Climate: Past, Present & Future

Climate: Past, Present & Future

Varves – Revealing the past layer by layer

Varves – Revealing the past layer by layer
Name of proxy

Varved glacial lake sediments

Type of record

Sedimentological structures


Ice marginal lake environments

Period of time investigated

Last Glacial Termination (LGT, c.21-14 thousands of years (ka)) to present times

How do varves work?

Proglacial lakes form in front of glaciers and act as sinks for water and sediment flowing from melting ice. Analyses of proglacial lake sediments enable continuous reconstructions of glacial and foreland environmental change, including annually resolved (varved) records.

Varves typically consist of two layers, a coarse sand or silt layer capped with a fine grained clay layer separated by a sharp contact (fig. 2). Varves form due to seasonal fluctuations in glacial environments. These include processes like meltwater and sediment input, lake ice cover, wind shear and precipitation. The relative age of a varved sequence can then be counted as this repeating cycle means each pair of layers represents a single years’ worth of deposition (Ashley, 1995).  The ability to count a single year from thousands of years ago far exceeds the resolution achievable from other dating techniques, which may have error bars of hundreds to thousands of years.

Another advantage of using varves is that they form in glacial lakes with very little biological activity.  Many other proxies and dating techniques rely on biologically produced matter and cannot be used to study environments very close to glaciers. These areas are often key in understanding ice-sheet behaviour and is another reason varves are an important tool in environmental reconstructions.

Figure 2. (a.) Varved sediment sequence from Central Ireland displaying rhythmic sedimentation of a coarse silt layer capped with a fine a clay layer. (b.) Scanning electron microscope image of the sharp contact separating the two laminae in a couplet. Source: Delaney et al. (2009)

Many glacial lakes formed during the Last Glacial Termination (LGT, c.21-14 ka) as large terrestrial ice-sheets retreated or melted entirely. In areas such as the British Isles there is no remnant of ice; however many sites of paleo-lake sediments are preserved. Analyses of these sediments allow the environmental impact and rate of ice retreat to be modelled. This can be used to constrain timing of periods of ice growth and decay or can be correlated with paleo-temperature records from ice cores to model ice-sheet response to temperature change.

Key findings

Varves allow for a continuous record of environmental change to be constructed and provide a method of calculating the rate of these changes. Due to this, they are an ideal proxy for reconstructing paleo-ice environments from the LGT and subsequent glacial re-advances at sites that no longer support ice-sheets.

Varved sediment sequences in the British Isles have been used to constrain dating of ice advances and retreat. During a period of glaciation biological activity shuts down and environmental indicators such as pollen or insect remains cannot be used. Varves are then used to bridge these gaps and provide a method of calculating how long glacial periods lasted. An example of this is the precise dating of the onset and duration of ice advance during the Younger Dryas cold period in Scotland (12.6-11.5 ka) using varved sequences (MacLeod et al., 2011).

As well as paleo-records, modern glacial lakes may also contain varved sediments. These records can provide insights into anthropogenic impacts on glaciers and glaciated environments. Modern varve records can also be correlated with meteorological records and lake monitoring to refine the model of varve formation and controls on glacial lake sedimentology.

Correlating meteorological conditions with varve characteristics has shown that local climate processes, such as precipitation (Cockburn and Lamoureux, 2007) and snow melt (Leemann and Niessen, 1994), can be recorded and reconstructed from varves. This means that records of climate covering the last hundreds or thousands of years can be extracted from lakes and used to determine how they have changed with increasing human impacts on the Earth’s climate. Through the use of models of paleo-environmental change and more recent records of lake dynamics, varves provide a method of prediciting the future behaviour of ice sheet response to climate warming.

Edited by the editorial board


Ashley, G. M. (1995) ‘Glaciolacustrine Environments.’ In Menzies, J. (ed.) Glacial Environments, vol. 1: Modern Glacial Environments. Processes Dynamics and Sediments,. Oxford: Butterworth Heinemann, pp. 417–444.

Cockburn, J. M. H. and Lamoureux, S. (2007) ‘Century-scale variability in late-summer rainfall events recorded over seven centuries in subannually laminated lacustrine sediments, White Pass, British Columbia.’ Quaternary Research, 67(2) pp. 193–203.

Delaney, C., Hambrey, M., Christoffersen, P., Glasser, N. and Hubbard, B. (2009) ‘Seasonal controls on deposition of Late Devensian glaciolacustrine sediments , Central ireland : Implications for the construction of a varve chronology for the British-Irish ice-sheet Seasonal Controls on Deposition of Late Devensian.’ In Glacial sedimentary processes and products. Oxford: Wiley-Blackwell, pp. 149–163.

Leemann,  a. and Niessen, F. (1994) ‘Varve formation and the climatic record in an Alpine proglacial lake: calibrating annually- laminated sediments against hydrological and meteorological data.’ The Holocene, 4(1) pp. 1–8.

MacLeod, A., Palmer, A., Lowe, J., Rose, J., Bryant, C. and Merritt, J. (2011) ‘Timing of glacier response to Younger Dryas climatic cooling in Scotland.’ Global and Planetary Change. Elsevier B.V., 79(3–4) pp. 264–274.

What is in the (European) air?

What is in the (European) air?

You thought that Mauna Loa was the only observatory to provide continuous measurements of atmospheric carbon dioxide concentration and were disappointed because Hawaii is way too far from your study area or because you wanted to know how bad  the air is in your hometown? The US have been monitoring the composition of the atmosphere since 1972, but what about Europe? Since 2008, Europe has its own measurement network that is managed by a research infrastructure called ICOS (Integrated Carbon Observation System).


Since the beginning of the industrial era (around 1750), atmospheric concentrations of greenhouse gases such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) have increased, mostly because of human activities. As a consequence, the climate is getting warmer, which could have dramatic impacts on our daily life. The evolution of the atmospheric composition should therefore be closely monitored.

To improve our understanding of the climate system and achieve good climate predictions, high-precision measurements of greenhouse gas sources and sinks are needed. A large amount of datasets already exists, but the problem is that these data are often too difficult to access, too scattered, not consistent or not reliable.

ICOS main objectives

This is why the main goal of ICOS is to provide scientists, citizens and decision makers with harmonized and high-quality measurements of greenhouse gases in Europe. But the scope of ICOS mission is wider because these data can further be used to:

  • quantify greenhouse gas budgets
  • improve climate predictions
  • check how well/badly European countries are doing in reducing their greenhouse gas emissions
  • adapt policies

ICOS also encompasses an educational dimension by training young scientists through summer schools, workshops and conferences and by spreading knowledge about the carbon cycle to the general public.


ICOS is subdivided in national networks managed by research institutes. Twelve countries are currently members of ICOS: Belgium, Czech Republic, Denmark, Finland, France, Germany, Italy, Netherlands, Norway, Sweden, United Kingdom and Switzerland. The regional dynamics of greenhouse gases is monitored thanks to a network of 126 measurement stations implemented across these countries. Among these stations, 71 are ecosystem stations, 34 are atmospheric stations and 21 are ocean stations (Figure 1). ICOS grows rapidly and 8 other countries are expected to become members soon: Poland, Ireland, Estonia, Portugal, Spain, Hungary, Greece and South Africa.

To be part of the ICOS standardized network, candidate sites have to follow strict specifications regarding equipment, measurement protocols and data processing in order to ensure a homogeneous dataset. Periodic measurements are also carried out across the network with independent instruments to limit systematic errors. Moreover, ICOS is planning to render its data products compatible with outputs from other international measurement networks by taking part in an intercomparison program.

Atmosphere stations (Figure 2)

Atmospheric CO2, CO and CH4 concentrations are continuously measured in atmosphere stations, together with a range of usual meteorological variables such as air temperature, atmospheric pressure, relative humidity, wind direction and speed.

Figure 2: Cabauw atmosphere station in the Netherlands (ICOS ERIC,

Ecosystem stations (Figure 3)

Flux towers measure the exchange of water vapour, greenhouse gases and energy between the different types of ecosystems and the atmosphere. The list of variables collected at ecosystem stations is available here.

Figure 3: Brasschaat ecosystem station in Belgium (ICOS ERIC,

Ocean stations (Figure 4)

Ocean stations include ships, fixed buoys and flux towers. Carbon fluxes are measured at the ocean-atmosphere interface together with other marine variables such as pH, temperature, or salinity. You can have a look at the exhaustive list of measurements here.

Figure 4: VLIZ data buoy ocean station (ICOS ERIC,

Data products

Data collected by national network stations are gathered, processed and stored by central facilities called Thematic Centers (TC): the Atmosphere Thematic Center (ATC), the Ocean Thematic Center (OTC) and the Ecosystem Thematic Center (ETC).

You can access all these precious data for free here on the carbon portal. Among many examples, you can find ecosystem fluxes time series, atmospheric methane observations or global carbon budget. It is easy to handle as you can apply filters to refine your search, click on the “eye” icon to preview the data, or just select the dataset to obtain its description.

These data are protected by a Creative Commons Attribution 4.0 international licence, which means you can share and even modify them provided that you document any change, mention the original data source and give a link to the licence text ( It is of course necessary to cite  ICOS when you use the data. To make this as easy as possible, the citation is provided when you download the data set.

On the website of the Atmosphere Thematic Center, you can also find near real time data that are computed from all ICOS atmospheric stations every day in the morning. For example, Figures 5 and 6 show time series of the fraction of CO2 (top plot) and CH4 (bottom plot) in the air mass coming from the European continent measured at Mace Head station (MHD). Depending on the wind direction, this atmospheric station, located on the west coast of Ireland, is exposed to either the North Atlantic Ocean air mass and or the European continental air mass, offering a unique way to study these very different air masses. Time series for the period 2011-2017 show a clear upward trend for both greenhouse gases in the continental air mass. These increases are mainly caused by growing emissions associated to human activities.

Figure 5: CO2 molar fraction in continental air mass between 2011 and 2017 at Mace Head atmospheric station (ICOS ERIC,

Figure 6: CH4 molar fraction in continental air mass between 2011 and 2017 at Mace Head atmospheric station (ICOS ERIC,


Hopefully, this post helped you to get to know ICOS better. Do not hesitate to use this great tool in the future!

Find out more about ICOS

For those interested, the 3rd ICOS Science Conference will take place between the 11th and the 13th of September 2018 in Prague, Czech Republic.

Edited by Gabriele Messori and Célia Sapart


Forams, the sea thermometers of the past!

Forams, the sea thermometers of the past!
Name of proxy

Mg/Ca-SST on planktonic foraminifera shell

Type of record

Sea Surface Temperature (SST)


Marine environments

Period of time investigated

55 Million years ago to recent times

How does it work ?

Foraminifera (or Forams) are single-celled organisms varying from less than 1 mm to several cm in size. They are very abundant in the ocean floor (benthic species) or floating amongst the marine plankton (planktonic species) where they produce their shells mostly using calcite (CaCO3). The oldest fossils of benthic foraminifera date back to the Cambrian period (older than 485 million year ago (Ma)) (Armstrong and Brasier, 2005). Planktonic species are younger than the benthic group. For instance, the species Globigerina bulloides (Figure 1) range from Middle Jurassic (180 Ma) to recent times (Sen Gupta, 1999).

A large spectrum of information can be provided by the analysis of foraminifera shells, based on the chemical composition and morphology of their shells as well as the species abundance patterns. One type of proxy is the ratio between the abundance of magnesium (Mg) and calcium (Ca) (Mg/Ca ratio) present in the calcite shell. During the formation of the shell, Mg is incorporated and may weaken the shells. In some cases, it seems that foraminifera expend energy to control the incorporation of Mg (Toler et al., 2001). The substitution of Mg into calcite depends on the temperature of the seawater, so that the amount of Mg in the shell exponentially increases from cold to warm water (Lea, 1999). This means that the Mg/Ca ratio of the shells is expected to rise with increasing temperature (Rosenthal, 2007). Measuring the Mg/Ca ratio of foraminifera shells therefore allows reconstructing the sea surface temperature (SST) of the past.

What are the key findings that have been done using Mg/Ca-SST?

Past SST determination is essential for understanding past changes in climate. An advantage of the Mg/Ca ratio measured on the shells of planktonic foraminifera is that the same sample can be used for different types of analyses in order to obtain a large set of information on the past sea conditions (Elderfield and Ganssen, 2000; Barker et al., 2005). Another advantage of this Mg/Ca proxy is the possibility to reconstruct changes of temperature within the water column using multiple species living at different depths and/or coming from different seasonal habitats (Barker et al., 2005). This can give us, for example, valuable information for describing seasons in the past.

Planktonic foraminifera can survive in a wide range of environments, from polar to tropical areas, thus the analysis of their shells allows reconstructing the ocean conditions all around the world. Moreover, foraminifera are very sensitive to temperature and environmental changes therefore it is possible to reconstruct climate changes of various amplitudes and timescales, e.g. the Paleocene-Eocene Thermal Maximum (55 Ma) or the more recent climate oscillations (Zachos et al., 2003; Cisneros et al., 2016). For instance, Figure 2 shows that Mg/Ca ratio allows reconstructing the ~2ºC warming observed from the Roman Period onset to higher frequency thermal variability like those observed in the Little Ice Age (LIA).

Figure 2: Sea Surface Temperature (SST) record stack for the last 2700 years reconstructed by means of Mg/Ca analysed on the shell of the planktonic foraminifera Globigerina bulloides in the central-western Mediterranean Sea. The different historical/climate periods are indicated: TP=Talaiotic Period, RP=Roman Period, DMA=Dark Middle Ages, MCA=Medieval Climate Anomaly, LIA=Little Ice Age, IE=Industrial Era. Years are expressed as Before Common Era (BCE) and Common Era (CE). The grey shaded area integrates uncertainties of average values and represents 1 sigma of the absolute values. This uncertainty includes analytical precision and reproducibility and the uncertainties derived from the G. bulloides core-top calibration developed in the original reference. (Modified from Cisneros et al., 2016).


This article has been edited by Célia Sapart and Carole Nehme
  • Armstrong, H. and Brasier, M., Foraminifera. In: Microfossils, Blackwell Publishing, pp. 142-187.
  • Barker, S., Cacho, I., Benway, H. and Tachikawa, K., 2005. Planktonic foraminiferal Mg/Ca as a proxy for past oceanic temperatures: A methodological overview and data compilation for the Last Glacial Maximum, Quat. Sci. Rev., 24, 821–
  • Cisneros, M., Cacho, I., Frigola, J., Canals, M., Masqué, P., Martrat, B., Casado, M., Grimalt, J. O., Pena, L. D., Margaritelli, G., and Lirer, F., 2016. Sea surface temperature variability in the central-western Mediterranean Sea during the last 2700 years: a multi-proxy and multi-record approach. Climate of the Past, 12, 849-869.
  • Elderfield, H. and Ganssen, G., 2000. Past temperature and δ18O of surface ocean waters inferred from foraminiferal Mg / Ca ratios, Nature, 405, 442–
  • Lea, D.W., 1999. Trace elements in foraminiferal calcite. In: Sen Gupta, B.K., (), Modern Foraminifera, Great Britain, Kluwer Academic Publishers, pp. 259-277.
  • Rosenthal, Y., 2007. Elemental proxies for reconstructing Cenozoic seawater paleotemperatures from calcareous fossils. In: Hillaire-Marcel, C. and de Vernal, A. (), Developments in Marine Gelology, Elsevier, pp. 765-797.
  • Sen Gupta, B.K., 1999. Introduction to modern Foraminifera. In: Sen Gupta, B.K., (), Modern Foraminifera, Great Britain, Kluwer Academic Publishers, pp. 3-6.
  • Toler, S.K., Hallock, P., and Schijf, J., 2001. Mg/Ca ratios in stressed foraminifera, Amphistergina gibbosa, from the Florida Keys, Marine Micropalentology, 43, 199-206.
  • Zachos, J. C., Wara, M. W., Bohaty, S., Delaney, M. L., Petrizzo, M. R., Brill, A., Bralower, T. J., and Premoli-Silva, I., 2003. A transient rise in tropical sea surface temperature during the Paleocene–Eocene thermal maximum, Science, 302, 1551–1554.

Decomposing algae have not said their last word yet!

Decomposing algae have not said their last word yet!
Name of proxy

Phytane, a compound resulting from the degradation of Chlorophyll-a (Chl a), a green pigment in plants and algae that is involved in photosynthesis

Type of record

Atmospheric carbon dioxide concentrations


Marine sediments and oils

Period of time investigated

Phanerozoic (last 540 million years)

How it works

Before we can start predicting the potential impact of human activities on climate change, we first need to study the behavior of atmospheric CO2 concentration (pCO2) in the past. This represents a challenge given that continuous atmospheric measurements started only about 60 years ago.

To reconstruct past pCO2, a single well-constrained, ubiquitous proxy would be ideal. Unfortunately, it does not exist and we have to combine the estimates from many different types of proxies, each with their own advantages and weaknesses.

Different potential compounds that can give information on the pCO2 over the last hundreds of millions of years have been explored. The idea is based on the fact that algae consume CO2 and use that carbon to build organic compounds. But carbon atoms are not all the same. They can have different masses depending on the composition of their nucleus. We therefore differentiate between heavy carbon (13C) and light carbon (12C). Algae rather use the lighter, more common carbon (12C), but when less CO2 is available, they must use both 12C and 13C– or go hungry.

Therefore, it is considered that the more abundant is the CO2, the smaller is the 13C/12C ratio of the algae. Likewise, when CO2 is lower, the 13C/12C ratio in algae is higher. Long after an algae died, the 13C/12C ratio of its organic compounds will remain preserved in ancient sediments and oils and reflect the environment in which they were first produced.

Consequently, this ratio is compared to the ratio in CO2 consumed by the algae in order to calculate the fractionation factor (Ɛp) due to CO2-fixation. The fractionation factor is a measure of the discrimination occurring between heavy and light carbon. It can therefore be used to estimate the abundance of CO2 at the time the organic compounds were first produced.

Key Findings

This pCO2 estimation method via Ɛp is generally applied to organic compounds that come from specific species of algae. However, this has limitations – a single species is limited by its evolutionary history and its global abundance.

Here instead, we decided to explore the possibilities of using a general biomarker – an organic compound that all algae have and that contributes to the ocean record. We have explored naturally occurring CO2 vents, pockets of bubbling CO2 caused by volcanic activity, to test several different general biomarkers using this Ɛp method.

Collecting water samples, plankton nets, and sediment at a CO2 vent in Japan

One of the biomarkers that seems to work well are phytol and phytane, which are byproducts of Chl – a green pigment in plants and algae involved in photosynthesis. Phytane was used in specific case studies (e.g. Bice et al., 2006; Damste et al., 2008) but has not been tested extensively. We are now calibrating this potential proxy, by comparing it with other well-established pCO2 proxies over different timescales, and forming a compilation that extends the entire Phanerozoic (the past 540 million years!).

This proxy could significantly increase how far we can reconstruct pCO2 – Chl a and its products have been found in samples over 2 billion years old and is found everywhere (both spatially and temporally) throughout the geologic record.

Edited by Caroline Jacques and Célia Sapart

Further readings

Bice, K. L., Birgel, D., Meyers, P. A., Dahl, K. A., Hinrichs, K. U., & Norris, R. D. (2006). A multiple proxy and model study of Cretaceous upper ocean temperatures and atmospheric CO2 concentrations. Paleoceanography21(2).

Damsté, J. S. S., Kuypers, M. M., Pancost, R. D., & Schouten, S. (2008). The carbon isotopic response of algae,(cyano) bacteria, archaea and higher plants to the late Cenomanian perturbation of the global carbon cycle: Insights from biomarkers in black shales from the Cape Verde Basin (DSDP Site 367). Organic Geochemistry39(12), 1703-1718.