At The Geological Society in September 2019, Professor Caroline Lear delivered a public talk entitled ‘The Big Antarctic Freeze’. Amy Woodward, a second year Geophysics student at Imperial College, wrote the following blog post about Carrie’s lecture whilst volunteering for the Society.
‘The Big Antarctic Freeze’ told the story of changing concentrations of CO2 in the atmosphere since the Eocene period and how these related to global sea level. To reconstruct the history of sea level and CO2, Carrie collects sediment cores from up to 4km below the sea bed on research vessels such as the JOIDES Resolution drill ship (pictured above and below). These cores are analysed to determine the age of each sedimentary layer and can enable scientists like Carrie to interpret changes in the environment during their formation.
How do we know what Antarctica was like in the past?
Foraminifera, single-celled marine organisms with a calcium carbonate shell, are found in sedimentary cores extracted from the ground. As these shells form, they take up different oxygen isotopes depending on the oxygen isotope composition of the seawater they grow in and the temperature of the surrounding ocean. By analysing foraminifera from different geological periods, Carrie can determine the temperature at the time they formed. Together with other geochemical proxies within the cores, such as boron isotopes and magnesium concentrations, the oxygen isotopes of the shells can be used to determine ocean acidity, ice volume and ocean salinity. Professor Lear and her team use this information to determine how the Antarctic ice sheet may have responded to changing environmental conditions in the past.
Three competing theories of ice sheet formation
The Antarctic ice sheet formed approximately 34 million years ago during the Eocene-Oligocene transition. In the 1990s, it was thought that a the establishment of of circumpolar current around Antarctica led to colder temperatures that initiated sustained ice formation, yet this hypothesis didn’t align with the global temperature record for this period.
Ice sheet growth relies on cold enough seasonal conditions to prevent melting in the summer and allow year on year ice accumulation in the winter. During the Eocene-Oligocene transition, there were cooler summers due to changes in the Earth’s tilt towards the sun. However this alone cannot account for ice sheet growth alone, as otherwise during similar periods of obliquity 50 and 20 million years ago ice sheets growth would match that of the Eocene-Oligocene transition.
Recent work suggests that a decrease in CO2 concentrations caused a global decrease in temperature, and once temperatures dropped below the threshold for ice to be sustained over summer months, the ice sheets grew rapidly. More recent proxy records show a gradual decrease in CO2 concentrations prior to ice sheet formation, which agrees with this theory.
What does this mean for the future?
The problem we are facing today is the rising global mean temperature, which is met with a rapid loss of ice. Meltwater drains from ice sheets and enters the oceans, causing sea levels to rise, which is coupled with thermal expansion of seawater due to higher global temperatures. If the Antarctic ice sheet melted completely, the global sea level would rise around 30 metres – enough to submerge most of London (see below). Go to http://sealevel.climatecentral.org/ to see how the effects differ around the world.
How much and how fast the Antarctic ice sheet changes in the future depends largely on human action. Since the start of the industrial revolution, humans have burned millions of years’ worth of fossilised carbon stored in coal and other fossil fuels. This has led to the emission of CO2 into the atmosphere, which contributes to the greenhouse effect and global warming.
Policies and strategies to reduce the global carbon output into the atmosphere are being implemented around the world to try to limit our impact. Currently, the Paris agreement commits countries to limit warming to 1.5°C above current temperatures, yet current policies in place would still lead to over 3°C of global warming. Importantly, if temperatures stabilise at 2-3°C warmer than today, sea levels are predicted to rise by 16.3 metres over the next few centuries.
Ice melt must be considered in the long-term. If we take a ‘business as usual’ approach, where we continue to emit CO2 at the same rate, the year on year change is small. By 2100 however, sea level rise, thermal expansion and ice melt rates are projected to be much larger – and geologically speaking this is an extremely short period of time.
You can watch Carrie’s public lecture in full on our YouTube channel: