupt/ has good sections on how we know about past climate and also some of the key events in recent history (recent in a geological sense that is!).
Figure 1- Moscow Top News
Figure 2- UCAR
Figure 3- Cycles Research Institute