Climate: Past, Present & Future

Climate: Past, Present & Future

Corals, the thermometers of the past!

Corals, the thermometers of the past!
Name of proxy:


Type of record:

Oceanic variability


Fringing reefs, barrier reefs, or atoll

Period of time investigated:

Mainly the last 200 years

How does it works ?

What we usually picture as a coral is actually a colony of tiny living animals called coral polyps, which are closely related to jellyfish or anemones. They live in symbiosis with photosynthetic algae called Zooxanthellae (Figure 1).

Figure 1: Schematic of a coral with its individual parts (modified from Veron, 1986).

Each polyp secretes a skeleton made of aragonite -a form of calcium carbonate- whose chemical composition depends on ambient oceanic and climatic conditions. Coral skeletons can therefore serve as monitors of the past oceanic and climatic variability through time (Figure 2).

Figure 2: X-radiographs and coral images (modified from DeLong et al., 2011).

Corals are distributed in the tropical belt mostly in the central and western Pacific, the Indian Ocean, and the Caribbean. These areas are also the most affected by climate variability such as the El Niño Southern Oscillation (ENSO) phenomenon. At interannual time scale, this phenomenon influences worldwide patterns of sea surface temperature (SST). Our present understanding of ENSO variability is limited by the short duration of instrumental records. In the current context of climate change, we need to understand the past variability of this phenomenon to be able to predict its future evolution. A proxy for past SST changes in the tropical oceans is therefore highly desirable to extend the length of the instrumental record.

Key Findings

Coral skeletal Sr/Ca have been shown to be an accurate tracer (“proxy”) of SST at many sites (Corrège, 2006). There is an inverse relationship between coral Sr/Ca values and SST conditions, with low Sr/Ca values corresponding to high SST environments and vice versa. Regression of coral Sr/Ca to instrumental SST (Figure 3) leads to a calibration equation that allows reconstruction of SST variability further back in time. SST records that span at least the last 200 years allow to differentiate the contributions of natural climate variability from those that are anthropogenically forced (Solomon et al., 2011). These results place coral as a perfect tool to reconstruct past oceanic variability which leads to a better understanding of past climate variability and a tremendously useful record to help predict future changes.

Figure 3: Time series of Sr/Ca from a living coral from New Caledonia and local SST (left). Calibration of Sr/Ca vs. SST. Sr/Ca appears to be a robust SST tracer (right).

Further readings

Corrège, T. (2006), Sea surface temperature and salinity reconstruction from coral geochemical tracers, Palaeogeography, Palaeoclimatology, Palaeoecology, 232(2-4), 408-428, doi:10.1016/j.palaeo.2005.10.014.

DeLong, K. L., J. A. Flannery, C. R. Maupin, R. Z. Poore, and T. M. Quinn (2011), A coral Sr/Ca calibration and replication study of two massive corals from the Gulf of Mexico, Palaeogeography, Palaeoclimatology, Palaeoecology, 307, 117–128, doi:10.1016/j.palaeo.2011.05.005.

Solomon A, et al. (2011), Distinguishing the roles of natural and anthropogenically forced decadal climate variability: Implications for prediction. Bull Am Meteorol Soc, 92:141–156.

Veron, J.E.N. (1986), Corals of Australia and the Indo-Pacific. Angus and Robertson:London/Sidney.


Edited by Caroline Jacques and Célia Sapart

Defrosting the freezer. Climate change and glacial meltwater

Defrosting the freezer. Climate change and glacial meltwater

 Why are glaciers important?

Glaciers cover around 10% of the global land surface. This includes the large ice sheets (e.g. in Greenland and Antarctica) as well as smaller ice caps and valley glaciers (e.g. in Iceland, Norway and New Zealand). Figure 1 shows the current distribution of glaciers around the world.

Figure 1 – The global distribution of glaciers around the world from the GLIMS glacier database. Source:


Glaciers play an important role in moderating global and local climate, but they are very sensitive to changes in climatic conditions. Currently, around 90% of the world’s glaciers are retreating. Under current IPCC predictions of future global warming and climatic changes, many glaciers will have disappeared by 2100. Figure 2 shows the temperature for different parts of the globe in 20167 relative to average (‘normal’) values. Red and yellow colours mean that temperatures are hotter than usual, and it is clear that most of the world is warming. The Arctic is warming especially quickly, and is several degrees (°C) warmer than normal. Glaciers here will therefore be especially sensitive to climate change.

Figure 2 – Global average (mean) surface temperature January-June 2016 relative to long-term conditions. Red and yellow colours indicate higher temperatures than normal. Source:


Glaciers contain around 75% of the world’s freshwater. Many of the world’s rivers are fed by meltwater from glaciers and mountain snowpacks. These include major rivers such as the Ganges and Brahmaputra, where meltwater from Himalayan glaciers and snow makes its way downstream and, together with river water from other sources such as monsoon rains, eventually supplies over 1 billion people.



What are the key issues?

As climate change continues, and global air temperature rise leads to enhanced glacier melt, there are a number of key considerations:

How will glaciers respond to climate change? – Will they disappear?

How will glacier melt affect water flow downstream?

How quickly might these changes happen?


How will glacier melt affect river systems?

Here we consider some of the impacts of glacier retreat on river flow, but there are also many other impacts, including: changes to river water chemistry, and impacts on ecosystems – the plants and animals living in and around the rivers

  1. Turning on the tap

Increased glacier melt produces more meltwater, which means that rivers will have a higher flow and more water will be transported downstream. However, this situation is likely to last only temporarily, because…

2. Turning off the tap 

Eventually (usually over several decades or longer), if a glacier melts fully, there will be no meltwater feeding into rivers downstream. Some rivers, that are fed by water from multiple sources (such as rainfall) do not rely on glacial meltwater and will not be greatly impacted by the disappearance of glaciers in their headwaters. Other rivers, especially those in mountain catchments, are supplied only by snow and ice melt. The disappearance of glaciers would therefore have major impacts on their water supply – the equivalent of turning off a tap. We know that many glaciers are melting rapidly, and some are predicted to have disappeared over the next few decades.

3. Changing lanes 

In some places, as a glacier retreats, the meltwater streams may change course entirely and flow in a different direction. This has been seen recently in Alaska, where meltwater from the Kaskawulsh glacier has undergone a major transformation in its drainage pathway in the space of only four days. Meltwater previously flowed northwards, supplying the Slims River, but recent glacier retreat has caused a shift in the drainage pathway, and it is no longer favourable for the water to flow north, and the Slims has almost entirely disappeared. Instead, meltwater has been diverted towards the south to the Alsek river. This event has highlighted that major transformations in glaciers and river systems, in response to climate change, can happen in the blink of an eye. See a full news report on the changes here and the full research article here.

4. The four seasons

Climate change can also affect seasonality – the timing and duration of the seasons in a year. For example, with increased global warming, we might expect some parts of the planet to experience a longer warm season. Climate change might also affect the duration and intensity of precipitation (e.g. rain and snowfall) events and storminess. Changes in seasonality are already being felt in some parts of the world. In some parts of the Arctic, the Spring melt season, and therefore the onset of river flow, is starting earlier than it has done in the past. Such changes will influence when and in what quantities meltwater is transported downstream. Continued monitoring of climatic conditions, glacier and river behaviour will allow us to more fully understand the changes that are occurring in glacial environments in response to global temperature rise.


In summary

  • We know that global climate change is influencing glacier behaviour. Some glaciers are responding rapidly to climate change – over years and decades – and many will have melted completely by 2100.
  • As glaciers melt they produce more meltwater, which increases the flow of river systems downstream.
  • But if glaciers melt entirely, the meltwater ‘tap’ will be switched off. This may have major impacts on river systems that rely on meltwater inputs – such as in high mountain regions where meltwater is the dominant source of river water.
  • We have seen recently in Alaska, that glacier retreat can cause meltwater drainage to change direction in a matter of days.
  • Understanding glacier and river response to climate change is therefore key for our ability to prepare for future scenarios.


Helpful resources

The following links provide information, data, graphics, and videos about glaciers, glacier melt, meltwater, and climate change. There is something suitable for all age groups.

National Snow and Ice Data Centre


INTERACT Arctic Monitoring programmes

NASA Climate




Ostracods, the sentinels of past oceanic circulation

Ostracods, the sentinels of past oceanic circulation
Name of the proxy


Type of proxy

Paleoenvironment proxy


All types of aquatic environments but here we will focus on marine waters

Period of time investigated


How does it work?

Ostracoda are crustacean of millimetre size which have inhabited all types of marine environments from the Ordovician to today (e.g. Salas et al. 2007) and colonized continental water bodies during the Carboniferous (Bennett et al. 2012). They are characterised by their bivalve calcified carapace articulated dorsally which encloses and protects the soft parts and appendages of the animal (Figure 1). The majority of Ostracoda live on or in the sediments: they are consequently highly sensitive to their environment.

What are the key findings that have been done using this type of proxy?

Throughout their history, marine Ostracoda inhabiting deep seas had very different morphologies from the contemporary shallow water species: thin shells, long, hollow and delicate spines and no eye spots (although this point is discussed; Figure 2). Based on the study of sediments, associated organisms and analogies with modern-days Ostracoda, ostracodologists concluded that those animals developed in low energy environments ranging from 500 to 5000 m depth in connection with global ocean cold water supplied by ice-caps (Lethiers & Feist 1991). This discovery provided a unique window into the oceanic circulation through geological times and the existence of a cold deep-water layer. The presence and characteristics of these Ostracoda have been cornerstones in understanding that the thermohaline circulation has not been constant through the Phanerozoic but rather existed only during the Late Ordovician, the Carboniferous-Permian interval and from the Eocene to today (Benson 1975).

Figure 2. Simplified geological time scale with Eras and Periods of the Phanerozoic. On the right are reported some archetypal deep-sea Ostracoda from the literature (for all photos, scale bar is 100 µm). A: Processobairdia spinanterocerata Bless & Michel, 1987; B: Cristanaria katyae Crasquin-Soleau, 2008; C: Gencella taurensis Forel, work in progress; D: Pedicythere klothopetasi Yasuhara et al., 2009.

Today, this field of research is very active as Ostracoda are the only metazoans regularly fossilized in deep-sea sediments over an extremely long period of the history of Earth. Their long fossil record spanning 5 mass extinctions and periods of extreme climatic changes make them precious tools to unravel the response of deep-water ecosystems to past climatic changes and the rhythms of their recovery. The extreme sensitivity and history of these peculiar animals make them sentinels of deep-sea ecosystems facing ongoing global temperature increase and acidification of marine waters.

  • Bennett, C.E., Siveter, D.J., Davies, S.J., Williams, M., Wilkinson, I.P., Browne, M., Miller, C.G. 2012. Ostracods from freshwater and brackish environments of the Carboniferous of the Midland Valley of Scotland: the early colonisation of terrestrial water bodies. Geological Magazine, 149, 366-396.
  • Benson, R.H. 1975. The origin of the psychrosphere as recorded in change of deep sea Ostracode assemblages. Lethaia, 8, 69-83.
  • Bless, M.J.M., Michel, M.P. 1967. An ostracode fauna from the Upper Devonian of the Gildar-Monto region (NW Spain). Leidse Geologische Mededelingen, 39, 269-271.
  • Crasquin-Soleau, S., Carcione, L., Martini, R., 2008. Permian ostracods from the Lercara Formation (Middle Triassic to Carnian?, Sicily, Italy). Palaeontology, 51, 537-560.
  • Lethiers, F., Feist, R. 1991. Ostracodes, stratigraphie et bathymétrie du passage Dévonien–Carbonifère au Viséen Inférieur en Montagne Noire (France). Geobios, 24, 71-104.
  • Salas, M.J., Vannier, J., Williams, M. 2007. Early Ordovician Ostracods from Argentina: their bearing on the origin of Binodicope and Palaeocope clades. Journal of Paleontology, 81, 1384-1395.
  • Yasuhara, M., Okahashi, H., Cronin, T.M. 2009. Taxonomy of Quaternary Deep-Sea Ostracods from the Western North Atlantic Ocean. Palaeontology, 52, 879-931.

Written by Marie-Béatrice Forel

Edited by Célia Sapart and Caroline Jacques

Hot towns, summer in the city!

Hot towns, summer in the city!

Cities obviously experience a different climate than natural landscapes. Already in 1810 the British meteorologist Luke Howard documented that the air temperature in the city of London was several degrees higher than in its surroundings. This so called urban heat island has several causes. In general the relatively dark surfaces of asphalt and roofs absorb solar radiation very efficiently and this heat is stored in building material during the day. At night this heat is released to the atmosphere, which keeps the city warm. Moreover, heat produced by air conditioning, traffic and industry contributes substantially to a city’s heat load. With a decreased amount of vegetation, cities also lose the shade and cooling effect of trees (Figure 1).

Currently already over 50% of the world’s population is residing in urban areas and this number is foreseen to increase even further in the future. Moreover, climate model projections indicate that heat waves will occur more often in the future. Together these developments will make many citizens potentially vulnerable to  urban heat. Although a slight temperature increase might look appreciable at first glance, elevated temperatures affect human health, since hospital visits and mortality are enhanced in warm conditions (about 2% per degree Celsius, e.g. Hajat et al., 2002). In addition, labour productivity in warm periods is reduced, resulting in economic losses. E.g. for Australia this was estimated to be about $650 per capita (Zander et al, 2015) which is a substantial contribution to the national income. Also, the urban energy demand needed for heating purposes in winter and cooling in summer is governed by urban weather. Finally cities are vulnerable to flooding in case of extreme precipitation by peak showers, when the sewage system capacity is hampered. Hence how do cities manage urban heat and keep dry feet?

Behind the general picture on the urban heat island, several scientific questions do remain. E.g. what is the temperature variability within a city, and how can we monitor temperatures? Also, can we make special weather forecasts for cities? Monitoring urban weather and climate is challenging since traditional weather stations are not suitable for urban areas since they require undisturbed terrain. Crowdsourcing, i.e. the collection of weather data by citizens has now become popular. Many hobby meteorologists have installed weather stations at home, and distribute their data directly via several websites as,, and These crowdsourced observations were crucial in estimation of the urban heat island effect in the Netherlands, a west-European country with a mild maritime climate were little attention was paid to urban climate. On hot summer days the urban heat island was over 6 degrees (Steeneveld et al., 2011, Heusinkveld et al., 2014)!

The urban climate can be efficiently monitored by tricycle traverse measurements (Figure 2). Bikes are excellent modules to measure the urban climate, since a wide variety of urban morphology (vegetation cover, building design and material) can be explored, especially outside the main roads. And it is carbon free, all driven by electricity generated by solar panels. Moreover, the bike is appealing for the general public.

Figure 2: A cargo tricycle equipped with a weather station to measure temperature, humidity, wind speed, solar and thermal radiation (source: Wageningen University; design Bert Heusinkveld).

What do we learn from such bike traverses? Figure 3 shows the variety of temperatures observed during a heat wave in a mid-size town in the Netherlands. Obviously, local temperature differences at the end of the afternoon may reach up to 3.5 ºC in this case. In general the town centre is relatively warm, though more surprisingly the relatively young neighbourhoods at the city edges appear to be warm too. In these neighbourhoods the vegetation is young, resulting in limited shadowing and therefore efficient heat absorption in roads and building walls. Local cool spots appear in parks and at small lakes. Luckily, enough room to escape from the heat!

Figure 3: Air temperature observations in the mid-size town Wageningen (the Netherlands, August 2nd 2013, 17.00 local time) obtained from two bike traverses. Source land cover maps:

No weather station in your garden? We still catch you via your smartphone! Smartphone users with the OpenSignal App that is intended to monitor wireless network capacity provide as a by product the smartphone battery temperature. Recently it was discovered that the temperature of your smartphone battery follows the air temperature outdoor (Overeem et al., 2013). Inversely this means that if we know the smartphone battery temperature, we can estimated the outdoor temperature. This insight offers a high potential for recording urban temperatures in areas where observations are rather scarce. The open question remains whether spatial and temporal scales these observations are applicable. Is it possible to get a reliable temperature record for your neighbourhood via available smartphones. Also, will these modern Big Data techniques change the paradigm on performing traditional measurements?

Weather forecasts on TV and radio never focus on the detailed weather for cities, which is somewhat surprising since many human activities, human health and critical infrastructures depend on the city temperature. Since computer power has rapidly grown the last decades, and still is, weather forecast models have refined their grid spacing. With this refinement urban areas “become visible” for these models. On one hand this offers a great potential for city specific forecasts. On the other hand information about the urban morphology is needed to feed these weather forecast models. For example they need to know whether urban districts contain skyscrapers or just three-story residential areas, and for example how much vegetation is present. Thereto the World Urban Database and Portal Tool is set up in which local experts document their city. You are welcome to join to describe your city and inform our weather forecast models how your city looks like!

  • Heusinkveld, B.G., G.J. Steeneveld, L.W.A. van Hove, C.M.J. Jacobs, and A.A.M. Holtslag 2014: Spatial variability of the Rotterdam urban heat island as influenced by urban land use, J. Geophys. Res, 119, 677–692.
  • Overeem, A.,  J. C. R. Robinson,  H. Leijnse,  G. J. Steeneveld,  B. K. P. Horn, and  R. Uijlenhoet (2013), Crowdsourcing urban air temperatures from smartphone battery temperatures, Geophys. Res. Lett., 40, 4081–4085, doi:10.1002/grl.50786
  • Steeneveld, G.J., S. Koopmans, B.G. Heusinkveld, L.W.A. van Hove, and A.A.M. Holtslag, 2011: Quantifying urban heat island effects and human comfort for cities of variable size and urban morphology in The Netherlands, J. Geophys. Res., 116, D20129, doi:10.1029/2011JD015988.
  • Zander, K.K., W.J.W. Botzen, E. Oppermann, T. Kjellstrom, S.T. Garnett, 2015: Heat stress causes substantial labour productivity loss in Australia, Nature Climate Change  5, 647–651.

Edited by Célia Sapart and Caroline Jacques