CR
Cryospheric Sciences

CO2 emissions

Image of the week – Learning from our past!

Image of the week – Learning from our past!

Understanding the climate evolution of our planet is not an easy task, but it is essential to understand the past if we are to predict the future! Historic climate cycles provide us with a glimpse into a period of time when the Earth was warmer than it was today. Our image of the week looks at these warmer periods of time to see what they can tell us about the future! For example, during the Pliocene, the global mean sea level was greater than 6 m higher than it is today… so what can these historic records tell us about the future of the ice sheets and their contribution to sea-level rise?


We will work forward in time from 3 million years ago to present-day and examine the evidence we have about the past climate of Earth. In this time period there have been cycles of warm and cool climate (glacials and interglacials – see our previous post). Here we will examine those interglacial periods where the climate was warmer than the preindustrial period (before 1750).

The Pliocene ~ 3 million years ago

Approximately 3 million years ago during the mid-Pliocene period, the earth experienced climate cycles every 41,000 years, and the atmospheric CO2 ranged from 350 to 450 ppm compared to around 400 ppm today and 250 ppm preindustrial. During the Pliocene period, peak global mean temperatures were on average 1.9ºC to 3.6ºC warmer than preindustrial temperatures. Ice sheet modelers have used these changes in climatic conditions to estimate the retreat of the Antarctic and Greenland ice sheets, which predicted the global mean sea level to rise ~6 and ~7 m, respectively (see our Image of the Week). Others have used geochemical methods to reconstruct historic sea level, which suggest that the global mean sea-level rise was 21 ± 10 m! While these studies provide great reason to be alarmed, they are unfortunately plagued with uncertainty that makes it challenging to provide any robust estimates of future sea-level rise based on the Pliocene period. Fortunately, more data is available from time periods closer to the present.

Marine Isotope Stage 11 (MIS 11) ~ 400 thousand years ago

Approximately 400,000 years ago, the earth experienced an unusually long period of warming, where the global mean temperature was estimated to be 1-2ºC warmer than preindustrial levels (see our Image of the Week). This period is known as MIS 11. Historic records such as pollen records, biomolecules, and ice-rafted debris suggest that the Greenland ice sheet severely retreated to the extent that forests developed on Southern Greenland! Ice sheet modelers estimate that this retreat in Greenland could have contributed 4.5 – 6 m to the global mean sea level rise. Paleoshoreline reconstruction at sites across the globe suggests that that the global mean sea level rise was ~6 – 13 m, which supports the large retreat experienced by the Greenland ice sheet and suggests that the Antarctic ice sheet likely experienced significant retreat as well if those higher estimates of sea level rise (~13 m) occurred.

Marine Isotope Stage 5 (MIS 5e) ~ 125 thousand years ago

Approximately 125,000 years ago, the earth experienced a period of warming approximately 1ºC warmer than preindustrial levels, known as MIS 5e. This warmer period has significantly more data available compared to the other time periods. It is often the case that more recent times have more abundant data and in this caseshorelines that developed during MIS 5e provide an excellent record of global mean sea level being an estimated 6 – 9 m higher than present.

Modeling studies suggest that at this time 0.6 – 3.5 m of sea level rise can be attributed to the retreat of the Greenland ice sheet and ~1 m can be attributed to thermal expansion and the melt of mountain glaciers (see Figure 2). Therefore, despite a lack of mass loss records of the Antarctic ice sheet at this time , it is likely that it underwent considerable retreat to enable contributing to the additional sea level rise.

Figure 2: Compilation of MIS 5e reconstructions for peak GMSL, the Greenland ice sheet contribution, and bets estimate of the total sea level budget [Credit: Dutton et al. (2015)].

What does this all mean for our future…

The further back in time, the larger the sources of uncertainty. Hence, there is fairly limited data regarding the Pliocene that may be used to help predict future conditions. Additionally, it’s important to remember that the climatic cycles in the Earth’s history resulted largely from changes in the Earth’s orbit. This is why the CO2 level associated with MIS 5e and 11 are similar to preindustrial levels, and yet these periods experienced significant increases in global mean temperature accompanied by rises in the global mean sea-level.

what we do know from the past is that both ice sheets experienced significant mass loss during these warm periods that directly impacted sea-level rise.

Today, the CO2 concentrations are around 407 ppm and the peak global mean temperature is approximately 1ºC warmer than preindustrial times (see the Image of the Week). For reference, the Paris Climate Accord is trying to bring our world leaders together to keep the peak global mean temperatures lower than 2ºC above pre-industrial levels. While the cause of the warming periods might be different, what we do know from the past is that both ice sheets experienced significant mass loss during these warm periods that directly impacted sea-level rise. Therefore, it’s very important to monitor and improve our future projections of mass loss from these ice sheets in order to better understand how sea-level rise will affect us.

Further Reading

  • Read the paper this article is based on here

Edited by Emma Smith 

Image of the Week – For each tonne of CO2 emitted, Arctic sea ice shrinks by 3m² in summer

Image of the Week – For each tonne of CO2 emitted, Arctic sea ice shrinks by 3m² in summer

Declining sea ice in the Arctic is definitely one of the most iconic consequences of climate change. In a study recently published in Science, Dirk Notz and Julienne Stroeve find a linear relationship between carbon dioxide (CO2) emissions and loss of Arctic sea-ice area in summer. Our image of this week is based on these results and shows the area of September Arctic sea ice lost per inhabitant due to CO2 emissions in 2013.


What did we know about the Arctic sea ice before this study?

Since the late 1970s, sea ice has been dramatically shrinking in the Arctic, losing 3.8% of its area per decade. Sea-ice area is at its minimum in September, at the end of the melting season.

The main cause of this loss is the increase in surface temperature over the recent years (Mahlstein and Knutti, 2012), which has been more pronounced in the Arctic compared to other regions on Earth (Cohen et al., 2014). The use of statistical methods involving both observations and climate models shows that the recent warming in the Arctic can be attributed to human activity, i.e. mainly greenhouse gas emissions (Gillett et al., 2008). This suggests a direct link between human activity and Arctic sea-ice loss, which is confirmed in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC).

How exactly is sea-ice loss related to CO2 emissions ?

Notz and Stroeve (2016) relate the Arctic sea-ice decline to cumulative CO2 emissions since 1850 (i.e. the total amount of CO2 that has been emitted since 1850) for both observations and climate models. Cumulative CO2 emissions constitute a robust indicator of the recent man-made global warming (IPCC, 2014).

The two quantities are clearly linearly related (see Figure 2). From 1953 to 2015, about 3.5 million km² of Arctic sea ice have been lost in September while 1200 gigatonnes (1 Gt = 10e9 tonnes) of CO2 have been emitted to the atmosphere. This means that for each tonne of CO2 released into the atmosphere, the Arctic loses 3 m² of sea ice.

Fig 2: Monthly mean September Arctic sea-ice area against cumulative CO2 emissions since 1850 for the period 1953-2015. Grey circles and diamonds show the results from in-situ (1953-1978) and satellite (1979-2015) observations, respectively. The thick red line shows the 30-year running mean and the dotted red line represents the trend of 3 m² sea-ice area loss per tonne of CO2 emitted. [Credit: D. Notz, National Snow and Ice Data Center ]

Starting from the relationship between cumulative CO2 emissions and sea-ice area, it is then easy to attribute to each country in the world their own contribution to sea-ice loss based on their CO2 emissions per capita. The countries that stand out in the map are thus the countries emitting the most in relation to their population.

Could the Arctic be ice-free in the future?

If this relationship holds in the future (in other words, if we extend the red dotted line to zero sea-ice area in Figure 2), adding 1000 Gt of CO2 in the atmosphere would free the Arctic of sea ice in September. Since we are currently emitting about 35 Gt CO2 per year, it would take less than 30 years to have the Arctic free of sea ice in the summer (which confirms previous model studies (e.g. Massonnet et al., 2012)).

Edited by Clara Burgard and Sophie Berger

Further reading

DavidDavid Docquier is a post-doctoral researcher at the Earth and Life Institute of Université catholique de Louvain (UCL) in Belgium. He works on the development of processed-based sea-ice metrics in order to improve the evaluation of global climate models (GCMs). His study is embedded within the EU Horizon 2020 PRIMAVERA project, which aims at developing a new generation of high-resolution GCMs to better represent the climate.