The relationship between atmospheric CO2 (red) and sea-level (blue) over the last 500 thousand years.  From Foster and Rohling (2013) — The relationship between atmospheric CO2 (red) and sea-level (blue) over the last 500 thousand years. From Foster and Rohling (2013)

four images showing the variations in orbit, tilt and precession

the Orbital cycles over the last 100 kyr and for 200 kyr in the future.  All these orbital images from here — the Orbital cycles over the last 100 kyr and for 200 kyr in the future. All these orbital images from here

Background

The Earth's climate is always changing. Over the last 600 thousand years (kyr) or so, it has oscillated, roughly every 100 kyr, between warm "interglacial" periods with climates similar to today, and cold "glacial" periods when several km's of ice blanketed North America and northern Europe. Indeed, so much water was locked up as ice on land, sealevel was around 130 m lower then than today. Bubbles of ancient atmosphere trapped in ice cores from Antarctica reveal these cyclical changes were partly driven by changes in the atmospheric concentration of CO2 - a well-known potent greenhouse gas (see here for e.g.).

These natural changes in climate are paced by regular changes in the way the Earths orbits around the sun and variations in the tilt and wobble of its axis. These changes are caused by the gravitational interaction of the Earth with the other planets and were first quantified by the Serbian astrophysicist Milutin Milankovitch in the 1920s.

These changes in orbit, known as orbital cycles, influence the distribution of solar energy so that some orbital configurations favour cold northern hemisphere summers which can trigger the start of glaciations, and then later the orbit changes and this new configuration favours warm summers in the northern hemisphere - triggering deglaciation. The Earth system is incredibly complex however and a cascade of feedbacks, some of which are only poorly known, amplify this orbital forcing to enact global climate change. One important amplifier that we work on in the FosterLab is how CO2 is partitioned between the atmosphere and ocean on these timescales (see here). For more details on these orbital cycles and Milankovitch himself go here.

Obliquity pacing climate over the last 2 million years.  Here climate is expressed as d18O - which is the oxygen isotopic composition of benthic foraminifera - up is warm and little or no ice in the North, down is cold and lots of ice in the no… — Obliquity pacing climate over the last 2 million years. Here climate is expressed as d18O - which is the oxygen isotopic composition of benthic foraminifera - up is warm and little or no ice in the North, down is cold and lots of ice in the north. From Huybers et al. (2007)

The MPT

During the more recent glacial cycles, cooling from peak interglacial was gradual, taking 90 kyr or so to reach glacial maximum. Deglaciation was then rapid (over around 10 kyr) so that each cylce had a "saw tooth" pattern (see figure above). Various theories abound as to exactly which orbital cycle is important for triggering deglaciation - for example the figure above suggests its the tilt of the axis (obliquity) that is key - with each of the last 10 or so big deglaciations lining up with obliquity maxima (increasing summer insolation in the Northern Hemisphere).

Before about 1.2 million years ago (before -1200 ky on figure above) there was on average less ice on the continents during glacials and the cycles were shorter (40 vs. 100 kyr), more symetrical and more closely followed an individual obliquity cycle. Gradually between 1.2 and 0.6 million years ago the character of the climate cycles shifted from these shorter, less intense cycles, to the larger and longer saw-toothed cycles. This transition, known as the Mid-Pleistocene Transition (or MPT), occured in the absence of any change in the nature of the orbital cycles. This is a major puzzle and has been said to be one of the main challenges of Milankovitch's orbital climate theory.

What are we going to do?

A number of models for the MPT exist and they fall broadly into two camps - (shown schematically in the figure above) - Scenario 1 where the larger glacials after the MPT were due to a change in ice dynamics such that for the same climate forcing (orbit and CO2) it was possible to grow larger ice sheets. Alternatively, in Scenario 2 some process kicks in at the MPT that causes CO2 to drop lower during glacials - thereby causing larger ice sheets. In both these scenarios the 100 thousand year cycles occur because these post-MPT ice sheets are then big enough to survive through multiple warm orbital cycles (great paper here and summary here).

Unfortunately the ice core CO2 record only goes back to 800 kyr - it stops before the MPT is reached. Given the key, and contrasting, role for atmospheric CO2 in these scenarios our aim is to use the boron isotope pH proxy (more info here) to reconstruct atmospheric CO2 across the MPT for the first time at a temporal resolution to fully pick out any changes in CO2 associated with the orbital cycles. Putting our current understanding of the MPT to the test in this way promises new insights into the coupling of climate change and the global carbon cycle, and therefore potentially also shedding light on how climate and polar ice sheets will respond in the near future in the face of anthropogenic climate change.

What have we found out so far?

Our first major output for this project is Chalk et al. (2017) where we looked at a snap shot of CO2 during the early MPT. This revealed that around 1050-1250 kyr interglacial CO2 values were similar to those of the late Pleistocene (i.e. ~280 ppm), whereas glacial CO2 values during the early MPT were ~220 ppm, nearly 40 ppm higher than in the late Pleistocene. This is illustrated below:

So this initial study shows that glacial-aged CO2 decreased substantially across the MPT. The relationship between CO2 and sea level indicates that some hybrid between scenario 1 and 2 was liklely responsible for the MPT (see figure below) - i.e. both the sensitivity of the ice sheets to forcing increased AND CO2 declined. In Chalk et al. (2017) we suggest this is due to some change in ice dynamics (regolith removal?) leading to bigger ice sheets which caused the climate to be dustier, this in turn led to the Fe fertilisation of the Southern Ocean, which then reduced atmospheric CO2.