There's glacials and interglacials, and they swap positions in a regular, predictable way, all over the globe, right? Just to name something: Salle Kroonenberg needs no more than 5 pages in his book "de menselijke maat" to reach the stage where he sees the need to point such a thing out. So we're in an interglacial now, we know how that happened, and we know when and how we will enter the next ice age. Or do we?
The whole idea of ice ages and interglacials is a fairly recent thing. If you have a damn long record of something that is an indication of climate you tend to have quite a mess until roughly 2 million years ago, when a fairly regular cycle appeared. Ice ages and interglacials came and went with a roughly 40.000 year period, the same frequency with which the Earth's tilt changes. That went on for quite a while, until roughly a million years ago, when the period changed to ~100.000 years, the frequency with which the Earth's orbit's ellipticity changes. This change is not yet entirely explained. Anyway, we've now had that new cycle for a while, and it's not particularly regular. It's not expected to be, really, as the earth is quite a complex system. There's lots of positive and negative feedback systems, there's chaos, there's all sorts of stuff to complicate things. Luckily some influences are well-known and predictable, most pronounced being the astronomical cycles. Check wikipedia in case you want a bit of an elaboration on them. I mentioned two already: eccentricity (ellipticity of the orbit) and obliquity (tilt). There's precession, too, and eccentricity is actually two cycles, the other one having a period of ~400.000 years. All these cycles matter, even though the 100.000 year cycle has been dominant for a while now. But we've hardly been through two periods of the (longer) eccentricity cycle, so it's difficult to really see how cyclic this cycle really is.
The present is key to the past, and the past is key to the future, as Lyell said. (At least he said half of that.) In order to understand the cycle we have to look back at previous interglacials, and the easiest one, of course, is the previous one. You don't have to drill too deep in order to get to these sediments, and they only had ~125.000 years to suffer from all sorts of disturbing processes that blur the signal. The best interglacial, however, is one three periods before that one, 400.000 years ago, as that one had all the astronomical cycles more or less the same as they are now. The previous interglacial does not at all have that. And as said before, we can hardly claim that what happens in a previous interglacial gives a solid guarantee on what will happen now, but it's the best we've got.
Unfortunately the sediment core I worked on for my PhD project did not cover that interglacial. It did cover the last 240.000 years. And I contended myself with this interglacial and the previous one. What did I want to find out? The development of the Indian monsoon, as this is a societally very relevant climate feature. Forget India if the monsoon fails. There's quite some Indians to forget if that happens. Finding out what really drives the monsoon will give us a chance to know what it will do in these recent times of anthropogenic climatic upheaval. And just studying the last decennia or centuries will not do; the system is now shaken up beyond anything we've seen in the Holocene. And CO2 levels actually are already way beyond anything we've seen ever since we've even had the glacial-interglacial cycle. We climate scientists have to work damn hard and be damn inventive to keep up with this, actually.
Anyway. We do what we can! And I studied the Indian monsoon. The monsoon, by the way, technically speaking is not torrential rainfall, as it is used in colloquial speech, but a system of seasonally reversing winds. As these blow from the land in the one season and from the sea in the other, and these latter evidently bring all the moisture, these summer monsoon rains have pushed over the original meaning of the word. But that aside. Reconstructing the monsoon requires some preparation. My first article hardly mentioned the monsoon. I basically presented the records I first constructed by means of framework.
What I did for my first article was tossing some dead plankton into some expensive machines that, as thanks, spat out two records, one of which providing (amongst others) a time frame, which we corroborated with 14C dating, and another one that gave a record of sea water temperature. These records spanned the last 20.000 years. The older part of the record is described in later articles. We had the end of the last ice age, the deglaciation, and most of the Holocene.
What did we find? The deglaciation started earlier than on most of the rest of the northern hemisphere, and it started with two distinct warm spells. The warmest period was, actually, the time of the the Younger Dryas, which is a strong cold spell in the middle of the deglaciation, found all over the place in the northern hemisphere. Strange! We compared our records with other regional records, and we added a record that gives a measure of organic production.
A map of the Arabian Sea, with in red the location of "my" core, and in white the locations of the cores I compared my results with.
We found out that the whole of the Arabian Sea tends to have this early onset of the deglaciation, but the records did not all show the same thing regarding the Younger Dryas. In some records this was a warm period indeed, but not in all. The record had low organic productivity in the warmest periods. Nowadays, plankton growth (the bulk of productivity) is strongest during the summer monsoon, as the summer monsoon winds mix the water column, and bring nutrients to the surface that otherwise would have stayed at such high depths that all light-dependent squirmy things living in the ocean could not reach them. And these low-productivity periods were at the same time as cold spells on the higher northern latitudes, and that was in line with other records from the area that contained productivity information.
So what does that mean? The Arabian Sea warms up before the ice age is over. This may mean it's the low latitudes that start the end of an ice age, and the ice caps don't go around melting on their own initiative. There's strange warm blips at the onset of the warming, which I will come back to. If the northern hemisphere is cold, for instance during the Younger Dryas, when massive ice melting weakened the gulf stream, the Arabian Sea may well lose lots of its productivity.
I realise I'm playing a dangerous game with cause and effect here. The Arabian Sea apparently starts things before the northern hemisphere ice sheets catch on, but that does not mean the Arabian Sea drives them. It does mean it's not the other way around. But you can always get positive feedbacks. So it's difficult (but probably not impossible!) to imagine that low productivity in the Arabian Sea would cause melting of Greenland ice. What is easy to imagine is heat from the Arabian Sea (which sneaks past South Africa into the Atlantic Ocean) melting Greenland ice, the fresh water weakening the gulf stream, that causing low temperatures over Europe, the cold somehow reaching the Tibetan Plateau (NB: weak point in the argument!), that weakening the summer monsoon, and that in turn leading to low productivity in the Arabian Sea, but not to a homogeneous Arabian Sea temperature effect as the summer monsoon does not influence the water temperature in the same way over the whole region. And from that description it is already clear we're not there yet, but maybe this illustrates how complex it all is, and how many records we need. I won't have to explain how many records from how many places you need to, for instance, figure out if the path I just described would indeed work that way. And think of all the things that are not mentioned here! For instance, if some nutter would cut down all the forests in Meso-America this may well hit the gulf stream hard too. And thus the rest of the world. And so on! Once you start, there's so much to consider.
Is what I found new? Sort of. There is no consensus yet on how we actually go from a glacial to an interglacial or back. There are records that suggest all kinds of things, but the picture is still incomplete, and this record gives its two cents. Two very detailed cents: when I published this record it had many times the resolution of all the other records from around there. These other records could not resolve things like my blips, let alone see how fast these things can happen.
The production of many records also helps to show how things relate to each other.Even before we figured out how this would work, we now have a warning, for instance, that if we exhaust lots of CO2, which leads to ice melting,we may be left with a warm and inproductive Arabian Sea. I have no idea how many people depend on Arabian Sea fisheries, and I actually do have an idea how much we western white rich people care about them anyway, but it's worth knowing. And science moves on: this was already published in 2007, and by now maybe somebody has come up with an explanation. And remember: this article is only the framework for the actual monsoon research that still is to come!
I thought I could summarise this paper in a few sentences. (One guy in Durham already suggested how: "I've done a lot of measurements, and it's all very boring, but don't worry!") How wrong! And I left so, so much out already. I hope that with leaving so much out it has remained understandable. And if not: shoot! This text can still be edited.
And another thing: some people who may have the patience to read this may also have my thesis. In its introduction I try to do something similar, digging deeper into things like methods and mechanisms, while in this text I try to place more emphasis on the relevance and the context. If anybody remembers the thesis well enough to reflect on to what extent the texts do or do not complete each other I would be grateful..