Choosing the future of Antarctica

In a new article in the journal Nature, Stephen Rintoul and colleagues present two very different visions of Antarctica’s future, from the perspective of an observer looking back from 2070. In one vision, humanity continues to exploit Earth’s natural resources (such as fossils fuels) and does little to protect the environment, and in the other, there is a global movement towards conservation. The article shows how Antarctica will change over the next 50 years, should either of these two situations occur.

Post by Jacob Bendle. Continue reading

Glacier response to climate change

Climate change on the Antarctic Peninsula | Glacier fluctuations on James Ross Island | Glacier modelling experiments | Summary and conclusions | Further reading |

Modelled glacier response to centennial temperature and precipitation trends on the Antarctic Peninsula

This article is a summary of the following paper:

Davies, B.J., Golledge, N.R., Glasser, N.F., Carrivick, J.L., Ligtenberg, S.R.M., Barrand, N.E., van den Broeke, M.R,. Hambrey, M.J., and Smellie, J.L., 2014. Modelled glacier response to centennial temperature and precipitation trends on the Antarctic Peninsula. Nature Climate Change. DOI: 10.1038/nclimate2369

Read the Press Release.

Climate change on the Antarctic Peninsula

It’s getting snowier on the Antarctic Peninsula. Glaciers are thickening in their accumulation centres, but are melting more and thinning in their lower portions. Snowfall is forecast to continue to increase over the next couple of decades, so what does this mean for glaciers on the Antarctic Peninsula? Will the increase in snow offset glacier melt?

We studied this phenomenon on James Ross Island, northern Antarctic Peninsula. Here, summer temperatures can reach above 0°C and glaciers are melting strongly. In 1995 AD, Prince Gustav Ice Shelf collapsed, which resulted in glacier acceleration, recession and thinning, which continues to this day.

We carried out extensive fieldwork on James Ross Island to map and analyse the changes to a glacier, which is currently 4km long, over the past 10,000 years. We used a combination of glacier and climate modelling, glacial geology and ice-core data. We found that small glaciers that end on land around the Antarctic Peninsula are highly vulnerable to slight changes in air temperature. Over the next few decades, they will be smaller than at any point during the last 10,000 years. Just small increases in air temperature increased melting so much that even large amounts of extra snowfall could not prevent glacier recession. These small glaciers around the Antarctic Peninsula are likely to contribute most to rising sea levels over the coming decades, because they can respond quickly to climate change.

James Ross Island on the NE Antarctic Peninsula. James Ross Isand ice core is denoted by a green star. The study area is Ulu Peninsula, denoted by the red box.

James Ross Island on the NE Antarctic Peninsula. James Ross Isand ice core is denoted by a green star. The study area is Ulu Peninsula, denoted by the red box.

Glacier fluctuations on James Ross Island

James Ross Island is well studied, being reasonably accessible by ship. There is a high resolution ice core, taken in 2007 AD, available from the summit of the Mount Haddington Ice Cap. The Ulu Peninsula is one of the largest ice-free areas in the region, which makes it ideal for glacial geological studies of past ice-sheet extent. Here, a large boulder train extends from Glacier IJR45 towards Brandy Bay, where it aligns with a large moraine. This moraine overlies reworked marine sediments with shells, which have been dated to ~5000 years ago. Because the shells are underneath the moraine, this suggests that the readvance that deposited the boulder train and the moraine occurred AFTER ~5000 years ago.

Ulu Peninsula on James Ross Island, showing modern glaciers, published areas, and glacial geomorphology. The reconstructed maximum Holocene extent is also shown. The model domain, line A-B, is shown in plan and profile view.

Ulu Peninsula on James Ross Island, showing modern glaciers, published areas, and glacial geomorphology. The reconstructed maximum Holocene extent is also shown. The model domain, line A-B, is shown in plan and profile view. Published radiocarbon (circles) and cosmogenic nuclide ages (diamonds) are shown.

The ice-core temperature records from Mulvaney et al. (2012) tell us that the air temperatures on James Ross Island 5000 years ago were about 0.5°C warmer than today. Prince Gustav Ice Shelf was absent from ~6000 to 2000 years ago, during this relatively warm period. Ice shelf absence is usually indicative of strong surface melt.

James Ross Island ice core temperature record over the last 14,000 years.

James Ross Island ice core temperature record over the last 14,000 years. Location is shown by a green star on the map of James Ross Island.

Warm air usually carries more water, so a readvance may have occurred because there may have been more precipitation at this time. People have therefore hypothesised that the glacier readvanced during a period that was WARMER but WETTER than today, in response to increased snowfall. This behaviour is contrary to that observed today, with glacier recession during a period of warming and ice-shelf collapse. This readvance is therefore a good analogue for future glacier behaviour in this sensitive region.

Glacier modelling experiments

We used a computer numerical model to test these hypotheses of glacier behaviour. The glacier flowline model was calibrated and tested using the temperature and precipitation ice-core record over the last 160 years. The glacier flowline model (along line A-B in the Ulu Peninsula map) was calibrated and tested using the temperature and precipitation ice-core record over the last 160 years. The modelled glacier replicated rates of recession from 1970 AD to present, and finished in 2006 with a similar geometry and velocity to the observed glacier in 2006 AD, thus increasing confidence in our model.

Results of the dynamic calibration. (a) air temperature and (b) precipitation from the ice core record. (c) Glacier length. (d) Glacier volume. (e) Glacier area.

Results of the dynamic calibration. (a) air temperature and (b) precipitation from the ice core record. (c) Glacier length. (d) Glacier volume. (e) Glacier area.

Sensitivity experiments

We conducted a number of sensitivity experiments on the glacier model, including precipitation (snowfall), temperature, how easily snow and ice melt (snow and ice degree day factors), and how much the glacier ice deforms and slides (flow enhancement coefficient, sliding factor). The results are shown in the plots below. You can see that for a small decrease in temperature there is a very large increase in glacier length and volume, but there are only small changes for a +20% to -20% change in precipitation, degree day factors or the ice deformation factor.

Sensitivity experiments for Glacier IJR45 on Ulu Peninsula, James Ross Island.

Sensitivity experiments for Glacier IJR45 on Ulu Peninsula, James Ross Island.

The profiles below show just how different the length changes are between temperature and precipitation sensitivity experiments.

Change in glacier profile following temperature and precipitation perturbations.

Change in glacier profile following temperature and precipitation perturbations.

Holocene simulations

The next stage was to use the Holocene climate record to drive the glacier through the last 10,000 years. Snowfall data are unavailable for this length of time. At first, we kept precipitation constant at modern values (0.65 m per year). This resulted in a fairly stable glacier for most of the Holocene, with a small retreat from 6000-2000 years ago, when Prince Gustav Ice Shelf was absent, and a large readvance out to the bay and Brandy Bay Moraine in the last millennium.

To test the hypothesis that it was a warmer but WETTER climate that forced a readvance 5000 years ago, we varied precipitation with temperature. We varied it by 5%, 7.3%, 15%, 20% and 100%, so that for every 1°C of warming, it became 5%, 7.3% (etc.) wetter. We were feeding the glacier during warm periods and starving it during cool periods.

Simulations of glacier length change over the last 10,000 years, driven by the James Ross Island ice core temperature record.

Simulations of glacier length change over the last 10,000 years, driven by the James Ross Island ice core temperature record.

You can see from the figure (b) above that all this did was successively dampen the glacier’s response to cooling and warming. The readvance occurred at the same time, but was smaller according to amount of precipitation was reduced by. You can see this happening in profile view in the animation below.

These modelling experiments show that the glaciers on Ulu Peninsula remained largely stable for most of the Holocene, with likely recession during the period of ice-shelf collapse (6000-2000 years ago), and a large readvance during climatic cooling 1500-300 years ago, when the ice-shelf reformed. The glacier then receded rapidly in response to warming temperatures. The ice shelf collapsed again in 1995 AD, and the glacier continues to retreat today. This modelled behaviour is therefore more in line with current glacier observations.

We surmise that the readvance occurred during the Neoglacial, or “Little Ice Age” period, and not during the warmer period ~5000 years ago. Evidence for this is patchy, and there are few terrestrial records of ice advance at this time. Our study is the first to convincingly show glacier advance during a period of strong cooling during the last millennium. This research also suggests that, rather than being more extensive during warm periods in the past, glacier minima similar to present have occurred multiple times during the last 10,000 years.

Future simulations

To assess the significance of these findings within the context of future climate change, we performed simulations from 1980 to 2200 AD, with climate outputs from regional climate models. All four scenarios used predict temperature rises over the region for the next two hundred years, but projections of snowfall vary.

All four experiments predicted a strong decrease in glacier volume over the next two centuries. Glacier volume reaches minima not experienced at any point in the last 10,000 years within the next century, and the glacier has almost disappeared after 200 years.

Simulations of air temperature (c), precipitation (d) and glacier volume (e) over the next 200 years.

Simulations of air temperature (c), precipitation (d) and glacier volume (e) over the next 200 years.

Summary and conclusions

Glacier IJR45 is typical of many of the small, land-terminating glaciers peripheral to the Antarctic Peninsula, where surface melting is strongly controlled by air temperature and the length of the melt season. Both of these are projected to increase over coming decades, and summer melting will become increasingly important. These glaciers will therefore contribute significantly to sea level rise over coming decades to centuries. These processes are likely to be representative of regional glaciers.

Our main conclusions are:

  • Glacier modelling, spanning a range of past, present and future time intervals, shows that Glacier IJR45 has a high sensitivity to temperature and is less sensitive to precipitation.
  • Glacier advance during past and future warm periods is unlikely. Increases in precipitation on the Antarctic Peninsula will not offset glacier melting and recession.
  • The most recent advance of the glacier likely occurred during the last millennium, peaking at ~300 years ago, during a period of strong cooling.
  • The currently observed trends of glacier melting, thinning and recession around the Antarctic Peninsula will continue over coming decades to centuries, with glacier minima not experienced at any time over the last 10,000 years occurring within the next century.

Citation:

Please read the original article and cite as:

Davies, B.J., Golledge, N.R., Glasser, N.F., Carrivick, J.L., Ligtenberg, S.R.M., Barrand, N.E., van den Broeke, M.R,. Hambrey, M.J., and Smellie, J.L., 2014. Modelled glacier response to centennial temperature and precipitation trends on the Antarctic Peninsula. Nature Climate Change. DOI: 10.1038/nclimate2369

Further reading

Coverage in the news

Ice core basics

Why use ice cores? | How do ice cores work? | Layers in the ice | Information from ice cores | Further reading | References | Comments |

Why use ice cores?

420,000 years of ice core data from Vostok, Antarctica research station. Current period is at left. From bottom to top: * Solar variation at 65°N due to en:Milankovitch cycles (connected to 18O). * 18O isotope of oxygen. * Levels of methane (CH4). * Relative temperature. * Levels of carbon dioxide (CO2). From top to bottom: * Levels of carbon dioxide (CO2). * Relative temperature. * Levels of methane (CH4). * 18O isotope of oxygen. * Solar variation at 65°N due to en:Milankovitch cycles (connected to 18O). Wikimedia Commons.

420,000 years of ice core data from Vostok, Antarctica research station. Current period is at right. From bottom to top: * Solar variation at 65°N due to en:Milankovitch cycles (connected to 18O). * 18O isotope of oxygen. * Levels of methane (CH4). * Relative temperature. * Levels of carbon dioxide (CO2). From top to bottom: * Levels of carbon dioxide (CO2). * Relative temperature. * Levels of methane (CH4). * 18O isotope of oxygen. * Solar variation at 65°N due to en:Milankovitch cycles (connected to 18O). Wikimedia Commons.

Ice sheets have one particularly special property. They allow us to go back in time and to sample accumulation, air temperature and air chemistry from another time[1]. Ice core records allow us to generate continuous reconstructions of past climate, going back at least 800,000 years[2]. By looking at past concentrations of greenhouse gasses in layers in ice cores, scientists can calculate how modern amounts of carbon dioxide and methane compare to those of the past, and, essentially, compare past concentrations of greenhouse gasses to temperature.

Ice coring has been around since the 1950s. Ice cores have been drilled in ice sheets worldwide, but notably in Greenland[3] and Antarctica[4, 5]. High rates of snow accumulation provide excellent time resolution, and bubbles in the ice core preserve actual samples of the world’s ancient atmosphere[6]. Through analysis of ice cores, scientists learn about glacial-interglacial cycles, changing atmospheric carbon dioxide levels, and climate stability over the last 10,000 years. Many ice cores have been drilled in Antarctica.

How do ice cores work?

This schematic cross section of an ice sheet shows an ideal drilling site at the centre of the polar plateau near the ice divide, with ice flowing away from the ice divide in all direction. From: Snowball Earth.

This schematic cross section of an ice sheet shows an ideal drilling site at the centre of the polar plateau near the ice divide, with ice flowing away from the ice divide in all direction. From: Snowball Earth.

The large Greenland and Antarctic ice sheets have huge, high plateaux where snow accumulates in an ordered fashion. Slow ice flow at the centre of these ice sheets (near the ice divide) means that the stratigraphy of the snow and ice is preserved. Drilling a vertical hole through this ice involves a serious effort involving many scientists and technicians, and usually involves a static field camp for a prolonged period of time.

Shallow ice cores (100-200 m long) are easier to collect and can cover up to a few hundred years of accumulation, depending on accumulation rates. Deeper cores require more equipment, and the borehole must be filled with drill fluid to keep it open. The drill fluid used is normally a petroleum-derived liquid like kerosene. It must have a suitable freezing point and viscosity. Collecting the deepest ice cores (up to 3000 m) requires a (semi)permanent scientific camp and a long, multi-year campaign[6].

Layers in the ice

If we want to reconstruct past air temperatures, one of the most critical parameters is the age of the ice being analysed. Fortunately, ice cores preserve annual layers, making it simple to date the ice. Seasonal differences in the snow properties create layers – just like rings in trees. Unfortunately, annual layers become harder to see deeper in the ice core. Other ways of dating ice cores include geochemisty, layers of ash (tephra), electrical conductivity, and using numerical flow models to understand age-depth relationships.

This 19 cm long of GISP2 ice core from 1855 m depth shows annual layers in the ice. This section contains 11 annual layers with summer layers (arrowed) sandwiched between darker winter layers. From the US National Oceanic and Atmospheric Administration, Wikimedia Commons.

This 19 cm long of GISP2 ice core from 1855 m depth shows annual layers in the ice. This section contains 11 annual layers with summer layers (arrowed) sandwiched between darker winter layers. From the US National Oceanic and Atmospheric Administration, Wikimedia Commons.

Although radiometric dating of ice cores has been difficult, Uranium has been used to date the Dome C ice core from Antarctica. Dust is present in ice cores, and it contains Uranium. The decay of 238U to 234U from dust in the ice matrix can be used to provide an additional core chronology[7].

Information from ice cores

Accumulation rate

The thickness of the annual layers in ice cores can be used to derive a precipitation rate (after correcting for thinning by glacier flow). Past precipitation rates are an important palaeoenvironmental indicator, often correlated to climate change, and it’s an essential parameter for many past climate studies or numerical glacier simulations.

Melt layers

Ice cores provide us with lots of information beyond bubbles of gas in the ice. For example, melt layers are related to summer temperatures. More melt layers indicate warmer summer air temperatures. Melt layers are formed when the surface snow melts, releasing water to percolate down through the snow pack. They form bubble-free ice layers, visible in the ice core. The distribution of melt layers through time is a function of the past climate, and has been used, for example, to show increased melting in the Twentieth Century around the NE Antarctic Peninsula[8].

Past air temperatures

It is possible to discern past air temperatures from ice cores. This can be related directly to concentrations of carbon dioxide, methane and other greenhouse gasses preserved in the ice. Snow precipitation over Antarctica is made mostly of H216O molecules (99.7%). There are also rarer stable isotopes: H218O (0.2%) and HD16O (0.03%) (D is Deuterium, or 2H)[9]. Isotopic concentrations are expressed in per mil δ units (δD and δ18O) with respect to Vienna Standard Mean Ocean Water (V-SMOW). Past precipitation can be used to reconstruct past palaeoclimatic temperatures. δD and δ18O is related to surface temperature at middle and high latitudes. The relationship is consistent and linear over Antarctica[9].

Snow falls over Antarctica and is slowly converted to ice. Stable isotopes of oxygen (Oxygen [16O, 18O] and hydrogen [D/H]) are trapped in the ice in ice cores. The stable isotopes are measured in ice through a mass spectrometer. Measuring changing concentrations of δD and δ18O through time in layers through an ice core provides a detailed record of temperature change, going back hundreds of thousands of years.

The figure above shows changes in ice temperature during the last several glacial-interglacial cycles and comparison to changes in global ice volume. The local temperature changes are from two sites in Antarctica and are derived from deuterium isotopic measurements. The bottom plot shows global ice volume derived from δ18O measurements on marine microfossils (benthic foraminifera) from a composite of globally distributed marine sediment cores. From Wikimedia Commons.

The figure above shows changes in ice temperature during the last several glacial-interglacial cycles and comparison to changes in global ice volume. The local temperature changes are from two sites in Antarctica and are derived from deuterium isotopic measurements. The bottom plot shows global ice volume derived from δ18O measurements on marine microfossils (benthic foraminifera) from a composite of globally distributed marine sediment cores. From Wikimedia Commons.

An example of using stable isotopes to reconstruct past air temperatures is a shallow ice core drilled in East Antarctica[10]. The presence of a “Little Ice Age”, a cooler period ending ~100 to 150 years ago, is contested in Antarctica. Disparate records often provide conflicting evidence. This ice core attempted to investigate the evidence for cooler temperatures during this period.

A 180 m deep ice core from the Ross Sea, Antarctica, was drilled by a team led by Nancy Bertler in 2001/2002[10]. The top 50 m of the ice core was analysed at 2.5 cm resolution using a continuous melting system. Ice core samples were analysed for stable isotope ratios, major ions and trace elements. An age model was extrapolated to the ice core using a firm decompaction model[10].  Deuterium data (δD) were used to reconstruct changes in summer temperature in the McMurdo Dry Valleys over the last 900 years. The study showed that there were three distinct periods: the Medieval Warm Period (1140 to 1287 AD), the Little Ice Age (1288 to 1807 AD) and the Modern Era (1808 to 2000 AD).

These data indicate that surface temperatures were around 2°C cooler during the Little Ice Age[10], with colder sea surface temperatures and possibly increased sea ice extent, stronger katabatic winds and decreased snow accumulation. The area was cooler and stormier.

Past greenhouse gasses

This photograph shows me (Bethan Davies) visiting Nancy Bertler and others in her ice core laboratory at GNS, New Zealand. The ice core is continuously melted and analysed by numerous automatic machines.

This photograph shows me (Bethan Davies) visiting Nancy Bertler and others in her ice core laboratory at GNS, New Zealand. The ice core is continuously melted and analysed by numerous automatic machines.

The most important property of ice cores is that they are a direct archive of past atmospheric gasses.  Air is trapped at the base of the firn layer, and when the compacted snow turns to ice, the air is trapped in bubbles. This transition normally occurs 50-100 m below the surface[6]. The offset between the age of the air and the age of the ice is accounted for with well-understood models of firn densification and gas trapping. The air bubbles are extracted by melting, crushing or grating the ice in a vacuum.

This method provides detailed records of carbon dioxide, methane and nitrous oxide going back over 650,000 years[6]. Ice core records globally agree on these levels, and they match instrumented measurements from the 1950s onwards, confirming their reliability. Carbon dioxide measurements from older ice in Greenland is less reliable, as meltwater layers have elevated carbon dioxide (CO2 is highly soluble in water). Older records of carbon dioxide are therefore best taken from Antarctic ice cores.

Other complexities in ice core science include thermal diffusion. Prior to becoming trapped in ice, air diffuses to the surface and back. There are two important fractionation processes: thermal diffusion and gravitational settling[11]. Thermal diffusion occurs if the surface is warmer or colder than the bottom boundary (the close-off depth). This temperature gradient occurs from climate change, which affects the surface first. The heavier components of the air (like stable isotopes) also tend to settle down (gravitational settling).

Thermal diffusion and gravitational settling can be measured and analysed because the fractionation of air follows well understood principles and relationships between different stable isotopes (namely, nitrogen and argon).

Other gasses

Other major gases trapped in ice cores (O2, N2 and Ar) are also interesting. The stable isotope concentration (δ18O) in ice core records mirrors that of the ocean. Oceanic δ18O is related to global ice volume. Variations of δ18O in O2 in ice core gasses are constant globally, making it a useful chronostratigraphic marker. It’s another way to relate ice-core chronologies.

Other ice-core uses

The vertical profile of an ice core gives information on the past surface temperature at that location[6].   In Greenland, glass shard layers from volcanic eruptions (tephra) are preserved in ice cores. The tephra ejected in each volcanic eruption has a unique geochemical signature, and large eruptions projecting tephra high into the atmosphere results in a very wide distribution of ash. These tephra layers are therefore independent maker horizons; geochemically identical tephra in two different ice cores indicate a time-synchronous event. They both relate to a single volcanic eruption. Tephra is therefore essential for correlating between ice cores, peat bogs, marine sediment cores, and anywhere else where tephra is preserved[12, 13].

Changes in sea ice concentrations can also be reconstructed from polar ice cores[14]. Ice core records of sea salt concentration reveal patterns of sea ice extent over longer (glacial-interglacial) timescales. Methane sulphonic acid in near-coastal ice cores can be used to reconstruct changes and interannual variability in ice cores.

Mineral dust accumulates in ice cores, and changing concentrations of dust and the source (provenance) of the dust can be used to estimate changes in atmospheric circulation[15]. The two EPICA ice cores (European Project for Ice Coring in Antarctica) contain a mineral dust flux record, showing dust emission changes from the dust source (glacial Patagonia). Changes in the dust emission is related to environmental changes in Patagonia.

Further reading

References


1.            Jouzel, J. and V. Masson-Delmotte, 2010. Deep ice cores: the need for going back in time. Quaternary Science Reviews, 29(27): 3683-3689.

2.            Augustin, L., C. Barbante, P.R.F. Barnes, J.M. Barnola, M. Bigler, E. Castellano, O. Cattani, J. Chappellaz, D. DahlJensen, B. Delmonte, G. Dreyfus, G. Durand, S. Falourd, H. Fischer, J. Fluckiger, M.E. Hansson, P. Huybrechts, R. Jugie, S.J. Johnsen, J. Jouzel, P. Kaufmann, J. Kipfstuhl, F. Lambert, V.Y. Lipenkov, G.V.C. Littot, A. Longinelli, R. Lorrain, V. Maggi, V. Masson-Delmotte, H. Miller, R. Mulvaney, J. Oerlemans, H. Oerter, G. Orombelli, F. Parrenin, D.A. Peel, J.R. Petit, D. Raynaud, C. Ritz, U. Ruth, J. Schwander, U. Siegenthaler, R. Souchez, B. Stauffer, J.P. Steffensen, B. Stenni, T.F. Stocker, I.E. Tabacco, R. Udisti, R.S.W. van de Wal, M. van den Broeke, J. Weiss, F. Wilhelms, J.G. Winther, E.W. Wolff, M. Zucchelli, and E.C. Members, 2004. Eight glacial cycles from an Antarctic ice core. Nature, 429(6992): 623-628.

3.            Johnsen, S.J., D. Dahl-Jensen, N. Gundestrup, J.P. Steffensen, H.B. Clausen, H. Miller, V. Masson-Delmotte, A.E. Sveinbjornsdottir, and J. White, 2001. Oxygen isotope and palaeotemperature records from six Greenland ice-core stations: Camp Century, Dye-3, GRIP, GISP2, Renland, and NorthGRIP. Journal of Quaternary Science, 16: 299-307.

4.            Mulvaney, R., N.J. Abram, R.C.A. Hindmarsh, C. Arrowsmith, L. Fleet, J. Triest, L.C. Sime, O. Alemany, and S. Foord, 2012. Recent Antarctic Peninsula warming relative to Holocene climate and ice-shelf history. Nature, 489: 141-144.

5.            Lambert, F., B. Delmonte, J.-R. Petit, M. Bigler, P.R. Kaufmann, M.A. Hutterli, T.F. Stocker, U. Ruth, J.r.P. Steffensen, and V. Maggi, 2008. Dust-climate couplings over the past 800,000 years from the EPICA Dome C ice core. Nature, 452(7187): 616-619.

6.            Brook, E.J., ICE CORE METHODS | Overview, in Encyclopedia of Quaternary Science, A.E. Scott, Editor. 2007, Elsevier: Oxford. 1145-1156.

7.            Aciego, S., B. Bourdon, J. Schwander, H. Baur, and A. Forieri, 2011. Toward a radiometric ice clock: uranium ages of the Dome C ice core. Quaternary Science Reviews, 30(19): 2389-2397.

8.            Abram, N.J., R. Mulvaney, E.W. Wolff, J. Triest, S. Kipfstuhl, L.D. Trusel, F. Vimeux, L. Fleet, and C. Arrowsmith, 2013. Acceleration of snow melt in an Antarctic Peninsula ice core during the twentieth century. Nature Geosci, advance online publication.

9.            Jouzel, J. and V. Masson-Delmotte, ICE CORE RECORDS | Antarctic Stable Isotopes, in Encyclopedia of Quaternary Science, A.E. Scott, Editor. 2007, Elsevier: Oxford. 1242-1250.

10.          Bertler, N.A.N., P.A. Mayewski, and L. Carter, 2011. Cold conditions in Antarctica during the Little Ice Age – implications for abrupt climate change mechanisms. Earth and Planetary Science Letters, 308: 41-51.

11.          Grachev, A.M., ICE CORE RECORDS | Thermal Diffusion Paleotemperature Records, in Encyclopedia of Quaternary Science, A.E. Scott, Editor. 2007, Elsevier: Oxford. 1280-1284.

12.          Abbott, P.M. and S.M. Davies, 2012. Volcanism and the Greenland ice-cores: the tephra record. Earth-Science Reviews, 115(3): 173-191.

13.          Hoek, W., Z. Yu, and J.J. Lowe, 2008. INTegration of Ice-core, MArine, and TErrestrial records (INTIMATE): refining the record of the Last Glacial – Interglacial Transition. Quaternary Science Reviews, 27(1): 1-5.

14.          Abram, N.J., E.W. Wolff, and M.A.J. Curran, 2013. A review of sea ice proxy information from polar ice cores. Quaternary Science Reviews, 79(0): 168-183.

15.          Fischer, H., F. Fundel, U. Ruth, B. Twarloh, A. Wegner, R. Udisti, S. Becagli, E. Castellano, A. Morganti, and M. Severi, 2007. Reconstruction of millennial changes in dust emission, transport and regional sea ice coverage using the deep EPICA ice cores from the Atlantic and Indian Ocean sector of Antarctica. Earth and Planetary Science Letters, 260(1): 340-354.

Climate Change

What is climate? | Quaternary climates | Is our climate changing? | The Role of Carbon Dioxide | Climate change in Antarctica | Observed impacts of climate change | References | Comments |

What is climate?

First, it is important here to note the differences between weather and climate. Weather is local and what you can see out of your window; weather is the cold British winter snows in 2011 and our 2012 wet and rainy summer (these are to do with short term variations in winds and atmospheric pressures), or heat waves in the USA in July 2012.

Climate is much broader scale; we are looking at long term statistical patterns in weather. An individual flood or storm is not caused by climate change, but climate change may make extreme weather events (like floods) occur more frequently. “Climate” encompasses data such as temperature, humidity, pressure, wind, precipitation (snow or rain), and other meteorological measurements.

This page is intended to be a short introduction, and by no means covers all of the vast topic of climate change. For more information, see: New Scientist; IPCC; Met Office.

Quaternary Climates

Throughout the last 2.6 million years (the “Quaternary Period”), the earth’s climate has oscillated many times, swinging between glacial and interglacial states (Figure 1). Over the last ~1 million years, we have experienced large ice ages and interglacials with a periodicity of around 100,000 years.

Figure 1. 5 million years of climate change – Global Warming Art Project. Wikimedia Commons

We are currently in an interglacial state, which began at the start of the Holocene, ~11,500 years ago. About 104 stages of these cold and temperate cycles have been recognised in deep ocean marine sediment cores (Figure 1) (Lowe and Walker, 1997). During glacials, large ice sheets developed in mid- to high-latitudes, including over Britain and North America.

Repeated climate oscillations

These large changes are driven by changes in the earth’s orbit around the sun – see The Quaternary Period (Table 1) (Jouzel et al., 2007). Glacials and interglacials can be further divided into stadials and interstadials, and within these we have smaller scale Dansgaard-Oeschger cycles, and then even smaller cycles, such as El-Nino and ENSO.

Climate data is therefore very noisy, and climate scientists must determine patterns in this data using complex statistical techniques. Throughout this time, carbon dioxide has mirrored temperature variations, which have formed a regular pattern.

Table 1. Geological timescale

Climate variations are a natural part of the earth system. It is therefore important, using instrumental records and proxies (such as ice cores, or microfossils in marine sediment cores), to compare current trends with those in the past (Bentley, 2010).

Is our climate changing?

Recent Change

The Earth warmed by an average of 0.6 ± 0.2°C during the twentieth century (Houghton et al., 2001). Rapid warming has been measured with global instrumental data since the 1800s (Figure 2). In 2019, the global average temperature was 0.95°C above the twentieth Century average, making it the second warmest year on record. The five warmest years have all occurred since 2015, and nine of the 10 warmest years since 2010 (Climate.Gov, accessed August 19, 2020).

Figure 2. This image shows the instrumental record of global average w:temperatures as compiled by the w:NASA’s w:Goddard Institute for Space Studies. (2006) “Global temperature change”. Proc. Natl. Acad. Sci. 103: 14288-14293. Following the common practice of the w:IPCC, the zero on this figure is the mean temperature from 1961-1990. This figure was originally prepared by Robert A. Rohde from publicly available data and is incorporated into the Global Warming Art project. Wikimedia Commons.

The 2001 IPCC report stated that most of this warming was likely to have been due to an increase in greenhouse gas emissions. Later IPCC reports have stated this with ever increasing confidence. However, this average rate hides considerable variations in the rate and magnitude of warming.

Regional Rapid Warming

Climate change varies seasonally, on decadal timescales, and is geographically patchy (Mann et al., 2008). Three areas in particular have been subject to recent regional rapid warming (sensu Vaughan et al., 2003), with rates of warming far faster than the average noted in the IPCC. These regions are: north-western North America, the Siberian Plateau in northeast Asia, and the Antarctic Peninsula and Bellingshausen Sea. These areas warmed by more than 1.5°C between 1950 and 2000 AD  (compared with a global mean of 0.5°C) (Mann et al., 2008).

More populated regions have atmospheric sulphate aerosols, that may mask warming. Urban meteorological stations may also record anomalous warming due to urban heat island effects. However, the Antarctic continent is free of these effects. Although direct meteorological observations are short (~60 years), trends in these areas are particularly important.

The Longer Term View

Although we have measured changing temperatures and carbon dioxide levels at short timescales (since the 1700s), surely this could just reflect natural variability over short timescales? In order to understand our climate, it is very important to look at the long-term view.

In many places, atmospheric temperatures are now warmer than they have been throughout the Holocene. Figure 3 shows that the temperature of the last 200 years is much higher than it was when Romans were making wine in southern England. However, what is really concerning is the rate of change. Temperatures have risen far more sharply in the last century than they have at any point in the last 2000 years.

2000 year temperature variations. Robert A. Rhode, Global Warming Art Project. Wikimedia Commons.
Figure 3. 2000 year temperature variations. Robert A. Rhode, Global Warming Art Project. Wikimedia Commons.

This conclusion is reached again and again, and a paper by Mann et al. (2008) in PNAS show that the Earth’s temperature is anomalous in a long-term time context. Mann et al. used stacked records from a variety of sources to create a global graph of temperature change. This famous ‘hockey stick’ graph (Figure 4) clearly demonstrates the rapid rate of change since the industrial revolution, with temperature rise sharply accelerating over the last 150 years.

From Mann et al., 2008. Original caption: Composite CPS and EIV NH land and land plus ocean temperature reconstructions and estimated 95% confidence intervals. Shown for comparison are published NH reconstructions, centered to have the same mean as the overlapping segment of the CRU instrumental NH land surface temperature record 1850–2006 that, with the exception of the borehole-based reconstructions, have been scaled to have the same decadal variance as the CRU series during the overlap interval (alternative scaling approaches for attempting to match the amplitude of signal in the reconstructed and instrumental series are examined in SI Text). All series have been smoothed with a 40-year low-pass filter as in ref 33. Confidence intervals have been reduced to account for smoothing.
Figure 4. From Mann et al., 2008. Temperature change over the last 2000 years. Composite land plus ocean temperature reconstructions and estimated 95% confidence intervals.

The Pages2k team have compiled global temperature records over the last 2000 years (Pages 2k Team, Nature Geoscience 12, 643-649). This reconstruction comes from many different kinds of proxy records, including tree rings, cave deposits, corals, etc.

Ed Hawkins (website: Climate Lab Book) has visualised the Pages2k climate record for the years 0 AD to 2019 AD using #warmingstripes (Figure 5). The colours highlight the recent very rapid warming.

Figure 5. PAGES 2k global temperature reconstruction, for the last 2019 years. By Ed Hawkins

The uncertainty estimates mean that you can also view the dataset in a more traditional way. The Medieval Warm Period and Little Ice Age are clearly shown, and the highly unusual recent rapid warming. The Medieval Warm Period and Little Ice Age are relatively small phenomena compared with the recent rapid warming.

Figure 6. PAGES 2k global temperature reconstruction. Image by Ed Hawkins.

Climate change over the last 11,300 years

A paper out in Science by Marcott et al. (2013) extends Figures 3 and 4 back to the last 11,300 years, with the Earth’s temperatures now warmer than the Earth was 4000 years ago (Figure 7). During the last 200 years, temperatures have rocketed up, confirming earlier reconstructions by Mann et al. 2008.

Figure 5. Climate change over the last 11,500 years from multiple proxies. From Marcott et al., 2013
Figure 7. Climate change over the last 11,500 years from multiple proxies. From Marcott et al., 2013

Although the Earth has not yet exceeded the temperatures of the Early Holocene (5000 to 11,000 years ago), global temperatures have risen from cooler than 95% of the Holocene at around 1900 to warmer than 72% of the Holocene in the last 100 years (Figure 7).

This means that, in the last 100 years, the Earth’s temperature has reversed a long-term cooling trend that began around 5000 years ago to become near the warmest temperatures during the last 11,000 years. Furthermore, climate models predict that the Earth’s temperature will exceed the warmest temperatures of the Holocene by 2100, regardless of which greenhouse gas emission scenario is used (Marcott et al., 2013).

However, Marcott et al. do note in their paper that the ‘uptick’ shown in the graph is not statistically robust, as the median resolution of all data is 120 years. However, this has been shown by other authors, such as Anderson et al. 2013, who demonstrate rapid warming over the last century from geological data.

Kaufman et al. (2020) provide an updated global temperature reconstruction, dating back to 12,000 years ago (Figure 8). They used a median from five different reconstruction methods, generating a consensus global mean surface temperature reconstruction. It follows the same approach as the PAGES 2k Consortium.

Figure 8. From Kaufman et al. 2020. Global mean temperature composites. The fine black line is instrumental data, 1900-2010. Inset shows the last 2000 years. Coloured lines are the medians of the five different reconstruction methods. Temperature anomalies are relative to 1800-1900.

Kaufman et al. (2020) found that the warmest period in the Holocene was centred at 6,500 years ago, and was 0.6°C warmer than the 1800-1900 reference period.

The decade of 2011-2019 averaged 1 °C higher than 1850–1900 (Kaufman et al., 2020). For 80% of the ensemble members, no 200 year interval in the last 12,000 years was warmer than the last decade. Temperatures projected in the rest of this century, and beyond, are very likely to exceed 1 °C above pre-industrial temperature (IPCC, 2013).

The temperatures reconstructed vary in the different latitudes and different hemispheres; the palaeoclimate of the Northern and Southern hemispheres is very different (Figure 9).

Figure 9. From Kaufman et al., 2020. Reconstructed mean annual temperatures from the Temperature 12k database using different reconstruction methods from the different latitude bands. Temperature anomalies are relative to 1800-1900.

Below, you can see the Kaufman et al. 2020 reconstruction, compared with other published proxy reconstructions over this time period. There are more proxy datasets from the Northern Hemisphere than the Southern Hemisphere.

Figure 10. From Kaufman et al., 2020. (a) multi-method median global surface temperature reconstruction compared with previous reconstructions. (b) locations of proxy data sites.

Going back to the Last Glacial Maximum

However, it doesn’t end there. Using more proxies to extend the temperature graph back to the last glacial and modelling scenarios from the IPCC for the next 100 years, the graph below (Figure 11) has been created. Dubbed “The Wheelchair”, it shows the current and future alarming rate of temperature increase, compared with temperature fluctuations over the last 20,000 years. This figure was created by Jos Hagelaars.

Global average temperatures from 20,000 BC up to 100 years into the future under a middle-ranking emissions scenario.
Figure 11. Global average temperatures from 20,000 BC up to 100 years into the future under a middle-ranking emissions scenario.

The role of carbon dioxide

Carbon dioxide is a key factor in climate change. Carbon dioxide concentrations fluctuate annually, as seen in Figure 12 from the longest direct measurements of CO2 from Mauna Loa in Hawaii.

Atmospheric carbon dioxide measurements at Mauna Loa. Published by the Global Warming Art Project and sourced from Wikimedia Commons.
Figure 12. Atmospheric carbon dioxide measurements at Mauna Loa. Published by the Global Warming Art Project and sourced from Wikimedia Commons.

You can explore the changing global levels of carbon dioxide, methane, the global temperature record over the last 2000 years and global mean sea level here.

Figure 13. Climate Levels is an interactive website where you can explore the levels of carbon dioxide, methane, temperature and mean sea level over the last 1000 years.

The seasonal fluctuation is caused by variations in uptake of carbon dioxide by land plants. However, looking at Figure 2 above, you can see that this is just part of a much longer record of carbon dioxide. The IPCC 2007 Synthesis Report stated:

“Global GHG emissions due to human activities have grown since pre-industrial times, with an increase of 70% between 1970 and 2004”.

IPCC 2007

These increases in carbon dioxide are primarily due to increased use of fossil fuels, but land use changes also provide a significant contribution. You can explore global energy production at the Global Electricity Review website.

Because humans are constantly adding more carbon dioxide to the atmosphere, we are changing its balance and affecting the Earth’s climate. The carbon dioxide and other atmospheric gasses in the atmosphere combine with changes in land cover and solar radiation to drive climate change. They affect the absorption, scattering and emission of radiation within the Earth’s atmosphere, resulting in a positive energy balance and warming influences on our climate.

The IPCC 2007 states that there is very high confidence that the world has warmed since 1750, and that the combined radiative forcing due to increases in carbon dioxide, methane and nitrous oxide is 2.3W/m2, and that the rate of increase during the industrial era is was unprecedented in the last 10,000 years.

Changes in solar irradiance since 1750 AD have caused a small radiative forcing of +0.12 W/m2.

But how has carbon dioxide changed over the last few thousand years?

Figure 14 shows the long-term temperature and carbon dioxide record from Antarctica. Carbon Dioxide (blue line) now approaches 400 parts per million, which is far higher than it has been at any point in the last 400,000 years.

420,000 years of ice core data from Vostok, Antarctica research station. Current period is at left. From bottom to top: * Solar variation at 65°N due to en:Milankovitch cycles (connected to 18O). * 18O isotope of oxygen. * Levels of methane (CH4). * Relative temperature. * Levels of carbon dioxide (CO2). From top to bottom: * Levels of carbon dioxide (CO2). * Relative temperature. * Levels of methane (CH4). * 18O isotope of oxygen. * Solar variation at 65°N due to en:Milankovitch cycles (connected to 18O). Wikimedia Commons.
Figure 14. 420,000 years of ice core data from Vostok, Antarctica research station. Current period is at left. From bottom to top: * Solar variation at 65°N due to en:Milankovitch cycles (connected to 18O). * 18O isotope of oxygen. * Levels of methane (CH4). * Relative temperature. * Levels of carbon dioxide (CO2). From top to bottom: * Levels of carbon dioxide (CO2). * Relative temperature. * Levels of methane (CH4). * 18O isotope of oxygen. * Solar variation at 65°N due to en:Milankovitch cycles (connected to 18O). Wikimedia Commons.

Methane (green line) is also higher than it has been before over this timescale. You can also see how closely temperature (red line) has tracked carbon dioxide and methane over this time. The temperature in Antarctica (red line) is expected to continue to rise with this increase in atmospheric greenhouse gases.

Climate change in Antarctica

Figure 15. Global climate change. Global climate change 2000-2009 and 1950-1980/Wikimedia Commons.

The western Antarctic Peninsula warmed by 2.5°C from 1950-2000 (Turner et al., 2005) (Figure 15). The mean annual temperature is rising, with the -9°C isotherm moving southwards, resulting in the collapse of several ice shelves (28,000 km (Turner et al., 2005) of ice shelf has been lost since the 1990s) (Morris and Vaughan, 2003). Climate records from the South Orkney islands suggest that warming began in earnest in the 1930s (Mann et al., 2008).

Southern Westerly Winds

This warming has been related to variations in the belts of upper atmosphere winds that encircle the Antarctic continent (the Circumpolar Vortex) (Figure 16).

Figure 16. Westerly Winds and ocean fronts around Antarctica

Periodic oscillations in this result in the periodic strengthening and weakening of this belt of winds. A strengthening of this vortex has been associated with changes in surface atmospheric pressures (van den Broeke et al., 2004).  This pressure pattern causes northerly flow anomalies, which in turn cause cooling over East Antarctica and warming over the Antarctic Peninsula.

Other factors contributing to the recent regional rapid warming over the Antarctic Peninsula include decreased sea ice in the Bellingshausen Sea, resulting in warmer air temperatures, and decreasing precipitation over the south western peninsula (van den Broeke and van Lipzig, 2004; van Lipzig et al 2004).

Finally, since the 1950s, the Antarctic Circumpolar Current has warmed by +0.2°C, with the warming greatest near the surface (Gille, 2008). Waters west of the Antarctic Peninsula have also warmed rapidly (Turner et al., 2005; Mayewski et al. 2009).

This video from NASA shows global warming since 1884, and you can really see the large and rapid warming around the Antarctic Peninsula over the last 50 years.

Observed impacts of climate change

The impacts of this recent regional rapid warming around the Antarctic Peninsula have been dramatic, with the collapse of ice shelves (Cook and Vaughan, 2010), and with 87% of glaciers in recession (Cook et al. 2005). The present-day ice loss from the Antarctic Peninsula is -41.5 giga-tonnes per year (Ivins et al. 2011). Ice-shelf tributary glaciers have become destabilised following ice-shelf collapse (Scambos et al. 2004; DeAngelis and Skvarca, 2003).

Other glaciers have thinned, accelerated and receded as a result of increased melting (see Recent Change) (Pritchard et al. 2009; Pritchard and Vaughan, 2007). The rapid shrinkage of glaciers around the Antarctic Peninsula, coupled with the potential for ice-shelf collapse and grounding line retreat, raises concerns for the future of the West Antarctic Ice Sheet, and this is an area of urgent current research (Bentley, 2010). This is covered in more detail under Marine Ice Sheet Instability.

Videos to watch

Abrupt Arctic Climate Change: Comparison of Today with Paleoclimate: Change Rates and Distribution. By Paul Beckworth

Further reading

Other websites

References


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Bentley, M.J., 2010. The Antarctic palaeo record and its role in improving predictions of future Antarctic Ice Sheet change. Journal of Quaternary Science, 2010. 25(1): p. 5-18.

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